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24rd EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 10 Apr 2011 - 14 Apr 2011
- Location: Charleston, USA
- Published: 10 April 2011
1 - 100 of 190 results
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Previous Excavations and Geophysical Discoveries at a Prehistoric Earthwork Site in Western Michigan
Authors Laura Sherrod, Jan Brashler and Donald GaffGeophysical surveying was performed during the summer of 2010 at the South Flats Earthwork Site (20MU2) located in Muskegon County, Michigan. This prehistoric earthwork site is a circular feature of raised earth material located in a present-day young oak forest. the enclosure is approximately 25-30m in diameter, has a width of 4m around the perimeter where the ground relief is altered approximately 0.5m. It is located within the Muskegon State Game Area on a bluff overlooking the Muskegon River. the site was first excavated by the Michigan archaeologist George Quimby in 1937 and has been only moderately disturbed by anthropogenic sources since that time. Recent excavation was performed in 2006 by a team of researchers from Grand Valley State University. This field work clarified the results and aided in the Interpretation of the records kept of the 1937 excavation. Additionally, the recent excavations increased overall knowledge of the role that earthwork structures played in the cultures of the time.
Due to the site location on State-owned land, excavation is restricted. Ground penetrating radar and magnetometer surveys were applied at this site to provide an image of the subsurface and facilitate further development of cultural Interpretations. Soil composition is predominantly sandy making ground penetrating radar a very effective geophysical tool. Previous excavation units and other recent anthropogenic disturbances are clearly visible in the geophysical results. the earthwork structure itself is identifiable through the ground penetrating radar images and several small-scale geophysical features aid in site Interpretation. Results from the surveys provide insight into the prehistoric use of earthworks structures and illustrate the complicated nature of interpreting geophysical surveys at previously excavated sites.
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Overview of the Workshop On Monitoring and Failure Detection in Earthen Embankments
Authors Craig Hickey and James SabatierThe Homeland Security Presidential Directive 7 of May 7, 2007, classified dams and levees as one of the 18 critical infrastructure and key resource sectors. these networks of dams and levees are vital to the security, national economic security, public health or safety of the United States. the U.S. National inventory of Dams currently lists more than 80,000 dams in the USA. Preliminary estimates by the National Committee on Levee Safety indicate there may be more than 100,00 miles of levees across the United States. Due to the extent of the embankment system and the high cost of failure, rapid, low-cost, highly reliable inspection and monitoring technologies are needed. A workshop was held at the US Army Corps of Engineers Waterways Experiment Station 9-12 February 2009 to explore the state-of-the-art in inspection and monitoring, identify technologies that might be applied in the near term, and to define a roadmap for research investment. in this presentation we will provide an overview of the results of the workshop. (Workshop sponsored by the US Army Research office and the USDA-ARS.)
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A Comprehensive validation of the Skytem System
Authors Esben Auken, Nikolaj Foged, Kurt Sorensen, Anders Vest Christiansen and Max HalkjaerAccuracy and stability of data from airborne electromagnetic systems is an issue addressed in numerous publications and conference presentations. We have recently done a comprehensive and well documented comparison of data from ground based measurements and repeated SkyTEM lines from different altitudes.
the ground based test line was made on the National Danish Test Site and consists in two perpendicular lines each of a length of 1.0 km. On each of these lines 25 soundings with a 40x40 m2 TEM system was made. the TEM system measures the first time gate in 8 micro sec from begin of ramp and was prior to the measurements calibrated at the test site.
Comparing Inversion results form the SkyTEM and the groundbased data shows that the model repeatability of the SkyTEM system is excellent in both the same altitude and in different altitudes. Also, there are no directional problems, i.e. it does not matter in which direction the lines are flown. the agreement to the groundbased reference sections is also very good, showing that the SkyTEM-system yields data of same high quality as the groundbased system.
A validation of the SkyTEM system at the data level was set up to examine the responses gate by gate, which, among other things, rules out the equivalent model issue. the different comparisons in the data space show that the SkyTEM system repeats the test lines equally good in different altitudes, and the agreement to the ground based responses is very good and in general within the data standard deviation of 5%.
the validation serves as a quality assurance of the high-precision modeling of the SkyTEM system, together with the robust processing and Inversion scheme developed for the data. the validation of the SkyTEM system now set the standard for the data quality expected for any airborne system to qualify for the Danish national groundwater mapping effort.
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Application of an Artificial Neural Network for Airborne Magnetic Data Discrimination
Authors Jeannie Norton, Les Beard and Jacob SheehanArtificial neural networks (ANN) are computational constructs that attempt to mimic the workings of the human brain with respect to the brain’s ability to detect patterns. ANNs can be trained to determine which class an object belongs to based on selected inputs. the objective of this work was to develop and apply an artificial neural network to discriminate ordnance from non-ordnance based on input derived from airborne magnetic data. A training set was constructed for the ANN from existing data acquired over known items: ordnance, exploded fragments, and non-ordnance. the goal was to be able to predict whether the source of a magnetic anomaly was produced by ordnance. the output classes fell into one of two categories. Either the item of interest was unexploded ordnance (UXO) or it was not unexploded ordnance (non-UXO). This latter category included geology, fragments from exploded ordnance, and non-ordnance items. the low-altitude magnetic data were inverted for source parameters (depth, magnetic moment, and orientation) to generate various parameters that served as input to the ANN. using this training data set, weighting coefficients were computed. the reliability of the ANN was verified and validated using a subset of data with known solutions that were not included in the original training set, and the ANN successfully reduced the number of false positives. in the original assessment of this dataset the analytic signal was to prioritize the anomalies and resulted in a ratio of false positives to true items of 42:75 at 50% target detection and 74:150 at 100% detection. using the ANN on this same dataset the ratio of false positives to true items was improved 5:75 at 50% detection and 68:150 at 100% detection. the use of this ANN will improve the efficiency of the airborne data through enhanced target picking and reduced sampling and excavation costs.
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New Generation TDR for Identifying and Characterizing Engineering Properties of Soils
Authors Vincent Drnevich, Majdi Najm and Sochan JungRecent work indicates that Time Domain Reflectometry (TDR), using signatures from currently available probes and equipment can be processed to identify soil type and determine water content and dry density. Several characteristics of the signatures in addition to the apparent dielectric constant and bulk electrical conductivity have been identified to support these determinations. the presentation will identify these characteristics, give the development of the process, and demonstrate their use for a variety of soils and compaction conditions. Implications will be provided for extension of the method to field applications for geotechnical/transportation construction control of soils.
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Characterizing Mississippi River Levee Segments using Soils and Geologic Data
Authors Khaled Hasan, James Aanstoos, Majid Mahrooghy, Lalitha Dabbiru and Joseph DunbarApproximately 150 miles of levees in the Lower Mississippi River valley bordering Mississippi, Arkansas and Louisiana are being studied to determine their vulnerability to slough slide failure and sand boil development. the study is designed to use Synthetic Aperture Radar (SAR) imagery from both aerial and satellite platforms as its primary investigative tool. Images have been acquired with NASA’s airborne UAVSAR imaging at L-band and DLR’s satellite borne TerraSAR-X imaging at X-band. Currently multi-faceted analysis of the acquired images is in progress by several of our team members and the results look promising. to aid the image analysis effort we are also looking at the physical processes of slides and sand boil formation which are highly influenced by subsurface movement of water which in turn is controlled by textural properties of the soil and sediments beneath. to characterize levee segments having unique soil and sediment condition with respect to water transmissivity, soil and geologic data have been collected from NRCS-USDA and USACE respectively. GIS analyses of the textural and geomorphic characteristics of these data have allowed classification of the levee into several zones where water transmission properties may make them more susceptible to failure than other areas. in these zones highly sandy soils on the riverside of levee allows water to move to the levee where clay rich soils restrict lateral flow, creating conditions where the water could move upward to weaken the levee segment. A majority of the known slide events occur in these zones, which supports this inference. the spatial distribution of these slide events also suggests that geomorphic features such as ridge and swale and ox-bow lakes tend to have a geographic association with these events. Vegetation patterns indicative of repaired slides derived from TerraSAR-X images also coincide with these zones based on soil criteria.
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Restoring a National Treasure: Recovering Airborne Geophysical Data from Antiquated Data Tapes for Geosurv Iraq
Authors David Smith, Benjamin Drenth, Jared Abraham and Janan SaffoThe State Company of Geological Survey and Mining (GEOSURV) is the ministry in Iraq responsible for the development of natural and mineral resources. As part of a Department of Defense initiative by the Task force for Stabilization and Business Operations in Iraq, the U.S. Geological Survey partnered with GEOSURV scientists to modernize GEOSURV’s geophysical capabilities. the objectives of the geophysical component were two-fold: train GEOSURV personnel in the use of state-of-the-art data processing techniques; and recover digital data from a 35-year old tape archive so that GEOSURV can use it to guide their exploration program. This talk focuses on the data recovery effort.
A national aeromagnetic and radiometric survey was flown in 1974 by Compagnie General de Geophysique (CGG). the data were of excellent quality, as evident from the detailed paper contour maps that were originally produced. A comparable survey today would cost millions of dollars. the digital data were stored on 9-track magnetic tapes. Over time, as computing equipment evolved, GEOSURV lost the capability to access the data on the tapes. GEOSURV agreed to convey the tape archive to the USGS for possible data recovery. We worked closely with a company near Denver, Colorado that specializes in reading old magnetic tapes using legacy equipment, in this case a 1970’s vintage Raytheon mainframe computer with original Kennedy tape drives. After a lengthy cleaning and conditioning process, the company managed to read Information from the tapes in the original EBCDIC format. the USGS converted the data to standard ASCII and painstakingly corrected innumerable data errors caused by drop-outs and bit-flips, achieving in the end 99.999 percent recovery. the data were imported into Geosoft Oasis montaj for filtering, gridding, and mapping. the final total field anomaly map is superior to the original magnetic field contour maps, as it reveals structural features not previously detected. GEOSURV can now access the CGG survey data and utilize it to benefit the economic development of Iraq. the money saved by not re-surveying the entire country can be redirected for field Investigations of targets of economic importance.
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Detecting Graves in a Lime Marl Environment: A Comparison of Soil Resistivity and Ground Penetrating Radar Methods
Locating unmarked graves is frequently necessary in older cemeteries that have poor or no records. Such is the case for a cemetery located on the fort Hood Army Base. the base is located in central Texas and is home to more than 2,000 archeological sites of varying ages. the goal of our Investigation was to determine if unmarked graves were present at a particular site. the Tools utilized to reach this goal were a soil resistivity meter and a Ground Penetrating Radar (GPR) system. the existing headstones indicate the cemetery was active at least from 1871 through 1964. However, it is suspected that the cemetery was used prior to 1871. Soils at the site consist of thin loam overlying limey clay. the Walnut formation (Cretaceous) crops out at the site and consists of lime marl with layers of fossiliferous limestone. the survey area was approximately sixty meters by fifty meters. Resistivity measurements were made using the Wenner array with an A-spacing of one meter and a transect spacing of two meters. the GPR system utilized a 270 MHz antenna with half meter transect spacing. Resistivity data indentified areas believed to be disturbed, but not at a scale useful for locating specific graves. GPR successfully located eighty-four percent of the marked graves. GPR also identified nine anomalies consistent with graves that were not marked. Data indicates that specific graves are not identifiable in a lime marl environment using soil resistivity alone. the use of GPR greatly enhanced the ability to identify individual graves, both marked and unmarked.
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Combining Tomography and Reverse Time Migration for Improved Near Surface Seismic Imaging
Authors Nedra Bonal and Lewis BartelSeismic traveltime Tomography is often used to attain velocity structure images but cannot resolve complex structures due to smoothing requirements in processing. Furthermore, imaging at depth may be restricted due to limited raypath density below a reflecting boundary. Consequently, imaging lateral discontinuities (e.g. steep dips, buried boulders) or beneath seismically reflective boundaries common in near surface environments can be difficult. Additionally, reflected signals of interest are regularly combined with direct arrivals so removing direct arrivals to observe only reflected signals is not possible. Reversed-time migration (RTM) uses the complete wave equation, making it possibly more accurate than the ray approximation Kirchhoff migration algorithm frequently used in Tomography. Furthermore, RTM can process arrivals from multiple waves and steep slopes. Though computationally expensive, multi-core machines commonly used today can process small data sets typical of near surface projects. We present an example of imaging below a seismically reflective boundary using Tomography followed by RTM. Tomography results show reflective layers at 10 and 17 meters below the ground surface. Most P-wave energy is reflected from these layers with little to no energy reflected from the deeper perched aquifer of interest. the Tomography results are used as the starting point for the high-quality velocity model required to produce an accurate image from the RTM method. the resulting RTM image has strong reflectors between 60-70 meters deep, which correspond with expected depth and range of the perched aquifer. Additionally, there appears to be a break in the continuity at a distance of x = 75 meters for depths in excess of 55 meters. This break may be related to the connection of the perched aquifer to the regional groundwater system, which is expected in this area. the case presented shows this two-step approach results in imaging complex structures and deeper depths than Tomography alone.
Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
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Monitoring Tree Roots over Time with GPR
By John ButnorFor over ten years, ground-penetrating radar (GPR) has been used in a variety of applications to detect tree roots, quantify their spatial distribution, and to estimate root diameter and biomass. Most surveys are performed once, offering a point-in-time assessment of root parameters. My own approach has been to link a small destructive sample using soil cores to GPR data collection, allowing direct comparison of target and acquired signal. All collection parameters (range, gain, dielectric, etc.) are held constant, since the data are collected over the course of a day or two, there is minimal temporal variation. for measures spread out over a longer period of time, differences in soil moisture, water usage, and drainage patterns will likely play havoc with root resolution and parameter estimation, making time series comparisons (months to years) problematic unless such environmental “noise” can be accounted for.
to better understand seasonal variation in root detection, several 20 m transects were established in a 25 year-old balsam fir (Abies balsamea) plantation in South Burlington, Vermont. the transects were located on a deep sandy loam and were scanned 20 times with a 1500 MHz antenna between March and November 2010 using identical settings. A reference transect was created by horizontally inserting 8 mm aluminum rods into undisturbed soil at several depths to accurately measure signal velocity and variation with depth. Soil moisture ranged from 7% to 32% creating dielectric values from 6 to 23. After processing to reduce clutter and isolate root biomass, the radargrams were analyzed with image analysis to quantify the area occupied by roots. Preliminary results are encouraging; GPR data (area) under one tree were compared to soil moisture and dielectric values for 20 different days. Though variation in GPR data was large (order of magnitude) over the season, it was exponentially related to both soil moisture (R2 0.48) and soil dielectric (R2 0.81).
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Seismic Refraction Shootout: Using Ray-Based Tomographic Inversion
Authors Nedra Bonal and Leiph PrestonRay-based Tomographic Inversion is a common technique used to process first-arrival travel times into structural images of the earth. We propose to use a 3-dimensional ray-based travel-time Tomographic Inversion program to obtain a velocity model from the synthetic data for the Seismic Refraction Shootout session at the SAGEEP 2011 meeting. Our code uses a ray-tracing scheme to calculate travel times for the forward problem and an iterative inverse procedure to find incremental changes in model parameters that optimally fit travel-time residuals (observed minus calculated) subject to model smoothness constraints. We first calculate the ray paths by following the finite-difference (eikonal) travel-time gradient from source to receiver to compute travel time residuals and fill the matrix of partial derivatives. We then solve for perturbations in slowness by conjugate gradient least squares. Due to regularization requirements in Tomographic processing, resolution of the data is important to consider especially for complex structures. Resolution tests (checkerboard tests) will be conducted on the data to aide in the Interpretation of the results of the tomogram. Standard error analysis tests (jackknife tests) will also performed on the Tomography model to obtain error estimates.
Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
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The Development of a 2-Dimensional Microtremor Survey Method Based on Spac Method using Sequential Linear Arrays
Authors Takaho Kita, Koichi Hayashi and Haldun BingolThe demand to quickly and economically obtain S-wave velocity profiles in geotechnical and seismic engineering fields has been increasing recently. in order to satisfy this need, many types of surface wave methods have been developed and have become widespread. It is, however, not easy to find survey methods which rapidly provide cross sections of the shear-wave velocities at depths of greater than about 30m. in order to overcome this limitation, we have developed a new 2-dimensional microtremor survey method based on the SPAC method using sequential linear arrays. We call this survey method “2-Dimensional Linear Array Microtremor Survey (2D-LAMS) Method”. We applied this new method to field surveys and evaluated the results. Consequently, we confirmed that the cross sections of the shear-wave velocities obtained by this new method were consistent with the cross sections obtained by such conventional geophysical survey methods as 2D-MASW, seismic reflection surveys and electrical resistivity surveys. the 2-Dimensional Linear Array Microtremor Survey Method has advantages in terms of the depths of Investigation in comparison with MASW. in addition, this method is less susceptible to ambient noises when compared with the other conventional survey methods.
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Nuclear Magnetic Resonance (NMR) Properties of Unconsolidated and Unsorted Materials Under Low-Field and Mid-Field Conditions
Authors Kathryn Martin, Ryan North, Jason McKenna and Yaoguo LiFailing dams and levees experience changes in porosity, permeability, and water content. Surface nuclear magnetic resonance (sNMR) may allow direct and noninvasive detection of water distribution in these dams and levees. in particular, the T2 response is dependent upon such hydrologic parameters as porosity and permeability that in turn affect water content. the properties of sNMR make it a potentially good technique for monitoring dams and levees. However, a better understanding of the NMR response to the subsurface under the conditions associated with leaky dams and levees is required to fully develop this approach. Though NMR measurements are routinely performed on unsaturated materials in the petroleum industry, these measurements are typically used to detect the locations of water or hydrocarbons in well sorted consolidated rock. Also, most laboratory measurements performed to date utilize a magnetic field that is much higher than the earth’s field, which is used for sNMR. Thus, there is a need to characterize the NMR response of materials typically used in dam and levee construction under the low-field condition and variable water saturations. A typical earthen dam is constructed of an impermeable clay core covered with a less impermeable layer of gravel, sand, and clay sized particles. Due to a lack of sorting and the wide distribution of grain sizes these sediments are typically unsaturated. We have carried out a series of laboratory sample measurements using a low-field and a mid-field performed on unsaturated, unsorted, and unconsolidated samples that are analogous to the permeable layer of dams so that we can better understand the sNMR responses under the conditions of a leaky dam. using spherical glass beads we prepared samples of varying porosities and permeabilities. We then performed both T2 and T2* lab measurements on the saturated samples using the low-field lab equipment. We then performed T2 measurements using mid-field laboratory equipment. these measurements were inverted to estimate the T2 time constantdistribution. Parallel to the saturated measurements, we repeated the same set of measurements on the gravity drained samples thereby simulating a naturally occurring condition of partial saturation. in this talk, we will present preliminary results obtained from these measurements.
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Assessment of the Cooperative Source Concept for Single Target Classification using EM63 Metal Detector
Authors Alfonso Benavides and Mark E. EverettThe objective is to assess UXO risk reduction capabilities of a new cooperative source concept. the basic requirement is to develop cost-effective technologies to discriminate munitions of various types from the wide range of non-hazardous items buried in the ground. Classification techniques based on diagnostic target attributes such as principal axis polarizabilities have made significant progress in recent years. However significant limitations remain due to the limited Information content on available sensors. Current development of EMI devices is based on adding transmitters or receivers. Our approach makes use of Geonics EM63 metal detector and a cooperative source. Cooperative source is an object of regular shape and known composition that operates synergistically with a conventional EMI transmitter to illuminate a buried target from a wide range of azimuth positions. the method is based on the mutual inductive coupling between the cooperative source and a buried target after both have been illuminated by the transmitter. Data obtained consists of EMI transients as a function of the cooperative source position. Datasets are dimension-reduced by extracting feature vectors based on the moments of the EMI transients. the feature vectors are loaded into an unsupervised statistical classifier, which in our case is based on a merge self—organizing map (MSOM). Upon output, the primary target is classified as either a target—of—interest (toI), warranting further attention, or else an item not of interest (non—toI) requiring no further attention. Preliminary results show that mutual coupling between cooperative source and target is measurable and strongly dependent on the geometry and orientation of the target, resulting in extraordinarily rich dataset. Unsupervised MSOM learning shows feature vectors clustering from which a-posteriori toI classification can be performed.
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Geophysical Investigations at a Potential Mass Grave Site in Bethlehem, Pa
Authors Charles Messler, Laura Sherrod and James HigginsDuring the winter of 1918-19 there was an outbreak of Spanish influenza in Bethlehem, PA. the city was based around the steel industry and had seen a large surge in production and therefore population during the First World War. the outbreak is documented to have claimed nearly two hundred lives, although the accuracy of this number has been questioned by some. Those hardest hit were the poor working class. This group of immigrant workers had few or no relatives living in the area, and thus no one available to make funeral arrangements. the task was appointed to the city, or done through the church of the deceased. Records from this time period are incomplete, but there is evidence that suggests a mass burial during the epidemic on South Mountain in Bethlehem. Geophysical surveying (GPR and magnetometer) was performed in the summer of 2010 at St. Michael’s Cemetery located in South Bethlehem, Lehigh County Pennsylvania. the section of the cemetery surveyed is 19m by 13m and located on the south end of the cemetery. the land was used by St. Michael’s Roman Catholic Church from the mid 19th through the early 20th centuries. the area surveyed is of particular interest because of the time period in which it was used. Grave markers show that a large number of burials took place during the Spanish influenza Outbreak of 1918-19. the terrain is sloped and marked by many physical obstructions. Due to vandalism, topography, and vegetation many of the grave markers are broken, shifted, or completely missing. A three dimensional dataset was created to image the subsurface burials.
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Monitoring the Spatial and Temporal Evolution of the Self-Potential Field Produced By Pumping in a Fractured Aquifer
Authors Aaron DesRoches and Karl ButlerSelf potential (SP) signals associated with groundwater flow through a fractured aquifer system were continuously monitored on the surface around a pumping well in the hydrogeologically complex Springdale well field, near Sussex, New Brunswick, Canada. the objective was to investigate whether anticipated azimuthal anisotropy in groundwater flow would be reflected in the SP anomaly expected to develop on surface. SP data were measured over a 40 by 70 m area using an irregular array of 34 non-polarizing Pb-PbCl2 electrodes referenced to a remote electrode placed 170 m away. SP data were logged at 3 second intervals using a high input impedance (20 Gohms) datalogger connected to the electrodes through a custom-built 36-channel passive filter box used to attenuate noise at frequencies higher than ~ 1 Hz. the pumping well operated for 300 minutes at a flow rate of 8 L/sec, generating drawdowns of 1.4 m in the pumping well and 1.0 m in a monitoring well 27 m away. SP increased monotonically during pumping with clear anomaly amplitudes ranging from ~ 1 to 8 mV amongst the 34 electrodes. Linear regression of the drawdown in the monitoring well versus SP of an adjacent electrode provides an apparent streaming potential correlation coefficient of -3.36 mV/m used to calculate the approximate drawdown over the field of the SP array. This drawdown approximation assumes a homogeneous and isotropic aquifer. Time-lapse contour maps illustrate the progression and intensification of the SP field over the study area, with signal variations greatest to the northwest through to the southwest; unlike a radially symmetric pattern one would expect in an isotropic, homogeneous aquifer. This study has revealed that SP signals generated during pumping tests have the potential to rapidly identify variations in the self-potential field associated with groundwater flow and aquifer drawdown.
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Classification of UXO By Principal Dipole Polarizability
By Karl KapplerData acquired by multiple-transmitter, multiple-receiver time-domain electromagnetic instruments show great potential for determining the geometry and composition of near surface conductive targets. Here is presented analysis of data from one such system; the Berkeley Unexploded-ordnance Discriminator (BUD) system. BUD multi-static data matrices data are succinctly reduced to obtain magnetic dipole polarizability tensors for each time gate. When viewed over all time gates, the projections of the data onto the principal polar axes yield so-called polarizability curves. these curves are especially well suited to discriminating anomalies corresponding to objects of rotational symmetry from those of irregularly shaped objects. the curves have previously been successfully employed as library elements in a pattern recognition scheme aimed at discriminating harmless scrap metal from dangerous intact unexploded ordnance. However, previous polarizability-curve matching methods have only been applied at field sites which are known a priori to be contaminated by a single type of ordnance, and furthermore, the particular ordnance present in the subsurface was known to be large. Thus signal amplitude was a key element in the discrimination process. the work presented here applies feature-based pattern classification techniques to BUD field data where more than 20 categories of object are present, spanning a broad range of sizes. Data from a calibration grid at the Yuma, AZ proving ground are used in a cross validation study to calibrate the pattern recognition method. the resultant method is then applied to the Yuma Blind Test Grid data. Results indicate that when lone UXO are present and SNR is reasonably high, Polarizability Curve Matching successfully discriminates UXO from scrap metal when a broad range of objects are present.
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Intergrated Well-Log, VSP, and Surface Seismic Analysis of Near-Surface Glacial Sediments: Red Lodge, Montana
Authors Jingqiu Huang, Robert Stewart, Joe Wong and Carlos MontanaWe conducted a series of geophysical surveys to characterize a glacial bench deposit and underlying strata near Red Lodge, Montana. Well logs and VSP data were acquired in a PVC-cased, 115m deep borehole. the multi-offset VSP was undertaken using surface sources (an accelerated weight drop and sledge hammer) with a hydrophone string and downhole, wall-clamping, 3-component geophone. the well logs included measurements of conductivity, radioactivity (gamma ray), temperature, and sonic velocity. Sonic and VSP velocities range from 1500m/s in the very near surface to 3000m/s at 85m depth. A distinct black clay (with high conductivity, high gamma ray, and low velocity) is penetrated at 85m. High-resolution 2D and 3D seismic surveys, using a sledge hammer source, show a number of reflectors to about 150ms two-way traveltime. An 80ms seismic reflection correlates to the 85m interface (via the composite or L-plot with well log, synthetic seismograms, and VSP processed to a corridor stack). Various other reflections in the VSP and surface seismic data are interpreted to represent glacial deposit layers and water zones (from the perforation logs).
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High Resolution Eri Evaluation of injectate used to Remediate a Dry Cleaning Site, Jackson, Tennessee
Authors Todd Halihan, Stuart McDonald and Phil PateyA former dry cleaning site in Jackson, Tennessee has undergone remediation to treat TCE and PCE impacted soil beneath the site. the dry cleaning operation closed in 1977. in 2002, a series of Injections were made at the site including corn syrup, vegetable oils, and Simple Green®. in 2004, approximately 200 cubic yards of soil was excavated and the bottom of the excavation covered with sodium lactate. in 2009, the site was characterized using high resolution electrical resistivity imaging (ERI) and follow up confirmation soil borings that targeted anomalies detected via the geophysical work. the results indicate an extremely conductive (< 1 ohm-m) vadose zone downgradient from the Injection wells, and extremely resistive areas (> 10,000 ohm-m) near the Injection area. Sample data indicate that the electrically resistive zones in the subsurface contain moderate to high concentrations of undegraded dry cleaning compounds, and the electrically conductive areas are believed to represent zones of degradation. Specifically, due to the extreme conductivity values detected, the chemical composition, and the dominant vadose zone location of the conductive zones, these are interpreted as likely areas of biological activity generated by the injected amendments to the site.
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MRS Tests in Eastern Canada - Coping with Magnetite Dissemination
From late 1990s to early 2000s several MRS (Magnetic Resonance Sounding) missions in Europe and Southern Africa have shown the appropriateness of the MRS technique for groundwater studies. However MRS tests in Eastern Canada in 2003, did not show significant NMR responses over shallow, saturated, clean sand targets. Upon examination, it was found that disseminated magnetite was likely the cause of high magnetic gradients at the scale of water containing pores. This was later confirmed at the Rice U. NMR lab (Hirasaki, personal communication 2004).
the 2003 tests were documented in a paper with a recommendation to focus on T2 rather than T2*. Thanks to IRD, a spin echo mode of MRS survey was implemented on a light version MRS tool: NUMISLITE. This implementation was field tested in August 2008 over most of the sites tested in 2003. Field NMR spin echo responses were observed over these sites. All the tested sites represented shallow aquifers.
in the mean time numerical modeling of the field-scale spin echo mode was developed at IRD, Grenoble, France.
in a subsequent MRS mission - summer 2010 - a full power MRS tool (NUMISPLUS) was used to demonstrate that no spurious surface effects were observed during the 2008 tests. Sites in the same geological environment, with magnetite dissemination, were systematically investigated with aquifers at least 10 m below surface. Also a key site made in 2008 was repeated to insure instrumental integrity of the procedure. Clear NMR spin echo responses were observed at all the tested sites.
As a side issue, field data acquisition was also modified to allow acquiring MRS data at locations where the Larmor frequency is an harmonic of the local power line frequency.
All in all, the MRS technique is gradually improving toward a maturity status.
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Long Term ERT Monitoring of Remediation at an LNAPL Site, Enid, Ok
Authors Shannon Jeffries and Todd HalihanMonitoring of the subsurface changes that take place during contamination remediation is difficult. One method that allows for stable long term monitoring is Electrical Resistivity Tomography (ERT). A contaminated site in Enid, Oklahoma, a saturated sand aquifer with overlying unsaturated clay, was evaluated to determine the distribution and migration of LNAPL. Fifteen direct push boreholes were drilled at ten meter spacing and 27 electrodes were grouted into place at a vertical spacing of 1.5 feet (0.46 meters) giving a total depth of about 40.2 feet (12.26 meters). Data were collected prior to remediation, during remediation and eight years after remediation began. the time lapse data shows the difference in resistivity values from one data set to the next and allows for the determination of background resistivity values and the movement of the contamination across the site. the data indicate that the LNAPL can be quantitatively evaluated and that the degradation over time appears to impact the unsaturated clay layer more strongly than the saturated sand. the clay layer is becoming more conductive with time suggestive of biological impacts in the clay.
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Time-Lapse Seismic Tomography of a Small Embankment Dam with Possible Zones of Weakness.
Authors Leti Teklu Wodajo, Craig Hickey, Gregory J. Hanson and Chung R. SongEarthen dam or levee failure can occur with little warning. Internal erosion, seepage and piping are among the major causes of failure in earthen embankment dams and levees. these Internal problems can only be detected visually after the damage has progressed significantly. Identifying such problems at an early stage requires a method of imaging the progressive change in the Internal structure of the dam. Seismic velocities depend on the elastic properties of the material; therefore, time-lapse seismic measurements have promise for monitoring changes in the Internal dam structure caused by piping, seepage or erosion. in this paper, results of time-lapse seismic measurements conducted on two small earthen embankments will be discussed. these dams were constructed at the USDA-ARS Hydraulic Engineering Research Unit; Stillwater, OK having two artificially built weak zones susceptible to Internal erosion. Seismic measurements were conducted on these dams at different stages from the start of construction up to failure. Time-lapse surveying of the experimental dams will provide insight on how the changes in the distribution of elastic properties manifest in the seismic Tomography. Comparing seismic Tomography images, an area of increased P-wave or lowered S-wave velocity could be used as an indication of possible seepage or wet area. While the increase of the P-wave velocity is due to the incompressibility of the water, the slight decrease in the S-wave velocity is associated with the increase in total density. A lower P and S wave velocity correlates with the weakening of the surrounding soil structure due to void formation caused by the erosion of fine materials from the dam. [This research was funded by the department of Homeland Security- sponsored Southeast Region Research initiative (SERRI) at the Department of Energy’s Oak Ridge National Laboratory.]
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Application of Geophysics for Environmental Characterization, Remedial Design, and Remedial Implementation at Three Fractured Bedrock Sites Impacted by Chlorinated Solvents in New England
Authors Peter Thompson, Kerry Tull and Scott CalkinOver the last sixteen years, geophysical methods have been used at multiple sites to characterize fractured bedrock impacted with chlorinated solvents and dense non-aqueous phase liquids (DNAPLs). Borehole geophysical data is essential to develop accurate site conceptual models (CSM) for bedrock contaminant source areas and plumes. these data must be integrated with other hydrophysical and chemical data in order for the CSM to support remedial design decisions. the importance of this integrated characterization approach lies in the recognition that at many older DNAPL impacted fractured bedrock sites, the original DNAPL mass has been transferred from the fracture network to the rock matrix through DNAPL dissolution and matrix diffusion. Three case studies are presented to illustrate how geophysical data including 1) borehole geophysics (caliper, temperature, electric logs, acoustic televiewer (ATV), digital borehole image processing systems (BIPs), heat pulse flow meter (HPFM) logging), 2) interwell test data (HPFM interference testing, electrical resistivity Tomography (ERT)), and 3) surface methods (azimuthal resistivity) have been successfully integrated with other primary data, including: straddle packer groundwater data, methanol extracted rock matrix data, conservative interwell tracer tests (CITT), and partitioning interwell tracer tests (PITT). the platform for the CSM is a dynamic three dimensional (3-D) visualization tool that allows a comprehensive Interpretation approach by Integrating these independent, multi-characteristic data. the visualizations relate the distribution of contaminants in specific fractures and rock matrix porosity to the hydraulic characteristics of the fracture network.
the three sites to be presented include two sites in Maine and one in Massachusetts. Geophysical data in one of these case studies was used to support regional scale fractured flow contaminant fate and transport models and two Technical Impracticality (TI) waiver evaluations. the geophysical data at the other two sites supported detailed source area characterizations for evaluating and implementing remedial actions supported by tracer studies.
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Determination of the Near-Surface Structure using Multimodal Ground-Roll Inversion
Authors Soumya Roy and Robert StewartThe near-surface often consists of unconsolidated, low-velocity layers which can cause time delays in passing seismic waves, especially S-waves. the high resolution near-surface S-wave velocity structure can be useful for the S-wave static corrections (adjustments required for those time delays) in multicomponent seismic analysis. the near-surface S-wave velocity structure can be obtained using the surface-wave Inversion method. in this paper, the Multichannel Analysis of Surface Waves (MASW) method has been used. This method uses the dispersion properties of ground-roll (Rayleigh wave) to create dispersion curves (phase velocity versus frequency plots). then, the Inversion of these dispersion curves for the fundamental (and/or multi) modes provide the near-surface S-wave velocity structure. A detailed study of the near-surface S-wave velocity structure has been performed using seismic data sets acquired at La Marque Geophysical Observatory, Texas. the site is flat and undisturbed by cultural activity. the sediments are from the Quaternary Beaumont formation and consist of clay and silty clay. Different source-receiver combinations have been used. for receivers, planted vertical geophones provide better results and better separation of modes compared to vertical land streamer. for sources, accelerated weight drop provides greater depth of Investigation compared to 10 lb sledgehammer. the near-surface S-wave velocity structure (obtained using only fundamental mode) varies 100-300 m/s for top 17 m and going up to 410 m/s at 23.5 m. One of the recent advances is the use of higher modes which have higher velocities than fundamental mode at a particular frequency. Thus, they can ‘see’ deeper for that particular frequency. Also, as higher modes appear more at higher frequencies, they can resolve the shallow near-surface better. Improvements in velocity resolution for first few meters have been observed when Inversion for multiple modes has been performed. An overall increase in the depth of Investigation (up to 5 m) and velocity (up to 50 m/s) have also been observed. the near-surface stratigraphy of the area obtained from a hydraulic conductivity study shows an embedded silty clay layer at about 5 m depth within the surrounding clay formation. This bed is better resolved when multimodal Inversion is applied.
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Estimation of Near-Surface Quality Factors By Constrained Inversion of Rayleigh-Wave Attenuation Coefficients
Authors Jianghai Xia, Richard Miller, Julian Ivanov and Shelby PeterieQuality factors of near-surface materials are as important as velocities of the materials in many applications. High-frequency (≥ 2 Hz) surface-wave data are generally inverted to determine near-surface shear (S)-wave velocities, in which only phase Information of surface-wave data is utilized. Amplitude Information of high-frequency surface-wave data can be used to determine quality factors of near-surface materials. Given S-wave velocity, compressional (P)-wave velocity, and Rayleigh-wave phase velocities, it is feasible to solve for S-wave quality factor QS and P-wave quality factor QP (for some specific velocity models) by inverting high-frequency Rayleigh-wave attenuation coefficients in a layered earth model down to 30 meters below the ground surface in many settings. Amplitude is an exponential function of attenuation coefficients. When calculating attenuation coefficients from changes in amplitude, this nonlinear nature would result in that small variations in amplitude cause huge changes in attenuation coefficients, which suggests data (attenuation coefficients) normally possess large errors that eventually transfer to a model (quality factors). therefore, constraints (a priori Information) on models become necessary. Because an Inversion system of this problem itself is unstable, a regularization parameter must be introduced into an Inversion algorithm to stabilize Inversion procedure. these characteristics of the Inversion problem lead us to solve a constrained and regularized linear system. Usually, a set of models that meet the constraints can be obtained by solving the system. Based on the linear nature of the Inversion system, we can pick a smooth model as a solution of the Inversion using the L-curve method, which is a trade-off solution between data misfit and model length. Several real-world examples demonstrate importance of constraints in finding acceptable quality factors from real data.
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Application of an Airborne HEM System on a Freshwater Lake
Authors David Smith, Maria Deszcz-Pan and Bruce SmithAirborne electromagnetic systems are superior to ground-based geophysical techniques in many respects. they afford rapid reconnaissance of large areas for producing images of earth electrical resistivity as two-dimensional maps and as pseudosections. Systems today are powerful, stable and precise. Adapting such a system for ground-based operation can provide increased sensitivity to the subtle variations in the resistivity of the ground and increase lateral resolution. We analyze the limits on a helicopter electromagnetic (HEM) system owned by the USGS using forward model analysis, and we present the results of a proof-of-principle test on Lake of the Arbuckles in Chicksaw National Recreation Area in south-central Oklahoma. the HEM system was mounted on a floating platform and towed behind a motorboat which carried the data acquisition electronics and a bathymeter.
Preliminary results show that the waterborne system is capable of mapping geology under shallow cover of water.
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Practical 3D Inversion of AEM Data for Environmental Applications in Complex Regolith Settings
Authors Glenn Wilson, Timothy Munday, Andrew Fitzpatrick, Leif Cox and Michael ZhdanovThis paper presents the application of an innovative approach to 3D Inversion of airborne electromagnetic (AEM) data which renders the routine Inversion of entire surveys realistic from the computational perspective. the method uses a moving footprint, and is based on the 3D integral equation method for computing data and sensitivities, and the re-weighted regularized conjugate gradient method for minimizing the objective functional. Specifically, we examine results from several well constrained AEM surveys that target the geometry, distribution and water quality associated with palaeochannel aquifers which are developed in contrasting, variably complex regolith settings. the TEMPEST and SkyTEM surveys targeted anastomozing coarse sands and gravels beds in a sandy clay background. the surveys also targeted the abruptly changing and varying basement (due to changes in lithology and structure) which defines the base of the alluvial aquifer and/or the weathered regolith. Results from the 3D Inversions are compared against drill hole geological and other geophysical data. the integrated Interpretations are able to define the location and groundwater salinity within the palaeochannels. As a result of the surveys and drilling, local farm water and irrigation systems are being developed.
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MRS as a Groundwater Investigation tool
By Jean RoyDuring the past 75 years, indirect measurements, e.g. surface geophysical techniques, contributed to groundwater Investigations. Such non-invasive techniques permit lowering costs and delays in acquiring subsurface Information while preventing invasion or interconnection between hydrostratigraphic units. However, at results exploitation step, some ambiguity is noticed. During the successive developments of the different indirect techniques, two goals have been targeted: (1) lower this level of ambiguity and (2) supply results directly useful for groundwater resource characterization. MRS (Magnetic Resonance Sounding) is a significant step in this direction.
MRS is NMR-based using the Earth's magnetic field as static field and provides its own excitation field which, contrary to usual laboratory case, is not an RF field but an audio-frequency field i.e. the hydrogen nuclei (1H+) Larmor frequency. the excitation field moment is used as sounding parameter i.e. as the controlled parameter for depth discrimination.
MRS supplies, as a function of depth, the in-situ water content and the NMR signal decay rate. This decay rate is linked to pore-size and therefore to the investigated medium water flow properties. Below the water table, the MRS-supplied water content is roughly equivalent to effective porosity under suitable conditions; bound water is usually not quantified with current generation MRS Tools. As any NMR technique, MRS is sensitive to the static field homogeneity which is the topic of another contribution.
MRS 'delivers the goods' on a commercial basis since 1996 in most places. It performs non-invasively down to about 150 m in a resistive medium; this bottom limit is reduced as the medium gets more electrically conductive.
the support of ITC and the collaboration of the following institutions and their personnel are greatly appreciated: AGEOS, BRGM, CSIR-Envirotek, DWA-B, DWA-N, École Polytechnique, GSD GSN, IGM, inRS-ETE, IRD, IRIS, MBG, QUALITAS, UQAC, UQAT, WCS, WRC.
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The Use of Microtremors to Map Subsurface Structure of Shear Wave Seismic Impedance Boundaries in Unconsolidated Overburden: Examples from Eastern Canada
Authors James Hunter, André Pugin, Susan Pullan, Didier Perret, Karl Butler, Heather Crow and Gregory BrooksIn overburden areas of the Eastern Canada, shear wave seismic impedance boundaries between soft and firm soils can exceed 20:1; such boundaries are commonly associated with Holocene post-glacial sediments overlying Pleistocene glacial sediments. in some locations, where post-glacial sediments overlie firm bedrock the impedance contrasts can be even larger. in all such cases horizontal-to-vertical spectral ratios (HVSRs) of microtremor ambient seismic noise yield narrow well-defined spectral peaks associated with the fundamental resonant frequency of these sites as governed by the thickness and shear wave velocity-depth function of the soft upper layer. Microtremor instruments using broad-band (0.2 – 60 Hz) 3-component velocity sensors have been utilized in a reconnaissance mode to estimate thethickness of soft soil at survey areas in Eastern Canada where groundwater Investigations or earthquake seismic vulnerability studies are being conducted. Many of these studies have been “ground-truthed” with multi-component Landstreamer reflection seismic lines or with shear wave refraction/reflection/downhole site Investigations. Regional site period-versus-thickness curves have been developed for isopach mapping and cross-section construction. Examples of microtremor applications include detection and delineation of buried valleys in soft Champlain Sea sediments of the Ottawa Valley-St. Lawrence Lowlands. these include: -areas where the subsurface topography of firm ground may have influenced earthquake shaking amplification with subsequent landsliding and other forms of ground deformation, -delineation of buried valleys for positioning of long-term earthquake monitoring facilities, and -mapping the presence of buried valleys for groundwater aquifer determination. Other similar studies have been conducted in the St. John River valley near Fredericton N.B. as an aid in the assessment of earthquake hazards.
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Correlation of High Resolution Eri Results to Geochemical Zonation at a Degraded LNAPL Site in Grand Junction, Colorado.
Authors Marcy Stonecipher and Stuart McDonaldGeochemical zonation in the subsurface has been imaged successfully at several environmental sites impacted by LNAPL using GeoTrax Survey™ technology (a high resolution form of electrical resistivity imaging). One of these sites is located in Grand Junction, Colorado, and shows a zone of decreased sulfate that is electrically distinct from zones containing oxygen and nitrate in the subsurface.
Overall, the site presents as electrically conductive (<8 ohm-m), with extremely conductive zones (< 1 ohm-m) in the eastern portion of the site, and very conductive zones (1 – 3 ohm-m) in the central and western portions of the site. RockWorks ™ visualization software was used to create a 3-D model of site, which integrated GeoTrax Survey™ images, groundwater analytical data, monitoring well locations, and groundwater elevation data. Bio-parameter data was also included in the 3-D model, and consisted of dissolved oxygen, sulfate, nitrate, and redox potential. the 3-D model demonstrates a strong correlation between the conductivity of the site and geochemical zones which are expected to indicate the type of bacteria present. Data in the 3-D model show oxygen and/or nitrogen zones near the eastern portion of the site that correlate well with extremely conductive zones (<1 ohm-m), and sulfate areas in the central and western portions of the site correlated well to the very conductive zones (1-3 ohm-m). Data obtained at this site indicate that it is possible to use GeoTrax Survey™ technology to identify geochemical zones that correlate to various nutrient reducing bacteria. these findings are significant to environmental professionals investigating and/or remediating LNAPL impacted sites. Specifically, this approach can be used to support natural attenuation arguments and/or identify areas where bioactivity is nutrient limited, thereby allowing efficient and focused deployment of bioamendment technologies to enhance existing levels of natural attenuation.
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Using Data from Multiple Loop Sizes Simultaneously in a 1D Surface Nuclear Magnetic Resonance Inversion
Authors Trevor Irons, Yaoguo Li and Jared AbrahamWhile the surface nuclear magnetic resonance (SNMR) method holds enormous promise, the technique has not been widely adopted due to its challenges. to be useful to hydrologists reliable Inversions for both porosity and decay parameters must be developed, and then relationships between these decay parameters and hydraulic permeability established. using field data collected around Lexington, Nebraska we first illustrate that the variability of the SNMR data is sufficient, even over short time intervals, to lead to dramatically different Inversion results with currently available Inversion schemes. these field sites are unique because a battery of geophysical and hydro-geologic tests have been done in the area, providing insight into the true aquifer characteristics. the same sites were returned to for SNMR data collection several times a year between 2007 and 2010. Most of the variability in Inversion results can be attributed to changing noise levels. often smaller diameter figure-eight loops were deployed to suppress noise and better illuminate the very near surface. However, these data are currently inverted independently of the deeper sensing large diameter loop data. We present a new 1D Inversion scheme that uses the complete dataset to simultaneously invert for T2* and partial water content in the Fourier domain. Electrical conductivity effects are taken into account and arbitrarily shaped transmitter and receiver wires are incorporated. the scheme has several advantages over existing Inversion algorithms in that the NMR signal is demodulated in this domain, and is narrow band. As such a generalized Inversion in this domain delivers significant compression compared to a time-domain formulation. Due in part to this compression, it becomes practical to combine multiple SNMR datasets into a single Inversion. By combining datasets from multiple loop sizes, more consistent results are realized as the sensitivity of each loop configuration contributes to the solution.
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Noise Reduction in a Multi-Channel SP Monitoring System
Authors Karl Butler, Aaron DesRoches and Peter SimpkinSelf-potential (SP) monitoring is attracting growing interest as a tool for inferring groundwater flow, both in saturated media and in the vadose zone. Multi-channel recording systems are being used to acquire data from large arrays of electrodes with the simultaneous temporal sampling desirable for data processing and Interpretation. We present an analysis of the noise performance of one relatively common commercially available datalogger (Campbell Scientific CR1000, with AM32B multiplexer) in SP monitoring applications and illustrate the improvements realized by incorporation of passive RC filters. Our filters, consisting of a 10 kOhm resistor (Rf) in series with each input and a 10 microfarad capacitor (Cf) across each differential input pair, provide a lowpass filter with a cut-off frequency of 0.8 Hz. This corner frequency is lowered further by the source impedance Rs (sum of the electrode contact resistances) giving a lowpass corner of approximately 1/(2pi(2Rf + Rs)Cf), or 0.5 Hz for Rs = 10 kOhm. We suspect that the principal improvement arises from the attenuation of powerline noise at 60 Hz and its harmonics prior to sampling. Such noise can commonly be hundreds of times stronger than the SP signals of interest. Although the datalogger uses analog Integration over 60 Hz periods to effectively attenuate stable powerline harmonics, it remains vulnerable to instabilities in harmonic amplitude or phase, and to settling time artifacts that can arise on switching between channels with vastly differing voltages. Our limited experience to date indicates that the benefits of pre-filtering are somewhat site dependent. the filters proved critical during recent SP monitoring of water well pumping where they reduced noise levels by at least a factor of 10, allowing the identification of coherent sub-milliVolt temporal variations in SP over an array of 34 electrodes distributed around the pumping well on surface.
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Multiple Receiver Loop SNMR Surveys Applied to Geo-Technical and Civil Engineering Problems
Authors Trevor Irons, Yaoguo Li and Jason McKennaWhile surface nuclear magnetic resonance (SNMR) has primarily been applied to hydrology problems -- particularity in 1D -- there exist myriad other applications for this technology. there is a large number of problems where directly detecting and/or imaging water is of primary concern. A few examples include geo-technical and civil engineering problems such as locating voids in roads that are potential sinkholes, monitoring or exploring for leaky dams, landslide hazard assessment and permafrost monitoring. Many of these problems are Inherently 3D in nature. Although the latest generation of multi-channel SNMR instruments has opened the door to 3D surveys, little work has gone into this area. Questions such as feasibility and survey design require 3D modeling capabilities.
We have developed a general forward modeling algorithm that supports arbitrarily shaped loops, with surface topography, as well as decoupled transmitter and receiver loops. the algorithm makes all calculations in the frequency domain and models the entire dataset. Three dimensional electrical conductivity and water distributions are supported and the loops may be embedded in or at the surface of the earth.
using our new algorithm, we investigate the feasibility of several novel SNMR applications. We consider the case of a leaky levee or dam structure with Internal erosion and piping. We seek to determine whether SNMR loops can be used to monitor the presence and progression of the erosion. We consider loops both on on the surface and embedded within the structure. We also examine the problem of using SNMR to image water filled voids that can turn into sinkholes.
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Important New Archaeological Finds Resulting from Shallow Seismic Anomalies at the Ancient Hierakonpolis Temple-town Site in Upper Egypt
Authors Shelton Alexander, Elizabeth Walters and Recep CakirProminent high-velocity, high-frequency signals from very shallow depths observed on contiguous and crossing seismic refraction profiles have been used to map anomalous areas within the perimeter wall of the ancient Hierakonpolis Temple-town in Upper Egypt, a site continuously occupied from at least 3800 BCE through Roman times. these shallow, high-velocity anomalies are characterized by laterally continuous, high-frequency (200-300 Hz) arrivals with velocities comparable to or exceeding the deeper water table refraction velocities. these anomalous areas are within a 1 to1.5 m-thick upper layer of unconsolidated, air-filled sediments that have extremely low velocities and very low Q.
Several localized excavations in the anomalous area in the western portion of the site have revealed the presence of a zone of closely spaced artifacts (dense in potsherds and stone fragments) that provide new evidence of human occupation in the ancient town as early as Dynasty I, c. 3200 BCE. in the northwest excavation an ‘in situ’ deposit of special pottery lies next to a bench, a large block of dressed limestone. Further to the north, layered occupation suggests a secular context with pottery of Dynasty II, 2900 BCE preserved ‘in situ.’ Recent field work in 2008-2010 has clearly designated these special finds as part of a site for offerings, dating to 3200 BCE, the time of the early rulers. Most exceptional is an ebony statuette leg, a masterpiece and the first preserved evidence of an ebony statue for this early time. these and additional findings from further planned excavations in early 2011 will be presented.
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An Integrated Geophysical Study to Characterize the Near-Surface of Barringer Meteorite Crater
Authors Soumya Roy, Robert Stewart, Arkadiusz Turolski, Susan Green and Li ChangAn integrated geophysical survey was conducted by University of Houston, University of Texas (Austin) and NASA in May, 2010 at Barringer (also known as Meteor) Crater, Arizona. the crater was excavated some 49,000 years ago by the impact of an iron-nickel meteorite. the crater has a diameter of 1.2 km with a rim rising some 30–60 m above the surrounding plain and 180 m deep bowl-shaped depression. the startigraphy of the area consists of Coconino sandstone overlain by the toroweap (sandstone and dolomite), followed by the Kaibab (dolomite and dolomitic limestone) and then the red Moenkopi (calcareous siltstone with iron-rich matrix and sandstone). An overturned sequence (Coconino underlain by Kaibab and then the Moenkopi above the bedrock Moenkopi) is observed at the rim and beyond as a result of the impact. the excavated debris also consists of this overturned sequence (collectively known as the ejecta blanket) and is found on the flank of the crater. the ejecta tapers as a function of distance from the crater rim. the aims of this survey were to unravel some of the existing mysteries related to the asymmetry of the crater, thickness of the ejecta blanket and its variation, depth and orientation of fractures. Different geophysical surveys (seismic, ground penetrating radar (GPR), gravity, magnetic) were conducted on the gently dipping flanks beyond the southern crater rim. A near-surface S-wave velocity structure has been obtained using the ground-roll Inversion technique. the near-surface S-wave velocity varies from 200-600 m/s within top 15 m. A change in the S-wave velocity at a depth of 10 m is observed from a seismic line at the southeast flank, which is interpreted as the transition from the ejecta blanket to the underlying Moenkopi sandstone. Ultrasonic measurements of Moenkopi hand samples indicate P-wave velocities up to 1600 m/s and Initial first-break picks show a near-surface P-wave velocity of 1700 m/s. Thus, S-wave velocities in the neighborhood of 600 m/s are reasonable. GPR surveys were also conducted at the southeast crater rim to probe the very shallow subsurface (3-5 m). Several anomalies in the GPR data were found indicating objects with a strong contrast in electrical properties with respect to the host material, possibly an iron concretion formed from the impact.
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Advances in GPR Imaging with Multi-Channel Radar Systems from Engineering to Archaeological Sites
Authors Dean Goodman, Alexandre Novo, Gianfranco Morreli, Doria Kutrubes and Henrique LorenzoAdvances in ground penetrating radar imaging with multi-channel systems have greatly improved the speed and areal coverage of the ground. Along with improved imaging software, datasets recorded with multi-channel systems can be processed at similar speeds to coarsely spaced single channel data that would normally require additional time for interpolation processes to fill in the gaps between lines. with the crossline spacing approaching a 1/4 wavelength or less of the transmitted microwaves into the ground, multi-channel systems have the advantage of complete coverage of a site with no need for interpolation in most cases except to fill in the gaps between adjacent tracks if so desired. Multi-channel systems do require additional RSP (radagram signal processes) in order to balance the channels and to condition the data prior to imaging. Spectral whitening and several other RSP methods are shown with their application to imaging of sites from bridgedecks for deterioration to the discovery of subsurface archaeological remains. Data processed from several different multi-channel GPR systems are shown.
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Prospecting for a Municipal Water Supply using Multiple Geophysical Methods, Long Lake, NY
Authors Doria Kutrubes and John F. KickAs part of a continuing effort to find a potable water supply that meets both hydrogeologic, legal and political criteria, a geophysical Investigation was conducted on the Stanton Hill property east of Long Lake, New York. Seismic refraction, GPR, gravity, VLF and magnetic surveys were conducted to characterize the properties of both unconsolidated materials and bedrock Two seismic lines, each 440 feet in length were established along gravel roads. Explosive energy sources were placed at five locations along each line and at 200 feet off-end locations. the 24 channel data was interpreted using “critical distance” and non-linear refraction travel time Tomography methods. GPR, gravity, VLF and magnetic surveys were completed along the seismic lines and over much of the area of Investigation. A major discontinuity was detected near the midpoint of Line 1. Northwest of the discontinuity, bedrock, with a velocity of 9000 ft/s, is detected; its surface slopes downward to the southeast. South east of the discontinuity bedrock with a velocity of 15,000 ft/s is found with surface sloping upwards to the southeast A third segment with velocity of 13,500 ft/s and surface parallel to the ground surface makes up the southeast end of the profile. the overburden has a velocity of 6000 ft/s which is likely a dense till, and appears thickest over the bedrock contact. Coincident with this interpreted contact. GPR depth-slice imaging indicates a zone of high-amplitude reflectors, caused by an increase in water content or change in mineralogy, 35 to 55 feet below grade. Bouguer gravity values show an increase of over 35 microgals coincident with the bedrock profile. An increase in conductivity was also observed in the VLF data coincident with the deep pocket of unconsolidated material and bedrock contact. Magnetic data were inconclusive, biased by iron-rich rock placed as a cover on the gravel road.
A bedrock well, drilled in proximity to, but unfortunately up-dip from this contact and tens of feet from the thickest low velocity area determined by the seismic, confirmed the presence of a highly weathered rusty gneiss overlying a highly competent black, iron rich rock. However, it did not yield more than 10 GPM. Information from the Geophysics did, however, enable our client to situate two other wells in thicker sections of the highly weathered gneiss to produce yields sufficient to meet the town's water supply requirements.
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An Improved Early-Channel Vtem Helicopter System for Near-Surface Applications
Authors Jean Legault, Pavel Tishin, Alexander Prikhodko and Harish KumarThe helicopter-borne Versatile Time Domain Electromagnetic System (VTEM) is a geophysical instrument which has been in continuous development, utilizing the most recent advances in digital electronics and signal processing for deeper penetration; higher spatial resolution; better resistivity discrimination; and increased detection of a broad variety of conductive targets. Since its inception in 2002, more than 1 300 000 line kilometers have been flown around the world, in a wide variety of geological environments, primarily involving mineral exploration, but also including near-surface groundwater applications. However, although the VTEM system has progressively achieved marked improvements in its deep penetration characteristics, at the same time its near-surface imaging capability has been limited by its early time data. This is significant because early time or high frequency airborne electromagnetic data (AEM) are desirable for shallow sounding or mapping of resistive areas. Yet many time-domain AEM system have problems obtaining quantitative early-time data due to a variety of issues, namely system bandwidth. Recently, workers and researchers in shallow electromagnetic sounding have confirmed the calibration accuracy of VTEM mid to late delay time data (>100μsec to 10msec) while at the same time highlighting the quantitative incompatibility issues with its early channel data and near-surface conductivity layering. Processing strategies, such as deconvolution, aimed at correcting system imperfections, such as bandwidth, have been proposed with promising results but are still not routinely implemented.
in an effort to address this issue, Geotech has embarked on a system design strategy aimed at improving the early-channel VTEM data and achieving fully calibrated, quantitative measurements closer to the transmitter current turn-off. This development has led to new model of VTEM system designed specifically for near-surface, high resolution applications, while maintaining reasonably optimal deep penetration characteristics. Results have shown a significant improvement in quantitative VTEM data at earlier times than previously achieved, approaching 5μseconds after the current turn-off.
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Geoelectrical and Hydrogeological Modeling of the Fresh Water / Saline Water Interface in the Lower Florida Keys.
Authors Albert Yeboah-forson, Dean Whitman, Danielle Ogurrcak and Mike SukopSalt water intrusion is a critical challenge for the management of fresh water resources on small oceanic islands like those in the lower Florida Keys, which have extensive coastal development including construction of canals. We present the use of the Direct Current Resistivity (DCR) method for investigating the location of the fresh water, saline water (FW/SW) interface. A Super-Sting R1 28-electrode resistivity imaging system was used to investigate the subsurface geology and the FW/SW interface on Big Pine Key. the survey consisted 54m long profile in the center of the island and a 250m long roll along profile extending inland from the western shore.
in the center of the island, resistivities range from 40 – 250 ohm-m near the surface and correspond to the freshwater lens which extends to a depth of 6 m below sea level. Below this depth, values of 20-30 ohm-m indicate brackish groundwater. On the western shore of the island, resistivities of 1-3 ohm-m corresponding to sea water are found at the surface and extend 25 m inland. Farther inland, resistivities increase to 50 ohm-m indicating a 2-3 m thick fresh water lens. the FW/SW interface slopes landward and 250 m inland from the shore, brackish resistivity of ~20 ohm-m extend to 4 m below sea level and saline found at 8 m depth.
Observed geoelectrical data were compared with a 2-D salinity profile simulated from the USGS SEAWAT model .Analysis of the data show that the largest depth to FW/SW interface occurs at 8m depth in the middle of the island with a gradual decrease in the FW towards the saline zone on the shore of the island and this is generally consistent with the geophysics. Ultimately, both geophysical methods and hydrogeological model Tools like SEAWAT can predict island sea water intrusion with reasonable accuracy.
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Tomographic Imaging of the Common Dataset
By Don ZhaoTomographic Imaging of the Common Dataset Don Zhao, Geogiga Technology Corp., Alberta, Canada Abstract A set of P-wave first-arrival times for a synthetic common dataset is analyzed with refraction Tomography to determine the two-dimensional seismic velocity structure. the refraction Tomography employs a hybrid ray-tracing scheme based on the shorted path method and the ray-bending method to efficiently obtain an accurate forward solution, and regularizes the non-linear iterative Inversion with vertical and horizontal smoothing constraints, damping contraints, and first-arrival picking errors. A vertcial gradient velocity model with lateral homogeneity is used as an Initial model to generate the velocity structure from the first-arrival times. the final result shows a non-flat bedrock structure overlaid with lower velocity layers and identifies two lower velocity zones within the structure.
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Factors in Determing the Survey Depth from Passive Surface Waves
By Don ZhaoFactors in Determing the Survey Depth from Passive Surface Waves Don Zhao, Geogiga Technology Corp., Alberta, Canada Abstract the passive surface wave technique has gained considerable attention for retrieving the subsurface shear velocity structure. Same as that in the active surface wave technique, the survey depth in the passive technique is affected by many factors. to study these factors and provide a guideline in designing the passive surface wave survey, we simulate a set of passive surface wave data and conduct several field tests with differenct lengths of arrays and different frequencies of geophones. the data are analyzed with the frequency-wavenumber (FK) method and the spatial auto-correlation (SPAC) method. the study shows that the larger length of array, the deeper the survey depth; the lower frequency of geophones, the deeper the survey depth. the result also demonstrates that the SPAC method is much better than the FK method in analyzing the passive surface waves, especially in the lower frequency range.
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An Overview on Geophysical Investigations of Earthen Dam Foundations in California for the U.S. Army Corps of Engineers
Authors Lewis Hunter, Michael Powers and Ronn RoseThe Sacramento District, U.S. Army Corps of Engineers (USACE) teamed with the Crustal Geophysics and Geochemistry Science Center of the U.S. Geological Survey in 2005 in order to conduct geophysical Investigations along the downstream toe of Success Dam. An underlying goal of these surveys was to demonstrate the effective use of geophysics for answering geotechnical questions for a skeptical audience of geotechnical engineers. the successful completion of that survey led to follow-on work at Isabella, Hidden and Martis Creek Dams, all located in California and operated by the Sacramento District. A range of techniques including P- and S-wave refraction and reflection seismic, DC resistivity, time-domain electromagnetic, electromagnetic induction, self potential, ground and airborne magnetics, magnetotelluric, and ground-penetrating radar have been used to characterize the subsurface in the vicinity of these dams. At Isabella Dam, water borne surveys including magnetics and seismic reflection were also used. At Isabella and Martis Creek Dams, airborne and ground based LiDAR have been used to locate and map fault related structures. This talk will review the work that we have done, provide a summary and demonstration of significant findings, and review some of the lessons learned over the past 5-years of these Investigations.
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Utilization of Integrated Geophysical Surveying for the Safety Assessment of Levee Systems
Authors Tomio Inazaki and Koichi HayashiIntegrated geophysical surveying plays an important role for the safety assessment of levee systems, because only the continuous 2-D surveying can detect anomalies effectively. It mainly consists of surface wave method using Land Streamer, capacitively-coupled resistivity method using OhmMapper, and supplemental Slingram electromagnetic survey method. We could successfully identify anomaly structures in and beneath levee systems by means of the combination of the above methods. the geophysical properties evaluated by these methods, resistivity and S-wave velocity, are useful to evaluate permeability and stiffness of levee systems. We also demonstrated the usefulness of crossplots of the measured data as criteria for the assessment of levee conditions.
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Comparison of MRS and Pumping Tests in Denmark
More LessIn Denmark hydrogeophysical mapping is performed intensively in accordance with the Danish Government's environmental plans of ensuring high quality drinking water supply based solely on naturally clean and chemically untreated groundwater. This national Danish groundwater mapping is performed by the Danish Ministry of the Environment as a part of the Act of Environmental Goals, which stipulates extensive groundwater mapping of particularly valuable groundwater abstraction areas comprising 40 % of Denmark. Since 2006 efforts have been made to implement the Magnetic Resonance Sounding method (MRS) in the Danish national groundwater mapping in accordance with the conventional requirements for geophysical methods in Denmark. from 2006 to 2010 approximately 160 MRS soundings have been performed in Denmark in different hydrogeological environments. Each campaign has been performed with a specific purpose and agenda as a part of the general groundwater mapping, and the method has been proven to improve the hydrogeological characterisation from geophysical surface measurements in Denmark. the MRS results from different Danish hydrogeological conditions are verified by comparing numerous MRS and pumping tests. When these comparisons are performed with attention on the investigated volume in each specific case, very good correlation is obtained. these comparisons of MRS and pumping tests have lead to the determination of MRS calibration coefficients from different Danish hydrogeological conditions. MRS calibration is essential for quantitative use of MRS transmissivities. A tendency of increasing calibration coefficients with increasing transmissivity and aquifer grain size is revealed, however overall only little dispersion around one average value is observed.
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Biogeophysical Monitoring of the Microbial Degradation of Crude Oil from the BP Deep Horizon Oil Spill
The BP Deep Horizon oil spill, although an unfortunate incident, presents a unique opportunity to utilize geophysical methods to investigate at the physical, chemical, and biological transformation of fresh crude oil in a highly saline environment and specifically investigate if the biotransformation of the oil will cause the electrical signature of the sediment to change over time. Based on the depictions of the contaminate concentration on the shores of the Gulf of Mexico, the island Grand Terra 1, located off of Louisiana was selected as the location of the survey site.
We are performing geophysical, geochemical and microbiological measurements. We have deployed an autonomous resistivity monitoring system that addressed thirty-two stainless steel electrodes at a half-meter spacing. This automated system was created to monitor and acquire electrical resistivity in the form of time-lapse geophysical measurements. Soil samples were collected at predefined time intervals for microbiological and geochemical analysis. Our aim is to determine the microbial community structure and dynamics in response to the crude oil spill, to capture the biodegradation of the crude oil, and link time evolving electrical geophysical signatures to biodegradation of the oil spill.
Initial data acquired from GT1 during a reconnaissance trip included electrical resistivity data and soil samples for microbial analyses. Post acquisition processing of the collected data produced a 2D resistivity model of the subsurface, which demonstrates that the electrical geophysical technique is not just able to detect the presence of hydrocarbons in the subsurface as a relatively small but still distinctive resistive anomaly, as would be expected of a fresh oil spill which has not yet undergone extensive biodegradation, but could also be utilized to delineate the oil-impacted sediments. This result suggests that electrical resistivity may be a useful technique in both the short term for characterizing the full extent of the oil impacted zones as well for the long term monitoring the biodegradation of the oil.
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Geophysical Characterization of Permafrost Distribution in the Yukon River Basin, Alaska
By James NolanGeophysical Tools including airborne electromagnetic (HEM), time domain electromagnetic (TDEM), DC resistivity and continuous resistivity profiling (CRP) are used to evaluate permafrost distribution in the Yukon River Basin in the area of fort Yukon, Alaska. Permafrost is a primary control on hydraulic processes in interior Alaska, but its distribution is poorly characterized. Major questions about the distribution of permafrost limit understanding of hydrology such as the thickness of the permafrost layer, the geometry of taliks and their role in the hydrogeologic framework, and the connection between surface water and groundwater. Electrical and electromagnetic geophysical technologies can be used to characterize permafrost distribution by exploiting the high resistivity contrast of frozen and liquid water phases. these techniques can provide improved spatial coverage at a higher resolution and lower cost than traditional direct sampling techniques, which remain sparse in the area. in an effort to develop geophysical techniques for characterizing permafrost and inform hydrologic models, Initial campaigns of geophysical data collection were conducted during the summer of 2010. HEM data was collected from a helicopter in the region surrounding fort Yukon, Alaska, including the Yukon River and the Porcupine River. using Initial results of the HEM surveys, sites were chosen to collect ground based geophysics to investigate anomalies that appear to relate to the hydrogeologic framework of the area. the ground based geophysical campaign included TDEM, DC resistivity, and CRP. Initial results from this campaign show that the techniques can be successfully used to map thickness and distribution of permafrost in the area.
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Geophysics Imaging and Ecological Mapping of the Lower Hackensack River System
The Meadowlands and the lower Hackensack River form a connected system that is an important asset for the greater New York City metropolitan region because of its economic, environmental, and recreational value. e4sciences|Earthworks LLC produced geophysical images and ecological maps of the lower 14 miles of the Hackensack River, New Jersey, for the Hudson-Raritan Estuary Ecosystem Restoration Study. the mapping program included sub-bottom seismic profiling, orthosonographs, multibeam bathymetry, magnetic field mapping, digital photographs, aerial photographs, cores, sediment profile images, morphology, stratigraphy, sedimentation, biological data, and benthos. the geophysical methods imaged the water-bottom morphology and subsurface stratigraphy to 100 feet depth with 1-foot spatial resolution. Going into these studies, it was presumed that the river bottom would be covered uniformly with recent industrial-age black silt. Our observations demonstrate otherwise. the black silt is concentrated in Harmon Cove and locally in point bars in the north. Elsewhere, there is tremendous variability in the river bottom. the Hackensack River system is a dynamic asset and resource that deserves continued monitoring and management.
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Digital Soil Mapping of Available Water Content in the Lower Macquarie Valley, Australia
Authors John Triantafilis and Liam GooleyTwo thirds of all irrigated agriculture in Australia is undertaken within the Murray-Darling Basin. However climate change predictions for this region suggest rainfall will decrease. in addition, environmental concerns and new industries are competing for water resources. in order to maintain profitability, more will need to be done by irrigators with less water. in this regard, irrigators need to be aware of the spatial distribution of the available water content (AWC) in the root-zone (i.e. 0.0-0.90 m). Owing to the expense of traditional soil survey methods, digital soil mapping techniques are being used with increasing frequency to map soil properties. This includes, soil properties related to AWC such as clay content and mineralogy. This paper aims to present the development of a digital soil map of the AWC at the district level. This is achieved by determining AWC by the difference between the permanent wilting point (PWP) and field capacity (FC) which were measured in the laboratory using a pressure plate apparatus. the PWP and FC was coupled with ancillary Information including; gamma ray spectrometry (i.e. dose rate, Potassium (K-%), Uranium (eU-ppm), Thorium (eTh-ppm)), electromagnetic induction data (i.e. EM38 and EM34) and two trend surface parameters using various multiple linear regression models (e.g. stepwise). using this Information we develop a hierarchical spatial regression (HSR) model to predict AWC in the irrigation areas of Warren and Trangie. the reliability of the models were compared using prediction precision (RMSE – root mean square error) and bias (ME–mean error). It was found that using EM38-v, EM34-10, eU, and eTh provided the best results (r2=0.55). the DSM maps are consistent with the known pedoderms and soil types and provide a basis for irrigation management and future research.
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The Effect of Smoothing First Arrival Times and the Initial Velocity Model on Refraction Tomography Results
By Curtis LinkTypically refraction data are collected to characterize a 2D heterogeneous subsurface. to accommodate a 2D subsurface, refraction Tomography has become a useful alternative to the more commonly used layer approach. Common to both methods is the problem of dealing with noisy data which makes picking first arrival times a subjective process. for a layered solution, break points related to refracting layers are chosen which is a subjective process as well. for the Tomographic approach, an Initial velocity model based on less than perfect Information must be chosen as the starting point in the iterative process.
using the data set consisting of P-wave travel times for a synthetic seismic refraction experiment created for the ‘refraction shootout’, we investigate the effect of 1) smoothing noisy first arrival time picks and 2) smoothing the Initial velocity model used in the refraction Tomography approach. Smoothing is applied to the first arrival times from the synthetic data set which have uncorrelated Gaussian noise added. We also investigate the effect of smoothing the Initial 2D velocity model produced by the delta T-V method on the final model results. We use a standard refraction Tomography package using Fresnel volume Tomography to produce the P-wave velocity images. Trials are run for varying degrees and types of smoothing of the first arrival times to investigate the time smoothing effect on the final models. Similarly, trials are run for smoothing of varying degree applied to the 2D delta T-V Initial model(s) to determine the Initial model smoothing effect on the final velocity models.
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Magnetic Susceptibility Measurements as a Proxy for Bioremediation at Hydrocarbon Contaminated Sites
Magnetic susceptibility (MS) of sediments affected by hydrocarbon contaminated groundwater was studied at two sites. the sites used in the study are Bermidji MN, and Carson City, MI. Two cores were retrieved from Bemidji site; one from the contaminated area and the other from the background area. Three cores were collected from the Carson site; two within the contaminated area and one from the background area. All the cores that we collected extended from the unsaturated zone into the saturated zone. for the Bemidji site, there is a 0.9m hydrocarbon smear zone due to groundwater level fluctuations. for the Carson site, hydrocarbon smear zone is approximately 1-2 m. MS and Grain size data were collected from the cores from both sites. Our results show that MS increased towards the top of the GWT within the contaminated area of both sites. in contrast, the MS does not show any changes around the GWT within the clean cores of both sites. We postulate that this increase in MS is due to iron reducing microbes creating magnetite as a byproduct of hydrocarbon breakdown. Based on these results we conclude that the MS measurements can be used as a tool to investigate microbial activity within hydrocarbon contaminated zones.
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An Intercomparison of Airborne Electromagnetic Systems for Hydrogeologic Studies
Authors Paul Bedrosian, Burke Minsley, Esben Auken, anders Vest Christiansen and Jared AbrahamMany of today’s airborne electromagnetic (AEM) systems arose out of the mining industry, where anomaly detection and geometric modeling were the primary interpretive products. the increasing use, however, of airborne electromagnetic surveys for hydrogeologic studies is driving increased demand for data that can be consistently inverted for accurate subsurface resistivity structure from the near surface through to depths of several hundred meters.
toward this goal, the USGS has been evaluating a number of commercial AEM systems over two test areas in Western Nebraska, USA. the selected test blocks have a conductive base-of-aquifer, the depth to which varies from zero to several hundred meters. the overlying aquifer shows significant lithologic heterogeneity, and is incised by both modern and ancient river systems. Finally, ground control from borehole lithologic and geophysical logs, as well as ground-based electromagnetic methods, is excellent. these test blocks are thus ideal for evaluating system resolution, sensitivity, depth-of-Investigation, top-of-Investigation, and the accuracy of inverted models in relation to ground truth.
We will show comparisons of several systems flown to date, and emphasize the importance of accurate system characterization (geometry, timing, waveform, bias, bandwidth, elevation, tilt). We will illustrate the effects of these parameters in the data space and also on inverted resistivity models using a consistent approach for all systems. the need for calibration is stressed and several approaches will be described.
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Estimating Liquid Water Content in Snow from Frequency Dependent Attenuation Analysis of Pulsed and FMCW Radars
Authors John Bradford and Hans Peter MarshallAt radar frequencies, dry snow is non-dispersive and the dielectric permittivity is primarily a function of snow density. Liquid water in the snow strongly alters the polarization properties and the dielectric permittivity becomes complex. the Complex Refractive index Method (CRIM) provides an accurate prediction of the complex dielectric permittivity in wet snow in the frequency range from 500 MHz – 4 GHz. Analysis of the CRIM equation shows that in wet snow, the real and imaginary components of permittivity are functions of both snow density and liquid water content. However, to a good approximation the frequency dependent attenuation is a function of water content only. Further, the slope of the attenuation coefficient vs frequency curve is a linear function of volumetric liquid water content. Given an appropriate reference spectrum, the method of spectral ratios provides a direct estimate of the slope of the attenuation vs frequency curve. We utilize a reflectivity method to investigate the radar response to wet snow. We have adapted the algorithm to simulate data acquired with both pulsed and frequency-modulated, continuous-wave (FMCW) radars. FMCW radars utilize a linear frequency sweep over a broad bandwidth and hold several important advantages over pulsed radar including a flatter spectrum which means that the signal-to-noise ratio will be higher over a broader frequency range. We have calculated the relationship between the slope of the attenuation coefficient curve and the liquid water content. We show that for a broad range of realistic snow conditions, this relationship reliably estimates the liquid water content in snow from both pulsed and FMCW radars operating in the 500 MHz – 2500 MHz range.
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A Feasibility Study on the Seismic Response of Select Nanoparticles in Saturated Granular Media
Authors Nihad Rajabdeen, Barbara Luke, Dale Werkema and Danney GlaserAs engineered nanomaterials become more prevalent, environmental releases of nanoparticles likewise increase. the environmental and human health impacts of these engineered nanoparticles are largely unknown. Currently, there are no proven methods of detecting their fate and transport in the subsurface. for this reason, new testing and detection techniques are being explored.
This research is part of the U.S. EPA National Exposure Research Laboratory (NERL) broad effort to conduct studies on the effects of engineered nanoparticles on ecosystems and human health. the seismic characteristics are used to establish defining features of select nanoparticles in saturated granular media. A bench-scale seismic testing apparatus is designed, built and optimized for measuring the seismic signatures of varying concentrations of engineered nanoparticles in saturated granular media. the ultimate goal of this research is to test the seismic method as a feasible approach to detecting nanoparticles in the subsurface under optimal conditions, while establishing the concentration detection limit for this method.
By actuating piezoceramic bender elements mounted inside a test column, shear and compression waves are generated and received through the saturated granular media within the column. the column system is Initially calibrated in air and water. the column is then filled with granular media and saturated with water. the seismic signature for this set of parameters establishes the baseline. Tests are then repeated with various nanoparticle dispersions. the dispersions to be tested include nano titanium dioxide, nano zinc oxide, nano silver, and nano zero valent iron. the seismic responses are to be analyzed for variances in travel time, amplitude and spectral content with respect to nanoparticle concentration. It is anticipated that the results will be applied to plot a “breakthrough curve” (BTC) of seismic response with respect to nanoparticle concentration levels. This BTC would reveal the nanoparticle concentration seismic detection point. the preliminary results of this research will guide future experiments investigating the utility of seismic methods in nanoparticle fate and transport studies.
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Geophysics for Rock Properties and Material Separation during Dredging and Expansion of the New York and New Jersey Harbors
The US Army Corps of Engineers and the Port Authority of New York and New Jersey are deepening 26 miles of navigation channels of New York harbor from -40 to -50ft MLW. the project requires dredging 50 million cubic yards of rock and sediment. At the halfway point, the project was ahead of schedule and under the US$1.6 billion budget.
Two key difficulties in the project are: (1) contaminated sediments and (2) undiggable rock. the cost of removing and placing contaminated black silt is 7 times the cost of removing and placing clean sands and silts. the cost of blasting and removing undiggable rock is 10 times the cost of removing sands and silts.
We have mapped the geology and the physical properties of the rocks and sediments throughout the project. All data, maps, and cross-sections are compiled into a single reference frame. All measurements are integrated in the Interpretation. the maps and cross-sections constitute the primary Information for managing the construction and engineering of the harbor deepening project.
We have developed geophysical techniques and operate them daily to determine the top-of-rock and to map and quantify the properties of the sediment, rock strata and quantify the area and thickness of black silt. Our techniques include calibration with core borings. Orthosonography yields aerial-photograph-like maps of the areal extent. Subbottom-seismic images, which are depth migrated, profile the thickness of the black silt and the approximate depth to undiggable rock. the black silt demonstrates anomalous properties and behavior from gray silt and other sediments. All images are georeferenced. We interpret the seismic sections through selection of horizons and estimation of physical properties. the seismic properties are correlated with mechanical properties to estimate diggability. the results are calibrated with core borings. We present the results as geological and geotechnical cross-sections with core borings.
Careful application of geology and geophysics to the New York Harbor Deepening Project allows for reduced uncertainty, improved project management, better planning, and reduced costs.
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Digital Soil Class Mapping at the Regional Level using Gamma-Ray Spectrometry and a Numerical Clustering Algorithm
Authors John Triantafilis and Nina EarlNatural resource management at the district level requires an understanding of the interrelationship between soil physical and chemical properties in order to identify soil management classes. However the collection of soil property Information at these levels and this purpose is cost-prohibitive. increasingly proximal and remotely sensed data sets are being used as surrogates to assist in identifying soil management classes. This includes the use of proximal sensors (e.g. electromagnetic (EM) induction instruments) and remotely sensed data (e.g. secondary terrain attributes derived from a DEM). However, even these have limitations owing to the need to cover large areas and their limited use on flat alluvial landscapes, respectively. Another option is the use of passive remote sensing techniques (e.g. gamma-ray spectrometry data) to map the regolith. Regardless of which ancillary data set is used an objective methodology is also required in order to identify structures or classes that may relate to soil properties and type. in this regard many researchers are increasingly using numerical clustering algorithms (e.g. fuzzy k-means analysis - FKM). in this research we demonstrate how gamma-ray spectrometry data can be used as a surrogate for the collection of soil property data. Structures are identified by passing the data through a FKM algorithm (FuzME v5.0). using indices such as the fuzziness performance index (FPI) and normalized classification entropy (NCE), we identify k = 11 classes and a fuzziness exponent ? = 2.0 for further Interpretation. the k = 11 classes correlate well with previously identified geological and geomorphological units in the Edgeroi district (i.e. eroded, alluvial and dust-mantled landforms). A wider Interpretation of the clustering showed that the k = 11 classes were also consistent with a broader soil survey of the lower Namoi valley. the inclusion of EM data could enhance the k =11 classes by providing detail on subsoil changes as well as top soil changes within the landscape.
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Impact of Soil Vapor Extraction On SIP Measurements
Authors Ryan Joyce, Estella Atekwana, Dale Werkema and Eliot AtekwanaThe Wurtsmith Air force Base (WAFB) and Carson City refinery sites have long been known as locations exhibiting active bioremediation of hydrocarbons. the geophysical response over the hydrocarbon contaminants has been well documented by numerous authors. Recently the WAFB site has undergone extensive remediation (soil vapor extraction) efforts to remove the contaminant mass above the water table. the WAFB data and the Carson City data have been compared to see the effect that remediation efforts have had on the geophysical signatures of the sites. in both studies, sediment cores were collected from locations inside and outside of the contaminant plumes: one outside the plume and two inside. the contaminated cores were collected at a location proximal to the contaminant source and distant from the source to determine the effect that distance plays on the results. Spectral Induced polarization measurements (0.1Hz – 10kHz) were then taken on the cores to develop a SIP depth profile of the contaminant plumes. When the WAFB and Carson City cores are compared, the Carson City data shows a clear increase in real and imaginary conductivity components when inside the contaminant mass while the WAFB data does not show any significant response. This lack of response at the WAFB site is due to removal of the contaminant body and carbon source for the microbial populations that were once at the site. the contaminant body and microbial activity is still active at the Carson City location and exhibits a strong SIP response.
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Integration of Seismic Surveys to investigate an Abandoned Mine Site
Authors Diego Arosio, Laura Longoni, Monica Papini and Luigi ZanziThe near-surface rock structure (about 50m thick) that covers an abandoned marl mine nearby the small village of Montevecchia (Italy) was investigated through a combination of seismic surveys. the mine was abandoned in 1958, after a massive collapse that involved all mine levels causing a large sinkhole on the top of the hill where the mine is placed. the seismic experiments had to be performed outside the mine because, at present, nearly all mine levels are flooded. the only accessible gallery is the upper one, but its direct inspection is considered unsafe and limited to a small section.
the methods selected for these Investigations were Refraction Seismics and Multichannel Analysis of Surface Waves (MASW). in addition, a sort of transillumination experiment was tried by posing two geophones inside the mine at the entrance of the upper gallery while the seismic source was moved along the road above the mine. All the methods were successful and gave complementary Information. Refraction seismics was used to characterize the upper low velocity layer and the second layer of the near-surface structure. the MASW method was necessary to assess the existence of a velocity Inversion revealing the presence of a low velocity layer trapped between the 2nd layer and a 4th high velocity layer covering the upper mine gallery. the transillumination experiment validated the presence of the 4th layer and gave an estimate of the average velocity that represents a lower boundary for the P-wave velocity within this layer. All the methods were also consistent in indicating the areas where the rock structure is more affected by fractures and discontinuities. Finally, both refraction and transillumination data were analyzed to derive average estimates of attenuation level and rock quality factor.
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Managing Residential Development in Karst Landscapes, Perth Metropolitan Area, South Western Australia
Authors Andrew Spyrou and Benjamin WilkinsAs the business and administration centre for the resource rich state of Western Australia, population growth in the Perth Metropolitan region has been consistently greater than the national average. the metropolitan area is located on the Swan Coastal Plain, a stretch of land which lies between the Indian Ocean and the Darling Scarp. Much of this region forms part of a karst belt which runs along the west Australian southern coast to 20 km inland. the belt is comprised of one main geological unit, the Tamala Limestone which is typically a soft porous rock that is highly susceptible to dissolution, with karst weathering often resulting in the development of sinkholes, open voids and caves.
Due to its geological setting, the pressure of new developments and the subdivision of land within the Perth Metropolitan region pose a risk with geohazards typical of karst environments. Urbanisation such as roads, buildings and construction can often acts of catalysts for karst formation by altering the flow of rain water runoff, applying increased loads on the surface, and increased vibrational noise. Urbanisation of karst environments presents a number of challenges for urban planners and engineers with structural failure of buildings and infrastructure a reality. This poses the question: How to manage urban development in karst landscapes in Western Australia’s fastest growing residential area?
This paper will present a number of case studies undertaken in residential developments in the Perth region detailing how geophysical methods have been used to map the limestone bedrock and identify karstic formations of potential risk. Ground penetrating radar and seismic methods have proven to be an affective combination of techniques over this environment with the sand dominant subsurface providing excellent transmission of radar wave energy and the basic two layer sand and limestone strata providing a distinct seismic refractive interface.
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Refuse Conductivity Variations Following Leachate Injection in a Bioreactor Landfill Cell: Modeling EM Results and Comparison with Well Logs
Authors Philip Carpenter and Krishna ReddyBioreactor landfills enhance waste degradation through recirculation of leachate inside the waste mass. in this study frequency-domain electromagnetic (EM) conductivity was measured along several profile lines before, during and after leachate Injection at the bioreactor cell of the Orchard Hills landfill, 15 km south of Rockford, Illinois. Electrical resistivity Tomography (ERT) measurements were also made before, during and after leachate recirculation. One profile line was located above and coincident with a leachate recirculation line (LRL). the other profiles were perpendicular to LRLs, which varied in depth from 5-15 m below the EM lines. Apparent conductivity increased along segments of the EM profile over the LRL during Injection of 17.8 m3 of leachate, when measured with a Geonics EM34 at 10 and 20 m vertical dipole separations. forward modeling of the EM data, using three layer models (cover, refuse above leachate Injection, and refuse below leachate Injection) suggest major increases in refuse conductivity below the Injection level. in some cases the resistivity of this lower layer decreases by 50% after Injection, from approximately 10 to 5 ohm-m. the apparent non-uniform nature of the Injection along the LRL is also examined through forward modeling of EM data, as is the lack of response during a second experiment in which a smaller leachate volume was injected. Well logging with a slim-line conductivity sonde adjacent to a gas extraction well also reveals a major conductivity increase below 8 m depth in the bioreactor cell. Natural gamma logs clearly show the depth extent of cover materials and a bentonite seal; refuse typically exhibits natural gamma values of 20-30 cps.
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Integrated Surface and Borehole Geophysics to Map Subsurface Cavities and Fractures Near a Superfund Site South of Rockford, Illinois
Authors Ryan Adams and Philip CarpenterThe ACME Superfund site is one of many Superfund sites in Northern Illinois. This 20 acre (8.1 ha) site was contaminated by various volatile organic compounds (VOC’s) and heavy metals during the 1960-1980s. in order to help formulate a conceptual hydrogeological model, subsurface lithology and structural features in the small valley immediately south of the site are examined using surface resistivity, frequency-domain electromagnetics (FDEM) and seismic surveys, as well as natural gamma, EM conductivity and DC conductivity logs. Natural gamma logs were most effective for defining subsurface lithology. Steel well casings on some wells and calibration problems with the EM sonde lead to poor quality conductivity logs. Bed thicknesses were determined from the gamma logs using the full-width-at-half- maximum method. Depth to bedrock, and existence of cavities, were verified by GeoprobeTM surveys. Results of the first phase of this project suggest hydraulic isolation of the surface and shallow groundwater (probably a perched aquifer), and the discovery of a network of karstic fractures and caverns in the deeper epikarstal bedrock (below 6 m depth), which contain contaminated water. the epikarst appears to be saturated, forming a highly transmissive zone. Recent GeoprobeTM conductivity logs agree qualitatively with 1D resistivity sounding models. GeoprobeTM surveys also revealed previously unmapped cavities in the northeastern part of the site which are probably connected to a larger fracture system. these cavities are also evident in FDEM surveys from this area. the hydrogeological system thus consists of a shallow perched flow system overlying a deeper fracture-controlled system in the epikarstal bedrock.
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Application of the Automatic Optimal Spline Smoothing Method to Optimizing Edges of Moroccan Bouguer Gravity Anomaly Map
More LessWe present a singular method that is capable to filter out noise as well as suppress outliers of sampled real functions under fairly general conditions. from an a priori selection of the number of points that define the adjusting spline, but not their location in that curve, the automatic optimal spline smoothing method automatically determines the adjusting cubic spline in a least-squares optimal sense. the method is fast and easily allows for selection of various possible number of knots, adding a desirable flexibility to the procedure. As an illustration, we apply the AOSS method to Moroccan Bouguer gravity data map. the AOSS smoothing technique is an efficient tool in the Interpretation of geophysical potential field data particularly suitable in denoising, filtering and analyzing gravity data singularities. the AOSS smoothing and filtering technique was found to be consistently useful for optimizing edges and contours of geophysical data maps as Moroccan Bouguer gravity anomaly data map.
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Modelling Phosphates “Disturbances” Depth using Analytical Signal Responses of Geoelectrical Resistivity Data (Sidi Chennane, Morocco)
By Saad BakkaliExploitation of the phosphate layers in Sidi Chennane deposit (Morocco) collides frequently with problems bound to the existence, in the phosphate series, of sterile bodies qualified as derangements. Our study shows that these bodies, masked by the Quaternary cover can be mapped using the geoelectrical prospecting survey. A Schlumberger resistivity survey over an area of 50 hectares was carried out. A new field procedure based on analytic signal response of resistivity data was tested to deal with the presence of phosphate deposit disturbances. Models of the geology were successfully obtained from surface modelling of 2D peaks of resistivity data. Image processing optimization was based on surface optimization Tools. Downward analytical continuation of the surface modelling along 30 meters depth was used for modelling Sidi Chennane phosphates “disturbances” distribution. Analytical procedures were found to be consistently useful. Optimization of phosphate reserves were improved and better constrained.
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Instrument Assisted Criteria for Freezing Damage Prevention
More LessFreeze-thaw damage is among the most detrimental factors on concrete durability. Cost of thermal heating also leads to high cost and delay during winter construction activities. This paper describes the development of mechanistic criteria assisted by instrument to prevent freezing damages for curing of early stage concrete. It is based on the mechanical physical and measurement criteria involving the determination of the free water content and the volume of air void. Preliminary results are provided.
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Noise Cancellation for Surface NMR: A Comparison of Time and Frequency Domain Approaches
Authors Mike Mueller-Petke and Ugur YaramanciEven though surface NMR is the only geophysical technique that provides hydrogeophysical rock properties on the base of direct sensitivity to subsurface water, in many cases surface NMR measurements suffer from bad signal-to-noise ratio, and measurements can be carried out only far from sources of electromagnetic noise.
to overcome these restriction some approaches using a reference loop based system were developed during the last years (Mueller-Petke and Yaramanci (2010), Radic (2006), Walsh (2008)). All approaches have demonstrated useful capability to improve S/N. But comparison that allows for determining properties, i.e., pro and contra of each approach is missing.
the scheme of Mueller-Petke and Yaramanci (2010) calculates a transfer function between detection loop and reference loop based on an optimal filter, i.e., working in the time domain. However, calculating transfer functions in the frequency domain (as proposed by Radic (2006) but not published as replicable and verifible) is close to the time domain approach. in order to test the frequency approach explicetely, we developed our own frequency domain code.
This now allows for evaluating and comparing important properties of the schemes such as (i) factor of improving S/N, (ii) appropriate filter length, (iii) accuracy and stability of the transfer function and (iv) numerical costs. Thus, for the first time we present a comprehensive comparison of the mathematical basis of the schemes and application of the time and frequency domain approach on synthetic and field data.
Finally we conclude that the frequency domain approach is to be preferred usually provided that the noise is stationary.
Mueller-Petke, M. and Yaramanci, U. 2010. Improving the signal-to-noise ratio of surface-NMR measurements by reference channel based noise cancellation. Proceedings of NearSurface 2010, Zurich.
Radic, T. 2006. Improving the signal-to-noise ratio of surface-NMR data due to the remote reference technique. Proceedings of NearSurface 2006, Helsinki.
Walsh, D.O. 2008. Multi-channel surface-NMR instrumentation and software for 1D/2D groundwater Investigations. Journal of Applied Geophysics, Vol. 66, 3-4, 2008.
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A Global Measure for Depth of Investigation
Authors Anders Vest Christiansen and Esben AukenDepth of Investigation (DOI) is a commonly requested parameter in geophysical surveys. for diffusive methods, such as groundbased or airborne EM, there is no specific depth below which there is no Information on the resistivity structure, but the question is to what depth the model is most reliable. We present a new robust concept for the calculation of DOI that is valid for any 1D EM geophysical model.
the method is based on the actual model output from the Inversion and includes the full system response, contrary to assuming e.g. planar waves over a homogeneous halfspace. Equally important, the data noise and the number of data points are integrated in our calculation. Our methodology is based on a recalculated sensitivity (Jacobian) matrix of the final model and it can thus be used on any model type for which a sensitivity matrix can be calculated.
Contrary to other sensitivity matrix methods we define a global and absolute threshold value contrary to defining a relative, say 5%, sensitivity limit. the threshold limit applies to all 1D inverted data and will thus produce comparable numbers of DOI.
in the presentation we will show examples from systems spanning from the ground conductivity scale over DC-equipment to large airborne EM systems. the DOI will be calculated using a global threshold enabling detailed comparison of resolution capabilities of very different systems and methods.
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Creating 3D Models of Clay Content - An Inversion Approach using Geophysics and Borehole Information
More LessWe present a concept to produce 3D-voxel models of clay content Integrating geophysical resistivity models and lithological Information from boreholes. the concept minimizes the difference between reported clay volumes in boreholes and calculated clay volumes based on geophysical resistivity models.
For sedimentary areas the clay content is the most important factor for the water infiltration speed and thereby the vulnerability of aquifers to pesticides, nitrate etc. with the methodology presented we produce 3D models of clay content in a manner that can be fed directly into hydrogeological models.
Borehole Information contains detailed Information on the clay contents, but borehole Information is too sparse for the detail level required in actual mapping situations. However, the clay contents are also reflected in the resistivity of the sediments and thereby in geophysical data measuring sensitive to resistivity.
in short, the concept estimates clay content in a 3D voxel model based on:
1. Clay distribution identified in boreholes and accompanying uncertainties.
2. Layered models obtained from Inversion of geophysical data including the parameter uncertainties.
3. A non-linear Inversion scheme minimizing the difference between observed clay contents (from boreholes) and calculated clay contents (from geophysics) using a spatial interpolation (kriging) to match the observation points in space.
the model which is the output of the Inversion scheme not only gives the best model for clay content, but it also describes the uncertainty of the model. the model greatly improves the overview of the huge data sets and it improves the basis for decisions regarding the area planning. The concept has been employed in a groundwater mapping campaigns in Denmark with promising results.
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Integration of Ground Penetrating Radar with Real Time Kinematic - Global Positioning System Receivers for Efficient Mapping of Drainage Pipe Systems Beneath Golf Course Greens
Authors Barry Allred and Robert FreelandThe U.S. alone has over 16,000 golf course facilities. the upkeep of these facilities requires continual maintenance and occasional remodeling. the superintendents and architects responsible for golf course maintenance and remodeling efforts need non-destructive Tools for obtaining shallow subsurface Information, particularly with regard to determining drainage pipe locations beneath golf course greens. Ground penetrating radar (GPR) has already been proven capable of finding buried drainage pipes under golf course greens; however, the use of this technology needs to be streamlined in order to map these drainage pipes in a timely manner with minimal disruption of golf course activities.
A case study was therefore initiated to evaluate the efficiency of Integrating GPR with real time kinematic - global positioning system (RTK-GPS) receivers to map drainage pipe systems beneath golf course greens. the Integration of GPR (using 400 MHz antennas) with RTK-GPS for drainage pipe mapping was tested at six different golf course green sites in central Ohio. Integration of GPR and RTK-GPS allowed data to be collected without a grid set-up of well defined measurement transects, and as a result, the time needed to conduct a golf course green GPR survey was reduced by half with the added benefit of no disruption to golf course activities. Furthermore, based on knowledge of typical golf course green drainage pipe installation practices, extremely simple and easy to use processing techniques can then be employed using the GPR/RTK-GPS data to generate accurate golf course green drainage pipe maps. Consequently, this case study provides strong evidence that Integration of GPR with RTK-GPS is a valuable tool that can be used by the golf course industry for efficient mapping golf course green drainage pipe systems.
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Employing Airborne Electromagnetics for Spatial and Temporal Hydrogeophysical Monitoring: A View from Opposite Ends of the Globe
Authors Andrea Viezzoli, Timothy Munday and anders Vest ChristiansenAlthough the notion of hydrogeophysical monitoring of natural landscapes using multi-temporal airborne EM data sets, has been around for some time, translating this into practice has been limited by several factors, including the availability of fully characterised systems and by accurate Interpretation procedures. there is also the issue of what we term “a will” – specifically a willingness to employ hydrogeophysical methods because they are trusted to yield quantitative Information on, for example, groundwater quality. This paper examines two contrasting cultures, namely an Australian and Danish one, and considers how hydrogeophysical monitoring is being developed and deployed there. in Denmark, over a period of nearly 5 years, large portions of the country have been surveyed with AEM. Derived datasets complement a large suite of ground- based TEM measurements (> 40000 soundings), dating back to the early 1990’s. Both data sets, after Interpretation, have been used to produce a seamless output covering acquisition with different instruments and configurations, often many years apart. This was made possible through the use of a national TEM test site, against which all instrumentation, had to be calibrated prior to use. This systematic approach provides the framework for hydrogeophysical monitoring, ensuring that the long term, quantitative measurement of change is a reality. By virtue of this national approach to groundwater management Denmark is uniquely placed to employ hydrogeophysical methods in spatio-temporal Investigations. in contrast, other countries such as Australia, who have a more fragmented, regional approach to groundwater management, with jurisdiction commonly the responsibility of individual States, have seen hydrogeophysical monitoring options confined to small areas and targeted, high value, assets. An example of this is the floodplains of the Murray River, where existing knowledge regarding floodplain behaviour during and after floods is limited and hydrogeophysical methods are seen as offering the promise of elucidating process.
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Automated Leak Detection of Buried Tanks using Geophysical Methods At the Hanford Nuclear Site
Authors Shawn Calendine, Dale Rucker, James Fink and Marc LevittAt the Hanford Nuclear Site in Washington State, the Department of Energy oversees the containment, treatment, and retrieval of high-level mixed radioactive waste. Much of the waste is currently stored in single-shelled tanks (SSTs) that do not meet current State criteria for waste storage. Currently, the waste is being retrieved from the SSTs and transferred into new double-shelled tanks (DSTs) for temporary storage before final treatment. Monitoring the tanks during the retrieval processes is critical in indentifying leaks and creating a warning system for the older SSTs.
An electrical geophysics monitoring program for Leak Detection and Monitoring (LDM) has been successfully deployed on several SSTs at the Hanford site since May of 2004. the monitoring program takes advantage of changes in resistivity that will occur when conductive tank liquid leaks into soil contiguous to the subsurface tank shell. During monitoring, voltages are measured and recorded between steel cased wells, surface electrodes, and the tanks themselves. Data acquisition hardware and software allow for continuous real-time monitoring of the received voltages through time-series data analysis, creating a highly sensitive method of leak detection and complementing drywell logging as a means to detect and quantify leaks. Working in an industrial environment such as the Hanford site involves many challenges for electrical monitoring, such as cathodic protection, grounded electrical infrastructure, lightning strikes, diurnal and seasonal temperature trends, and precipitation, to create a complex environment for leak detection analysis. in this discussion we present solutions to the complicated challenges of working in the tank farms of the Hanford site.
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Spectral Induced Polarization Monitoring during Microbial Enhanced Oil Recovery
Authors Jeffrey Heenan, Dimitrios Ntarlagiannis and Lee SlaterMicrobial Enhanced Oil Recovery (MEOR) has been established as a cost effective method for enhancing tertiary oil recovery. Although not commonly used for shallow heavy oils, it could be a viable alternative since it can offer sustainable economic recovery and minimal environmental impact. Successful MEOR treatments require accurate, real time monitoring of the biodegradation processes resulting from the Injection of microbial communities into the formation. Results of recent biogeophysical research suggest that minimally-invasive geophysical methods could significantly contribute to such monitoring efforts.
Here we present results of laboratory experiments to assess the sensitivity of the spectral Induced polarization method (SIP) to MEOR treatments. We used heavy oil from a shallow oilfield in SW Missouri to saturate three sand columns. We then followed common industry procedures and used a commercially available microbial consortia (Para-Bac/STM) to treat the oil columns. the active MEOR experiment was performed in duplicate while a control column maintained similar conditions, without promoting microbial activity. We monitored the SIP signatures, between 0.001 Hz and 1000 Hz, for a period of six months. to support the geophysical measurements we also monitored geochemical parameters, including pH, Eh and fluid conductivity, and collected weekly fluid samples from the outflow and inflow which were analyzed to confirm that microbes actively degraded the heavy oils in the column. Destructive analysis of the solid materials was performed upon completion of the experiment,
Preliminary analysis of the results suggests that SIP is sensitive to MEOR processes. in both inoculated columns, we recorded an increase in the low frequency polarization with time, where there were measurable changes up to 3.5 mrads in the phase shift recorded for both active columns, while no change was observed in the control column for the duration of the experiment. these results may indicate that remote geophysical methods could successfully complement current MEOR monitoring schemes for promoting sustainable oil recovery.
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Integration of Geological and Geophysical Data to Improve the Understanding of A Deep-Seated Landslide
Authors Diego Arosio, Davide Brambilla, Laura Longoni, Monica Papini, Giovanni Savazzi and Luigi ZanziHeavy rainfalls in late 2002 reactivated an ancient deep-seated landslide in the Varrone valley (Northern Italy). the unstable body threatens a hydroelectric power plant as well as a new industrial district placed along the valley line. Though the sliding surface is believed to be deeper than 100m, landslide reactivation concerned just a shallow portion of the slope close to the landslide toe. A detailed geological survey was undertaken after the events in 2002. inclinometers and piezometers were deployed in some boreholes, which were also used to assess rock layering and quality, along with the presence of discontinuities and of groundwater. to this final aim a preliminary Electrical Resistivity Tomography survey was also carried out. Recently, geophysical surveys were performed in order to validate geological assumptions and to assess the condition of the slope over a wider area. Ground penetrating radar profiles helped to determine the dip as well as the persistence of the discontinuities that are considered preferential paths for groundwater flow. A combined P- and S-wave refraction survey delineated the interface between shallow glacial sediments and underlying gneiss layer, and was used to infer elastic parameters of the subsurface. the outputs of the geophysical surveys were used to run a numerical model with the aim of evaluating the kinematics of the slide, possible future scenarios and the associated mitigation measures.
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Field Demonstration of Nuclear Magnetic Resonance (NMR) Logging Tools for Groundwater and Environmental Investigations
Nuclear magnetic resonance (NMR) measurements provide direct sensitivity to pore fluids enabling estimation of key petrophysical properties including porosity, bound/free fluid content, and permeability. NMR logging instruments have been used for decades in the oil and gas applications, but until recently, the size and high cost of oil-field Tools have limited the availability of this powerful technology for groundwater and environmental Investigations. We present results from field tests of a newly-developed suite of economical, small-diameter NMR logging Tools, specifically designed to meet the unique needs of near-surface Investigations. Principal field tests of two NMR probes were conducted at the Geohydrologic Experimental and Monitoring Site near Lawrence Kansas. One of the probes, is optimized for logging measurements in very small-diameter holes that are commonly available for environmental studies. with an outer diameter of 43 mm, this probe operates in small PVC or open-cased holes ranging from 50 to 100 mm. the second probe has a diameter of 64 mm and is designed for dynamic and minimally-invasive operation with a Geoprobe ® direct push machine. Both probes were operated with a minimum echo time spacing of 2.0 ms; the depth resolution is1.0 m for the 45 mm probe and 0.5 m for the direct-push probe. the NMR logs provide continuous distributions of the transverse (T2) relaxation time versus water content at each depth interval, which are used to derive estimates of porosity, bound/free water content, and hydraulic conductivity. Results from the two Tools were found to be consistent with one another and with auxiliary subsurface Information available at the extensively characterized site. the availability of these newly-developed and unique Tools open vast opportunities for utilizing NMR to enhance groundwater and environmental Investigations.
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Applications and Advantages of a Short Dead-Time for Surface Nuclear Magnetic Resonance (SNMR) Measurements
Authors David Walsh, Elliot Grunewald and Peter TurnerSurface nuclear magnetic resonance (SNMR) measurements provide non-invasive detection and characterization of groundwater by measuring the relaxation decay of hydrogen nuclei following excitation by a surface loop. A key factor controlling the sensitivity of the SNMR measurement to groundwater is the so-called dead-time, which describes the delay between the end of the excitation pulse and the first reliable recording of the NMR signal. the dead-time will fundamentally limit the shortest signals that can be detected and thus effectively controls the range of environments in which SNMR can be successfully used. We describe the recent development of very short dead-time SNMR instrumentation and application of these measurements in a series of case studies. We demonstrate the achievement of dramatically reduced dead-times, as short as 4 milliseconds, which enable detection of groundwater in environments previously considered inaccessible by SNMR. Specifically, we illustrate detection of very short NMR decay signals associated with water in magnetic geology, bound water in silts and clays, and capillary water in the vadose zone. the availability of a new small diameter NMR logging tool provides an opportunity to ground-truth the surface NMR data and confirms that these short signals are reliably detected using short dead-time surface NMR capabilities. Based on our field results, we discuss the advantages and implications of a short-dead time SNMR instrument for quantifying short signals, improving signal to noise ratios, and characterizing aquifer properties.
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A New Marine Acoustics Prospecting instrument with the Advantage of Sub-Bottom Profiler and Shallow Seismic Reflection Method On River and Sea
More LessIt is required for prospecting of subaqueous to explore the thickness fluctuation of subaqueous silt seam, sand layer, pebble bed, and riprap layer, fluctuation of subaqueous bedrock surface, burial depth of bedrock, fault in the bedrock and fracture zone. Sub-bottom profiler and seismic reflection method are the most common. Generally, the Sub-bottom profiler is adopted for exploration of thinks of silt seam, sand layer of bottom with high resolution from the high frequency band however the relatively poor penetrating power; while the low frequency band is adopted for the water shallow seismic reflection method, resulting high penetrating ability but low resolution, which makes it suitable for exploration of rock stratum and geological structure in straight measuring line. Is there any method or instrument combining the advantages of there two methods above? That is the very hope of the workers engaged in geophysical prospecting. KS marine acoustics prospecting instrument is designed to address such problems on the base of the aforementioned ideal, and was awarded the patent of invention. It is basically aimed at penetrating pebble and riprap layer, with enough sensitivity to measure the layer thickness above 1m, and replacing in whole or in part the shallow seismic reflection method on water to penetrate underwater rock strata. the author is striving to achieve breakthrough which are adopting the very wide frequency-band based receiving system to receive all reflection signals between tens of Hertz to 10kHz, adopting receiving detector with strong directing property and adopting the spark source that can control emission energy. It is possible for the instrument to explore silt seam, sand layer, pebble bed, top and bottom surface of riprap layer which are thicker than 1m, and deep stratum through collecting the reflection wave with a reflex time above 150ms.
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A New Version of Seismic Wave Reflection Method Fit for Urban Exploration —Landsonar
More LessThere are two problems on shallow exploration in urban:(a) frequent exposure to vibration noise and terrestrial electricity peculiar to city. Seismic methods are usually highly embarrassed by the vibration from the electric cars, pedestrians, engineering construction and operating metro lines; (b) Many geophysical exploration methods cannot meet the requirements on desired exploration depth and resolution. Landsonar method is the shortened form of an elastic reflection continues profiling with extremely small offset and very wide frequency-band, bringing the characteristics of zero-offset into full play. Adopting single-channel collection with almost zero-offset; increasing signal-to-noise ratio through vertical stacking ; Through combination of wave detector with very wide frequency-band and main unit of receiver, it is able to stimulate and receive the wave with 5Hz to 4000Hz without any frequency distortion (no compressing or magnifying wave with any frequency). with these features Lansonar can fit for urban shallow high resolution exploration , and several applications have got the correct feedback over deep of 150 m in China.
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Recent Developments in the Skytem System
Authors Kurt Sorensen, Esben Auken and Max HalkjaerThe SkyTEM system has originally been developed for groundwater mapping and is capable of measuring in a wide time range from very early decay times to late times without any bias correction applying high altitude data sets. the system has been extensively used since its introduction in 2002 and have proven its superior performance in ground water mapping as well as mineral exploration when near surface as well as deeper resolution is required. Further developments including higher transmitting moment by segmented transmitter techniques, higher signal to noise ration by motion noise suppression and adaptive signal processing and reduction of coil responses at very early times by on-time measurements have expanded the measurable time decay interval, the deep of penetration and the accuracy of the measurements, thus meeting the increasing demands for detailed and accurate Interpretation of data sets. Selected topics from the developments will be presented together with examples of applications.
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Integrating Hydrology and Geophysics to Evaluate the Impact of Artificial Recharge on Groundwater in Rural India
The monsoonal climate of India coupled with the complex geology and low storage capacity of the Deccan basalts contribute to water scarcity in central India during the dry season. One of the primary Tools proposed to manage this problem is the artificial recharge of runoff captured during the monsoon to enhance groundwater availability throughout the year. One common approach for artificial recharge is the construction of small dams to generate percolation ponds, as exemplified by a small reservoir in the Salri watershed of Mahdya Pradesh, India. We use this specific example to illustrate how the Integration geophysical and hydrologic data can be used to understand the influence of the dam on groundwater in the watershed. Electrical resistivity and electromagnetic induction surveys are used to assist in developing a geologic conceptual model for the watershed consisting of a thick sequence of basalt flows overlain in the lowland portion of the watershed by weathered basalt and alluvium for a depth of up to 10m. This geologic model has guided our understanding of the local flow system. A shallow flow system in the near-surface weathered basalts and alluvium is the primary source of water for agriculture. in contrast, vertical variability in the competent basalt flows is expected to create a highly anisotropic flow system with high horizontal permeability and low vertical permeability. As a result, the geophysical data help to form a conceptual model where the dam primarily impacts the shallow aquifer and has limited impact on deeper regional flow systems. to assess this hypothesis and quantify the impact of the dam on the overall hydrology of the watershed a hydrologic monitoring program was implemented. By Integrating the geophysically-based conceptual model with this hydrologic data we are able to provide a quantitative assessment of the role of the dam within the watershed.
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Estimation of Aquifer Transport Parameters from Resistivity Monitoring Data within a Coupled Inversion Framework
Authors Dylan Fowler and Stephen MoyseyDespite the fact that electrical resistivity surveys are being increasingly used to image solute migration in porous media, there is limited experience in how these data can be used to characterize aquifer transport parameters. in this study we evaluate whether electrical resistivity monitoring data can constrain the hydraulic conductivity, porosity, and dispersivity of a homogeneous aquifer within the framework of coupled Inversion. We focus on responses from a single set of current electrodes to provide insights into the Information content of these measurements and motivate future work in designing optimal surveys for monitoring groundwater transport processes. We have found that even in this simple system two distinct types of tradeoffs exist that could confound the direct estimation of transport parameters. First, different values of porosity and hydraulic conductivity can produce the same plume velocity, thereby leading to identical concentration distributions in the subsurface. in ideal cases this hydrologic non-uniqueness can be resolved by electrical data because electrical resistivity is dependent on both solute concentration and porosity, but this discrimination will only possible when the rock physics relationship between concentration and resistivity can be accurately determined. the second type of non-uniqueness occurs when different concentration plumes produce equivalent electrical responses. in this case, tradeoffs between plume velocity and dispersivity cause shifts in the position and mass of solute relative to the electrodes that result in equivalent values of subsurface apparent resistivity. This non-uniqueness can be overcome by explicitly recognizing that different Information about transport processes is captured by the timing versus magnitude of the electrical response in monitoring data. Though we have investigated a homogeneous system, we argue that the insights here can also be applied to the estimation of effective transport parameters for heterogeneous systems.
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Time-Lapse Imaging of Dynamic Systems using Multi-offset GPR Reflection Data
Authors Adam Mangel and Stephen MoyseyGround-penetrating radar is widely recognized as a powerful tool for mapping the subsurface to image structures and locate targets. However, velocity variations resulting from heterogeneity in soil texture and water content can greatly degrade the coherency of radar responses. This issue is particularly problematic in dynamic environments where the background environment may change over time, e.g., as a result of rainwater infiltration. to address this problem we investigate the radar signature of reflectors embedded in homogenous and layered soils during the course of a simulated infiltration event. the data are collected using multiple antenna offsets to allow for evaluation of traveltime and amplitude changes with offset. the result of this survey design is a rich data cube with dimensions of GPR traveltime, experiment time, and antenna offset. We demonstrate the value of this data by performing normal moveout velocity analysis for arrivals obtained over the course of the experiment. We show that we are able to perform time to depth corrections of the GPR data that provide a consistent image of the subsurface over the course of the experiment as moisture conditions change. for example, in experiments performed in a homogenous tank we show that the depth to the reflection produced at the bottom of the tank can be resolved with an error of ±5cm (±10%), where the error is mainly associated with picking errors. While this error can be somewhat reduced by improved traveltime picking, we have also found that hydrologic processes, e.g., the buildup of water at a seepage face formed at the bottom of the tank, contribute to complexity in the GPR data that requires analysis of wave interference for improved Interpretation. We suggest that multi-offset reflection GPR shows promise for both estimating velocity variations in the shallow subsurface and improving subsurface imaging.
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Integrating Geophysical Surveys with Geotechnical Investigations at offshore Wind Facilities
By John MadsenThree-dimensional variation in sediments due to their geologic setting is a common occurrence in marine environments. Adequate geophysical data, including that from acoustic, side-scan sonar and high-resolution seismic reflection surveys constrained by grab sampling, coring and video ground-truth, must be available to define the types and distribution of these sediments. Due to their having differing geotechnical properties, the three-dimensional variable distribution of sediments has significant implications for the siting and construction of offshore wind facilities along the East Coast of the US.
Geophysical surveys need to be integrated with geotechnical requirements at regional, local and site specific scales when developing offshore wind facilities. At the regional scale, geophysical, coupled with geological, data provide Information that along with first-order considerations such as wind, wave, and/or tidal conditions, water depths, access to onshore grid infrastructure, and ecological and human impacts, must be considered in the selection of a location for an offshore wind facility. the geologic setting is relevant because coastal marine areas have evolved under conditions of sea level rise and fall during the past several hundred thousand years. these conditions have determined the present and past sedimentary environments where facilities will be sited. At the local scale, geophysical data can be used to determine the distribution of surficial and sub-surface sediments. for example, paleochannels, ancient river valleys formed during periods of lower sea-level, are in-filled with coarser- to finer-grained sediments and preserved in the sub-surface. Shallow, high-resolution, seismic surveys can image networks of these paleochannels thus providing constraints on grain size and spatial variations of sediments. At the site specific scale, bottom and sub-bottom sediment types and their distribution play a role in selection of the type of wind turbine foundation (e.g., monopile, tripod pile structure, or gravity base), and in the subsequent location and installation of these foundations.
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High-Density Electromagnetic Induction Survey: Mapping the Archaeological Landscape At Ocmulgee National Monument, Georgia
More LessThis paper presents the results of a large-scale (8 ha), high-density electromagnetic induction survey carried out in 2010 at the famous Ocmulgee National Monument archaeological site located in central Georgia. Ocmulgee is the fourth largest (70 ha) mound site in the eastern United States and there is evidence of human occupation dating from the Paleo-Indian period (approximately 11,000 BC) to post American Civil War. High-density data collection methods were used to identify micro-variation in the subsurface since the archaeological targets (postholes, hearths, and pits) were often small. the continuous collection mode allowed me to gather data at rapid speed with close spacing between data points. I am able to distinguish between areas of high occupational density, areas of moderate occupational density, and areas that were relatively free of occupation. Anomalies representing different pre-contact and historic architectural types are discernable in some cases. the survey results indicate that EM conductivity is an effective technique for rapidly mapping large archaeological landscapes.
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Vs Comparison of Near Surface MASW to SCPT Data from Ontario
Authors Peter Hutchinson and Maggie BeirdShear wave velocities measured through Seismic Cone Penetration Tests (SCPT) at several locations in glacial sediments in south-central Ontario, Canada show poor correlation with MASW shear wave velocities to 8 to 9 meters below grade. SCPT near-surface measurements display Vs in the “rock” range; inconsistent with site geology. the correlation below 8 to 9 m is excellent, suggesting that the elastic wave front for the SCPT test is exploiting the drive pipe in the near-surface measurements. the prediction of Vs from MASW is more robust than those derived from the shallow portion of an SCPT test.
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In-Situ Measurements of Pore Pressure Generation and Nonlinear Shear Modulus Behavior at the Wildlife Liquefaction Array
Authors Brady Cox, Kenneth Stokoe and Ellen RathjeAn active, strain-based test that may be used to directly evaluate the liquefaction resistance of soils in-situ has been developed. the test is based on the premise of dynamically loading a native soil deposit in a manner similar to an earthquake, while simultaneously measuring its deformation and pore pressure response with push-in sensors. Dynamic loading is performed via a large hydraulic shaker (vibroseis) that is used to generate vertically propagating (downward), horizontally polarized shear waves (Svh-waves) of varying amplitude within an instrumented portion of a saturated soil deposit.
the new test method has been used to conduct field experiments in soft, saturated soil deposits 3 to 4m below the ground surface at the Wildlife Liquefaction Array (WLA) in Imperial Valley, California. WLA has been intensely studied over the past 25 years and has also been designated as a Network for Earthquake Engineering Simulation (NEES) site for the study of soil liquefaction. the extensive site characterization, the documented occurrence of earthquake-Induced soil liquefaction at the site twice in the 1980’s (1981, Mw =5.9 Westmorland earthquake; and 1987, Mw =6.6 Superstition Hills earthquake) and the likelihood for re-liquefaction of the site during subsequent earthquakes make WLA an ideal location for verifying the in-situ dynamic liquefaction test method.
in-situ liquefaction tests at WLA were successful at measuring: (1) excess pore water pressure generation, and (2) nonlinear shear modulus behavior in native silty-sand deposits as a function of Induced cyclic shear strain and number of loading cycles. these results are compared to pore pressure generation curves and nonlinear shear modulus curves previously developed for WLA soils from laboratory testing methods. Variations in the dynamic soil response across the site are also discussed. these accomplishments represent a large step forward in the ability to accurately evaluate the susceptibility of a soil deposit to earthquake-Induced liquefaction.
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Seismic Refraction Shootout: Tomographic Inversion using Geogiga Seismic Pro
More LessThis submission to the Seismic Refraction Shootout uses the Tomographic Inversion software from Seismic Pro 6.0, by Geogiga. the software is very interactive, giving the user extensive real-time Information relating to the activity being conducted.
One example of this is that the travel time curves are displayed alongside the shot record while picking first arrivals, allowing the user maintain a good sense of overall trends while picking.
Another example of the interactive nature of the software is the display options that are available during the Inversion process. the user can watch the ray paths superimposed on the current model, along with measured and modeled travel time curves updated in real time. This not only allows the user to stop the Inversion if they notice something going awry, but could provide users with a more intuitive understanding about the Tomographic process itself.of course the most important feature of any Tomographic Inversion package is the effectiveness of the Inversion algorithm used. This shoot-out should provide a good Initial check on the capabilities of the Inversion algorithm.
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Artificial Neural Networks Models for Determining the Basic Geotechnical Properties of Soils from Electrical Measurements
By Fred BoaduNon-invasive and cost effective geophysical techniques can be used to predict some useful petrophysical and engineering properties of unconsolidated geomaterials that are known to affect their geotechnical behavior, that is, stability and strength. We investigate the correlation of fundamental petrophysical and geotechncial engineering parameters of unconsolidated near surface materials with their electrical measurements in a laboratory environment using artificial neural networks and multivariate regression analysis. Spectral electrical response (SER) resistivity measurements are performed at a constant effective stress level on thirty two samples of varying textures and compositions. Engineering parameters which affect the mechanical behavior of soils, that is, fines content, specific surface area, pore size and fractal dimension of the particle size distribution are determined. Simultaneously, the electrical parameters which describe their electrical responses, that is, resistivity amplitude, phase shift, percent frequency effect, loss tangent and the normalized phase, are extracted from the SER measurements. the neural network is used to analyze the data and infer relationships between the engineering parameters describing the soils and their measured electrical parameters. these relationships are compared with the traditional multivariate regression models as a way of evaluating the strengths and weaknesses of the neural network in predicting the engineering parameters from the electrical measurements.
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Nitrate Contamination in Drinking Water from Farming Activities in Rural Communities in the NSAWAM District, Ghana: Environmental Geoscience, Education and Civic Engagement
Authors Fred Boadu and Frederick Owusu-NimoThe main source of water for drinking and domestic needs for inhabitants (mostly farmers) in the Nsawam District, Ghana is groundwater. However, these fractured groundwater reservoirs in this district pose to be vulnerable to nitrate contamination, and have been of growing environmental and health concern in recent times. Water samples collected from several groundwater wells in the study area indicate high nitrate concentration levels that significantly exceed the permissible limits for human consumption set by the World Health Organization (WHO). Azimuthal resistivity surveys (ARS) were conducted on exposed rocks with mapped fracture parameters in the vicinity of the seven wells where anisotropic coefficient, real and imaginary parts of the conductivity were measured. the specific surface area unexposed subsurface fractures were estimated using the regression model and the fracture porosity was estimated from the anisotropic coefficient. Fracture parameters, fracture porosity, specific surface area correlated with nitrate concentration. the results serve to establish the role of fractures in groundwater contamination by nitrates in the study area. in the summer of 2008, a group of students from Duke University from different majors (engineering, chemistry, biology, geology and global health), visited the study area to applying their knowledge gained in the classroom to address such societal problems with the intent of making a difference in the world. the students educated the inhabitants on potential health dangers regarding environmental pollution by direct interaction with locals, chiefs, school children and political representatives.
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Use of the Skytem Airborne Electromagnetics (AEM) System and Item Fast Approximate Inversion Software to Provide Robust and Rapid Data for Groundwater and Salinity Management, Ord River Irrigation Area, Western Australia
Authors Ken Lawrie, Kok Tan, Jon Clarke, Timothy Munday, Andrew Fitzpatrick, Larysa Halas and Heike AppsThe Ord Valley Airborne Electromagnetics (AEM) Interpretation Project was established to underpin salinity and groundwater management in the Ord River Irrigation Area (ORIA), in NW Australia. While covering the existing irrigation areas, it is also the first time in Australia that AEM data have been acquired to inform potential irrigation expansion. A total of 5,936 line km of AEM data were acquired using the SKYTEM time domain system. Conductivity signatures for key components of the hydrogeological system are non-unique. Interpretation therefore required the use of borehole and landscape analysis constraints. the study has provided greater spatial detail on critical elements of the hydrostratigraphy in the alluvial aquifer. This includes palaeochannel delineation, sand, gravel and clay distribution, the mapping of salt stores and groundwater quality, and the generation of salinity hazard maps. the project has demonstrated the potential for ‘calibrated’ AEM systems and Fast Approximate Inversion software to shorten AEM project timelines significantly. for example: - AEM data inverted using the fast approximate Inversion (iTEM) were obtained within 24 hrs of data acquisition. the early availability of high quality estimates of ground conductivity facilitated the early design of a drilling program, the extension of the survey whilst it was in progress, and the initiation of data Interpretation immediately. - the Initial multi-layer iTEM Inversions had high correlation coefficients (>0.8) when comparing FID points to adjacent borehole induction logs, and the Inversions have proven to be very robust.
Overall, this project has demonstrated the benefits of a phased or staged approach to assessing the methods and technologies used as part of a ‘hydrogeological systems’ approach when developing the framework for salinity and groundwater management. the AEM-based products are being used to assist with planning future irrigation infrastructure and strategies, the development of salinity mitigation and groundwater management strategies, and to protect environmental assets.
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Using Airborne Electromagnetics (AEM) for Rapid Identification and Assessment of Managed Aquifer Recharge (MAR) Options in the Darling Floodplain, N.S.W., Australia
Authors Ken Lawrie, Ross. S. Brodie, David Gibson, Kok Tan, Aaron Davis, Yusen Ley-Cooper, Larysa Halas and Jon ClarkeThe Broken Hill Managed Aquifer Recharge (BHMAR) project is part of a larger strategic effort aimed at securing the town of Broken Hill’s water supply and identifying significant water-saving measures for the Darling River system. to meet the challenge of rapidly identifying and assessing potential MAR targets and groundwater resources over a large area (>7,500 km2) within a short timeframe (18 months), it was concluded that the only cost-effective method was airborne electromagnetics (AEM). the SkyTEM time domain system was chosen and 31,834 line km of data was acquired in 9 weeks. the AEM data was validated by a 4,500m drilling program. the study has confirmed the location of several large potential MAR targets in close proximity (<40km) to the existing water supply infrastructure. the AEM survey has also mapped key confining aquitard units. ‘Holes’ in the overlying clay unit appear to provide preferential recharge of the underlying Pliocene aquifers along losing reaches of the Darling River and Talyawalka Creek. the hydrogeological complexity revealed by AEM mapping and validated by drilling provides a diversity of potential MAR options. At this same time, this brings with it a challenge of understanding a range of complex hydrogeological and hydrogeochemical processes. the provision of robust Inversions in short timeframes, Initially using the iTEM software and subsequently LCI and SCI Inversions using the Aarhus Workbench, greatly assisted with targeting drilling and rapid hydrogeochemical assessment of targets.
Overall, the BHMAR project is the first application of AEM technology and an integrated hydrogeological systems approach to MAR target detection and assessment. It has demonstrated the potential for large data-poor areas to be assessed rapidly, with a high degree of confidence. the success of the approach is dependent on the use of a phased approach within a broader risk assessment framework.
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Use of the Resolve Airborne Electromagnetic (AEM) Data in the Assessment of the Salinity Hazard and Risk to Iconic River and Wetland Ecosystems, Murray River, Se Australia
Authors Ken Lawrie, Kok Tan, Larysa Halas, Ross C. Brodie, Heike Apps and Laura GowAn AEM survey using the RESOLVE frequency domain system has been acquired along a 450 km reach of the Murray River in SE Australia. the AEM data were inverted using the holistic Inversion method, enabling key elements of the hydrogeological system in the shallow sub-surface (top 20-50m) to be mapped with high confidence levels. the AEM data have been used in conjunction with remote sensing, and hydrogeological and hydrogeochemical data obtained from drilling, to determine that healthy vegetation along the Murray River is generally associated with the presence of significant river ‘flush zones’ where fresh groundwater is present at shallow depths, and groundwater salinity is low. the study has also found that the corollary is true: where the river is ‘gaining’, and salt stores are high, vegetation health is generally in decline. Similarly, the AEM data show there is a marked decline in vegetation health towards the western edge of the iconic Gunbower State forest. This appears to be associated with salt being mobilised from irrigation districts on the western margins of the Gunbower forest. In the areas where the river flush zones are discontinuous, and the salt stores and water tables are closer to surface, there is also a risk of salt ingress to the river. in these areas, the data identify areas for targeted salinity management, including sites for potential Salt interception Schemes. This study fills important knowledge gaps particularly the distribution of key elements of the hydrostratigraphy, salinity extent, and the relationships between vegetation health, salinity and groundwater processes. in particular, the project has successfully integrated AEM, remote sensing, and lithological and hydrogeological data from drilling, to identify reaches of the River Murray and areas of iconic wetland ecosystems at risk from groundwater salinisation. these datasets provide geospatial context for targeted salinity and groundwater management actions.
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Surface Water Extent Trends in interior Alaska (1979-2009)
Authors Jennifer Rover, Lei Ji, Bruce Wylie and Larry TieszenRecent reports suggest that during the last few decades the surface extents of many high-latitudinal inland water bodies appear to be decreasing at alarming rates. Widespread declines in surface water have local and global impacts on ecosystems including carbon, methane, heat, and water fluxes. Because northern areas are susceptible to climate change and permafrost warming, we require an accurate account and understanding of surface water extent changes. in remote northern areas, remotely sensed data provide an efficient tool for monitoring water bodies and measuring their extents at multiple dates. However, a few remotely sensed measurements over a several-decade period may not produce reliable estimates of changes occurring to dynamic water bodies. Surface water extents can vary by water body, both seasonally and interannually due to functional differences in hydrology. in this study, we show that surface water extent trends in interior Alaska, developed from multi-date Landsat data, are highly influenced by the quantity and timing of the data collected. By taking natural variations into account through the use of 20 remote measurements that span multiple decades and seasons, we found that declines in surface water extents are not as prevalent as previously reported. Further, we found that most water bodies exhibited no statistically significant drying trends. Our results suggest that reliable assessments regarding the sustainability of northern lakes and ponds require a sufficient number of remotely sensed measurements to adequately account for natural interannual and seasonal variations. This clear documentation is essential for estimating the environmental consequences of changing surface water extents at northern latitudes.
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Alltem Uxo Discrmination Results from the Aberdeen Proving Ground using a Hybrid Generalized Neural Analysis and Standard Dipole Inversion and Classification Scheme
Authors Ted Asch and Michael FriedelAn advanced multi-axis electromagnetic induction system, ALLTEM, has been specifically designed for detection and discrimination of unexploded ordnance (UXO). This work has been funded by ESTCP (Project MM-0809). ALLTEM uses a continuous triangle-wave excitation that measures the target step response rather than the more common impulse response. Ferrous and non-ferrous metal objects have distinct characteristic responses. the system multiplexes through all three orthogonal (Hx, Hy, and Hz axes) transmitting loops and records a total of 19 different transmitting (Tx) and receiving (Rx) loop combinations with a spatial data sampling interval of 20 cm. This paper presents some of the results of a demonstration and validation survey at the Aberdeen Proving Ground in March 2010. the U.S. Geological Survey operated ALLTEM with a Leica 1200 GPS over the Army’s UXO Calibration and Blind Test Grids and the Direct Fire and indirect Fire areas. Custom data analysis is conducted from within Oasis Montaj including importing survey data, gridding, noise analysis for threshold determination, automatic selection of targets, and Inversion and classification. Batch Inversion of selected targets with a prolate spheroid starting model followed by the application of neural SOM (self organizing map) algorithms is used to automatically classify the objects into targets of interest and those of clutter. the SOM process is unsupervised and completely data driven. the goal of learning in the SOM is to cause different parts of the network (the data) to respond similarly to certain input patterns. the result of this process is clustering of similar target data in different parts of the ‘map’. This SOM analysis, used in conjunction with the standard numerical dipole Inversion results, provides a better understanding of what data is being provided to the SOM and to the numerical Inversion with the result being classification of UXO with greater confidence.
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Implimentation of Rapid SASW Method
More LessThe SASW method is the first broadly utilized surface method for characterizing the stiffness profiles of soil deposits and pavement layers. in the geotechnical engineering application, this method is particularly appealing as the new codes require the average shear wave velocity of the site down to a depth of about 30 m. in the pavement engineering, the method provides a more reliable modulus profile for the asphalt and base layers.
In the past twenty years several custom-made devices have been developed for rapid field-testing based on this approach.
The limitations of the surface methods, in general, and SASW method, in particular, are well-documented in the literature.
The main technical improvements of the SASW method in terms of data acquisition, construction of the dispersion curve and the Inversion process are discussed.
Case studies comparing the ease of use and the accuracy of the results from the traditional and improvements SASW program and are also included.
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Some Applications of Near Surface Geophysics to Earthquake Geohazards Investigations: Examples from Eastern Ontario Canada
Authors James Hunter, Heather Crow, André Pugin and Dariush MatazedianThe presence of thick soft soils is a major contributing factor to earthquake shaking response of structures. in the last several years, many national building codes have recognized the importance of soil effects, including shear strength, damping, amplification and soil-rock resonance. these parameters can now be measured or estimated in-situ using modern near-surface geophysical techniques and commonly the preferred measurement technique for seismic zonation is based on shear wave velocity structure of soil and bedrock; however, other active and passive, surface or invasive techniques, can also contribute valuable ancillary data for assessment of soil parameters.
in many areas of the Eastern Canada earthquake hazard zones, “gradient” amplification can occur when earthquake shear waves (body waves) travel from depth through increasingly low velocity zones towards ground surface. This can be further altered by focussing or de-focussing effects (from variations in subsurface soil stratigraphy) and from generation of local large amplitude Rayleigh and Love waves in soils. If a significant shear wave impedance boundary exists between soft soils and competent bedrock at depth, it is possible that very large resonance amplification at a “fundamental” site period can occur. the degree of amplification in soil is modified by the effect of soil damping. A low level of damping implies that sediments will not significantly attenuate small-strain earthquake ground motions, thereby resulting in increased surface motion.
in many areas of Eastern Canada, relatively inexpensive surface refraction and reflection analyses are most efficient for mapping lateral changes in subsurface structures with reasonable accuracy. invasive techniques such as the seismic cone penetrometer (SCPT) and downhole shear wave vertical seismic profiling (VSP), can yield very detailed vertical variations in velocities. Ancillary geophysical Tools such as surface electrical and EM profiling as well as slim hole geophysical logging can also help to detail other geotechnical parameters.
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A Guided Pilot Point Bayesian Inversion Approach for Monitoring Spatio-Temporal Dielectric Permittivity Variation in the Shallow Subsurface using GPR
Authors Neil Terry and Zhangshuan HouA method for estimating dielectric permittivity variations in the shallow subsurface using Tomographic radar first arrival times is presented. We expand upon our previous Minimum-relative-entropy (MRE)-Bayesian method by implementing a pilot point framework. We employ the principle of MRE as a means of non-subjectively choosing prior probability distribution functions (pdfs) for Inversion targets, and utilize a Quasi-Monte Carlo sampling technique to draw samples from these prior pdfs. in our previous study, we treated dielectric parameters as zonal unknowns on a coarse grid, in order to reduce the dimensionality of the inverse problem. in this study, the targets of Inversion are dielectric permittivity at chosen points selected from a fine-grid field and the parameters describing the correlation structure between these points. the number and locations of these points are selected in proportion to correlation length in order to avoid redundancy and capture areas likely to exhibit large contrasts in dielectric permittivity. for each numerical simulation, the dielectric and structural parameters are used to assign a dielectric permittivity value to each node on the grid through sequential Gaussian simulation. the output of the Inversion includes parameter estimates in the form of pdfs that can be used as priors for further updating when more datasets are available. the MRE-Bayesian method can profit from time-lapse datasets, handle nonlinearities between data and model parameters, and quantify uncertainty. the Integration of the pilot point method offers substantial improvement in the resolution of estimates and profits from knowledge regarding spatial correlation, such as known Information about the lithology of the study area.
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Fourth Year of Subsurface Drip Irrigation Monitoring using Gem2 Electromagnetic Surveys, Powder River Basin, Wyoming
Authors James Sams, Bruce Smith, Garret Veloski, Burke Minsley and Bethany BurtonThe National Energy Technology Laboratory and the U.S. Geological Survey are collaborating with BeneTerra LLC to comprehensively monitor a sub-surface drip irrigation (SDI) system at a site in the Powder River Basin (PRB) of Wyoming. Irrigation water for the SDI system is coalbed natural gas (CBNG) co-produced water. the study is being conducted at the Headgate Draw area, located approximately 17 km south of Arvada, Wyoming at the confluence of Crazy Woman Creek and the Powder River. the study site encompasses six alfalfa fields and covers an approximate area of 1.2 km2 (Figure 1).
Subsurface drip irrigation (SDI) is an emerging technology being applied in the PRB basin for the beneficial use of CBNG co-produced water. the pre-treated CBNG waters are applied to the root zones of agricultural land to aid in irrigation. Water drains from the perforated laterals at a rate of 0.75 to 1.5 gallons per day. This style of irrigation is capable of applying two to three times more water on a particular site than traditional surface irrigation (Engle and others 2009a and b). the method is designed to minimize environmental impacts by parking potentially detrimental salts in the vadose zone. This research project investigates the transport and fate of the water and salts from the injected CBNG produced waters at the SDI site, adjacent to the Powder River.
Ground, borehole, and helicopter electromagnetic conductivity surveys were conducted at the site prior to the installation of the SDI system (Sams and others 2008a and b; Smith and others, 2009). After the installation of the subsurface drip irrigation system, ground electromagnetic conductivity surveys have been performed quarterly (weather permitting). We summarize geophysical survey results from the first 4 years of a planned 5-year study of the SDI system.
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Assessing Water Storage Changes on the Field Scale Combining Superconducting Gravimeter Observations with a Hydrological Model
Authors Benjamin Creutzfeldt, Andreas Güntner, Hartmut Wziontek and Bruno MerzInformation on water storages is crucial for many different applications, like agricultural production, groundwater recharge or transport of contaminants. Limitations of observation techniques and high spatio-temporal variability make the estimation of water storage challenging, especially for deeper zones. Temporal gravimeter observations are significantly influenced by water storage changes (WSC) at the field scale and hence may provide valuable Information about the state of the hydrological system.
in this study, we assess the benefit of temporal gravimeter measurements as an integral signal for hydrological application by evaluating a hydrological model using residuals time series of a superconducting gravimeter (SG). A simple conceptual model is used to estimate local WSC in the snow, soil, unsaturated saprolite, and saturated aquifer storage. the model is calibrated and evaluated against SG data on the one hand and several groundwater and/or soil moisture data on the other. the model is validated against independently estimated WSC derived from a state-of-the-art lysimeter.
the results show that using an SG as calibration constraint improves the model results substantially in terms of predictive capability and variation of the behavioral model runs in comparison to classical hydrological point measurements. Gravity measurements integrate over different hydrological storage components and the sampling volume is several orders of magnitude larger than that for the point measurements. the general problem of specifying the Internal model structure or individual parameter sets can, however, not be solved with gravimeters alone. Additionally, the results show that also WSC in the deep vadose zone contribute significantly to the hydrological cycle, so SG might provide a tool to continuously and non-invasively monitor WSC also in this zone.
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Investigating the Deformational and Microstructural Changes in Unconsolidated Earth Materials using Complex Resistivity Measurements
Authors Frederick Owusu-Nimo and Fred BoaduMicro-structural modifications of near surface earth materials influence their geotechnical engineering behavior and stability conditions. the ability to characterize and monitor these modification non-invasively using geophysical techniques would be useful in geotechnical engineering Investigations. in this study, we conduct controlled laboratory experiments on sand-clay mixtures subjected to external loading and concurrently measure their deformational characteristics and frequency-dependent electrical resistivity (complex resistivity). the experimental setup consists of a modified oedometer with outlets for sample vacuuming, saturation and drainage, and is also fitted with pore pressure transducer, strain gauge and electrodes for electrical measurements. the samples are placed in the oedometer and saturated with water beyond their liquid limits and each subjected to an axial stress of 500kPa. the experiment is conducted under controlled drainage which allows measurements of changes in sample pore pressure and volume of water drained. At each effective stress level, the complex resistivity of the sample is measured over a frequency range (0.5Hz – 512Hz). the results show that electrical parameters obtained from the electrical response measurements are sensitive to the characteristic micro-structural changes occurring in the soil during the consolidation process. the relationship between the electrical parameters (resistivity amplitude, phase, capacitance, loss tangent) and the deformational and hydraulic parameters including axial strain, coefficient of compressibility, void ratio, dry density, pore size and hydraulic conductivity, influences the engineering properties and behavior of earth materials are also investigated. This study provides a good understanding of how changing state of soil and deformational characteristics can be predicted from electrical response measurements. the results will be useful in the use of geo-electrical methods in engineering, environmental and geohazard Investigations.
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Seismic Refraction Shootout: Presentation of True Model and Comparison with Estimated Models
Authors Colin Zelt, Seth Haines, Michael Powers, Jacob Sheehan and William DollThe purpose of this session is to provide an opportunity for all interested scientists to better understand the options and trade-offs involved in creating seismic velocity models from refraction travel time data. Submissions were encouraged from students new to seismic work, along with experienced experts. in this presentation, the true model will be revealed, along with quantitative comparisons with the estimated models presented during the session at the same scale and with the same display parameters. the accuracy of each final model will be discussed in some detail. the synthetic data consist of P-wave travel times for a seismic refraction experiment through a velocity model that represents a realistic near-surface geologic situation. Though not specifically intended to represent a karst environment, aspects of the model are consistent with a karst environment. there are unconsolidated, or semi-consolidated, sediments overlying a non-flat bedrock surface. the water table depth varies considerably in the area, between 20 and 100 m. there is zero surface topography along the seismic transect. the survey objective is to identify low velocity zones within the sedimentary layers, as well as features of the bedrock surface and within the bedrock. the travel times correspond to P-wave first arrivals, and were calculated assuming a 100 Hz wave. Uncorrelated Gaussianly-distributed noise with a standard deviation of 1 ms was added to the synthetic data. there are 101 shots from 0 to 300 m, and 100 receivers from 1.5 to 298.5 m. the shot and receiver spacing is 3 m. the total number of travel times is 10100.
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The Development of Geophysical Surveying Techniques in Permafrost at the Geological Survey of Canada: A Historical Perspective
By James HunterSince the late 1960’s, considerable effort has been directed towards oil and gas exploration in the Canadian arctic. to support this development and to provide regulatory Information, the Geological Survey of Canada has conducted extensive surficial geology mapping activities including detailed examination of permafrost occurrence and associated geotechnical parameters.
Central to such permafrost research is the regional geothermal regime. in the 1960s, in cooperation with various oil companies, geothermal Investigations and long term temperature monitoring were conducted in selected wildcat wells throughout the arctic. these wells were held in suspended status for many years for that purpose. Such Information gave useful data on the thickness limits and thermal susceptibility of permafrost.
in 1971, the GSC began testing various near-surface geophysical techniques at sites in the Western Arctic (Mackenzie River and Delta, Tuktoyaktuk Penninsula) where much of the early exploration had occurred and where pipeline routes were proposed. Later, surveying was extended to some areas of the arctic islands. Most test sites were associated with on-going Investigations of sensitive terrain features such as massive ground ice, river and lake thawing (taliks), as well as mapping discontinuous ice-bonding zones at lower latitudes. Electrical, electromagnetic, GPR and seismic techniques were tested. New surface and borehole techniques were developed.
As oil exploration moved northward into the Beaufort Sea in the mid 1970s, ice-bearing sub-seabottom permafrost was discovered by shallow marine seismic refraction profiles and geotechnical drilling. with industry collaboration, the GSC compiled a map of ice-bearing permafrost distribution from seismic refraction Interpretations. these results have been used as a guide for industry reflection processing corrections as well as for drill site and pipeline route planning.
Current GSC geophysical testing in permafrost is directed towards mining-related activities as well as geotechnical problems associated with northern community infrastructure.
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Integration of Surface and Borehole Geophysical Methods to Develop a Bedrock Model
Authors Mario Carnevale, Alex Buller and Jutta HagerA multi-method geophysical survey was performed at a contaminated facility in New Jersey to update the subsurface geological model for the project area, in order to advance the understanding of hydrogeologic conditions and constrain the placement of additional borings. Anomalous saprolite thicknesses, permeable stratigraphic contacts, and overturned stratigraphy introduced geologic complexities inhibiting the progress of the project.
More than 16,000 linear feet of geophysical data were collected over a survey area approximately 9 acres in size. GPR was selected as the primary method for modeling the saprolite and bedrock elevations, as well as profiling fill and other soil horizons above the saprolite.
Seismic reflection and DC resistivity methods were used to obtain data to confirm saprolite of variable thickness, competent bedrock surfaces, geologic fault structures, and other geophysical anomalies. Seismic and resistivity data were integrated within common depth intervals to emphasize bedrock structures detected by both methods as well as those unique to each method. Resistivity profiles also assisted in mapping the spatial distribution of different geologic units.
Borehole geophysical log data, obtained a year earlier, were integrated with the surface seismic data and provided key Information for deciphering complex reverse fault structures.
the data collected by these three complementary geophysical methods were used to create bedrock and saprolite topographic models. the bedrock models were used to provide a better understanding of the spatial extent of older and younger units and possible structural contacts between them due to reverse faulting.
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