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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 - 50 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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