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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
51 - 100 of 190 results
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An Intercomparison of Airborne Electromagnetic Systems for Hydrogeologic Studies
Authors Paul Bedrosian, Burke Minsley, Esben Auken, anders Vest Christiansen and Jared AbrahamMany of today’s airborne electromagnetic (AEM) systems arose out of the mining industry, where anomaly detection and geometric modeling were the primary interpretive products. the increasing use, however, of airborne electromagnetic surveys for hydrogeologic studies is driving increased demand for data that can be consistently inverted for accurate subsurface resistivity structure from the near surface through to depths of several hundred meters.
toward this goal, the USGS has been evaluating a number of commercial AEM systems over two test areas in Western Nebraska, USA. the selected test blocks have a conductive base-of-aquifer, the depth to which varies from zero to several hundred meters. the overlying aquifer shows significant lithologic heterogeneity, and is incised by both modern and ancient river systems. Finally, ground control from borehole lithologic and geophysical logs, as well as ground-based electromagnetic methods, is excellent. these test blocks are thus ideal for evaluating system resolution, sensitivity, depth-of-Investigation, top-of-Investigation, and the accuracy of inverted models in relation to ground truth.
We will show comparisons of several systems flown to date, and emphasize the importance of accurate system characterization (geometry, timing, waveform, bias, bandwidth, elevation, tilt). We will illustrate the effects of these parameters in the data space and also on inverted resistivity models using a consistent approach for all systems. the need for calibration is stressed and several approaches will be described.
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Estimating Liquid Water Content in Snow from Frequency Dependent Attenuation Analysis of Pulsed and FMCW Radars
Authors John Bradford and Hans Peter MarshallAt radar frequencies, dry snow is non-dispersive and the dielectric permittivity is primarily a function of snow density. Liquid water in the snow strongly alters the polarization properties and the dielectric permittivity becomes complex. the Complex Refractive index Method (CRIM) provides an accurate prediction of the complex dielectric permittivity in wet snow in the frequency range from 500 MHz – 4 GHz. Analysis of the CRIM equation shows that in wet snow, the real and imaginary components of permittivity are functions of both snow density and liquid water content. However, to a good approximation the frequency dependent attenuation is a function of water content only. Further, the slope of the attenuation coefficient vs frequency curve is a linear function of volumetric liquid water content. Given an appropriate reference spectrum, the method of spectral ratios provides a direct estimate of the slope of the attenuation vs frequency curve. We utilize a reflectivity method to investigate the radar response to wet snow. We have adapted the algorithm to simulate data acquired with both pulsed and frequency-modulated, continuous-wave (FMCW) radars. FMCW radars utilize a linear frequency sweep over a broad bandwidth and hold several important advantages over pulsed radar including a flatter spectrum which means that the signal-to-noise ratio will be higher over a broader frequency range. We have calculated the relationship between the slope of the attenuation coefficient curve and the liquid water content. We show that for a broad range of realistic snow conditions, this relationship reliably estimates the liquid water content in snow from both pulsed and FMCW radars operating in the 500 MHz – 2500 MHz range.
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A Feasibility Study on the Seismic Response of Select Nanoparticles in Saturated Granular Media
Authors Nihad Rajabdeen, Barbara Luke, Dale Werkema and Danney GlaserAs engineered nanomaterials become more prevalent, environmental releases of nanoparticles likewise increase. the environmental and human health impacts of these engineered nanoparticles are largely unknown. Currently, there are no proven methods of detecting their fate and transport in the subsurface. for this reason, new testing and detection techniques are being explored.
This research is part of the U.S. EPA National Exposure Research Laboratory (NERL) broad effort to conduct studies on the effects of engineered nanoparticles on ecosystems and human health. the seismic characteristics are used to establish defining features of select nanoparticles in saturated granular media. A bench-scale seismic testing apparatus is designed, built and optimized for measuring the seismic signatures of varying concentrations of engineered nanoparticles in saturated granular media. the ultimate goal of this research is to test the seismic method as a feasible approach to detecting nanoparticles in the subsurface under optimal conditions, while establishing the concentration detection limit for this method.
By actuating piezoceramic bender elements mounted inside a test column, shear and compression waves are generated and received through the saturated granular media within the column. the column system is Initially calibrated in air and water. the column is then filled with granular media and saturated with water. the seismic signature for this set of parameters establishes the baseline. Tests are then repeated with various nanoparticle dispersions. the dispersions to be tested include nano titanium dioxide, nano zinc oxide, nano silver, and nano zero valent iron. the seismic responses are to be analyzed for variances in travel time, amplitude and spectral content with respect to nanoparticle concentration. It is anticipated that the results will be applied to plot a “breakthrough curve” (BTC) of seismic response with respect to nanoparticle concentration levels. This BTC would reveal the nanoparticle concentration seismic detection point. the preliminary results of this research will guide future experiments investigating the utility of seismic methods in nanoparticle fate and transport studies.
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Geophysics for Rock Properties and Material Separation during Dredging and Expansion of the New York and New Jersey Harbors
The US Army Corps of Engineers and the Port Authority of New York and New Jersey are deepening 26 miles of navigation channels of New York harbor from -40 to -50ft MLW. the project requires dredging 50 million cubic yards of rock and sediment. At the halfway point, the project was ahead of schedule and under the US$1.6 billion budget.
Two key difficulties in the project are: (1) contaminated sediments and (2) undiggable rock. the cost of removing and placing contaminated black silt is 7 times the cost of removing and placing clean sands and silts. the cost of blasting and removing undiggable rock is 10 times the cost of removing sands and silts.
We have mapped the geology and the physical properties of the rocks and sediments throughout the project. All data, maps, and cross-sections are compiled into a single reference frame. All measurements are integrated in the Interpretation. the maps and cross-sections constitute the primary Information for managing the construction and engineering of the harbor deepening project.
We have developed geophysical techniques and operate them daily to determine the top-of-rock and to map and quantify the properties of the sediment, rock strata and quantify the area and thickness of black silt. Our techniques include calibration with core borings. Orthosonography yields aerial-photograph-like maps of the areal extent. Subbottom-seismic images, which are depth migrated, profile the thickness of the black silt and the approximate depth to undiggable rock. the black silt demonstrates anomalous properties and behavior from gray silt and other sediments. All images are georeferenced. We interpret the seismic sections through selection of horizons and estimation of physical properties. the seismic properties are correlated with mechanical properties to estimate diggability. the results are calibrated with core borings. We present the results as geological and geotechnical cross-sections with core borings.
Careful application of geology and geophysics to the New York Harbor Deepening Project allows for reduced uncertainty, improved project management, better planning, and reduced costs.
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Digital Soil Class Mapping at the Regional Level using Gamma-Ray Spectrometry and a Numerical Clustering Algorithm
Authors John Triantafilis and Nina EarlNatural resource management at the district level requires an understanding of the interrelationship between soil physical and chemical properties in order to identify soil management classes. However the collection of soil property Information at these levels and this purpose is cost-prohibitive. increasingly proximal and remotely sensed data sets are being used as surrogates to assist in identifying soil management classes. This includes the use of proximal sensors (e.g. electromagnetic (EM) induction instruments) and remotely sensed data (e.g. secondary terrain attributes derived from a DEM). However, even these have limitations owing to the need to cover large areas and their limited use on flat alluvial landscapes, respectively. Another option is the use of passive remote sensing techniques (e.g. gamma-ray spectrometry data) to map the regolith. Regardless of which ancillary data set is used an objective methodology is also required in order to identify structures or classes that may relate to soil properties and type. in this regard many researchers are increasingly using numerical clustering algorithms (e.g. fuzzy k-means analysis - FKM). in this research we demonstrate how gamma-ray spectrometry data can be used as a surrogate for the collection of soil property data. Structures are identified by passing the data through a FKM algorithm (FuzME v5.0). using indices such as the fuzziness performance index (FPI) and normalized classification entropy (NCE), we identify k = 11 classes and a fuzziness exponent ? = 2.0 for further Interpretation. the k = 11 classes correlate well with previously identified geological and geomorphological units in the Edgeroi district (i.e. eroded, alluvial and dust-mantled landforms). A wider Interpretation of the clustering showed that the k = 11 classes were also consistent with a broader soil survey of the lower Namoi valley. the inclusion of EM data could enhance the k =11 classes by providing detail on subsoil changes as well as top soil changes within the landscape.
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Impact of Soil Vapor Extraction On SIP Measurements
Authors Ryan Joyce, Estella Atekwana, Dale Werkema and Eliot AtekwanaThe Wurtsmith Air force Base (WAFB) and Carson City refinery sites have long been known as locations exhibiting active bioremediation of hydrocarbons. the geophysical response over the hydrocarbon contaminants has been well documented by numerous authors. Recently the WAFB site has undergone extensive remediation (soil vapor extraction) efforts to remove the contaminant mass above the water table. the WAFB data and the Carson City data have been compared to see the effect that remediation efforts have had on the geophysical signatures of the sites. in both studies, sediment cores were collected from locations inside and outside of the contaminant plumes: one outside the plume and two inside. the contaminated cores were collected at a location proximal to the contaminant source and distant from the source to determine the effect that distance plays on the results. Spectral Induced polarization measurements (0.1Hz – 10kHz) were then taken on the cores to develop a SIP depth profile of the contaminant plumes. When the WAFB and Carson City cores are compared, the Carson City data shows a clear increase in real and imaginary conductivity components when inside the contaminant mass while the WAFB data does not show any significant response. This lack of response at the WAFB site is due to removal of the contaminant body and carbon source for the microbial populations that were once at the site. the contaminant body and microbial activity is still active at the Carson City location and exhibits a strong SIP response.
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Integration of Seismic Surveys to investigate an Abandoned Mine Site
Authors Diego Arosio, Laura Longoni, Monica Papini and Luigi ZanziThe near-surface rock structure (about 50m thick) that covers an abandoned marl mine nearby the small village of Montevecchia (Italy) was investigated through a combination of seismic surveys. the mine was abandoned in 1958, after a massive collapse that involved all mine levels causing a large sinkhole on the top of the hill where the mine is placed. the seismic experiments had to be performed outside the mine because, at present, nearly all mine levels are flooded. the only accessible gallery is the upper one, but its direct inspection is considered unsafe and limited to a small section.
the methods selected for these Investigations were Refraction Seismics and Multichannel Analysis of Surface Waves (MASW). in addition, a sort of transillumination experiment was tried by posing two geophones inside the mine at the entrance of the upper gallery while the seismic source was moved along the road above the mine. All the methods were successful and gave complementary Information. Refraction seismics was used to characterize the upper low velocity layer and the second layer of the near-surface structure. the MASW method was necessary to assess the existence of a velocity Inversion revealing the presence of a low velocity layer trapped between the 2nd layer and a 4th high velocity layer covering the upper mine gallery. the transillumination experiment validated the presence of the 4th layer and gave an estimate of the average velocity that represents a lower boundary for the P-wave velocity within this layer. All the methods were also consistent in indicating the areas where the rock structure is more affected by fractures and discontinuities. Finally, both refraction and transillumination data were analyzed to derive average estimates of attenuation level and rock quality factor.
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Managing Residential Development in Karst Landscapes, Perth Metropolitan Area, South Western Australia
Authors Andrew Spyrou and Benjamin WilkinsAs the business and administration centre for the resource rich state of Western Australia, population growth in the Perth Metropolitan region has been consistently greater than the national average. the metropolitan area is located on the Swan Coastal Plain, a stretch of land which lies between the Indian Ocean and the Darling Scarp. Much of this region forms part of a karst belt which runs along the west Australian southern coast to 20 km inland. the belt is comprised of one main geological unit, the Tamala Limestone which is typically a soft porous rock that is highly susceptible to dissolution, with karst weathering often resulting in the development of sinkholes, open voids and caves.
Due to its geological setting, the pressure of new developments and the subdivision of land within the Perth Metropolitan region pose a risk with geohazards typical of karst environments. Urbanisation such as roads, buildings and construction can often acts of catalysts for karst formation by altering the flow of rain water runoff, applying increased loads on the surface, and increased vibrational noise. Urbanisation of karst environments presents a number of challenges for urban planners and engineers with structural failure of buildings and infrastructure a reality. This poses the question: How to manage urban development in karst landscapes in Western Australia’s fastest growing residential area?
This paper will present a number of case studies undertaken in residential developments in the Perth region detailing how geophysical methods have been used to map the limestone bedrock and identify karstic formations of potential risk. Ground penetrating radar and seismic methods have proven to be an affective combination of techniques over this environment with the sand dominant subsurface providing excellent transmission of radar wave energy and the basic two layer sand and limestone strata providing a distinct seismic refractive interface.
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Refuse Conductivity Variations Following Leachate Injection in a Bioreactor Landfill Cell: Modeling EM Results and Comparison with Well Logs
Authors Philip Carpenter and Krishna ReddyBioreactor landfills enhance waste degradation through recirculation of leachate inside the waste mass. in this study frequency-domain electromagnetic (EM) conductivity was measured along several profile lines before, during and after leachate Injection at the bioreactor cell of the Orchard Hills landfill, 15 km south of Rockford, Illinois. Electrical resistivity Tomography (ERT) measurements were also made before, during and after leachate recirculation. One profile line was located above and coincident with a leachate recirculation line (LRL). the other profiles were perpendicular to LRLs, which varied in depth from 5-15 m below the EM lines. Apparent conductivity increased along segments of the EM profile over the LRL during Injection of 17.8 m3 of leachate, when measured with a Geonics EM34 at 10 and 20 m vertical dipole separations. forward modeling of the EM data, using three layer models (cover, refuse above leachate Injection, and refuse below leachate Injection) suggest major increases in refuse conductivity below the Injection level. in some cases the resistivity of this lower layer decreases by 50% after Injection, from approximately 10 to 5 ohm-m. the apparent non-uniform nature of the Injection along the LRL is also examined through forward modeling of EM data, as is the lack of response during a second experiment in which a smaller leachate volume was injected. Well logging with a slim-line conductivity sonde adjacent to a gas extraction well also reveals a major conductivity increase below 8 m depth in the bioreactor cell. Natural gamma logs clearly show the depth extent of cover materials and a bentonite seal; refuse typically exhibits natural gamma values of 20-30 cps.
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Integrated Surface and Borehole Geophysics to Map Subsurface Cavities and Fractures Near a Superfund Site South of Rockford, Illinois
Authors Ryan Adams and Philip CarpenterThe ACME Superfund site is one of many Superfund sites in Northern Illinois. This 20 acre (8.1 ha) site was contaminated by various volatile organic compounds (VOC’s) and heavy metals during the 1960-1980s. in order to help formulate a conceptual hydrogeological model, subsurface lithology and structural features in the small valley immediately south of the site are examined using surface resistivity, frequency-domain electromagnetics (FDEM) and seismic surveys, as well as natural gamma, EM conductivity and DC conductivity logs. Natural gamma logs were most effective for defining subsurface lithology. Steel well casings on some wells and calibration problems with the EM sonde lead to poor quality conductivity logs. Bed thicknesses were determined from the gamma logs using the full-width-at-half- maximum method. Depth to bedrock, and existence of cavities, were verified by GeoprobeTM surveys. Results of the first phase of this project suggest hydraulic isolation of the surface and shallow groundwater (probably a perched aquifer), and the discovery of a network of karstic fractures and caverns in the deeper epikarstal bedrock (below 6 m depth), which contain contaminated water. the epikarst appears to be saturated, forming a highly transmissive zone. Recent GeoprobeTM conductivity logs agree qualitatively with 1D resistivity sounding models. GeoprobeTM surveys also revealed previously unmapped cavities in the northeastern part of the site which are probably connected to a larger fracture system. these cavities are also evident in FDEM surveys from this area. the hydrogeological system thus consists of a shallow perched flow system overlying a deeper fracture-controlled system in the epikarstal bedrock.
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Application of the Automatic Optimal Spline Smoothing Method to Optimizing Edges of Moroccan Bouguer Gravity Anomaly Map
More LessWe present a singular method that is capable to filter out noise as well as suppress outliers of sampled real functions under fairly general conditions. from an a priori selection of the number of points that define the adjusting spline, but not their location in that curve, the automatic optimal spline smoothing method automatically determines the adjusting cubic spline in a least-squares optimal sense. the method is fast and easily allows for selection of various possible number of knots, adding a desirable flexibility to the procedure. As an illustration, we apply the AOSS method to Moroccan Bouguer gravity data map. the AOSS smoothing technique is an efficient tool in the Interpretation of geophysical potential field data particularly suitable in denoising, filtering and analyzing gravity data singularities. the AOSS smoothing and filtering technique was found to be consistently useful for optimizing edges and contours of geophysical data maps as Moroccan Bouguer gravity anomaly data map.
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Modelling Phosphates “Disturbances” Depth using Analytical Signal Responses of Geoelectrical Resistivity Data (Sidi Chennane, Morocco)
By Saad BakkaliExploitation of the phosphate layers in Sidi Chennane deposit (Morocco) collides frequently with problems bound to the existence, in the phosphate series, of sterile bodies qualified as derangements. Our study shows that these bodies, masked by the Quaternary cover can be mapped using the geoelectrical prospecting survey. A Schlumberger resistivity survey over an area of 50 hectares was carried out. A new field procedure based on analytic signal response of resistivity data was tested to deal with the presence of phosphate deposit disturbances. Models of the geology were successfully obtained from surface modelling of 2D peaks of resistivity data. Image processing optimization was based on surface optimization Tools. Downward analytical continuation of the surface modelling along 30 meters depth was used for modelling Sidi Chennane phosphates “disturbances” distribution. Analytical procedures were found to be consistently useful. Optimization of phosphate reserves were improved and better constrained.
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Instrument Assisted Criteria for Freezing Damage Prevention
More LessFreeze-thaw damage is among the most detrimental factors on concrete durability. Cost of thermal heating also leads to high cost and delay during winter construction activities. This paper describes the development of mechanistic criteria assisted by instrument to prevent freezing damages for curing of early stage concrete. It is based on the mechanical physical and measurement criteria involving the determination of the free water content and the volume of air void. Preliminary results are provided.
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Noise Cancellation for Surface NMR: A Comparison of Time and Frequency Domain Approaches
Authors Mike Mueller-Petke and Ugur YaramanciEven though surface NMR is the only geophysical technique that provides hydrogeophysical rock properties on the base of direct sensitivity to subsurface water, in many cases surface NMR measurements suffer from bad signal-to-noise ratio, and measurements can be carried out only far from sources of electromagnetic noise.
to overcome these restriction some approaches using a reference loop based system were developed during the last years (Mueller-Petke and Yaramanci (2010), Radic (2006), Walsh (2008)). All approaches have demonstrated useful capability to improve S/N. But comparison that allows for determining properties, i.e., pro and contra of each approach is missing.
the scheme of Mueller-Petke and Yaramanci (2010) calculates a transfer function between detection loop and reference loop based on an optimal filter, i.e., working in the time domain. However, calculating transfer functions in the frequency domain (as proposed by Radic (2006) but not published as replicable and verifible) is close to the time domain approach. in order to test the frequency approach explicetely, we developed our own frequency domain code.
This now allows for evaluating and comparing important properties of the schemes such as (i) factor of improving S/N, (ii) appropriate filter length, (iii) accuracy and stability of the transfer function and (iv) numerical costs. Thus, for the first time we present a comprehensive comparison of the mathematical basis of the schemes and application of the time and frequency domain approach on synthetic and field data.
Finally we conclude that the frequency domain approach is to be preferred usually provided that the noise is stationary.
Mueller-Petke, M. and Yaramanci, U. 2010. Improving the signal-to-noise ratio of surface-NMR measurements by reference channel based noise cancellation. Proceedings of NearSurface 2010, Zurich.
Radic, T. 2006. Improving the signal-to-noise ratio of surface-NMR data due to the remote reference technique. Proceedings of NearSurface 2006, Helsinki.
Walsh, D.O. 2008. Multi-channel surface-NMR instrumentation and software for 1D/2D groundwater Investigations. Journal of Applied Geophysics, Vol. 66, 3-4, 2008.
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A Global Measure for Depth of Investigation
Authors Anders Vest Christiansen and Esben AukenDepth of Investigation (DOI) is a commonly requested parameter in geophysical surveys. for diffusive methods, such as groundbased or airborne EM, there is no specific depth below which there is no Information on the resistivity structure, but the question is to what depth the model is most reliable. We present a new robust concept for the calculation of DOI that is valid for any 1D EM geophysical model.
the method is based on the actual model output from the Inversion and includes the full system response, contrary to assuming e.g. planar waves over a homogeneous halfspace. Equally important, the data noise and the number of data points are integrated in our calculation. Our methodology is based on a recalculated sensitivity (Jacobian) matrix of the final model and it can thus be used on any model type for which a sensitivity matrix can be calculated.
Contrary to other sensitivity matrix methods we define a global and absolute threshold value contrary to defining a relative, say 5%, sensitivity limit. the threshold limit applies to all 1D inverted data and will thus produce comparable numbers of DOI.
in the presentation we will show examples from systems spanning from the ground conductivity scale over DC-equipment to large airborne EM systems. the DOI will be calculated using a global threshold enabling detailed comparison of resolution capabilities of very different systems and methods.
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Creating 3D Models of Clay Content - An Inversion Approach using Geophysics and Borehole Information
More LessWe present a concept to produce 3D-voxel models of clay content Integrating geophysical resistivity models and lithological Information from boreholes. the concept minimizes the difference between reported clay volumes in boreholes and calculated clay volumes based on geophysical resistivity models.
For sedimentary areas the clay content is the most important factor for the water infiltration speed and thereby the vulnerability of aquifers to pesticides, nitrate etc. with the methodology presented we produce 3D models of clay content in a manner that can be fed directly into hydrogeological models.
Borehole Information contains detailed Information on the clay contents, but borehole Information is too sparse for the detail level required in actual mapping situations. However, the clay contents are also reflected in the resistivity of the sediments and thereby in geophysical data measuring sensitive to resistivity.
in short, the concept estimates clay content in a 3D voxel model based on:
1. Clay distribution identified in boreholes and accompanying uncertainties.
2. Layered models obtained from Inversion of geophysical data including the parameter uncertainties.
3. A non-linear Inversion scheme minimizing the difference between observed clay contents (from boreholes) and calculated clay contents (from geophysics) using a spatial interpolation (kriging) to match the observation points in space.
the model which is the output of the Inversion scheme not only gives the best model for clay content, but it also describes the uncertainty of the model. the model greatly improves the overview of the huge data sets and it improves the basis for decisions regarding the area planning. The concept has been employed in a groundwater mapping campaigns in Denmark with promising results.
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Integration of Ground Penetrating Radar with Real Time Kinematic - Global Positioning System Receivers for Efficient Mapping of Drainage Pipe Systems Beneath Golf Course Greens
Authors Barry Allred and Robert FreelandThe U.S. alone has over 16,000 golf course facilities. the upkeep of these facilities requires continual maintenance and occasional remodeling. the superintendents and architects responsible for golf course maintenance and remodeling efforts need non-destructive Tools for obtaining shallow subsurface Information, particularly with regard to determining drainage pipe locations beneath golf course greens. Ground penetrating radar (GPR) has already been proven capable of finding buried drainage pipes under golf course greens; however, the use of this technology needs to be streamlined in order to map these drainage pipes in a timely manner with minimal disruption of golf course activities.
A case study was therefore initiated to evaluate the efficiency of Integrating GPR with real time kinematic - global positioning system (RTK-GPS) receivers to map drainage pipe systems beneath golf course greens. the Integration of GPR (using 400 MHz antennas) with RTK-GPS for drainage pipe mapping was tested at six different golf course green sites in central Ohio. Integration of GPR and RTK-GPS allowed data to be collected without a grid set-up of well defined measurement transects, and as a result, the time needed to conduct a golf course green GPR survey was reduced by half with the added benefit of no disruption to golf course activities. Furthermore, based on knowledge of typical golf course green drainage pipe installation practices, extremely simple and easy to use processing techniques can then be employed using the GPR/RTK-GPS data to generate accurate golf course green drainage pipe maps. Consequently, this case study provides strong evidence that Integration of GPR with RTK-GPS is a valuable tool that can be used by the golf course industry for efficient mapping golf course green drainage pipe systems.
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Employing Airborne Electromagnetics for Spatial and Temporal Hydrogeophysical Monitoring: A View from Opposite Ends of the Globe
Authors Andrea Viezzoli, Timothy Munday and anders Vest ChristiansenAlthough the notion of hydrogeophysical monitoring of natural landscapes using multi-temporal airborne EM data sets, has been around for some time, translating this into practice has been limited by several factors, including the availability of fully characterised systems and by accurate Interpretation procedures. there is also the issue of what we term “a will” – specifically a willingness to employ hydrogeophysical methods because they are trusted to yield quantitative Information on, for example, groundwater quality. This paper examines two contrasting cultures, namely an Australian and Danish one, and considers how hydrogeophysical monitoring is being developed and deployed there. in Denmark, over a period of nearly 5 years, large portions of the country have been surveyed with AEM. Derived datasets complement a large suite of ground- based TEM measurements (> 40000 soundings), dating back to the early 1990’s. Both data sets, after Interpretation, have been used to produce a seamless output covering acquisition with different instruments and configurations, often many years apart. This was made possible through the use of a national TEM test site, against which all instrumentation, had to be calibrated prior to use. This systematic approach provides the framework for hydrogeophysical monitoring, ensuring that the long term, quantitative measurement of change is a reality. By virtue of this national approach to groundwater management Denmark is uniquely placed to employ hydrogeophysical methods in spatio-temporal Investigations. in contrast, other countries such as Australia, who have a more fragmented, regional approach to groundwater management, with jurisdiction commonly the responsibility of individual States, have seen hydrogeophysical monitoring options confined to small areas and targeted, high value, assets. An example of this is the floodplains of the Murray River, where existing knowledge regarding floodplain behaviour during and after floods is limited and hydrogeophysical methods are seen as offering the promise of elucidating process.
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Automated Leak Detection of Buried Tanks using Geophysical Methods At the Hanford Nuclear Site
Authors Shawn Calendine, Dale Rucker, James Fink and Marc LevittAt the Hanford Nuclear Site in Washington State, the Department of Energy oversees the containment, treatment, and retrieval of high-level mixed radioactive waste. Much of the waste is currently stored in single-shelled tanks (SSTs) that do not meet current State criteria for waste storage. Currently, the waste is being retrieved from the SSTs and transferred into new double-shelled tanks (DSTs) for temporary storage before final treatment. Monitoring the tanks during the retrieval processes is critical in indentifying leaks and creating a warning system for the older SSTs.
An electrical geophysics monitoring program for Leak Detection and Monitoring (LDM) has been successfully deployed on several SSTs at the Hanford site since May of 2004. the monitoring program takes advantage of changes in resistivity that will occur when conductive tank liquid leaks into soil contiguous to the subsurface tank shell. During monitoring, voltages are measured and recorded between steel cased wells, surface electrodes, and the tanks themselves. Data acquisition hardware and software allow for continuous real-time monitoring of the received voltages through time-series data analysis, creating a highly sensitive method of leak detection and complementing drywell logging as a means to detect and quantify leaks. Working in an industrial environment such as the Hanford site involves many challenges for electrical monitoring, such as cathodic protection, grounded electrical infrastructure, lightning strikes, diurnal and seasonal temperature trends, and precipitation, to create a complex environment for leak detection analysis. in this discussion we present solutions to the complicated challenges of working in the tank farms of the Hanford site.
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Spectral Induced Polarization Monitoring during Microbial Enhanced Oil Recovery
Authors Jeffrey Heenan, Dimitrios Ntarlagiannis and Lee SlaterMicrobial Enhanced Oil Recovery (MEOR) has been established as a cost effective method for enhancing tertiary oil recovery. Although not commonly used for shallow heavy oils, it could be a viable alternative since it can offer sustainable economic recovery and minimal environmental impact. Successful MEOR treatments require accurate, real time monitoring of the biodegradation processes resulting from the Injection of microbial communities into the formation. Results of recent biogeophysical research suggest that minimally-invasive geophysical methods could significantly contribute to such monitoring efforts.
Here we present results of laboratory experiments to assess the sensitivity of the spectral Induced polarization method (SIP) to MEOR treatments. We used heavy oil from a shallow oilfield in SW Missouri to saturate three sand columns. We then followed common industry procedures and used a commercially available microbial consortia (Para-Bac/STM) to treat the oil columns. the active MEOR experiment was performed in duplicate while a control column maintained similar conditions, without promoting microbial activity. We monitored the SIP signatures, between 0.001 Hz and 1000 Hz, for a period of six months. to support the geophysical measurements we also monitored geochemical parameters, including pH, Eh and fluid conductivity, and collected weekly fluid samples from the outflow and inflow which were analyzed to confirm that microbes actively degraded the heavy oils in the column. Destructive analysis of the solid materials was performed upon completion of the experiment,
Preliminary analysis of the results suggests that SIP is sensitive to MEOR processes. in both inoculated columns, we recorded an increase in the low frequency polarization with time, where there were measurable changes up to 3.5 mrads in the phase shift recorded for both active columns, while no change was observed in the control column for the duration of the experiment. these results may indicate that remote geophysical methods could successfully complement current MEOR monitoring schemes for promoting sustainable oil recovery.
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Integration of Geological and Geophysical Data to Improve the Understanding of A Deep-Seated Landslide
Authors Diego Arosio, Davide Brambilla, Laura Longoni, Monica Papini, Giovanni Savazzi and Luigi ZanziHeavy rainfalls in late 2002 reactivated an ancient deep-seated landslide in the Varrone valley (Northern Italy). the unstable body threatens a hydroelectric power plant as well as a new industrial district placed along the valley line. Though the sliding surface is believed to be deeper than 100m, landslide reactivation concerned just a shallow portion of the slope close to the landslide toe. A detailed geological survey was undertaken after the events in 2002. inclinometers and piezometers were deployed in some boreholes, which were also used to assess rock layering and quality, along with the presence of discontinuities and of groundwater. to this final aim a preliminary Electrical Resistivity Tomography survey was also carried out. Recently, geophysical surveys were performed in order to validate geological assumptions and to assess the condition of the slope over a wider area. Ground penetrating radar profiles helped to determine the dip as well as the persistence of the discontinuities that are considered preferential paths for groundwater flow. A combined P- and S-wave refraction survey delineated the interface between shallow glacial sediments and underlying gneiss layer, and was used to infer elastic parameters of the subsurface. the outputs of the geophysical surveys were used to run a numerical model with the aim of evaluating the kinematics of the slide, possible future scenarios and the associated mitigation measures.
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Field Demonstration of Nuclear Magnetic Resonance (NMR) Logging Tools for Groundwater and Environmental Investigations
Nuclear magnetic resonance (NMR) measurements provide direct sensitivity to pore fluids enabling estimation of key petrophysical properties including porosity, bound/free fluid content, and permeability. NMR logging instruments have been used for decades in the oil and gas applications, but until recently, the size and high cost of oil-field Tools have limited the availability of this powerful technology for groundwater and environmental Investigations. We present results from field tests of a newly-developed suite of economical, small-diameter NMR logging Tools, specifically designed to meet the unique needs of near-surface Investigations. Principal field tests of two NMR probes were conducted at the Geohydrologic Experimental and Monitoring Site near Lawrence Kansas. One of the probes, is optimized for logging measurements in very small-diameter holes that are commonly available for environmental studies. with an outer diameter of 43 mm, this probe operates in small PVC or open-cased holes ranging from 50 to 100 mm. the second probe has a diameter of 64 mm and is designed for dynamic and minimally-invasive operation with a Geoprobe ® direct push machine. Both probes were operated with a minimum echo time spacing of 2.0 ms; the depth resolution is1.0 m for the 45 mm probe and 0.5 m for the direct-push probe. the NMR logs provide continuous distributions of the transverse (T2) relaxation time versus water content at each depth interval, which are used to derive estimates of porosity, bound/free water content, and hydraulic conductivity. Results from the two Tools were found to be consistent with one another and with auxiliary subsurface Information available at the extensively characterized site. the availability of these newly-developed and unique Tools open vast opportunities for utilizing NMR to enhance groundwater and environmental Investigations.
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Applications and Advantages of a Short Dead-Time for Surface Nuclear Magnetic Resonance (SNMR) Measurements
Authors David Walsh, Elliot Grunewald and Peter TurnerSurface nuclear magnetic resonance (SNMR) measurements provide non-invasive detection and characterization of groundwater by measuring the relaxation decay of hydrogen nuclei following excitation by a surface loop. A key factor controlling the sensitivity of the SNMR measurement to groundwater is the so-called dead-time, which describes the delay between the end of the excitation pulse and the first reliable recording of the NMR signal. the dead-time will fundamentally limit the shortest signals that can be detected and thus effectively controls the range of environments in which SNMR can be successfully used. We describe the recent development of very short dead-time SNMR instrumentation and application of these measurements in a series of case studies. We demonstrate the achievement of dramatically reduced dead-times, as short as 4 milliseconds, which enable detection of groundwater in environments previously considered inaccessible by SNMR. Specifically, we illustrate detection of very short NMR decay signals associated with water in magnetic geology, bound water in silts and clays, and capillary water in the vadose zone. the availability of a new small diameter NMR logging tool provides an opportunity to ground-truth the surface NMR data and confirms that these short signals are reliably detected using short dead-time surface NMR capabilities. Based on our field results, we discuss the advantages and implications of a short-dead time SNMR instrument for quantifying short signals, improving signal to noise ratios, and characterizing aquifer properties.
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A New Marine Acoustics Prospecting instrument with the Advantage of Sub-Bottom Profiler and Shallow Seismic Reflection Method On River and Sea
More LessIt is required for prospecting of subaqueous to explore the thickness fluctuation of subaqueous silt seam, sand layer, pebble bed, and riprap layer, fluctuation of subaqueous bedrock surface, burial depth of bedrock, fault in the bedrock and fracture zone. Sub-bottom profiler and seismic reflection method are the most common. Generally, the Sub-bottom profiler is adopted for exploration of thinks of silt seam, sand layer of bottom with high resolution from the high frequency band however the relatively poor penetrating power; while the low frequency band is adopted for the water shallow seismic reflection method, resulting high penetrating ability but low resolution, which makes it suitable for exploration of rock stratum and geological structure in straight measuring line. Is there any method or instrument combining the advantages of there two methods above? That is the very hope of the workers engaged in geophysical prospecting. KS marine acoustics prospecting instrument is designed to address such problems on the base of the aforementioned ideal, and was awarded the patent of invention. It is basically aimed at penetrating pebble and riprap layer, with enough sensitivity to measure the layer thickness above 1m, and replacing in whole or in part the shallow seismic reflection method on water to penetrate underwater rock strata. the author is striving to achieve breakthrough which are adopting the very wide frequency-band based receiving system to receive all reflection signals between tens of Hertz to 10kHz, adopting receiving detector with strong directing property and adopting the spark source that can control emission energy. It is possible for the instrument to explore silt seam, sand layer, pebble bed, top and bottom surface of riprap layer which are thicker than 1m, and deep stratum through collecting the reflection wave with a reflex time above 150ms.
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A New Version of Seismic Wave Reflection Method Fit for Urban Exploration —Landsonar
More LessThere are two problems on shallow exploration in urban:(a) frequent exposure to vibration noise and terrestrial electricity peculiar to city. Seismic methods are usually highly embarrassed by the vibration from the electric cars, pedestrians, engineering construction and operating metro lines; (b) Many geophysical exploration methods cannot meet the requirements on desired exploration depth and resolution. Landsonar method is the shortened form of an elastic reflection continues profiling with extremely small offset and very wide frequency-band, bringing the characteristics of zero-offset into full play. Adopting single-channel collection with almost zero-offset; increasing signal-to-noise ratio through vertical stacking ; Through combination of wave detector with very wide frequency-band and main unit of receiver, it is able to stimulate and receive the wave with 5Hz to 4000Hz without any frequency distortion (no compressing or magnifying wave with any frequency). with these features Lansonar can fit for urban shallow high resolution exploration , and several applications have got the correct feedback over deep of 150 m in China.
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Recent Developments in the Skytem System
Authors Kurt Sorensen, Esben Auken and Max HalkjaerThe SkyTEM system has originally been developed for groundwater mapping and is capable of measuring in a wide time range from very early decay times to late times without any bias correction applying high altitude data sets. the system has been extensively used since its introduction in 2002 and have proven its superior performance in ground water mapping as well as mineral exploration when near surface as well as deeper resolution is required. Further developments including higher transmitting moment by segmented transmitter techniques, higher signal to noise ration by motion noise suppression and adaptive signal processing and reduction of coil responses at very early times by on-time measurements have expanded the measurable time decay interval, the deep of penetration and the accuracy of the measurements, thus meeting the increasing demands for detailed and accurate Interpretation of data sets. Selected topics from the developments will be presented together with examples of applications.
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Integrating Hydrology and Geophysics to Evaluate the Impact of Artificial Recharge on Groundwater in Rural India
The monsoonal climate of India coupled with the complex geology and low storage capacity of the Deccan basalts contribute to water scarcity in central India during the dry season. One of the primary Tools proposed to manage this problem is the artificial recharge of runoff captured during the monsoon to enhance groundwater availability throughout the year. One common approach for artificial recharge is the construction of small dams to generate percolation ponds, as exemplified by a small reservoir in the Salri watershed of Mahdya Pradesh, India. We use this specific example to illustrate how the Integration geophysical and hydrologic data can be used to understand the influence of the dam on groundwater in the watershed. Electrical resistivity and electromagnetic induction surveys are used to assist in developing a geologic conceptual model for the watershed consisting of a thick sequence of basalt flows overlain in the lowland portion of the watershed by weathered basalt and alluvium for a depth of up to 10m. This geologic model has guided our understanding of the local flow system. A shallow flow system in the near-surface weathered basalts and alluvium is the primary source of water for agriculture. in contrast, vertical variability in the competent basalt flows is expected to create a highly anisotropic flow system with high horizontal permeability and low vertical permeability. As a result, the geophysical data help to form a conceptual model where the dam primarily impacts the shallow aquifer and has limited impact on deeper regional flow systems. to assess this hypothesis and quantify the impact of the dam on the overall hydrology of the watershed a hydrologic monitoring program was implemented. By Integrating the geophysically-based conceptual model with this hydrologic data we are able to provide a quantitative assessment of the role of the dam within the watershed.
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Estimation of Aquifer Transport Parameters from Resistivity Monitoring Data within a Coupled Inversion Framework
Authors Dylan Fowler and Stephen MoyseyDespite the fact that electrical resistivity surveys are being increasingly used to image solute migration in porous media, there is limited experience in how these data can be used to characterize aquifer transport parameters. in this study we evaluate whether electrical resistivity monitoring data can constrain the hydraulic conductivity, porosity, and dispersivity of a homogeneous aquifer within the framework of coupled Inversion. We focus on responses from a single set of current electrodes to provide insights into the Information content of these measurements and motivate future work in designing optimal surveys for monitoring groundwater transport processes. We have found that even in this simple system two distinct types of tradeoffs exist that could confound the direct estimation of transport parameters. First, different values of porosity and hydraulic conductivity can produce the same plume velocity, thereby leading to identical concentration distributions in the subsurface. in ideal cases this hydrologic non-uniqueness can be resolved by electrical data because electrical resistivity is dependent on both solute concentration and porosity, but this discrimination will only possible when the rock physics relationship between concentration and resistivity can be accurately determined. the second type of non-uniqueness occurs when different concentration plumes produce equivalent electrical responses. in this case, tradeoffs between plume velocity and dispersivity cause shifts in the position and mass of solute relative to the electrodes that result in equivalent values of subsurface apparent resistivity. This non-uniqueness can be overcome by explicitly recognizing that different Information about transport processes is captured by the timing versus magnitude of the electrical response in monitoring data. Though we have investigated a homogeneous system, we argue that the insights here can also be applied to the estimation of effective transport parameters for heterogeneous systems.
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Time-Lapse Imaging of Dynamic Systems using Multi-offset GPR Reflection Data
Authors Adam Mangel and Stephen MoyseyGround-penetrating radar is widely recognized as a powerful tool for mapping the subsurface to image structures and locate targets. However, velocity variations resulting from heterogeneity in soil texture and water content can greatly degrade the coherency of radar responses. This issue is particularly problematic in dynamic environments where the background environment may change over time, e.g., as a result of rainwater infiltration. to address this problem we investigate the radar signature of reflectors embedded in homogenous and layered soils during the course of a simulated infiltration event. the data are collected using multiple antenna offsets to allow for evaluation of traveltime and amplitude changes with offset. the result of this survey design is a rich data cube with dimensions of GPR traveltime, experiment time, and antenna offset. We demonstrate the value of this data by performing normal moveout velocity analysis for arrivals obtained over the course of the experiment. We show that we are able to perform time to depth corrections of the GPR data that provide a consistent image of the subsurface over the course of the experiment as moisture conditions change. for example, in experiments performed in a homogenous tank we show that the depth to the reflection produced at the bottom of the tank can be resolved with an error of ±5cm (±10%), where the error is mainly associated with picking errors. While this error can be somewhat reduced by improved traveltime picking, we have also found that hydrologic processes, e.g., the buildup of water at a seepage face formed at the bottom of the tank, contribute to complexity in the GPR data that requires analysis of wave interference for improved Interpretation. We suggest that multi-offset reflection GPR shows promise for both estimating velocity variations in the shallow subsurface and improving subsurface imaging.
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Integrating Geophysical Surveys with Geotechnical Investigations at offshore Wind Facilities
By John MadsenThree-dimensional variation in sediments due to their geologic setting is a common occurrence in marine environments. Adequate geophysical data, including that from acoustic, side-scan sonar and high-resolution seismic reflection surveys constrained by grab sampling, coring and video ground-truth, must be available to define the types and distribution of these sediments. Due to their having differing geotechnical properties, the three-dimensional variable distribution of sediments has significant implications for the siting and construction of offshore wind facilities along the East Coast of the US.
Geophysical surveys need to be integrated with geotechnical requirements at regional, local and site specific scales when developing offshore wind facilities. At the regional scale, geophysical, coupled with geological, data provide Information that along with first-order considerations such as wind, wave, and/or tidal conditions, water depths, access to onshore grid infrastructure, and ecological and human impacts, must be considered in the selection of a location for an offshore wind facility. the geologic setting is relevant because coastal marine areas have evolved under conditions of sea level rise and fall during the past several hundred thousand years. these conditions have determined the present and past sedimentary environments where facilities will be sited. At the local scale, geophysical data can be used to determine the distribution of surficial and sub-surface sediments. for example, paleochannels, ancient river valleys formed during periods of lower sea-level, are in-filled with coarser- to finer-grained sediments and preserved in the sub-surface. Shallow, high-resolution, seismic surveys can image networks of these paleochannels thus providing constraints on grain size and spatial variations of sediments. At the site specific scale, bottom and sub-bottom sediment types and their distribution play a role in selection of the type of wind turbine foundation (e.g., monopile, tripod pile structure, or gravity base), and in the subsequent location and installation of these foundations.
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High-Density Electromagnetic Induction Survey: Mapping the Archaeological Landscape At Ocmulgee National Monument, Georgia
More LessThis paper presents the results of a large-scale (8 ha), high-density electromagnetic induction survey carried out in 2010 at the famous Ocmulgee National Monument archaeological site located in central Georgia. Ocmulgee is the fourth largest (70 ha) mound site in the eastern United States and there is evidence of human occupation dating from the Paleo-Indian period (approximately 11,000 BC) to post American Civil War. High-density data collection methods were used to identify micro-variation in the subsurface since the archaeological targets (postholes, hearths, and pits) were often small. the continuous collection mode allowed me to gather data at rapid speed with close spacing between data points. I am able to distinguish between areas of high occupational density, areas of moderate occupational density, and areas that were relatively free of occupation. Anomalies representing different pre-contact and historic architectural types are discernable in some cases. the survey results indicate that EM conductivity is an effective technique for rapidly mapping large archaeological landscapes.
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Vs Comparison of Near Surface MASW to SCPT Data from Ontario
Authors Peter Hutchinson and Maggie BeirdShear wave velocities measured through Seismic Cone Penetration Tests (SCPT) at several locations in glacial sediments in south-central Ontario, Canada show poor correlation with MASW shear wave velocities to 8 to 9 meters below grade. SCPT near-surface measurements display Vs in the “rock” range; inconsistent with site geology. the correlation below 8 to 9 m is excellent, suggesting that the elastic wave front for the SCPT test is exploiting the drive pipe in the near-surface measurements. the prediction of Vs from MASW is more robust than those derived from the shallow portion of an SCPT test.
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In-Situ Measurements of Pore Pressure Generation and Nonlinear Shear Modulus Behavior at the Wildlife Liquefaction Array
Authors Brady Cox, Kenneth Stokoe and Ellen RathjeAn active, strain-based test that may be used to directly evaluate the liquefaction resistance of soils in-situ has been developed. the test is based on the premise of dynamically loading a native soil deposit in a manner similar to an earthquake, while simultaneously measuring its deformation and pore pressure response with push-in sensors. Dynamic loading is performed via a large hydraulic shaker (vibroseis) that is used to generate vertically propagating (downward), horizontally polarized shear waves (Svh-waves) of varying amplitude within an instrumented portion of a saturated soil deposit.
the new test method has been used to conduct field experiments in soft, saturated soil deposits 3 to 4m below the ground surface at the Wildlife Liquefaction Array (WLA) in Imperial Valley, California. WLA has been intensely studied over the past 25 years and has also been designated as a Network for Earthquake Engineering Simulation (NEES) site for the study of soil liquefaction. the extensive site characterization, the documented occurrence of earthquake-Induced soil liquefaction at the site twice in the 1980’s (1981, Mw =5.9 Westmorland earthquake; and 1987, Mw =6.6 Superstition Hills earthquake) and the likelihood for re-liquefaction of the site during subsequent earthquakes make WLA an ideal location for verifying the in-situ dynamic liquefaction test method.
in-situ liquefaction tests at WLA were successful at measuring: (1) excess pore water pressure generation, and (2) nonlinear shear modulus behavior in native silty-sand deposits as a function of Induced cyclic shear strain and number of loading cycles. these results are compared to pore pressure generation curves and nonlinear shear modulus curves previously developed for WLA soils from laboratory testing methods. Variations in the dynamic soil response across the site are also discussed. these accomplishments represent a large step forward in the ability to accurately evaluate the susceptibility of a soil deposit to earthquake-Induced liquefaction.
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Seismic Refraction Shootout: Tomographic Inversion using Geogiga Seismic Pro
More LessThis submission to the Seismic Refraction Shootout uses the Tomographic Inversion software from Seismic Pro 6.0, by Geogiga. the software is very interactive, giving the user extensive real-time Information relating to the activity being conducted.
One example of this is that the travel time curves are displayed alongside the shot record while picking first arrivals, allowing the user maintain a good sense of overall trends while picking.
Another example of the interactive nature of the software is the display options that are available during the Inversion process. the user can watch the ray paths superimposed on the current model, along with measured and modeled travel time curves updated in real time. This not only allows the user to stop the Inversion if they notice something going awry, but could provide users with a more intuitive understanding about the Tomographic process itself.of course the most important feature of any Tomographic Inversion package is the effectiveness of the Inversion algorithm used. This shoot-out should provide a good Initial check on the capabilities of the Inversion algorithm.
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Artificial Neural Networks Models for Determining the Basic Geotechnical Properties of Soils from Electrical Measurements
By Fred BoaduNon-invasive and cost effective geophysical techniques can be used to predict some useful petrophysical and engineering properties of unconsolidated geomaterials that are known to affect their geotechnical behavior, that is, stability and strength. We investigate the correlation of fundamental petrophysical and geotechncial engineering parameters of unconsolidated near surface materials with their electrical measurements in a laboratory environment using artificial neural networks and multivariate regression analysis. Spectral electrical response (SER) resistivity measurements are performed at a constant effective stress level on thirty two samples of varying textures and compositions. Engineering parameters which affect the mechanical behavior of soils, that is, fines content, specific surface area, pore size and fractal dimension of the particle size distribution are determined. Simultaneously, the electrical parameters which describe their electrical responses, that is, resistivity amplitude, phase shift, percent frequency effect, loss tangent and the normalized phase, are extracted from the SER measurements. the neural network is used to analyze the data and infer relationships between the engineering parameters describing the soils and their measured electrical parameters. these relationships are compared with the traditional multivariate regression models as a way of evaluating the strengths and weaknesses of the neural network in predicting the engineering parameters from the electrical measurements.
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Nitrate Contamination in Drinking Water from Farming Activities in Rural Communities in the NSAWAM District, Ghana: Environmental Geoscience, Education and Civic Engagement
Authors Fred Boadu and Frederick Owusu-NimoThe main source of water for drinking and domestic needs for inhabitants (mostly farmers) in the Nsawam District, Ghana is groundwater. However, these fractured groundwater reservoirs in this district pose to be vulnerable to nitrate contamination, and have been of growing environmental and health concern in recent times. Water samples collected from several groundwater wells in the study area indicate high nitrate concentration levels that significantly exceed the permissible limits for human consumption set by the World Health Organization (WHO). Azimuthal resistivity surveys (ARS) were conducted on exposed rocks with mapped fracture parameters in the vicinity of the seven wells where anisotropic coefficient, real and imaginary parts of the conductivity were measured. the specific surface area unexposed subsurface fractures were estimated using the regression model and the fracture porosity was estimated from the anisotropic coefficient. Fracture parameters, fracture porosity, specific surface area correlated with nitrate concentration. the results serve to establish the role of fractures in groundwater contamination by nitrates in the study area. in the summer of 2008, a group of students from Duke University from different majors (engineering, chemistry, biology, geology and global health), visited the study area to applying their knowledge gained in the classroom to address such societal problems with the intent of making a difference in the world. the students educated the inhabitants on potential health dangers regarding environmental pollution by direct interaction with locals, chiefs, school children and political representatives.
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Use of the Skytem Airborne Electromagnetics (AEM) System and Item Fast Approximate Inversion Software to Provide Robust and Rapid Data for Groundwater and Salinity Management, Ord River Irrigation Area, Western Australia
Authors Ken Lawrie, Kok Tan, Jon Clarke, Timothy Munday, Andrew Fitzpatrick, Larysa Halas and Heike AppsThe Ord Valley Airborne Electromagnetics (AEM) Interpretation Project was established to underpin salinity and groundwater management in the Ord River Irrigation Area (ORIA), in NW Australia. While covering the existing irrigation areas, it is also the first time in Australia that AEM data have been acquired to inform potential irrigation expansion. A total of 5,936 line km of AEM data were acquired using the SKYTEM time domain system. Conductivity signatures for key components of the hydrogeological system are non-unique. Interpretation therefore required the use of borehole and landscape analysis constraints. the study has provided greater spatial detail on critical elements of the hydrostratigraphy in the alluvial aquifer. This includes palaeochannel delineation, sand, gravel and clay distribution, the mapping of salt stores and groundwater quality, and the generation of salinity hazard maps. the project has demonstrated the potential for ‘calibrated’ AEM systems and Fast Approximate Inversion software to shorten AEM project timelines significantly. for example: - AEM data inverted using the fast approximate Inversion (iTEM) were obtained within 24 hrs of data acquisition. the early availability of high quality estimates of ground conductivity facilitated the early design of a drilling program, the extension of the survey whilst it was in progress, and the initiation of data Interpretation immediately. - the Initial multi-layer iTEM Inversions had high correlation coefficients (>0.8) when comparing FID points to adjacent borehole induction logs, and the Inversions have proven to be very robust.
Overall, this project has demonstrated the benefits of a phased or staged approach to assessing the methods and technologies used as part of a ‘hydrogeological systems’ approach when developing the framework for salinity and groundwater management. the AEM-based products are being used to assist with planning future irrigation infrastructure and strategies, the development of salinity mitigation and groundwater management strategies, and to protect environmental assets.
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Using Airborne Electromagnetics (AEM) for Rapid Identification and Assessment of Managed Aquifer Recharge (MAR) Options in the Darling Floodplain, N.S.W., Australia
Authors Ken Lawrie, Ross. S. Brodie, David Gibson, Kok Tan, Aaron Davis, Yusen Ley-Cooper, Larysa Halas and Jon ClarkeThe Broken Hill Managed Aquifer Recharge (BHMAR) project is part of a larger strategic effort aimed at securing the town of Broken Hill’s water supply and identifying significant water-saving measures for the Darling River system. to meet the challenge of rapidly identifying and assessing potential MAR targets and groundwater resources over a large area (>7,500 km2) within a short timeframe (18 months), it was concluded that the only cost-effective method was airborne electromagnetics (AEM). the SkyTEM time domain system was chosen and 31,834 line km of data was acquired in 9 weeks. the AEM data was validated by a 4,500m drilling program. the study has confirmed the location of several large potential MAR targets in close proximity (<40km) to the existing water supply infrastructure. the AEM survey has also mapped key confining aquitard units. ‘Holes’ in the overlying clay unit appear to provide preferential recharge of the underlying Pliocene aquifers along losing reaches of the Darling River and Talyawalka Creek. the hydrogeological complexity revealed by AEM mapping and validated by drilling provides a diversity of potential MAR options. At this same time, this brings with it a challenge of understanding a range of complex hydrogeological and hydrogeochemical processes. the provision of robust Inversions in short timeframes, Initially using the iTEM software and subsequently LCI and SCI Inversions using the Aarhus Workbench, greatly assisted with targeting drilling and rapid hydrogeochemical assessment of targets.
Overall, the BHMAR project is the first application of AEM technology and an integrated hydrogeological systems approach to MAR target detection and assessment. It has demonstrated the potential for large data-poor areas to be assessed rapidly, with a high degree of confidence. the success of the approach is dependent on the use of a phased approach within a broader risk assessment framework.
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Use of the Resolve Airborne Electromagnetic (AEM) Data in the Assessment of the Salinity Hazard and Risk to Iconic River and Wetland Ecosystems, Murray River, Se Australia
Authors Ken Lawrie, Kok Tan, Larysa Halas, Ross C. Brodie, Heike Apps and Laura GowAn AEM survey using the RESOLVE frequency domain system has been acquired along a 450 km reach of the Murray River in SE Australia. the AEM data were inverted using the holistic Inversion method, enabling key elements of the hydrogeological system in the shallow sub-surface (top 20-50m) to be mapped with high confidence levels. the AEM data have been used in conjunction with remote sensing, and hydrogeological and hydrogeochemical data obtained from drilling, to determine that healthy vegetation along the Murray River is generally associated with the presence of significant river ‘flush zones’ where fresh groundwater is present at shallow depths, and groundwater salinity is low. the study has also found that the corollary is true: where the river is ‘gaining’, and salt stores are high, vegetation health is generally in decline. Similarly, the AEM data show there is a marked decline in vegetation health towards the western edge of the iconic Gunbower State forest. This appears to be associated with salt being mobilised from irrigation districts on the western margins of the Gunbower forest. In the areas where the river flush zones are discontinuous, and the salt stores and water tables are closer to surface, there is also a risk of salt ingress to the river. in these areas, the data identify areas for targeted salinity management, including sites for potential Salt interception Schemes. This study fills important knowledge gaps particularly the distribution of key elements of the hydrostratigraphy, salinity extent, and the relationships between vegetation health, salinity and groundwater processes. in particular, the project has successfully integrated AEM, remote sensing, and lithological and hydrogeological data from drilling, to identify reaches of the River Murray and areas of iconic wetland ecosystems at risk from groundwater salinisation. these datasets provide geospatial context for targeted salinity and groundwater management actions.
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Surface Water Extent Trends in interior Alaska (1979-2009)
Authors Jennifer Rover, Lei Ji, Bruce Wylie and Larry TieszenRecent reports suggest that during the last few decades the surface extents of many high-latitudinal inland water bodies appear to be decreasing at alarming rates. Widespread declines in surface water have local and global impacts on ecosystems including carbon, methane, heat, and water fluxes. Because northern areas are susceptible to climate change and permafrost warming, we require an accurate account and understanding of surface water extent changes. in remote northern areas, remotely sensed data provide an efficient tool for monitoring water bodies and measuring their extents at multiple dates. However, a few remotely sensed measurements over a several-decade period may not produce reliable estimates of changes occurring to dynamic water bodies. Surface water extents can vary by water body, both seasonally and interannually due to functional differences in hydrology. in this study, we show that surface water extent trends in interior Alaska, developed from multi-date Landsat data, are highly influenced by the quantity and timing of the data collected. By taking natural variations into account through the use of 20 remote measurements that span multiple decades and seasons, we found that declines in surface water extents are not as prevalent as previously reported. Further, we found that most water bodies exhibited no statistically significant drying trends. Our results suggest that reliable assessments regarding the sustainability of northern lakes and ponds require a sufficient number of remotely sensed measurements to adequately account for natural interannual and seasonal variations. This clear documentation is essential for estimating the environmental consequences of changing surface water extents at northern latitudes.
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Alltem Uxo Discrmination Results from the Aberdeen Proving Ground using a Hybrid Generalized Neural Analysis and Standard Dipole Inversion and Classification Scheme
Authors Ted Asch and Michael FriedelAn advanced multi-axis electromagnetic induction system, ALLTEM, has been specifically designed for detection and discrimination of unexploded ordnance (UXO). This work has been funded by ESTCP (Project MM-0809). ALLTEM uses a continuous triangle-wave excitation that measures the target step response rather than the more common impulse response. Ferrous and non-ferrous metal objects have distinct characteristic responses. the system multiplexes through all three orthogonal (Hx, Hy, and Hz axes) transmitting loops and records a total of 19 different transmitting (Tx) and receiving (Rx) loop combinations with a spatial data sampling interval of 20 cm. This paper presents some of the results of a demonstration and validation survey at the Aberdeen Proving Ground in March 2010. the U.S. Geological Survey operated ALLTEM with a Leica 1200 GPS over the Army’s UXO Calibration and Blind Test Grids and the Direct Fire and indirect Fire areas. Custom data analysis is conducted from within Oasis Montaj including importing survey data, gridding, noise analysis for threshold determination, automatic selection of targets, and Inversion and classification. Batch Inversion of selected targets with a prolate spheroid starting model followed by the application of neural SOM (self organizing map) algorithms is used to automatically classify the objects into targets of interest and those of clutter. the SOM process is unsupervised and completely data driven. the goal of learning in the SOM is to cause different parts of the network (the data) to respond similarly to certain input patterns. the result of this process is clustering of similar target data in different parts of the ‘map’. This SOM analysis, used in conjunction with the standard numerical dipole Inversion results, provides a better understanding of what data is being provided to the SOM and to the numerical Inversion with the result being classification of UXO with greater confidence.
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Implimentation of Rapid SASW Method
More LessThe SASW method is the first broadly utilized surface method for characterizing the stiffness profiles of soil deposits and pavement layers. in the geotechnical engineering application, this method is particularly appealing as the new codes require the average shear wave velocity of the site down to a depth of about 30 m. in the pavement engineering, the method provides a more reliable modulus profile for the asphalt and base layers.
In the past twenty years several custom-made devices have been developed for rapid field-testing based on this approach.
The limitations of the surface methods, in general, and SASW method, in particular, are well-documented in the literature.
The main technical improvements of the SASW method in terms of data acquisition, construction of the dispersion curve and the Inversion process are discussed.
Case studies comparing the ease of use and the accuracy of the results from the traditional and improvements SASW program and are also included.
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Some Applications of Near Surface Geophysics to Earthquake Geohazards Investigations: Examples from Eastern Ontario Canada
Authors James Hunter, Heather Crow, André Pugin and Dariush MatazedianThe presence of thick soft soils is a major contributing factor to earthquake shaking response of structures. in the last several years, many national building codes have recognized the importance of soil effects, including shear strength, damping, amplification and soil-rock resonance. these parameters can now be measured or estimated in-situ using modern near-surface geophysical techniques and commonly the preferred measurement technique for seismic zonation is based on shear wave velocity structure of soil and bedrock; however, other active and passive, surface or invasive techniques, can also contribute valuable ancillary data for assessment of soil parameters.
in many areas of the Eastern Canada earthquake hazard zones, “gradient” amplification can occur when earthquake shear waves (body waves) travel from depth through increasingly low velocity zones towards ground surface. This can be further altered by focussing or de-focussing effects (from variations in subsurface soil stratigraphy) and from generation of local large amplitude Rayleigh and Love waves in soils. If a significant shear wave impedance boundary exists between soft soils and competent bedrock at depth, it is possible that very large resonance amplification at a “fundamental” site period can occur. the degree of amplification in soil is modified by the effect of soil damping. A low level of damping implies that sediments will not significantly attenuate small-strain earthquake ground motions, thereby resulting in increased surface motion.
in many areas of Eastern Canada, relatively inexpensive surface refraction and reflection analyses are most efficient for mapping lateral changes in subsurface structures with reasonable accuracy. invasive techniques such as the seismic cone penetrometer (SCPT) and downhole shear wave vertical seismic profiling (VSP), can yield very detailed vertical variations in velocities. Ancillary geophysical Tools such as surface electrical and EM profiling as well as slim hole geophysical logging can also help to detail other geotechnical parameters.
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A Guided Pilot Point Bayesian Inversion Approach for Monitoring Spatio-Temporal Dielectric Permittivity Variation in the Shallow Subsurface using GPR
Authors Neil Terry and Zhangshuan HouA method for estimating dielectric permittivity variations in the shallow subsurface using Tomographic radar first arrival times is presented. We expand upon our previous Minimum-relative-entropy (MRE)-Bayesian method by implementing a pilot point framework. We employ the principle of MRE as a means of non-subjectively choosing prior probability distribution functions (pdfs) for Inversion targets, and utilize a Quasi-Monte Carlo sampling technique to draw samples from these prior pdfs. in our previous study, we treated dielectric parameters as zonal unknowns on a coarse grid, in order to reduce the dimensionality of the inverse problem. in this study, the targets of Inversion are dielectric permittivity at chosen points selected from a fine-grid field and the parameters describing the correlation structure between these points. the number and locations of these points are selected in proportion to correlation length in order to avoid redundancy and capture areas likely to exhibit large contrasts in dielectric permittivity. for each numerical simulation, the dielectric and structural parameters are used to assign a dielectric permittivity value to each node on the grid through sequential Gaussian simulation. the output of the Inversion includes parameter estimates in the form of pdfs that can be used as priors for further updating when more datasets are available. the MRE-Bayesian method can profit from time-lapse datasets, handle nonlinearities between data and model parameters, and quantify uncertainty. the Integration of the pilot point method offers substantial improvement in the resolution of estimates and profits from knowledge regarding spatial correlation, such as known Information about the lithology of the study area.
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Fourth Year of Subsurface Drip Irrigation Monitoring using Gem2 Electromagnetic Surveys, Powder River Basin, Wyoming
Authors James Sams, Bruce Smith, Garret Veloski, Burke Minsley and Bethany BurtonThe National Energy Technology Laboratory and the U.S. Geological Survey are collaborating with BeneTerra LLC to comprehensively monitor a sub-surface drip irrigation (SDI) system at a site in the Powder River Basin (PRB) of Wyoming. Irrigation water for the SDI system is coalbed natural gas (CBNG) co-produced water. the study is being conducted at the Headgate Draw area, located approximately 17 km south of Arvada, Wyoming at the confluence of Crazy Woman Creek and the Powder River. the study site encompasses six alfalfa fields and covers an approximate area of 1.2 km2 (Figure 1).
Subsurface drip irrigation (SDI) is an emerging technology being applied in the PRB basin for the beneficial use of CBNG co-produced water. the pre-treated CBNG waters are applied to the root zones of agricultural land to aid in irrigation. Water drains from the perforated laterals at a rate of 0.75 to 1.5 gallons per day. This style of irrigation is capable of applying two to three times more water on a particular site than traditional surface irrigation (Engle and others 2009a and b). the method is designed to minimize environmental impacts by parking potentially detrimental salts in the vadose zone. This research project investigates the transport and fate of the water and salts from the injected CBNG produced waters at the SDI site, adjacent to the Powder River.
Ground, borehole, and helicopter electromagnetic conductivity surveys were conducted at the site prior to the installation of the SDI system (Sams and others 2008a and b; Smith and others, 2009). After the installation of the subsurface drip irrigation system, ground electromagnetic conductivity surveys have been performed quarterly (weather permitting). We summarize geophysical survey results from the first 4 years of a planned 5-year study of the SDI system.
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Assessing Water Storage Changes on the Field Scale Combining Superconducting Gravimeter Observations with a Hydrological Model
Authors Benjamin Creutzfeldt, Andreas Güntner, Hartmut Wziontek and Bruno MerzInformation on water storages is crucial for many different applications, like agricultural production, groundwater recharge or transport of contaminants. Limitations of observation techniques and high spatio-temporal variability make the estimation of water storage challenging, especially for deeper zones. Temporal gravimeter observations are significantly influenced by water storage changes (WSC) at the field scale and hence may provide valuable Information about the state of the hydrological system.
in this study, we assess the benefit of temporal gravimeter measurements as an integral signal for hydrological application by evaluating a hydrological model using residuals time series of a superconducting gravimeter (SG). A simple conceptual model is used to estimate local WSC in the snow, soil, unsaturated saprolite, and saturated aquifer storage. the model is calibrated and evaluated against SG data on the one hand and several groundwater and/or soil moisture data on the other. the model is validated against independently estimated WSC derived from a state-of-the-art lysimeter.
the results show that using an SG as calibration constraint improves the model results substantially in terms of predictive capability and variation of the behavioral model runs in comparison to classical hydrological point measurements. Gravity measurements integrate over different hydrological storage components and the sampling volume is several orders of magnitude larger than that for the point measurements. the general problem of specifying the Internal model structure or individual parameter sets can, however, not be solved with gravimeters alone. Additionally, the results show that also WSC in the deep vadose zone contribute significantly to the hydrological cycle, so SG might provide a tool to continuously and non-invasively monitor WSC also in this zone.
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Investigating the Deformational and Microstructural Changes in Unconsolidated Earth Materials using Complex Resistivity Measurements
Authors Frederick Owusu-Nimo and Fred BoaduMicro-structural modifications of near surface earth materials influence their geotechnical engineering behavior and stability conditions. the ability to characterize and monitor these modification non-invasively using geophysical techniques would be useful in geotechnical engineering Investigations. in this study, we conduct controlled laboratory experiments on sand-clay mixtures subjected to external loading and concurrently measure their deformational characteristics and frequency-dependent electrical resistivity (complex resistivity). the experimental setup consists of a modified oedometer with outlets for sample vacuuming, saturation and drainage, and is also fitted with pore pressure transducer, strain gauge and electrodes for electrical measurements. the samples are placed in the oedometer and saturated with water beyond their liquid limits and each subjected to an axial stress of 500kPa. the experiment is conducted under controlled drainage which allows measurements of changes in sample pore pressure and volume of water drained. At each effective stress level, the complex resistivity of the sample is measured over a frequency range (0.5Hz – 512Hz). the results show that electrical parameters obtained from the electrical response measurements are sensitive to the characteristic micro-structural changes occurring in the soil during the consolidation process. the relationship between the electrical parameters (resistivity amplitude, phase, capacitance, loss tangent) and the deformational and hydraulic parameters including axial strain, coefficient of compressibility, void ratio, dry density, pore size and hydraulic conductivity, influences the engineering properties and behavior of earth materials are also investigated. This study provides a good understanding of how changing state of soil and deformational characteristics can be predicted from electrical response measurements. the results will be useful in the use of geo-electrical methods in engineering, environmental and geohazard Investigations.
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Seismic Refraction Shootout: Presentation of True Model and Comparison with Estimated Models
Authors Colin Zelt, Seth Haines, Michael Powers, Jacob Sheehan and William DollThe purpose of this session is to provide an opportunity for all interested scientists to better understand the options and trade-offs involved in creating seismic velocity models from refraction travel time data. Submissions were encouraged from students new to seismic work, along with experienced experts. in this presentation, the true model will be revealed, along with quantitative comparisons with the estimated models presented during the session at the same scale and with the same display parameters. the accuracy of each final model will be discussed in some detail. the synthetic data consist of P-wave travel times for a seismic refraction experiment through a velocity model that represents a realistic near-surface geologic situation. Though not specifically intended to represent a karst environment, aspects of the model are consistent with a karst environment. there are unconsolidated, or semi-consolidated, sediments overlying a non-flat bedrock surface. the water table depth varies considerably in the area, between 20 and 100 m. there is zero surface topography along the seismic transect. the survey objective is to identify low velocity zones within the sedimentary layers, as well as features of the bedrock surface and within the bedrock. the travel times correspond to P-wave first arrivals, and were calculated assuming a 100 Hz wave. Uncorrelated Gaussianly-distributed noise with a standard deviation of 1 ms was added to the synthetic data. there are 101 shots from 0 to 300 m, and 100 receivers from 1.5 to 298.5 m. the shot and receiver spacing is 3 m. the total number of travel times is 10100.
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The Development of Geophysical Surveying Techniques in Permafrost at the Geological Survey of Canada: A Historical Perspective
By James HunterSince the late 1960’s, considerable effort has been directed towards oil and gas exploration in the Canadian arctic. to support this development and to provide regulatory Information, the Geological Survey of Canada has conducted extensive surficial geology mapping activities including detailed examination of permafrost occurrence and associated geotechnical parameters.
Central to such permafrost research is the regional geothermal regime. in the 1960s, in cooperation with various oil companies, geothermal Investigations and long term temperature monitoring were conducted in selected wildcat wells throughout the arctic. these wells were held in suspended status for many years for that purpose. Such Information gave useful data on the thickness limits and thermal susceptibility of permafrost.
in 1971, the GSC began testing various near-surface geophysical techniques at sites in the Western Arctic (Mackenzie River and Delta, Tuktoyaktuk Penninsula) where much of the early exploration had occurred and where pipeline routes were proposed. Later, surveying was extended to some areas of the arctic islands. Most test sites were associated with on-going Investigations of sensitive terrain features such as massive ground ice, river and lake thawing (taliks), as well as mapping discontinuous ice-bonding zones at lower latitudes. Electrical, electromagnetic, GPR and seismic techniques were tested. New surface and borehole techniques were developed.
As oil exploration moved northward into the Beaufort Sea in the mid 1970s, ice-bearing sub-seabottom permafrost was discovered by shallow marine seismic refraction profiles and geotechnical drilling. with industry collaboration, the GSC compiled a map of ice-bearing permafrost distribution from seismic refraction Interpretations. these results have been used as a guide for industry reflection processing corrections as well as for drill site and pipeline route planning.
Current GSC geophysical testing in permafrost is directed towards mining-related activities as well as geotechnical problems associated with northern community infrastructure.
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Integration of Surface and Borehole Geophysical Methods to Develop a Bedrock Model
Authors Mario Carnevale, Alex Buller and Jutta HagerA multi-method geophysical survey was performed at a contaminated facility in New Jersey to update the subsurface geological model for the project area, in order to advance the understanding of hydrogeologic conditions and constrain the placement of additional borings. Anomalous saprolite thicknesses, permeable stratigraphic contacts, and overturned stratigraphy introduced geologic complexities inhibiting the progress of the project.
More than 16,000 linear feet of geophysical data were collected over a survey area approximately 9 acres in size. GPR was selected as the primary method for modeling the saprolite and bedrock elevations, as well as profiling fill and other soil horizons above the saprolite.
Seismic reflection and DC resistivity methods were used to obtain data to confirm saprolite of variable thickness, competent bedrock surfaces, geologic fault structures, and other geophysical anomalies. Seismic and resistivity data were integrated within common depth intervals to emphasize bedrock structures detected by both methods as well as those unique to each method. Resistivity profiles also assisted in mapping the spatial distribution of different geologic units.
Borehole geophysical log data, obtained a year earlier, were integrated with the surface seismic data and provided key Information for deciphering complex reverse fault structures.
the data collected by these three complementary geophysical methods were used to create bedrock and saprolite topographic models. the bedrock models were used to provide a better understanding of the spatial extent of older and younger units and possible structural contacts between them due to reverse faulting.
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