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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
81 - 100 of 190 results
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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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