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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
101 - 150 of 190 results
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Integrated Geophysical Surveys in Support of Environmental Site Assessment at a former Pintsch Gas Plant
Authors Raye Lahti, Andri Hanson, Joe Renier and Micah HolzbauerA Phase II Environmental Site Assessment was performed to investigate a former Pintsch Gas Manufacturing Plant (FPGP) and an artesian well location. This survey is within an operating rail yard that is used for maintenance and fueling. the Phase II ESA was conducted in two work stages including an integrated geophysical Investigation using Geonics EM31, EM38 and EM61 surveys. in addition, a limited magnetometer survey was conducted to assist in determining the location of an abandoned Artesian Well. All work was conducted in the estimated vicinity of the FPGP and the artesian well location identified in a Phase I ESA conducted in June 2010. the second stage of the survey involved a trenching Investigation based upon the geophysical results. the FPGP was within the operational rail yard since development in the early 1900’s and was used for locomotive maintenance and fueling as well as rail car classification. Since development there have been a number of structures at the rail yard including a round house with functional turn table, machine shop, engine shop, car shop, passenger and freight depots, storage buildings and a Pintsch gas manufacturing plant. All surface structures were removed and the area graded allowing little or no evidence of the FPGP structures. the geophysical Investigation surveys covered an approximate 1.8 acre polygonal area that encompasses the estimated locations of the FPGP. the results of the three EM surveys closely matched the historic site plan maps and blue prints. Each survey provided independent and complementary data for the intrusive trenching Investigation. the trenching Investigation confirmed the location of many of the abandoned FPGP structures including tanks, piping and contaminants. This presentation will provide the results of the geophysical surveys and the subsequent intrusive confirmation conducted using trenches in strategic locations at the site. the use of complementary geophysical methods provided significant Information for this Investigation rather than conducting a blind trenching Investigation of the site and saved valuable Investigation budget dollars.
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Blind Test of Methods for Obtaining Velocity Models from First-Arrival Travel Times using Refraction Tomography Methods
Authors Julian Ivanov, Richard Miller and Shelby PeterieThe possibility to apply different approaches to first-arrival time analysis of seismic data was examined using a sample synthetic first-arrival data set calculated from an unknown velocity model. This model was intentionally created for the purposes of a blind test and it was designed to represent a realistic near-surface target. using the provided synthetic first-arrival data set various velocity models were estimated. the variety of models was acquired based on the understanding of the wide range of possible solution that can exist when dealing with the inverse refraction-traveltime problem (IRTP). Solutions to the IRTP were obtained using the refraction-Tomography Inversion technique. the variety of different solutions was provided by using different Initial models and regularization parameters. Efforts were applied to obtain realistic appearing solutions that are noticeably different from one another. the estimated velocity models will be compared with the synthetic velocity model when it becomes available. Such a comparison is expected to provide evidence about the possible degrees of nonuniqueness of the IRTP for this particular model.
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Using Time-Lapse Near Surface Geophysics to Image the Natural Capital and Ecosystem Services of Soils and the Vadose Zone
Authors David Robinson, Oliver Kuras, Barry Rawlins, Andrew Tye, Michael Raines, Inma Lebron and Aidan KeithThe Government in the United Kingdom (UK) is adopting an ecosystems approach in order to more fully integrate the environment, and its ‘goods and services’, into decision making at the highest levels. the Millennium Ecosystem Assessment (MEA, 2005) provided stark warning that the earth’s ecosystems are rapidly degrading, and that more effort to incorporate the goods and services they provide for human benefit is needed. One of the priorities for research raised by the MEA was the need for mapping ecosystem goods and services, to identify ‘service hotspots’. Near surface geophysics offers great potential for Integrating a range of measurement methods for mapping soil goods and services, which can be thought of as the soil stocks and fluxes from which we benefit. This presentation will present a range of data, including current collaborative research between the Centre for Ecology and Hydrology and the British Geological Survey to map soils at the field scale, using single pass and time-lapse geophysical imaging. By using a range of geophysical techniques with data fusion, we hope to better determine soil properties with a minimum of disturbance. We aim to quantify both ‘static’ and ‘dynamic’ properties and processes in the vadose zone with both one-off surveys, and time-lapse imaging. the use of a time-lapse approach is especially useful for determining hydrological processes in the vadose zone and differentiating these from soil or sediment texture.
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Use of the Utexas Vibroseis in the Neesr Sand Aging Field Experiment
Authors David Saftner, Russell Green, Roman Hryciw and Christopher BaxterThe objective of this study is to develop a better understanding of the mechanisms and implications of time-dependent changes in the state and properties of recently disturbed sands (i.e., sand “aging”). Particular emphasis is placed on the use of the UTexas NEES vibroseis in this study. Aging effects in sand, such as increases in penetration resistance with time after deposition, densification, and/or liquefaction, are known to occur in-situ, but the causes of these effects are not fully understood. As part of this study, saturated sand deposits at two sites were disturbed using explosives, an Impact PierTM mandrel, or the UTexas NEES vibroseis (T-Rex). One site is located in Griffin, in, where explosives at T-Rex were used, and the other site is in New Madrid, MO, where the Impact PierTM mandrel was used. Subsequent to the disturbance, a series of in-situ tests were performed, to include cone penetration tests (CPT), at varying time intervals (weeks, months, years) to monitor for aging effects. the reason for using the three methods to disturb the deposits is because each method induces different levels of strains, affects the soil state differently, and/or introduces foreign elements into the soil (e.g., blast gases from explosives). Consequently, further insights into the underlying mechanisms of sand aging can be gained. in the area where explosives and the Impact PierTM mandrel were used to disturb the soil, increases in CPT tip resistances were observed as a function of time after disturbance. However, in the area where the vibroseis was used, no increase in CPT tip resistance was observed as a function of time after disturbance. This is thought to be due to the relatively low level of strain Induced in the soil during the vibroseis shaking. However, a laboratory study is currently ongoing to more fully understand these field observations.
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Application of Informational and Wavelet Approaches for Integrated Processing of Geophysical Data in Complex Environments
Authors Lev Eppelbaum, Leonid Alperovich, Valery Zheludev and Alex PecherskyAt a specified accuracy of geophysical methods and characteristics of the objects, technique of integrated Interpretation is of primary importance for developing geophysical methods Integration. the same refers to obtaining particular Information from the separate method results and to the formulation of requirements to this particular Information. the particular Information can be obtained in terms of physical sources corresponding to various classes of geological (environmental or technical) objects. the integrated Interpretation providing intersection of classes makes it possible to single out one or several geological targets, which form a solution of the geological objective sought for. in some cases only the comparison of fields measured by various geophysical methods leads to an “Informational jump”, which results in singling out the objects of search. Sometimes only estimation of the amount of the corresponding Information containing in observations may substantiate the presence of an object of the desired class. these estimations are especially convenient for the integrated Interpretation, as they provide means for operating on the data obtained by various methods and expressed in common (Informational) unit. the target is singled out by maximum Information obtained from the set of geophysical methods.Another applied approach is advanced wavelet packet analysis (WPA). We chooseto characterize the geometric events in the merged data curves via the representation of the curves in the Fourier domain. the assumption is that each of data sets is characterized by the number of dominating scale bands. A perfect tool to reveal these characteristic scale bands is provided by the WPA. Once implemented, the wavelet packet transform of a signal yields highly redundant partitions of the frequency domain. the transform consists of an iterative application to a signal (curve) a pair (highpass - lowpass) of quadrature mirror filters. Developed WPA has been applied to synthesized gravity, magnetic and GPR data computed for geological models affiliated to two classes. the first one contains cavity (e.g. karst terrain), and the second one the models without cavity. the data were embedded into 3-dimensional space where data related to the C (cavity) subsurface are well separated from the N (non-cavity) data. This 3D set of the data representatives can be used as a reference set for the classification of newly arriving data.
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Self Calibrating Remote Monitoring System
Authors Trent Armstrong, Candice Jackson, Gail Heath, Clark Scott, Casey Smith and Jesse BennettSelf-calibrating remote monitoring systems are designed to reduce the cost and time needed to properly monitor acid mine drainage issues as well as assist in the remedial efforts. Self-calibrating systems allow real time data at low cost. A self-calibrating system has been installed at the Barite Hill Pit Lake, an acid mine pit lake with a stratified water column consisting of nearly ‘clean’ water at the top and highly acidic, heavy metal rich water below. the purpose of the monitoring system is to automatically sample and analyze daily for pH, ORP, DO, temperature, and conductivity at 5 foot increments down the profile of the pit lake water column. the four wells located in the waste rock repository south of the lake are also autonomously monitored for the same properties as the pit lake. the system also includes a sonic water level sensor to monitor the pit lake level. This sensor along with a camera mounted to observe the spillway and floating chemical system is used as an early warning system to monitor Lake Overflow. Stainless steel jacketed thermocouples are used to collect temperature every ten minutes along the vertical profile in the lake as well as the monitoring wells. A 3-D resistivity system is employed to monitor water chemistry interactions between the pit lake and the waste rock piles. Remote site communications is provided by a dedicated two-way cellular modem that allows continuous access to the system for data download software updates, and eventually remote control. the data is recorded on site as well as automatically grabbed by a remote server and parsed into a relational database. A secure web interface provides access to the data from any online computer, has graduated levels of access with multiple permissions levels, basic data analysis, and plotting Tools.
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Investigation of Source of Seeps Beneath Earthen Dam, Central Massachusetts
Authors Mario Carnevale, Jutta Hager and Alex BullerAlong with design and construction issues related to earthen dams, seeps have been implicated in numerous dam breaches. Geophysics is one technique that can be used to identify areas of possible existing “water seeps” and other high permeability zones within or beneath existing dams. HGI performed such an Investigation at a dam in central Massachusetts using a combination of seismic and GPR methods. Close to 7300 linear feet of GPR and 700 linear feet of MASW data were collected during the survey.
in order to provide the greatest resolution and to ensure sufficient signal penetration depth, a multiple frequency GPR program was implemented using 200-MHz, 100-MHz, 70-MHz, and 40-MHz antenna systems. the multi-channel analysis of surface wave (MASW) seismic method was used to obtain data for analyzing the Vs anomalies of soil layers and to provide stratigraphic Information to constrain the GPR Interpretation, including that for bedrock.
the geophysical survey identified areas of possible shallow and deep seepage both within and beneath the dam. these areas were further investigated during a borehole phase of the project. A stratigraphic analysis was also completed to assist in the dam remediation analysis. Till, bedrock surface trends, and other horizons were mapped along the length of the dam.
Anomalous shallow seepage zones were detected at a number of locations along the survey traverses, as well as deep seepage potential at and above the bedrock surface. Additional anomalies of interest included a possible former stream channel beneath the dam and an area with high concentration of boulders.
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Detailed GPR Mapping of Catacombs at Early Christian Cemetery on Malta
Authors Goran Skelac, Bozo Padovan and Marko MaricicDuring the period from 03th of March to 12th of March, 2010, team of researchers specialized in archaeological geophysical surveying from company Geoarheo d.o.o. from Zagreb, Croatia (and associates) carried out geophysical survey at location St.Paul’s and St. Agathas Catacombs and the surrounding streets at Rabat, Malta. Survey was based on principles of archaeological geophysics i.e.with dense grid of profiles producing high resolution 3D results with possibility to present and interpret such results to archaeologists.
Objective of GPR survey was to detect and map extends of catacombs and other underground features for further archaeological research. St. Paul’s catacombs are largest early Christian cemetery at Malta, dating roughly from 4 – 9 century AD, presenting one of most significant archaeological sites from this period.
Carved in a soft in (upper) coralline limestone which is the topmost formation of the Maltese layer cake. This formation is mostly made up of shallow marine sediments which have characteristics of several different marine or intertidal environments.
Catacombs consist of a system of corridors and walls covered with rectangular hollows for a burial of the death bodies. Depth of ceiling appears aprox. 1 – 4 m under surface. Results of GPR survey presents a detection of known catacombs as well as some unknown extends and catacomb corridors that have not been detected before.
Geophysical survey at Malta was taken with (GPR) model SIR 3000 GSSI and antennas of 1600, 400, 270 and 200 MHz. At the site, total of 651 GPR profiles were recorded within 24 fields and the 15 individual profiles. Area covered with survey reached approximately 5600 m2.
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The Importance of the Apparent Dispersion Curve and the Higher Modes of Rayleigh Waves in MASW-Remi Methods for Seismic Geotechnical Site Characterization
Authors Vitantonio Roma and Mario ForestaThe interest of both the scientific community and the professionals towards the MASW method (Multichannel Spectral Analysis of Surface Waves) has been increasing for the last years.
in the ’90 several researchers have realized that, when dealing with inversely dispersive sites, the MASW method based only on the fundamental mode can really cause erroneous Vs profiles, hence an erroneous seismic site characterization. When dealing with inversely dispersive sites ( i.e. sites where stiffness discontinuities exist, soft layers trapped between stiffer layers or viceversa stiff layers trapped between softer layers) higher modes of Rayleigh waves must be combined together with the fundamental mode to calculate the effective or apparent dispersion curve (Lai 1998, Roma 2001-2002-2006), in order to achieve a reliable Vs profile and a reliable seismic site characterization. It is not sufficient to calculate the numerical higher modes and use them separately for the Inversion process, because it is practically impossible to distinguish the experimental higher modes from the field data in the geotechnical scale. It is well known that the apparent experimental dispersion curve that is determined from the field data is the result of a superposition of the several higher modes.
In this article the potentialities of a new algorithm (www.MASW.it, Roma 2001) that calculates the apparent dispersion curve using all higher modes are shown into an application to a real case
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Use of Time-Domain Induced Polarization for Lithology Identication: Case Study from the Hanford 300-Area
The use of the Induced polarization (IP) method in environmental Investigations offers advantages over resistivity in yielding Information about the physicochemical characteristics of the subsurface. the IP response in rocks and soils results from a surface polarization that is largely determined by lithology. for non metallic soils, IP typically shows a weak dependence on fluid conductivity, but strong near linear relationships to textural parameters such as surface area and grain size. However, compared to the resistivity method, IP has much smaller signal to noise ratio, hence requiring additional care during data acquisition and data processing. in particular, appropriate data weight needs to be considered for successful Inversion of IP datasets.
We report on field-scale IP measurements at the U.S. Department of Energy Hanford 300 site for lithologic discrimination between two key units that control the hydrogeology of this site. the lower unit, Ringold formation is a finer, highly polarizable and electrically conductive compared to the upper, coarse-grained Hanford formation. A high-resolution IP survey was conducted along ten 2D lines covering a total of ~2500 m in an effort to map the distribution of these two geological units inland from the river and to determine potential geological controls on transport of uranium rich groundwater. Complete resistivity and IP reciprocal datasets were collected for measurement error analysis. Following full examination of the IP data errors, we developed IP error models that describe the dependence of the errors on measured chargeability in order to provide appropriate data weights in the Inversion. A 2D Inversion was performed to image the spatial distribution of complex resistivity across the site.
the Hanford-Ringold contact is clearly identifiable from the sharp contrasts in conductivity and polarizability of the two units. Although images from Inversion of IP data do not provide the spatial resolution obtainable with resistivity they do confirm that the subsurface structure being mapped with geophysics is controlled by lithology and not variations in fluid chemistry driven by groundwater-surface water exchange. Although IP images do not provide as much Information on spatial variation as the resistivity images, they significantly increase confidence in the Interpretation of the site. the results suggest that, given appropriate care to quantify and model measurement errors, IP is an effective technique in mapping the key lithologic units at this site.
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Human Riverine and Lacustrine Adaptations
Authors Gail Heath, Clayton Marler, Julie Williams, Holley Gilbert, Brenda Pace and Joshua KeeneThis multi-year project is intended to explore human adaptations to riverine and lacustrine environments in the eastern Snake River Plain high desert with focus on adaptations along river valleys and the margins of glacial Lake Terreton during the Pleistocene/Holocene transition. Past research (Marler 2004) has shown changes in site distribution and artifact assemblages in this period affecting diet, land use, and mobility patterns. Magnetics and Electromagnetics will be used to optimize the excavation efforts. the goal of this project is to better understand the role of changing paleo-environments on the archaeological record as well as the distribution of archaeological sites across the Idaho National Laboratory. to answer these questions, a testing project is underway to identify areas where sites dating to the terminal Pleistocene exist in a preserved context. Initial testing has tentatively identified a terrace formation with low-energy deposition beginning in the terminal Pleistocene and continuing through the early Holocene, providing an ideal environment for the preservation of archaeological horizons.
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“True” 3D Acquisition using GPR Over Small Areas: A Cost Effective Solution
More LessGPR 3D Investigations ensure the correct and proper reconstruction of an object within a surveyed space, and the final image is effectiveand understandable by the end-user. using GPR, a new positioning system called PSG (Pad System for Georadar) was developed for “true” 3D acquisitions over small areas. the system allows the easy and fast acquisition of a geo-referred, regular and dense matrix of GPR traces that satisfies the Nyquist theory for both in-line and cross-line directions. Two prototype GPR systems were assembled by combining the PSG with a dual-polarization antenna (HH and VV). One prototype was equipped with a bipolar high frequency 2GHz antenna and was tested on a concrete slab to map the rebar and determine how the marble slabs covering the investigated structure were anchored. the results were excellent, and a “true“ 3D GPR survey of a 100cm*100cm area was made in about 20 minutes, surveying both the vertical and plane surfaces in situ. This prototype acquired the minimum number of GPR traces at regular intervals in both in-line and cross-line directions, thus satisfying the Nyquist theory. Furthermore, all the traces had the same polarization as the grooved tracks of the PSG ensure that the GPR antenna always faces the same direction at every point of the investigated area. the second prototype had two 200MHz antennas assembled perpendicularly to simulate the HH and VV antenna dual-polarization. Also this prototype resulted in the profile acquisitions being taken in only one direction, and the result was a regular, and sufficiently dense, data grid that made it possible to acquire 3D images of linear objects independently of their direction. to assess the utility of the PSG system it was tested on 16 small urban sites (3m*3m); the GPR results of these tests were then compared with observations made during maintenance operations that involved the digging up of all the pipes at the sites. the reconstruction was found to be correct in all directions.
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Seismic Methods for Quality Control and Quality Assurance of Civil Engineering Materials: Opportunities and Challenges
More LessModulus is one of the primary parameters to qualify civil engineering materials and plays an important role in structural design of structures and pavements. for portland cement concrete structural members, concrete strength is another major parameter of concern. in-situ determination or estimation of these parameters has been an essential and challenging task for quality control/quality assurance during construction. Seismic nondestructive testing technology based on the use of stress waves has shown to be a useful and convenient tool to achieve this goal. This paper provides the fundamentals, capabilities and limitations of stress wave methods and testing devices, and represents the results obtained from their recent applications in a numbers of construction projects. these applications include: evaluation of hot-mixed asphalt layers, characterization of base courses and subgrade with different treatments and monitoring of early-age modulus/ strength development of concrete. This presentation presents a process that can be potentially used by agencies for a comprehensive quality of materials based on seismic technology. Seismic-based laboratory and field tests were briefly explained and the process of combining them to evaluate the appropriateness of compacted soil and asphalt layers, and concrete pavements is discussed. Issues in understanding the results from stress-wave tests and how to incorporate them to the existing specifications or common practices are also discussed. A site for each case is thoroughly discussed to demonstrate the effectiveness, reliability and weaknesses of the process.
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Hydraulic Parameter Estimation using Geophysically-Constrained Analytical Flow Models
Authors Dale Rucker and Andrew HinnellHydraulic parameters used in constitutive relationships describe how water moves through the soil and are necessary to formulate hydrogeological models that track infiltration, evaporation, and redistribution. the hydraulic parameters are often difficult to obtain directly at a scale that is useful for modeling engineering projects such as aquifer recharge, contaminant remediation, or waste repository performance. Geophysical methods sensitive to changes in water content, however, can be used more easily to indirectly obtain the hydraulic parameters at a variety of scales. in particular, methods that measure the electrical conductivity or dielectric permittivity can be used in a time lapse sequence to track the position of the wetting front during an infiltration event. When directly coupled with a hydrogeological model with the appropriate boundary conditions, the need for extraneous geophysical modeling such as Tomography becomes unnecessary, as the measured, unprocessed geophysical data become the constraining variable for inverse hydraulic parameter estimation. This paper shows two geophysical methods that, when coupled with analytical flow models, can be used to obtain estimates of the parameters that define the unsaturated hydraulic conductivity function. Both model development and field examples are discussed to show the practical applications of the approach.
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Utilizing Continuous Resistivity Profiling for Characterization of Canal Seepage in El Paso, Texas
Authors Wesley Brown, Amanda Cegon and Zhuping ShengAnnually, billions of gallons of water are lost through seepage within irrigation networks, therefore it is of critical importance to characterize seepage patterns along the canals and water ways. Water scarcity in semi-arid and arid regions calls for water conservation best management practices. El Paso is located in south bound of Chihuahua desert, where the local irrigation district operates and maintains unlined canals and laterals to deliver the Rio Grande water for agricultural irrigation. This project utilized Continuous Resistivity Profiling (CRP) techniques (two different multiple channel resistivity meters the “OhmMapper” and the “Super Sting R8”) to survey soil profiles below canal beds. This paper presents survey results along two half-mile sections of the Franklin Canal during the non-irrigation and irrigation seasons. the survey presented here shows a correlation between resistivity, grain size and water seepage along the canal. these research findings will provide valuable Information that can help the irrigation district develop best management strategies to conserve water and secure water supply for sustainable community development and economic growth.
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Non-Uniqueness in Seismic Refraction Analysis
More LessAny seismic refraction analysis is essentially non-unique. in most cases, it is difficult to obtain a true velocity model from traveltimes observed only at the ground surface. Even with a one-dimensional two-layer model, it is impossible to obtain true thickness and velocity without a priori knowledge that the model is two layers. Non-uniqueness is a problem not only for the seismic refraction method but also for most geophysical methods in which underground physical property models are estimated from geophysical data observed at the ground surface. All seismic practitioners and algorithm developers must admit this fundamental problem and try to develop alternative approaches. there are many approaches to reduce the non-uniqueness. using primary Information is the one of the most promising approach. It is well known that the result of a non-linear least squares Inversion depends highly on the Initial model. for instance, in the one-dimensional case, if we have a priori knowledge (perhaps from a downhole survey) that there are two layers, we can easily estimate a 1D true velocity function from a traveltime curve that indicates two layers. On the contrary, it is very difficult to estimate a true velocity function from the same traveltime curve if we know from other sources that they are actually three layers. Constraints during an Inversion are also very important. Most of the geophysical Inversions need spatial regularization in order to obtain stable results. During the Inversion of the surface seismic refraction method, a constraint that velocity is increasing with depth is generally very effective. in this presentation, we demonstrate the non-uniqueness of the seismic refraction method and the effectiveness of using appropriate primary Information and constraints.
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Satellite-Based INSAR Measurements: A New Approach to Groundwater Monitoring
Authors Jessica Reeves, Rosemary Knight and Howard ZebkerThroughout the western U.S., and in numerous other regions around the world, there is a growing need to monitor and regulate groundwater levels. Remotely sensed data offer spatially and temporally dense measurements over very large areas. of particular relevance to the characterization of groundwater systems is interferometric Synthetic Aperture Radar (inSAR), a remote sensing method that maps relative ground surface deformation. We present a methodology, using an advanced inSAR processing technique known as Small Baseline Subset Analysis (SBAS), that allows us to observe seasonal deformation and relate it to changes in the thickness of the confined aquifer due to recharge and withdrawal of groundwater.
We have applied our methodology in the San Luis Valley (SLV), an 8000 km2 region in southern Colorado that is home to a thriving agricultural economy. the state has developed a hydrogeologic database and a groundwater flow model in order to quantitatively study possible management schemes for water resources in the SLV. Because well-sampled hydrographs, accurate lithologic logs and high quality aquifer tests are scarce, the model has not yet been able to characterize the spatially heterogeneous, time-varying behavior of the groundwater system. the more complete coverage offered by inSAR allows us to augment these sparse measurements. We infer hydraulic head change in the confined aquifer system by combining inSAR measurements with aquifer test data and lithologic logs. Conversely, where well-sampled hydrographs exist we use inSAR data in conjunction with the thickness of the producing zone to estimate the specific storage of the producing sediments. We propose a framework to combine all existing data together with the inSAR deformation estimates in order to improve the groundwater flow model. Ultimately all data types can be incorporated within this mathematical framework to better inform the decision-making process for groundwater management.
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A Vertical Electrical Sounding Method for Agricultural Soil Survey
Authors Larisa Golovko, Antonina Pozdnyakova and Anatoly PozdnyakovSoil survey of agricultural lands requires a quick and, possibly, non-disturbing estimation of numerous soil properties, such as salinity, texture, stone content, groundwater depths, and horizon sequences in soil profiles. Most of geophysical methods currently used in agriculture, such as electromagnetic (EM conductivity) and four-electrode (resistivity) mapping are non-disturbing, but fail to provide Information about soil property variations within a profile and at the best arrangement measure EC/ER at only two distinct soil depths. A method of vertical electrical sounding (VES) adapted by us to use with LandMapper ER measuring hand-held device can distinguish differences in electrical resistivity or conductivity at the multiple (10+) layers in soil profiles. these differences reveal the changes in soil properties within the profiles. the paper shows various applications of the modified geophysical VES method in agricultural soil survey. Information is provided for the VES array assembly, field measuring procedure and Interpretation of sounding results. the applications of the method included soil morphology evaluation and soil mapping, assessment of stone content in soil, estimation of the groundwater table and salinity layers in the soil profile, monitoring of drying and defrosting in soils; and outlining hydrocarbon pollutant distributions in soil profiles. This modified VES method has also a great potential for other soil applications.
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Seismic Surface Wave Technique for Agricultural Applications
Authors Zhiqu Lu, Glenn Wilson and Tianyu ZhangA laser Doppler vibrometer (LDV) based multi-channel analysis of surface wave (MASW) method has recently been developed. in the method, an electro-mechanical shaker was used as a seismic source operating in a frequency sweeping mode to excite Rayleigh waves propagating through the surface. the surface vibrations along a straight line were detected by a moving LDV. Unlike conventional MASWs that explore soil profile from a few meters to a few tens of meters below the ground, the present MASW investigates soil depth from a few centimeters to a few meters due to its high frequency excitation and high spatial resolution of moving LDV. It is therefore more appropriate for agricultural soils applications. the method could be applied to the studies of soil surface crusting, soil moisture variation, compaction effects, layering, and underground water flow. Several case studies will be presented.
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Locating Water Accumulated in Tête Rousse Glacier (French Alps) using Large-Scale Magnetic Resonance Imaging
In 1892, about 80000 m3 of water accumulated within Tête Rousse glacier (Massif of Mont Blanc, French Alps) rushed down from the altitude of 3200 m to Saint Gervais village, causing death of 175 people. in 2007, a study carried out by French glaciologists and geophysicists suggested that the glacier may contain dangerous amount of water.
the Magnetic Resonance Sounding (MRS) is a geophysical method that allows estimating water volume in the subsurface without expensive drilling. However, water within glacier represents 3D target and we used a further development of MRS: the large-scale 3D magnetic resonance imaging (LSMRI) method. We present results of fieldwork carried out in September 2009 and in September 2010.
in 2009, we performed nine soundings with 80×80 m2 square loop that covered the major part of the glacier. 3D Inversion of field data allowed us to locate the principal reservoir in the central part of the glacier. This result was confirmed by twenty boreholes drilled following LSMRI survey. Estimated with LSMRI volume of accumulated water was not less than 45000 m3. in September 2010, water was pumped out the glacier. in the mid-point of the pumping, LSMRI was applied for locating possible non-connected sub-reservoirs. the volume of remaining water was estimated as about 20000 m3. Following pumping revealed 16000 m3 of water pumped out. the total volume of pumped water was about 48000 m3. Taking into account that some amount of water rested in the glacier we consider that LSMRI predictions were fully confirmed.
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Investigating Hillslope Hydrology with Electrical Resistivity Tomography
Authors Erika Gasperikova, Rohit Salve, Daniella Rempe and John PetersonSubsurface hydrologic dynamics in hillslopes remain poorly understood, in part because it is not directly observable, and in part because existing techniques for measuring the movement of water in rock fractures are not easily or effectively deployed in subsurface environments. in effect, the subsurface profile in hillslopes—which links atmospheric, hydrologic, geochemical, ecological, and geomorphic processes, and which may help to regulate the impact of climate change—remains unmeasured and poorly understood. An understanding of the physical makeup of this fractured continuum, and how water is stored and transmitted through it during and between precipitation events, will lead to improved predictive capabilities of trends in water quality and quantity, as dictated by changes in climate.
Along a hillslope in the Angelo Coast Range Reserve in northern California we are exploring the use of Electrical Resistivity Tomography (ERT) to reveal the hydrologic response, in the vadose zone of the subsurface domain, to seasonal precipitation. Surface ERT is a non-invasive geophysical technique that is sensitive to rock properties, which are related (directly or indirectly) to hydrological processes.
We have instrumented the 4000 m2 hillslope with numerous sensors to continuously track saturation changes in shallow soil and the underlying, fractured argillite. in addition, over a period of 18 months, which encompasses both a prolonged wet and dry season, we have conducted six ERT surveys along two transects laid across the hillslope. Results from time-lapse ERT Inversions agree with other measurements we have made of the subsurface hydrologic response. they imply that changes in resistivity are influenced by temperature, amount of rain, evaporation, and fluid movement into deeper sections of the subsurface profile. these Initial preliminary observations suggest that ERT could be a viable technique for exploring moisture dynamics in fractured rock underlying hillslopes.
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Blind Testing of Deformable Layer Tomography using Near-Surface First Arrivals
Authors Hua-wei Zhou, Fang Yuan, Zhihui Zou and Hui LiuIt is beneficial to both the developers and users of seismic imaging algorithms to learn their applicability and limitations. using the first-arrival dataset generated by the session chairs for an unknown synthetic near-surface velocity model, we have started to conduct a series of tests using deformable layer Tomography (DLT). in this method we parameterize the target model with a number of thickness-varying layers and the velocity function of each layer can be constant, gradient, or laterally varying. We use traveltimes of first arrives and reflections to invert for either the layer geometry, or the layer velocity function, or both simultaneously. We use a multi-scale scheme to regularize the Inversion for layer geometry and velocity functions. Details of the DLT method and field data examples are available in our publications.
in this blind test, the objective is to estimate a 300-meter-wide near-surface P-wave velocity model using 10,100 first arrivals. We designed the velocity model building work in several steps. Firstly, we estimated the long-wavelength model by restricting the DLT to invert for the layer geometry of some constant-velocity layers. This step has resulted in a multi-layer model whose traveltime residues have a standard deviation of less than 1.2 ms. Secondly, we fixed the layer geometry, and invert for the laterally varying velocity functions. the solution from this step has reduced the standard deviation of the traveltime residuals to about 1.05 ms, which is close to the 1-ms standard deviation of the added noise. We plan to quantify the resolution and to characterize the multiple models of similar levels of data fitness. together with other researchers in this session, we expect to learn a great deal about the behaviors of different methods and characteristics of all nonunique solutions.
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Evaluation of Surface Wave Method for Assessment of Heterogeneous Improved Ground
Authors Chun-Hung Lin, I-Lun Chen and Chih-Ping LinSoft ground is often improved to increase strength and stiffness by methods such as jet grouting and stone column which result in heterogeneous ground with improved columns. Experimental methods (Standard Penetration Test, sampling and laboratory testing, etc.) used to assess such ground improvement are subjected to several limitations such as small sampling volume, time-consuming, and cost ineffectiveness. the seismic surface wave method (i.e. multi-station analysis of surface wave, MASW) can non-destructively measure shear-wave velocity depth profile over wider lateral sampling range. It appears to be a good alternative or complement for efficient evaluation of overall quality of improved ground. However, current surface wave method is essentially a 1-D method assuming horizontally-layered medium. What MASW measures in the highly heterogeneous improved ground remains to be investigated. Field testings before and after ground improvements were performed to evaluate the feasibility of MASW method in the heterogeneous ground with improved columns. the performance and effect of lateral sampling of the surface wave testing were investigated by field testings with different survey line locations relative to the improved columns. the MASW testing in the heterogeneous improved ground was numerically modeled by finite difference method. Parametric studies were performed to investigate how the heterogeneous shear wave velocities were homogenized by the surface wave measurement. the engineering Information that can be extracted from the improvement rate of shear wave velocity obtained by surface wave testing is discussed based on the field experience and numerical Investigations.
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An Assessment of Sodium- and Salt-Affected Soils in the Northern Great Plains of the USA with EMI
Authors James Doolittle, Michael Ulmer, Eric Brevik, Jeanne Heilig and John KempenichSodium- and salt-affected soils extend well over 10 million hectares in the Northern Great Plains (NGP) of the USA. Levels of soil sodicity and salinity vary spatially across the landscape; vertically with soil depth, and temporally; making it difficult to characterize, classify, and manage these soils. Laboratory measurements are relatively time-consuming and costly to obtain, and are therefore limited in number. Surrogate field measures of apparent electrical conductivity (ECa) obtained with electromagnetic induction (EMI) are being evaluated to improve the classification, mapping, Interpretation and management of these soils in the NGP. This paper examines the use of EMI with the ESAP (ECe Sampling, Assessment, and Prediction) software program to predict and map the variability of soil sodicity and salinity at field scales. Large ECa data sets were collected across four fields located within the Rolling Soft Shale Plain resource area of southwestern North Dakota. these data sets were used with the ESAP-RSSD (response surface sampling design) program to locate a minimum number of soil sampling (calibration) sites within each field where soils were sampled for laboratory characterization. Geo-referenced ECa and soil profile characterization data were used to calibrate appropriate predictive equations for soil sodicity and salinity and to estimate the spatial variability of both properties. At all sites, soils became more sodium-affected and saline with increasing depths. for the 0 to 90 cm depth interval (primary rooting zone), soils were dominantly saline (>4 dS/cm) non-sodic (SAR <13) and saline-sodic (>4 dS/m and SAR >13). However, inclusions of non-saline (<4 dS/m) and non-sodic (SAR <13) soils occurred in each unit of management. Simulations were use to characterize spatial patterns of soil sodicity and salinity. This methodology shows promise for improving the understanding of soil-landscape relationships and impacting the classifications and Interpretations for sodium-affected and saline soil map units in the NGP.
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Image Appraisal Tools for Electrical Resistivity Tomography
Authors David Caterina, Jean Beaujean, Tanguy Robert and Frédéric NguyenImage appraisal is a problem frequently encountered in electrical resistivity Tomography (ERT), and more generally in non-linear geophysical Inversion. It may include several aspects such as the identification of the geometry of buried structures, the detection of numerical artefacts, the estimation of the depth of Investigation or the exactitude of inverted parameters. Geophysicists can rely on several Tools published in the literature to address these issues. However, few studies offer a quantitative comparison on the performance of these Tools concerning the different mentioned aspects. Moreover, to our knowledge, there is no commonly accepted methodology to handle image appraisal.
in this contribution, we compared quantitatively the ability of different image appraisal indicators to reach different objectives (geometry, artefacts, depth of Investigation, parameter resolution). Among possible image appraisal Tools, the model resolution matrix (MRM), the cumulative sensitivity matrix (CSM) and the depth of Investigation index (DOI) are the most cited ones and were studied here. We compared them first on numerical models representing different geological situations. This numerical benchmark showed that indicators based on the MRM and CSM were the more appropriate to appraise ERT images in terms of the geometry of structures and the exactitude of inverted parameters, DOI providing mainly qualitative Information.
On this basis, we propose a methodology to appraise field ERT images focusing on the resolution and geometric aspects (others being implicitly studied). First, True Synthetic Models (TSM), representing simplified cases of field ERT images, are built using available Information. then, through forward modelling, synthetic ERT data are computed and inverted to provide the inverted Synthetic Models (ISM). Afterwards, a comparison between TSM and ISM (or their gradients for geometry) is made in order to define the errors on inverted parameters. This discrepancy is then plotted with respect to resolution indicator values and points out in every tested cases a resolution range over which the errors abruptly increase allowing the definition of threshold values. the final step consists in applying the threshold values on the field ERT images and to validate the results with a posteriori knowledge.
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Shallow Geophysical Methods: Helping to Solve Geotechnical Problems, an Australian Perspective.
Authors ASndrew Spyrou and Simon WilliamsIn response to the Global Financial Crisis the Australian Government implemented a series of Economic Stimulus Packages designed to provide decisive action to support jobs and insulate the Australian economy from the worst of the global downturn. Part of the packages allocated funding for major infrastructure in the transport, communications, water and energy sectors of the economy. This has seen much development being undertaken throughout the country with many development sites located in urban areas. Although not routinely employed, shallow geophysical methods have played an important role at a number of these development sites throughout Australia where specific geotechnical and geoenvironmental problems have been encountered. This is especially the case at brownfield sites where a mixed history has resulted in unknown subsurface characteristics having a potential negative impact on the structures being built above it. Geophysical techniques including electro-magnetics, resistivity, ground penetrating radar, and seismic refraction and surface wave methods have been successfully applied at such sites providing valuable subsurface Information in addressing potential problems before they occur. This paper will present a number of case studies at development sites throughout Australia detailing how various shallow geophysical techniques have been applied and helped to manage potential problems encountered at these sites. Case studies presented include transport infrastructure projects occurring over former landfill sites, where a multi-technique approach to geophysical Investigations have provided subsurface Information critical to the design and construction of the project. This paper will show that whilst playing a niche role, geophysical Investigations are cost effective and can provide valuable subsurface Information at development sites where geotechnical and geoenvironmental problems occur.
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Detailed Low-Induction-Number EM Sounding to 9-m Depth
Authors Richard Taylor and Scott HolladayLow-induction-number electromagnetic instruments have been used widely since the 1980s to measure apparent conductivity of the near surface. in 2010, an instrument incorporating 6 rigidly mounted arrays between 2-m and 6-m in length was developed to provide 6 simultaneous soundings of apparent conductivity. the depth of exploration of the deepest-sounding array exceeds 9-m. Where there is conductive layering of sufficient contrast and continuity within this depth range, estimates of layer conductivity and thickness can be made from the simultaneous data. A profile of data was acquired across a previously mapped plume of leachate from a landfill at Canadian forces Base Borden. Estimates of vadose-zone conductivity, aquifer conductivity and aquifer depth derived from the EM data are in substantial agreement with well logs and samples.
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Panama Canal Expansion Project: How Marine Electrical Resistivity was used in Support of Canal Dredging
Authors Gillian Noonan and Dale RuckerThe Panama Canal is currently being dredged to deepen and widen the waterway for passage of larger vessels. Dredging efficiency relies heavily on knowledge of the rock type and hardness, as this will determine the dredging method and machinery selected. for example, loose sediment can be removed via suction dredging, whereas more complex drilling and blasting techniques are needed to remove harder rock types. to support this effort, a detailed marine electrical resistivity survey was completed within the Canal to produce a spatial distribution of electrical resistivity values along the canal floor to understand overall rock and sediment distribution. Resistivity data were acquired along a series of parallel, two-dimensional transects, nominally spaced 25 meters apart across the width of the navigable portion of the waterway. Alone, resistivity values could not determine rock type; it was found in this survey that differing geological units can show similar ranges of resistivity values. Although weathered and competent rock generally displays lower resistivity values and loose sediment exhibit higher resistivity, the porosity, water saturation and ionic constituents could potentially affect specific values in localized areas. This survey effort showed however, that when resistivity data are combined with known data from geological maps and Canal boreholes, rock types and geologic boundaries can be identified successfully in the resistivity data.
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Frequency Based Interpretation of offshore Passive Electro Magnetic Survey Data
Authors Marius J. Mes, David R. Bruns and Howard BarrieAn airborne 3D high resolution magnetic and electric (EM) survey was conducted in the Norwegian Sea offshore. This survey included survey lines over the Helgeland Basin, the Nyk High area, an area with known oil seeps, as well as three benchmark lines in the oil producing Norne area and three benchmark lines east of Norne where a dry well was located. the survey data were interpreted with unconventional frequency-based algorithms.
Interpretations of these data sets validated the survey system and confirmed that:
1) the 3D passive airborne system recognized areas with and without large hydrocarbon deposits,
2) Relative depth Information was obtained from a parallel Interpretation with geophysical data,
3) Multiple hydrocarbon bearing strata were identified,
4) Consistent survey results were found in 1200 meters of water.
Data and Interpretations supporting these claims will be presented.
Based on these encouraging results improved economic commercial grade data collection systems and a survey data Interpretation system were developed. the purpose of this development is to make it possible to optimize existing data (such as geological and seismic data) Interpretations with independent EM data Interpretations. Such combined Interpretations not only can reduce the exploration risk but also may come in useful for basin evaluations, the evaluation of farm-in prospects, license applications and drill site optimizations.
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Assessing the Potential of GPR AVO/A Analysis for Fracture Characterization
Authors Aisha Kana, Jared West and Roger ClarkThe ability of fractures to reflect radar waves and the dependence of this reflectivity on fracture properties (aperture and fill) makes the GPR method a promising tool for rock fracture characterization. Successful characterisation requires quantification of the reflectivity (reflection coefficient (R)) of individual fractures. the AVO/A method applied to fractures involves measuring the reflection amplitude variation with offset (incidence angle) with the aim of characterizing fracture properties such as aperture and fill material. Measuring the full AVO/A response using CMP (Common Mid Point) surveys enables estimation of these fracture properties which has proved difficult to achieve with normal-incidence data such as that typically collected in Common offset (CO) profiling.
We carried out analysis of CMP data acquired over a bedding plane fracture in Carboniferous Limestone in Yorkshire, United Kingdom. the reflection amplitude measured in the field is related to the reflection coefficient but however, is also influenced by other factors including the source amplitude, antenna coupling with the ground surface, transmission losses from overlying interfaces, source and receiver radiation patterns, and attenuation. Raw amplitudes were corrected for spreading and antenna radiation/receiver patterns; offsets were further converted to incidence angles to obtain a diagnostic AVA curve. the antenna patterns were measured through transillumination surveys across limestone boulders and through numerical modelling using the GPRMax 3D modelling code. Being a low loss formation, conductive attenuation was considered insignificant over the ray-path length. Because of the difficulty in characterizing the source pulse strength, both theoretical and measured amplitudes were normalized to the minimum offset amplitude. Fracture properties are then obtained by comparing the normalised field derived AVA curve with normalized theoretical reflection coefficient curves for thin fractures with various apertures and fills.
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A Practical Tool to Quantify and Map the Effect of Magnetic Noise on the Detectability of Unexploded Ordnance (UXO)
Authors David White, Brian Barrett and Asger EriksenSurface magnetometer surveys are frequently used to certify that a site is clear of a particular class of unexploded ordnance (UXO). However, the depth to which this can be guaranteed is highly dependent on the background noise encountered in a given survey. Understanding the relationship between background noise and target detectability is therefore critical to providing robust guarantees of UXO clearance from surface magnetometer data.
in this study, example data were analysed from a total magnetic field survey showing highly variable noise characteristics across the site. It was found that computing the experimental variogram of a region of noise provided an effective means of quantitatively measuring its properties. Specifically, the sill and range obtained from fitting a Gaussian model to the experimental variogram accurately characterised the amplitude and wavelength of the noise respectively. A series of experiments were carried out to determine the relationship between the noise, as measured by these parameters, and the probability of detecting a target. to facilitate these experiments, an algorithm was implemented to generate synthetic noise from specified model variograms. Modelled target anomalies were superimposed on the synthetic noise and the resulting grids were subjected to an automatic picking process. the frequency with which the correct target location was picked was used to estimate detection probability as a function of the noise characteristics. This Investigation revealed a strong relationship between the noise variogram and the probability of detecting a target. in particular, the results suggest that the maximum depth at which detection of a given target can be guaranteed can be quantitatively determined from the noise variogram, providing a practical tool to map the limits of detection across a site.
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Analysis of the Velocity Dispersion and Attenuation Behavior of Multi-Frequency Sonic Logs
Authors Ludovic Baron and Klaus HolligerModern sonic logging Tools designed for environmental and engineering applications allow for measurements of the phase velocity and the attenuation of P-waves at multiple emitter frequencies over a bandwidth covering 5 to 10 octaves. We have explored the possibility of estimating the permeability of saturated surficial alluvial sediments based on the poro-elastic Interpretation of the velocity dispersion and frequency-dependent attenuation of such broadband sonic log data.
Methodological considerations indicate that, for saturated unconsolidated sediments in the silt to sand range and typical nominal emitter frequencies ranging from approximately 1 to 30 kHz, the observable P-wave velocity dispersion is sufficiently pronounced to allow for reliable first-order estimations of the underlying permeability structure based on the theoretical foundation of poro-elastic seismic wave propagation. theoretical predictions also indicate that the frequency-dependent attenuation behavior should show detectable variations over the entire range unconsolidated lithologies.
the corresponding predictions have been tested for a borehole penetrating a typical surficial alluvial aquifer. with regard to the P-wave velocity dispersion, our results do indeed indicate that, even without any additional corrections or calibrations, the thus obtained permeability values as well as their variabilities within the pertinent lithological units are remarkably close to those expected based on the corresponding granulometric characteristics.
Conversely, the results of our attenuation measurements are more difficult to interpret because (i) the inferred attenuation values are systematically higher than the theoretically predicted ones and because (ii) the form of their dependence on frequency is variable and only partially consistent with theoretical expectations. This is surprising, even enigmatic, since the overall quality of our data is uniformly high to very high and the currently available evidence indicates that the classical theoretical framework of poro-elasticity should be largely valid in the considered permeability and frequency ranges.
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Application of Geophysics to Refine the Hydrogeologic Framework of the Elkhorn-Loup Groundwater Model in Central Nebraska
Authors Ted Asch, Paul Bedrosian and Christopher HobzaA regional groundwater model is being developed in the Elkhorn and Loup River Basins of Nebraska to evaluate the impact of groundwater irrigation on streamflow and assess the impact of specific management scenarios on ground- and surface-water resources. Early in the development of the Elkhorn-Loup model (ELM), the refinement of the existing hydrogeologic framework was identified as a critical need. within the ELM region, available test hole Information includes lithologic and geophysical logs, but these test holes are often separated from one another by 30 to 40 kilometers. Groundwater managers and the Natural Resource districts were intimately involved in the planning and the implementation of the geophysical data collection. in lieu of drilling additional test holes at great expense, ground-based geophysical methods including time-domain electromagnetics (TDEM) and audiomagnetotellurics (AMT) have been utilized as ‘virtual’ boreholes at specific locations to improve the interpreted base of the principal aquifer and identify the regional confining unit where sparse data exist. 72 TDEM soundings and 132 AMT soundings were acquired from 2008 through 2010. in 2008 and 2009 the soundings were intentionally located near existing or planned test holes in order to satisfy the participating Natural Resources Districts that the geophysical sounding data reflected the character of the lithologic and geophysical logs acquired in each test hole and, in a sense, to calibrate the sounding data to the test hole log data. in 2010 the TDEM sounding locations were scattered throughout the ELM area and the AMT data were acquired along one 22 km long profile to delineate subsurface paleochannel features. the use of ground-based geophysics has filled a critical data need over a very large area at a substantially lower cost than drilling test holes. Additional virtual borehole TDEM soundings are scheduled for the summer of 2011.
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Multi-Channel Analysis of Surface Waves (MASW) in Karst Terrain, Southwest Georgia: Implications for Detecting Anomalous Features and Fracture Zones
Authors Elias Parker Jr. and Robert B. HawmanMulti-channel analysis of surface waves (MASW) was used to map bedrock topography, detect anomalies in the soil overburden, and differentiate between competent and weathered limestone in covered, karst terrain near Albany, Georgia. Surface-wave data were acquired along seven parallel lines and two perpendicular cross-lines. Two surveys using a hammer and moving van-source were conducted to image depth ranges of 4, 12, and 23 meters to address the trade-off between lateral resolution and depth penetration. A steep S-velocity gradient marked by the 350 m/s contour at depths of 9-12 meters is interpreted as the bedrock surface, in agreement with coincident borehole data. the observed range in shear-wave velocities for limestone (S-velocity: 350-700 m/s) below 10 meters is likely due to variations in fracture density, weathering, and open or in-filled dissolution features. Standard penetration tests at two locations 45 meters apart confirm variations in limestone rigidity suggested by the velocity models. At one location, a dissolution feature within highly weathered, saturated limestone (N-values: 12 and 19) at a depth of 11-13 meters correlates with low S-velocities (<350 m/s) extending to 13 meters. Borehole data from the second location shows rigid limestone (N-values: 25 and 34) at a depth of 9 meters that correlates with S-velocities between 350-400 m/s at a depth of 8-10 meters. A zone of weathered rock at depths of 12-20 meters imaged along a suspected fracture zone is consistent with borehole data and sinkhole formation at the ground surface. An anomaly associated with a burn pit at the intersection of two perpendicular profiles was resolved with MASW at two different scales. S-velocity models and distinct changes in apparent velocity across shot gathers constrain both the lateral and vertical extent of the anomaly, and borehole data verify changes in soil composition and depth (5m) to the base of the anomaly.
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Groundwater Management Based on Sound Groundwater Modeling Tools
More LessWater managers in the Nebraska Central Platte River basin have worked to better understand future and long-term effects related to integrated Water Management by doing a Cooperative Hydrology Study (COHYST). the study has collected data and developed data bases to build effective groundwater modeling Tools to do predictive analysis on potential water management. the COHYST ground-water flow model has been constructed to simulate current and/or future ground-water and surface-water conditions. the predictive accuracy of this model depends upon the quality and quantity of data available in the study area. Model updates and refinements are a priority item for the water managers as groundwater management practices get implemented. Collecting and understanding the hydrogeology of the groundwater system is a major part of developing a groundwater modeling tool. Spatial distribution of aquifer properties is a major input to the groundwater model. these aquifer properties are typically derived from test holes and aquifer tests. the existence of testhole data in the Central Platte River basin is limited to Conservation Survey Division holes drilled every 1 to 2 miles on township lines across the basin starting in 1932 thru present. these data are currently being used to develop the hydrogeology for the regional scale groundwater models. As we plan to update the models and scale down to sub-regional areas the need for additional aquifer properties data becomes very important. We are looking to use near surface geophysics data collection to help supplement existing dataset. Magnetic Resonance Sounding (MRS) is a quick, non-intrusive surface geophysical technique that directly measures ground-water to gather Information similar to that gained by aquifer pump tests, specifically hydraulic conductivity and water in storage. These are valuable parameters that can improve the accuracy of ground-water models, therefore enabling water-resource managers to make more informed decisions.
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Use of a Resistance Meter to Define Depth Effects of Manure
Authors Roger Eigenberg, Bryan Woodbury, David Parker and Mindy SpiehsMineral constituents (i.e., salts) contained in beef feedlot manure alter soil conductivity. Additionally, runoff from feedlots and manure used in land application increases soil conductivity due to manure derived salts. Researchers at USMARC have adapted Tools such as electromagnetic soil conductivity meters and mapping/modeling software to identify areas affected by manure from beef cattle feedlots. these Tools have proven beneficial for a broad range of research associated with manure management from beef cattle feedlots. However, electromagnetic soil conductivity meters poorly define soil depth profile conductivities. Thus, a question arises regarding well-defined depth of impact of manure at various manure management sites. Traditional methods of soil coring can be laborious, time-consuming, and costly as well as risky in areas populated with buried water and electrical lines. Preliminary shallow soundings using a standard four-probe resistivity meter have been conducted at several waste management sites with results presented in this paper. This is an introductory work and examines the potential for future development and use for cattle feedlot waste management research.
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Electrical Resistivity Tomography used to Inspect Earthen Embankment Dams
Authors Jared Case, Craig Hickey and Gregory J. HansonTwo quarter-scaled earthen embankment dams were built at the USDA Hydraulic and Engineering Research Lab in Stillwater, Oklahoma. these two dams were constructed with Internal compromised zones that would be vulnerable to seepage and piping. the interior compromised zones consist of a sandy region and a region compacted at lower moisture content. During the construction process, dry state preliminary electrical resistivity measurements show that the sand zone had values that were approximately five times larger than the dam material, and the dryer compacted zone had resistivity values that were about three times larger than the dam material. Electrical resistivity surveys were conducted on the completed dam using a 56 electrode spread with dipole-dipole and inverted Schlumberger electrode configurations. the collected data was processed using electrical resistivity Tomography (ERT) and evidence of these two compromised zones were easily visible. Additional surveys are currently being conducted in order to evaluate the changes in electrical signatures associated with changes in these zones due to filling of the reservoir, and due to environmental/climate changes. [This research was funded by the Department of Homeland Security-sponsored Southeast Region Research initiative (SERRI) at the Department of Energy’s Oak Ridge National Laboratory.]
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Evaluation of Surface-NMR Spin Echo Measurements of T2
Authors Elliot Grunewald, Rosemary Knight and David WalshIn order to improve estimation of hydrogeologic properties from NMR relaxation measurements, we must improve our ability to measure the NMR relaxation parameters that are most directly sensitive to the underlying properties of interest. While standard surface-NMR measurements of the free induction decay can yield accurate estimates of the relaxation time parameter T2*, it has been shown that this parameter exhibits limited sensitivity to pore-size and permeability. in this study, we evaluate a modified surface-NMR acquisition scheme that uses spin echo measurements to estimate the more robust and readily utilizable relaxation parameter T2. We present a series of field experiments from the Central United States in which we have used surface-NMR to measure spin echo signals and estimate T2 with variable echo times ranging from 100 ms to 400 ms. NMR logging measurements in a nearby borehole provide a unique opportunity to compare the T2-values estimated by surface-NMR to ground-truth T2-values determined from the logging data. We find very close agreement between these two forms of measurement; we note, however, that we cannot detect spin echo signals from depth intervals where T2 is much shorter than the shortest echo time (100ms). Comparing the surface-NMR measurements of T2 to measurements of T2* acquired at the same time, we find that T2 is generally much longer than T2*. We explain the observed differences between these relaxation parameters by considering the effects of inhomogeneity in the background magnetic field. We find that T2 exhibits greater variation and sensitivity to pore size than T2* in coarse-grained materials; however, T2* provides greater sensitivity in fine-grained materials where no echo signal is detected. Given these distinct advantages of the T2 and T2* measurement, we propose a general framework for using both of these measurement types to enable more detailed characterization of groundwater aquifers.
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Monitoring Vertical Soil Moisture Dynamics using GPR Reflection Traveltimes
Authors Colby Steelman and Anthony EndresHigh-frequency ground-penetrating radar (GPR) surveys were used to investigate temporal soil moisture variations within a shallow vadose zone environment over multiple annual cycles between 28 August 2006 and 22 October 2008. Reflection profiling and common-midpoint (CMP) soundings were concurrently performed using 900 MHz antennas along a single 2.0 m transect characterized by stratified clean sand deposits. Surveys were conducted at intervals ranging from one day to four weeks across a wide range of wet, dry and frozen soil periods. Soil conditions were evaluated using measured traveltimes to four seasonally coherent stratigraphic interfaces. the presence of fixed stratigraphic events allowed us to monitor soil moisture changes within well-defined intervals, which significantly improves our ability to characterize hydraulic processes (e.g., infiltration, redistribution, drainage). Measured traveltimes were converted to velocity using the average interval thicknesses calculated from normal-moveout velocity analysis of CMP data which was then converted to an equivalent volumetric water content using a complex refractive index model. the GPR measurements effectively characterized temporal changes in vertical soil moisture distribution and migration along a 3.0 m vertical profile; the magnitude and duration of long-term seasonal moisture trends and short term fluctuations due to major precipitation events were quantified in terms of integrated water equivalents contained within each soil interval. in addition, direct ground wave velocity measurements from CMP soundings were used to examine the effects of soil wetting/drying and freezing/thawing cycles on the migration of moisture within the active root zone which significantly improved our ability to characterize dynamic processes at the air-ground interface and its coupling with deeper moisture profile variations.
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Geophysical Exploration for Water Resources in Buried Valleys in Western Canada
Authors Paul Bauman, Brad Hansen and Christeen NahasGeophysical Exploration for Water Resources in Buried Valleys in Western Canada Paul Bauman, Brad Hansen, Christeen Nahas, and Dan Parker WorleyParsons [email protected] Most of Western Canada is devoid of extensive regional aquifers. in the Prairie Provinces, while most large cities rely on surface water, smaller municipalities generally rely on groundwater. in such cases, buried valley aquifers are often the only viable source of a large yield groundwater source. Other than municipalities, many mines, power plants, farmers, oil and gas operations, and various industrial facilities also rely on water supplies derived from buried valley aquifers. Most of these aquifers are difficult to identify and delineate. Usually, they are covered by a significant thickness of till varying from 10 m to greater than 50 m, making air photo identification usually ambiguous, and often impossible. Even where the valleys can be approximately delineated from existing borehole Information, air photo Interpretation, airborne geophysical data, or legacy oil and gas seismic reflection data, most of the valley fill may be silt and clay. As such, easily accessed Information may not be sufficient to identify high yielding aquifers within the buried valleys. This paper will describe the systematic evolution and integrated use of a variety of ground geophysical techniques in exploring for, and delineating buried valley aquifers, including high resolution seismic reflection, time domain EM, fixed frequency EM, and electrical resistivity Tomography. Case studies will be presented from Saskatchewan, Alberta, and British Columbia. Case study applications will include municipal and rural water supply, oil and gas source water, agricultural supply, and mine dewatering.
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Identifying and Protecting Native American Graves using Electromagnetic induction: A Case Study from Central Georgia
More LessThis paper presents the results of an electromagnetic induction survey conducted at Ocmulgee National Monument in Macon, GA in 2010. the site wThis paper presents the results of an electromagnetic induction survey conducted at Ocmulgee National Monument in Macon, GA in 2010. the Ocmulgee National Monument Geophysical Survey Project mapped the monument using a soil conductivity meter. One of the project's goals is to identify sensitive cultural areas with Native American graves for future conservation. My sampling density ranged between 6 and 9 samples per meter. the project identified numerous anomalies that may be interpreted as Native American burials. Small anomalies having low apparent conductivity were located near the Funeral Mound (Mound C); a known cemetery. the data collected during the 2010 field season will aid the national monument in its conservation efforts and help the National Park Service avoid these sensitive materials during future archaeological work.
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Evaluation of Vibration Characteristics at Improved Soft Ground by Surface Wave Method
Authors Tatsunori Ikeda, Toshifumi Matsuoka, Takeshi Tsuji and Koichi HayashiSoil improvement becomes important for constructing buildings at soft ground. Casting rigid bodies into the soft ground at periodic distance is known as one of popular approaches for improving soft soil ground. When we construct an establishment over this improved ground, it is crucial to estimate the shear wave velocity on this field in order to evaluate the effect of vibration characteristics. in this study, we propose a new approach combining surface wave method and homogenization method to estimate macroscopic vibration characteristics for the improved ground. to study the effectiveness of this approach, the simulation study is carried out as follows. Firstly, simulated waveforms for the grid-form improved ground model are synthesized by 3D finite difference method. from this simulated waveforms, the dispersion curve of surface waves is then obtained from multichannel analysis of surface waves (MASW). to evaluate whether this dispersion curve represents macroscopic characteristics of treated soft ground properly, simulated waveforms are synthesized by using the homogenized ground model which is made by homogenization method. the dispersion curve for the homogenized model is also evaluated. the homogenization method is based on the assumptions that an intended composite ground is composed of periodic structures and the ratio between the length of intended structure and that of periodic structure is close to zero. therefore, the wavelength of surface waves should be long to some extent in order to represent macroscopic characteristics. We investigated the relation between the wavelength of surface waves and the periodic length of rigid body by comparing the dispersion curve of improved ground model with that of homogenized model. As a result of simulation for various improved models, it is concluded that the surface wave dispersion is consistent with the homogenized model when its wavelength is about twice longer than the periodic length of rigid body.
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Site Resonance Frequency Assessment Through Dynamic Load Testing (DLT): Early Results of a Test Carried Out in the Experimental Site of Fivizzano (Ms, Italy)
The assessment of the local seismic response is one of the fundamental phases when it comes to defining the levels of seismic hazard at a detailed scale. these are typically known as seismic zonation studies. the estimation of the site specific resonance frequency is a parameter of relevant interest within such studies as it is associated with the maximum seismic amplification which has to be related to the frequency of resonance of buildings. the ambient noise based methods and other experimental methods for measuring this parameter are not always applicable because they are strongly influenced by contrasts of impedence and by lateral subsurface heterogeneities. Furthermore, they sometimes present limits that are not always clearly known. On the other hand, the classic analytical approach that makes use of integrated and multidisciplinary Investigation methods, which estimate the local amplification parameters, mainly in terms of amplification factors and response spectrums, currently presents uncertainties and limitations. these are essentially associated with the difficulty of modelling the mechanisms of propagation of the seismic waves within a 3D system. the authors of this paper propose the experimental measurement of the site resonance frequency by means of Dynamic Load Testing (DLT). This is done by presenting the operative procedures, the data processing and the results obtained from a test undertaken at the experimental site of Fivizzano (Massa-Carrara, Italy) where a detailed study of the local seismic response was also undertaken. the test involved the delivery of sinusoidal forces with maximum amplitude of 20 kN and a range of frequencies between 0 and 15 Hz by means of an electro-mechanical generator. Simultaneous seismic monitoring was undertaken through a multichannel acquisition system. Velocimeters were placed on the ground and in a nearby strategic building. the measured frequency of resonance was compared to the predicted value obtained from empirical-analytical solutions and application of horizontal to vertical spectral ratios (HVSR) method and to the value measured by a seismic station equiped with two coupled accelerometers, one at the surface and one at depth; these sensors allowed an accurate analysis of the effects of the drift deposits overlying the bedrock, that is the transfer function of the stratigraphic column at the site.
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Characterizing Permafrost Thaw Anomalies with GPR and Electrical Resistivity - Field and Model Studies
Authors Emily Park, John Bradford and William BowdenPrevious work indicates that increasing Arctic temperatures are leading to permafrost degradation and increased numbers of thermokarst features. Our goal was to use ground-penetrating radar (GPR) to investigate depth-to-permafrost in areas with developing thermokarst features and to study how surface expression (ex: slope failure, thaw depressions) and the permafrost table are related. We also conducted a model study to investigate the potential for monitoring subsurface flow with GPR and electrical resistivity. We investigated sites in arctic Alaska with a range of surface morphologies. Using 250 MHz antennas, we conducted 3-D GPR surveys over three features: a large thermokarst failure, a thaw slump, and an area of poorly developed patterned ground. We also conducted 2-D GPR surveys at two adjacent small drainages: one in which thermokarst features and slope failure were evident, and one in which anomalous surface features were absent. We found depth-to-permafrost was often greatest in the vicinity of slump or slope failures. In some cases, well-developed linear thaw depressions were evident along the thaw table, though there was no surface expression. This suggests the possibility of preferential flow within the subsurface, which could substantially alter the permafrost before surface features become evident. We created a simplified model of groundwater flow based on our field observations. To determine the best method for detecting subsurface flow, we simulated salt tracer tests coupled with time-lapse electrical resistivity surveys over the thaw anomaly, using a variety of electrode configurations, GPR antenna frequencies, different concentrations of tracer, and different levels of saturation. We found that combined dipole-dipole and Wenner arrays were suitable for monitoring tracer movement through a small (10 cm depth) thaw table melt channel.
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Phase Inversion of Refraction Traveltimes
Authors Bethany Burton and Karl EllefsenPhase Inversion can process the traveltimes from either P?wave or S?wave refraction surveys. for this type of Inversion, the observed traveltimes of the first-arriving waves are transformed to their frequency-domain equivalents, which are the observed phases. If the sources and the receivers are aligned, then the Inversion model is 2D. Otherwise, the model is 2.5D. for both models, the parameterization is the logarithm of the slowness. the calculated phases are obtained from the solution of the scalar Helmoltz equation. the model is updated with a backpropagation method that is based on Rytov wavepaths. Because the objective function for the Inversion is nonlinear, this procedure must be applied repeatedly until convergence occurs.
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Three- Dimensional Characterization of Archaeological Sites using Multi-Channel Ground Penetrating Radar Arrays; Benefits and Examples
Authors Kevin Hon, Scott Harris, Maureen Hays and Kimberly PyszkaApplying geophysical techniques to archaeological Investigations is an ideal, non-destructive means for obtaining preliminary Information on a site prior to excavations. Recent advances in multi-channel ground penetrating radar (GPR) can now provide archaeologists with one hundred percent true coverage of an open site in a fraction of the time of previous GPR data collection and processing methods.
Although conventional two-dimensional and pseudo three-dimensional GPR surveys with single-antenna deployment (one transmitter and one receiver) can yield useful Information about a site, these types of systems do have limitations. Typically, archaeological sites require very dense data acquisition to enable a relatively high level of detail and to ensure the potential of delineating smaller targets. On larger sites, this dense data requirement is very time consuming to both set up (usually in the form of a very tightly spaced grid) and collect the GPR profiles. Accurate positioning may also be hindered by site parameters and rugged terrain. in regards to three-dimensional site representation, there is also a fair amount of data interpolation between each of the collected GPR profiles; which directly correlates to a possible loss of detail and Information.The Integration of multiple GPR transmitters and receivers into a single array eliminates the limitations of single-antenna systems to produce a true three-dimensional data set from a site. using a built-in marking system and coupled with a very accurate (centimeter precision) robotic total station, these arrays require little to no set up time given that a grid is not required. Since these arrays also cover a swath of approximately 1.5 meters across and can collect data at relatively fast speeds (up to 20 km/hr), acres of data can be collected in a single day.
The added benefits of using a true GPR array is expressed through examples collected from sites in Europe and from the Dixie Plantation located in Charleston, SC. Dixie Plantation is owned by the College of Charleston and is a cooperative testing site for the first ever deployment of a 1.3 GHz array at an archaeological site.
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High Frequency Ground Penetrating Radar Monitoring of Ethanol Released over Existing Gasoline Residuals
Authors John Mosquera, Anthony Endres and Juliana FreitasHigh-frequency (450 and 900 MHz) ground penetrating radar (GPR) was used to monitor the migration of an ethanol release and the redistribution of existing gasoline residuals within an unconfined sand aquifer. in September 2009, 185L of denatured ethanol mixture (E95) was released directly over a year old gasoline (E10) spill zone. GPR profiling prior to the ethanol release indicated that the gasoline impacted zone was largely confined to the vicinity of its Initial release point. GPR profiles collected one month after the ethanol release showed the formation of strong reflection events which propagate laterally away from the trench. After this Initial ethanol expansion, decreases in event amplitude were seen until the onset of winter conditions. the effects of the ethanol were also monitored using the apparent velocity “pull-up” of an underlying stratigraphic reflector; variations in the velocity pull-up correlated well with the behaviour of the ethanol-Induced reflection.
the presence of ethanol inhibited the freezing process of the pore water in the contaminated zone, resulting in a difference in the dielectric properties between the unfrozen and frozen zones. It was found that the unfrozen zone area was significantly greater than the spatial extent delineated by GPR during pre-winter conditions. the spatial extent and depth of the unfrozen zone imaged by GPR was been confirmed by soil probing with a steel rod. After seasonal thaw, evidence of the ethanol or gasoline was absent until mid summer when the water table dropped to approximately 80cm below ground surface. At this point, strong reflection events were again observed throughout the contaminated zone until the end of the experiment in September 2010 when the site was excavated.
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Determination of Main Barton Springs Groundwater Flow Path: 2-D Resistivity and Natural Potential Surveys At Barton Springs Swimming Pool, Austin, Texas
More LessThe Main Barton Springs is a major discharge site for the Edwards Aquifer and is located in Zilker Park in Austin, Texas. the spring discharges into the Barton Springs pool near the diving board at an obvious fault line (Barton Springs fault). the thin bedded unit on the southwest side of the fault is the Regional Dense Member and the lower Georgetown formation of the Edwards Group is exposed on the northeast side of the fault. the offset of the fault is in between 40 and70 feet. It was geologically assumed that the groundwater recharged from the Barton Spring Segment, which is located several miles to the south-west of the Barton Springs pool area, follows the Barton Springs fault strike and empties into the pool. to test this hypothesis geophysical surveys [resistivity imaging and natural potential (NP)] were performed across the Barton Springs fault and in the southern part of the Zilker Park. Only NP surveys were allowed within the boundaries of the pool because of endangered species of Barton Springs Salamander. the purpose of the surveys was multi-folded: 1) to locate the precise location of the Main Springs on the south banks of the Barton Springs pool; 2) to determine the potential location of caves and active flow paths beneath the spring; 3) to characterize the geophysical signature of the fault crossing the Barton Springs pool. We collected 5 N-S and 5 E-W resistivity and 5 NP profiles across and in the vicinity of the Barton Springs fault. Resistivity results to the east and southeast of the fault indicate a low resistivity zone caused by clay and/or clay-filled voids or caves at a depth of 25 feet extending down to 80 feet. Results also indicate that the low resistivity zone has a lense-shaped geometry and it is limited to the east by a significant fracture or fault. the NP results show high anomalies over the low resistivity zone indicating possible location of caves and/or groundwater flow. Geophysical results altogether, thus, suggest that significant amount of groundwater flow follows the path to the south of the fault along a fracture/fault zone that appears to coincide with the Springs location emptying into the pool within the Georgetown formation.
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Characterization of a Shallow Gasoline Release Over an Annual Cycle using High Frequency GPR
Authors Anthony Endres, Cameron McNaughton, John Mosquera and Juliana FreitasOur hydrogeophysical field experiment evaluated the ability of high frequency (450 & 900 MHz) ground penetrating radar (GPR) to characterize the release of gasoline over an annual cycle of in situ conditions. in August 2008, 200 liters of E10 gasoline were released into the unconfined sand aquifer at CFB Borden. the 900 MHz profiling clearly shows the development of shallow (i.e., above 10 ns) high reflectivity in the vicinity of the trench immediately after the release. Additional lateral extension of high reflectivity zone was observed over the following 20 days until the seasonal water table low stand occurred, after which no further lateral movement was observed. Throughout the remainder of the monitoring, the 900 MHz profiling observed a long-term dimming of reflectivity at the periphery of the impacted zone. While direct imaging of the shallow impacted zone by the 450 MHz antennas was significantly obscured by the superposition with the direct air-ground wave arrival; its improved depth of penetration allow the measurement of a velocity “pull-up” of an underlying stratigraphic interface resulting from the displacement of low velocity water by high velocity gasoline. the maximum pull-up was observed during the water table low stand. the ongoing changes in the pull-up magnitude during the remainder of the observation period suggest that the continued redistribution of fluids in the impacted zone. Because of the shallow depth of the gasoline impacted zone, the effects of freezing during the winter period were observed in the GPR imaging. the presence of the gasoline impacted zone appears to have affected the depth of freezing, causing a depression of the frozen soil base. the dimming of the direct air-ground wave complex indicates that the contaminant phase brought to the surface by the water table fluctuations have impacted the nature of the near-surface freezing.
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Electrical Monitoring of Pressurized Injections into a Heap
Authors Michael McNeill, Dale Rucker, James Fink, Chris Baldyga, Thom Seal and Jeffrey WintertonGold and copper mining of low grade ore typically involves an open pit circuit with a heap leaching facility. A heap consists of blasted rock placed in a large pile, which is subject to surface irrigation with a reagent to dissolve and transport the metal out of the rock. Pressurized Injections (Hydro-Jex) into the heap leach pad at the Cripple Creek &Victor (CC&V) Gold Mine were also conducted to further enhance the recovery of gold. Hydro-Jex is operated by injecting fluid at very high pressures from a centralized well to targeted zones within the heap. Real-time electrical monitoring, where electrical current and voltage potential measurements were made on a 96-channel system to track the Injections, provided both direct and indirect Information to understand fluid movement during the Injections. Although additional data processing would have provided valuable Information, the strategy of focusing on the monitoring of time series data allowed for a quick turn-around. During this trial application of the Hydro-Jex technology, the timing of wetting front arrivals were noted in real-time by monitoring the electrical current output of the borehole electrodes; a sharp increase in current was a result of an increase in saturation, and hence a decrease in contact resistance with the heap. Flow direction was an important aspect during this project because the Injections were near the slope of the heap and monitoring for stability of the rock pile was a necessary condition of the operating permit. Directionality of the wetting front was determined by mapping the shape of the apparent resistivity contours using a time-lapse mise-a-la-masse methodology. Overall, the monitoring showed no impact to the side slopes and that the recovery of gold could be improved through simple redesign of the well field and Injection timing.
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