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17th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 22 Feb 2004 - 26 Feb 2004
- Location: Colorado Springs, Colorado, USA
- Published: 22 February 2004
41 - 60 of 165 results
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Cross-Hole Complex Resistivity Survey For Pce At The Srs A-014 Outfall
Authors Robert E. Grimm and Gary R. OlhoeftCross-hole complex-resistivity imaging of the vadose zone was performed at the A-014
Outfall at the Savannah River Site, Aiken, SC. Five vertical electrode arrays (VEAs) were
installed with ~15-ft separations in and around a suspected DNAPL source zone to depths of 72
feet. Amplitude and phase data were edited for quality and then inverted to form threedimensional
(3D) images of the target volume. The comparatively small magnitude of the
nonlinear resistivity Hilbert distortion allowed approximate linearized imaging of the 3D
distribution of these effects. Laboratory analysis of nearby soil contaminated in situ indicated
that the CR response to the PCE-clay reaction was maximized near 50 mHz. PCE occurrence
was predicted to track phase and Hilbert-distortion indicators. PCE concentrations were
measured at three drilling locations and were compared to predictions a receiver-operating
characteristic analysis. The optimum performance at 1000 mg/kg in situ PCE was >80%
detection (true positives) with <30% false alarms (false positives) at an effective resolution of 4
ft, ~1/3 of the interwell separation. The CR survey successfully predicted the general distribution
of PCE at parts-per-thousand concentrations, specifically widespread near-surface contamination
and a zone of discontinuous pods or stringers immediately below the source.
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Three Dimensional Self-Potential Inversion For Subsurface Contaminant Detection And Mapping At The Doe Savannah River Site, South Carolina
Self-potential (SP) data are collected using a 3D array of non-polarizing electrodes, consisting of
a surface grid and four borehole arrays, over an area known to be contaminated with DNAPLs (Dense
Non-Aqueous Phase Liquids). The self-potential method is commonly used to measure the electric field
produced by electrokinetic, thermoelectric, or electrochemical coupling processes that take place in the
subsurface. DNAPLs are known to undergo oxidation-reduction (redox) reactions in the environment,
and are proposed as an electrochemical source for this investigation. Electrical currents that exist due to
the redox reactions at depth traverse the resistive Earth materials and are manifested as a potential field
that is measured at the surface and borehole locations. A 3D inversion algorithm is used to find the
electrical current source model that supports the measured data, taking into account the resistivity
structure derived from an induced polarization survey at the same field location. The sources and sinks
of electrical current are related to the zones of redox activity, and therefore to the areas of
contamination. These results are correlated with chemical concentration data obtained from a series of
ground-truth well measurements taken at the site.
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Mapping Of Tce And Pce Contaminant Plumes Using A 3-D Induced Polarization Borehole Data
In-situ complex resistivity (CR) or Induced Polarization (IP) data are collected using a 3D array
of surface and borehole electrodes, over an area known to be contaminated with DNAPLs (Dense Non-
Aqueous Phase Liquids). The contaminants include Tetrachloroethylene (TCE) and Trichloroethylene
(PCE), which, until recent years have been disposed of directly into the environment. The design of the
surface and cross-borehole array allows for a 3D IP inversion. Data are measured at two frequencies (1/4
and 1/16 Hz), and are inverted for resistivity magnitude and phase. The inversion results are compared
with PCE and TCE contaminant concentrations measured from core samples taken from three ground
truthing wells drilled within the region of interest. The phase and imaginary resistivity are shown to be
well correlated with the concentration data from two of the three ground truthing boreholes where the
TCE and PCE concentrations are above 1mg/kg.
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Sediment Sampling At The A-014 Outfall For Comparison With Complex Resistivity Measurements
Authors Joseph Rossabi, Brian D. Riha and Dennis G. JacksonChlorinated solvents were usually released as dense non-aqueous phase liquids (DNAPLs) to the
subsurface where they move in an unstable fashion driven by gravitational and capillary forces. They are
often retained in small discrete blobs (<< 1 m3) in fine grain materials particularly in the vadose zone
and contaminate ground water by slow continuous release through dissolution and diffusion. Locating
these small sources is a difficult but crucial part of remediating a contaminated site. Several methods
have been developed for subsurface DNAPL location but nearly all are intrusive and can only identify
DNAPL in close proximity to the access hole. Minimally invasive geophysical methods to locate
residual DNAPL have been proposed and developed but few methods are capable of the spatial
resolution required. Complex resistivity measurements sensitive to DNAPL (tetrachloroethylene)
interactions with clay (smectite) have recently been shown to have promise in laboratory experiments.
Based on these laboratory results, field tests of the complex resistivity technique were performed at the
A-014 outfall of the Savannah River Site.
The Savannah River Site (SRS) continues to use both innovative and baseline methods to
characterize sites including DNAPL contaminated areas. To support the field testing of complex
resistivity techniques an area (approximately 10m x 15m) was selected with residual DNAPL confirmed
by both the NAPL FLUTe (Ribbon NAPL Sampler) and cone penetrometer soil sampling and analysis.
After complex resistivity measurements were made, approximately 400 depth-discrete soil samples at 5
selected locations within the test area were collected and analyzed to compare with the geophysical
results. Despite inherent differences in spatial resolution and coverage, and the limited number of
baseline samples this comparison can be used to provide some measure of performance of the
geophysical technique.
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Geophysical Characterization Of The Fuel Producing Zones Near The Bedrock At Campbell Army Airfield, Fort Campbell, Ky
Authors M.D. Thompson, Wayne Mandell, Bill Davies, Steve Miller and Pat WilkeyFuel leakage adjacent to the Campbell Army Airfield, Fort Campbell, Kentucky has
introduced jet fuel (JP-4), a light, non-aqueous phase liquid, into soils and possibly the underlying
karst system where it poses a threat to the ground water supply. Previous geotechnical
investigations conducted over a 15-year period using traditional drilling and sampling methods
showed varied success in locating the presence of LNAPL contamination. Two contaminated
regimes were identified from this effort; a shallow zone that corresponds to perched water intervals,
and a deeper zone within the epikarst where free product is currently being recovered.
Surface geophysical data (seismic and electrical) were used to augment the established
network of wells by identifying changes in bedrock topography and overburden character likely to
be associated with the contamination regimes. A mapped bedrock depression appears to control the
spatial extent of the deeper contaminated zone, as extraction rates are greatest within the vicinity of
the rim of this depression. A weak correlation between fuel producing areas and zones of lowresistivity
(higher electrical-conductivity) can be established at this site. If true, this would imply
that active degradation of the fuel is occurring and producing an electrically conductive pore fluid.
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Reprocessing Gpr Data From The Cfb Borden Experiment Using Apvo/Gpr Techniques
Authors Thomas E. Jordan and Gregory S. BakerThis paper presents reprocessed GPR data from a 1991 Canadian Forces Base (CFB) Borden
experiment conducted by Brewster and Annan (1994). The purpose of our research is to
determine if a modified amplitude and phase variation with offset analysis of the ground
penetrating radar (APVO/GPR) data is a feasible technique for monitoring a dense non-aqueous
phase liquid (DNAPL) release. Forward models using the Fresnel reflection coefficient equation
accounting for low loss conditions indicate that conductivity is inversely related to the maximum
absolute value reflection coefficient max R observed for incidence angles of 0 through 89
degrees. The injection of a low conductivity DNAPL into a saturated sand decreases
conductivity by displacing some ground water. Conductivity values then increase as the DNAPL
migrates vertically downward. It is anticipated that values of max R increase as DNAPL is
injected followed by a decrease as the DNAPL drains. The ability to detect changes in the
amplitude of reflectors due to zones of anomalously conductive liquids may be an additional
method for monitoring NAPL releases. The APVO/GPR technique may also be useful for
determining the conductivity of subsurface reflectors.
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3D Multi-Offset, Multi-Polarization Acquisition And Processing Of Gpr Data: A Controlled Dnapl Spill Experiment
More LessDense non-aqueous phase liquid contaminants (DNAPL) typically have much lower electric
conductivity and electric permittivity than water. The bulk electric properties of the subsurface can be
significantly altered when these contaminants replace water in the pore space. Ground-penetrating radar
(GPR) is sensitive to permittivity contrasts and provides the potential to identify zones of low permittivity
associated with the presence of DNAPL. To test 3D multi-fold GPR techniques for quantifying DNAPL
induced permittivity anomalies, my research team conducted a small (107 cm x 122 cm), controlled DNAPL
spill experiment. The model was confined within a cylindrical polyethylene tank; model material consisted
of medium to coarse grained sand with a thin gravel layer near the base. My team injected twenty liters of
a chlorinated solvent solution into the vadose zone just below the surface, and monitored contaminant
migration into and through the water saturated zone to the bottom of the tank. I compiled a comprehensive
dataset for testing a variety of data processing and analysis techniques including 900 MHz, multi-offset, 3D
surface datasets in both TE and TM polarizations, 2D GPR transmission data, downhole TDR probe data,
and post-injection soil samples for chemical analysis. Both reflection tomography from TE polarized surface
data and crosswell tomography from transmission data reveal significant velocity anomalies associated with
pooled DNAPL that approaches a saturation of 40%. Further, thinbed offset-dependent reflectivity analysis
of TM surface data suggests the formation of a thin, highly saturated (80-100%) DNAPL zone at the top of
the main DNAPL pool. This work demonstrates that detailed analysis of multi-offset, multi-polarization GPR
data can significantly improve our ability to quantify subsurface permittivity anomalies.
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A Laboratory Study Of The Complex Electrical Resistivity Response Of Soils
Authors Stephen R. Brown, Jason R. Sorenson and Thackery I. BrownThere is an increasing need for new non-invasive geophysical techniques to locate DNAPLs in the
subsurface. G. Olhoeft and colleagues have published several reports which indicate that organic solvents,
notably toluene, PCE, and TCE, residing in clay-bearing soils have distinctive electrical signatures. These
results suggest a new measurement technique for remote characterization of DNAPL pollution.
As with any new research result we note the importance of reproducing the work of previous researchers
to ensure that any effects observed are due to the physical phenomena occurring in the specimen
and not due to the particular experimental apparatus or method used. To this end, we independently
designed and built a laboratory system for the measurement of the complex electrical resistivity properties
of contaminated soil. After careful characterization and calibration of our equipment, we attempted
numerous times to reproduce the seminal results of Olhoeft and Sadowski on the response of toluenecontaminated
clay-rich samples. While we observe similar responses to theirs for plain clays with brine,
the addition of toluene does not produce the effects they described.
Our results indicate, at best, a low sensitivity of the complex electrical resistivity method to organic
contamination in rocks and soils.
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Using The Depth Of Investigation Index Method In 2D Resistivity Imaging For Civil Engineering Surveys
Authors Laurent Marescot and Meng Heng LokeTo assess whether features in 2D imaging results are demanded by the data or are artefacts of the
inversion process, a special inversion algorithm was applied to process DOI (Depth Of Investigation)
index maps. This method carries out two inversions of the same data set using different values of the
reference resistivity. The two inversions reproduce the same resistivity values in areas where the data
contain information about the resistivity of the subsurface whereas the final result depends on the
reference resistivity in areas where the data do not constrain the model. This calculation can be also
performed for borehole-to-borehole or borehole-to-surface surveys. In this case, regions of investigation
can be outlined. Without DOI maps, interpretation of models can be sometimes difficult, nonrepresentative
and dangerous. As can be inferred from field examples, the DOI maps prevent overinterpretation
or misinterpretation of inversion results in electrical imaging studies. The DOI map helps
explaining the occurrence of erratic and non-geologic structures at depth. It also says how deep we can
see into an inverted resistivity profile. In this paper, the implementation of the algorithm is first
described and the methodology is then illustrated with 2D surface and borehole electrical resistivity
imaging applied to civil engineering and hydrogeological surveys.
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Comparision Of Different 2D And 3D Geoelectric Survey Layouts For Detection Of Saltwater Convection Cells In The Okavango Delta (Botswana)
Authors R. Supper, P. Bauer, W. Kinzelbach and S. ZimmermannThe Okavango Delta is a huge wetland ecosystem in northwestern Botswana, Southern
Africa. Water from the moist tropical highlands of southern Angola flows down the Okavango
river, spills into the Kalahari basin and forms an alluvial fan of the size of approximately 30 000
km2. Some 6000 km2 of permanent swamps form the core of the Delta and towards the fringes,
the environment is gradually becoming drier.
Although the delta has no outflow, the swamps sill consist of fresh water with low
salinity. Density driven convection is regarded to be the phenomenon responsible for that. The
goal of the field campaign was to determine the salinity distribution below selected islands in the
Okavango Delta to verify this theory. We have tested and compared different geoelectrical
configurations (2d, 3d, borehole to surface) to resolve the phenomenon of density fingering under
such unfavourable situation of a full 3d problem and a very low resistive top layer covering high
resistive structures.
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New Achievements In Developing A High Speed Geoelectrical Monitoring System For Landslide Monitoring (Geomonitor2D)
More LessLandslides are one of the major threats to human settlements and infrastructure, causing over time enormous human suffering and property losses than any other geological hazard. Therefore much effort has to be centred on risk detection, risk reduction and development of timely warning systems to prevent future loss of life and property. During recent years the dc-geoelectric method has been used widely to investigate the structure of landslide areas, thus gaining the status of a state-of-the-art-method in civil engineering for this. However so far the dc-geoelectric method was hardly used to detect structural changes with time in active landslide areas. Within this talk we present the development of an innovative, high speed geoelectrical monitoring system, which allows remote controlled automatic measurements of geoelectric pseudo-sections. Each section, using a new kind of configuration, consists of 3000 single measurements, each one sampled for 1000 times. The acquisition time for such a section is about 30 minutes thus allowing to derive a point shot of the subsurface structure. This system was installed on an active landslide area. Within a period of two years, 6 pseudosections were measured each day. The results within this monitoring period will be discussed.
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Applications For Capacitively Coupled Resistivity Suvreys In Florida
Authors K. Michael Garman and Scott F. PurcellThe use of capacitively coupled resistivity (CCR) as a geophysical method has historically been of
limited use in Florida due to the shallow water table and the time necessary to make multiple passes to
collect resistivity data at depth. The induced current used by CCR instruments is stronger and can
penetrate to greater depth if the surface materials are resistive, because the voltage measured at the
receiver equals the current in the transmitter multiplied by the resistivity of the earth materials. The
presence of shallow groundwater increases conductivity thereby reducing the CCR signal strength. The
availability of a multi-channel CCR instrument, the Ohm-Mapper by Geometrics, Inc. has eliminated the
need for multiple passes for a study. Subsurface Evaluations, Inc. has investigated two applications for
multi-channel CCR surveys in Florida:
1. CCR has been successfully used to delineate buried depressions in sandy upland areas producing
results remarkably similar to ground penetrating radar (GPR) data but easier to interpret; and
2. CCR has been successfully tested on road projects for identifying shallow deleterious soil
conditions such as clay and peat lenses that might be missed by a standard drilling program and
that are not readily detectable by GPR.
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Comparison Of Different Electrode Materials For Induced Polarization Measurements
Authors Gareth Morris, Andrew M. Binley and Richard D. OgilvyIn recent years, there has been a marked increase in the use of the induced polarization (IP) and
spectral induced polarization (SIP) techniques in environmental surveys. As a result, the required
measurement sensitivity of this method has increased significantly from their original application in
mineral exploration. In order to obtain the best results from IP and SIP data, considerable work has been
done on the data inversion procedures whilst the field procedure has remained essentially the same.
Standard IP survey practice is to record data using non-polarizing electrodes. However, recent work has
concentrated on the use of different materials as potential electrodes. Of particular interest has been the
use of standard metal electrodes in the place of non-polarizing electrodes since this would greatly
improve the speed and ease of field surveys. Here we focus on the initial results of trials assessing the
use of different electrode materials in both laboratory and field-analogue environments. The
experiments compare and contrast the use of a range of electrode materials, including non-polarizing, for
IP measurements. Electrode performance assessment focussed on the strength, stability, noise levels,
and frequency response of the signal recorded with each material. The results have implications on the
design of future electrode arrays and field surveys.
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Forward And Inverse Resistivity Modelling On Complex Three Dimensional Structures Using The Finite Element Method
The finite element method has been used by several authors in the context of direct current
forward and inverse modelling. This versatile numerical method is particularly useful for engineering
applications in which very varied structures (e.g. walls, dams, concrete piles) can be modelled for nondestructive
investigations. Nevertheless, there is currently a need in developing forward and inverse
resistivity modelling codes that are better suited for imaging structures with complex geometries.
Therefore, various original approaches must be used to create a well-adapted program. This paper
presents the adaptation and the use of the CESAR-LCPC finite element code for the forward and inverse
modelling of 3D resistivity data. Firstly, the forward modelling code uses an electrode-independent
mesh that allows to place the electrodes at their exact locations and to use a coarse mesh at the same
time. To calculate apparent resistivity values, a normalisation approach is used that gives significantly
better results than the use of the geometrical factor and allows the modelling of any kind of complex 3D
structure. An inversion code was also implemented for the processing of resistivity tomographies on
complex 3D structures using any electrode arrangement. This algorithm uses an original strategy to
avoid high-computation costs since it does not involve the explicit calculation of a sensitivity (Jacobian)
matrix. Synthetic results are presented to illustrate the efficiency of these forward and inverse modelling
codes.
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A Fast Forward Modeling Algorithm For Ert Inversion
Authors Shan Wei and Roelof VersteegThe standard approach in electrical resistivity tomography (ERT) forward modeling is to find the
potentials resulting from a current dipole by solving a set of linear equations for this dipole. For N
electrodes, the number of sets of linear equations to solve can be as large as N*(N-1)/2. For large
numbers of current dipoles this approach is prohibitively inefficient. By first computing only single
current source (pole) data, then combining these data into dipole-dipole data, the maximum number of
sets of linear equations is N. Compared to conventional methods, this approach is much more efficient in
cases where we have large numbers of dipole current sources. Examples using this approach are given to
show that there is no loss of accuracy and it greatly reduces the computation cost.
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Ert Inversion On Sharp Contrast Boundaries
Authors Shan Wei and Roelof VersteegSmoothness constraint is one of commonly used regularization terms during the geophysical
inversion process to find a possible model fitting the data. However, if sharp contrasts boundaries exist
within the earth, inversions using smoothness constraints may fail to converge or yield suboptimal
results. As presence or location of sharp contrast boundaries is normally unknown, it is hard to apply the
corresponding constraints prior to the inversion. An algorithm to locate the possible sharp contrast
boundaries as part of the inversion through an iterative update of the smoothness constraints allows
significant improvement of the final image of subsurface properties. To demonstrate this algorithm,
electrical resistivity tomography (ERT) examples are given.
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Non-Invasive 3D Conductivity Measurements During Flow Experiments In Columns With Merit
Authors Axel Tillmann, Roy Kasteel, Arre Verweerd, Egon Zimmermann, Andreas Kemna and Harry VereeckenThe non-invasive measurement of physical parameters in soils is an important tool for observation and sustainable management of soils and aquifers so as to preserve or to restore groundwater quantity and quality from natural or anthropogeneous e ects. We present the Magneto-Electrical Resistivity Imaging Technique (MERIT) in theory and application to investigate the variation of the three-dimensional distribution of the electrical conductivity during fluid flow. Numerical experiments were performed to simulate water flow in cylindrical soil columns, the effect of saturation changes on electrical conductivity, and the changes in the resulting electrical potential and magnetic field due to current injection. As result we can state that MERIT is a non-invasive method that is capable of monitoring fluid flow and solute transport processes in soils, if the accuracy of the magnetic eld measurements is 0:1% and the resolution is 50 pT minimum.
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Towards Three And Four Dimensional Data Collection For Electrical Resistivity Data
Authors Douglas LaBrecque, Roger Sharpe and Melissa StubbenAdvances in hardware and software have made the collection of fully three-dimensional and four
dimensional resistivity data not only possible but practical. To fully utilize these new capabilities
requires new approaches to collecting data. This paper shows both theoretical and field comparisons of
data collection schemes comparing more traditional one- and two- dimensional data collection strategies
with fully three-dimensional strategies. These new approaches include array patterns, such as horizontal
subsurface dipoles, that were not possible in older schemes. Finding optimal arrays requires tradeoffs in
resolution, acquisition time, and signal levels. A new statistical based approach is shown that can be
used to determine optimal array types under various constraints. Tests of these new array types show
that they can provide substantial improvements in the resolution of subsurface resistivity and the change
in resistivity with time.
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Transient Audio-Magnetotelluric Imaging Of A Buried Valley
Authors David K. Goldak and Shawn M. GoldakThunderstorm activity produces large amounts of electromagnetic energy which is
trapped within the earth-ionosphere waveguide. The random sum of energy from activity
on a near global scale produces a low-level quasi-continuous source field. Very large, or
equivalently, relatively nearby lightning discharges produce individual transient events whose
amplitude are significantly larger than that of the low-level background field. Therefore, the
best possible signal-to-noise ratio is realized by recording exclusively sources of a transient
nature. However, the transient events are strongly linearly polarized, the polarization diversity
of which can affect the estimation of earth response curves.
It has been shown that an adaptive time domain averaging of the transient waveforms
results in earth response curves whose bias converges to zero super-exponentially in
stacked signal-to-noise ratio (Goldak et al., 2001). The efficacy of the algorithm is shown
in the results of a transient audio-magnetotelluric (TAMT) survey conducted over a buried
valley system in southern Manitoba, Canada. Twenty three sites at 200 m spacing were
collected with the impedance tensor ˜Z and the magnetic field tipper ˜T estimated over the
bandwidth 8 Hz - 32 kHz.
The results of the TAMT survey agree very well with those of a time domain
electromagnetic (TEM) survey conducted by the Saskatchewan Research Council with a
Geonics EM-47 over nearly the same profile.
Two dimensional OCCAM inversion of the TAMT data reveal the buried valley
to be approximately 1 km wide, 70 m deep with a resistivity of approximately 12 ¡ m,
incised into Cretaceous sediments of approximately 4 ¡ m resistivity.
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Depth Of Investigation For Small Broadband Electromagnetic Sensors
More LessThe depth of investigation (DI) in electromagnetic (EM) soundings is related to many factors
such as the sensor sensitivity, resolution, coil configuration, ambient noise level, and the techniques used
in data processing and interpretation. Quantitative understanding of the relationships between the DI and
these factors will help the users to meet their geological objectives, avoid unnecessary survey expense, and
display meaningful geologic features.
Simple equations to calculate the DI for hand-held EM sensors have been derived from the analysis
of the EM response based on layered models. The DI is proportional to the target conductivity and
inversely to the frequency, overburden conductivity, and detection threshold. We present field examples
to illustrate how to use the DI in data interpretation.
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