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9th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 28 Apr 1996 - 02 May 1996
- Location: Keystone, Colorado, USA
- Published: 28 April 1996
61 - 80 of 134 results
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Radar Tomograms At Mirror Lake, New Hampshire: 3-D Visualization And A Brine Tracer Experiment
Authors David L. Wright, Thomas P. Grover, Karl J. Ellefsen, John W. Lane and Paul G. KaseIn this paper we examine radar tomograms produced from data acquired in the FSE well field
at the fractured rock hydrology site at Mirror Lake, Grafton County, New Hampshire. We first
show an example of 3-dimensional tomogram visualization with superimposed projections of
hydraulically significant fractures. We find a generally good, but not one-to-one, correlation
between electromagnetic velocity tomograms and fracture projections from individual wells. One
reason why the velocity tomograms do not always provide unambiguous indications of flow paths
is that the tomograms reflect rock electrical properties which vary because of lithologic
heterogeneity at the site. Another is that hydraulic permeability in fractured media may be
controlled by small-aperture “choke points” that are probably not well imaged by the tomograms.
In addition, hydraulic flow may be through a network of interconnected fractures, many of which
may be oriented differently from any particular fracture observed at a borehole.
We then show a difference attenuation tomogram produced from a brine tracer experiment carried
out at the Mirror Lake site in 1995. An objective of this experiment was to distinguish between
the mere presence of water and hydraulic permeability. When interpreting tomograms between
two hydraulically connected wells, an unrecognized assumption is often made that hydraulic flow
paths are in the plane of the wells. This assumption may be invalid, especially in crystalline rock
where flow is dominantly through fractures, rather than through the rock matrix. Our difference
attenuation tomogram shows that the primary flow path left the plane of the injection and pumped
wells.
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Nondestructive Assessment Of Pile Tip Elevation Using Flexural Waves
Authors Glenn J. Rix, Laurence J. Jacobs, Raymond Q. Raparelli and Patrick B. RhodesFor approximately 6000 of Georgia’s 14,500 bridges, pile tip elevations are unknown because design and
construction records no longer exist. This lack of information is critical because it is diEcult to determine the
capacity of these piles - particuhuly in the presence of scour. The unknown bridge foundation problem is an
important problem for federal and state agencies and has been the focus of several studies (Olson et al., 1995;
Douglas and Halt, 1993) The unknown bridge foundation problem is one in which either the type and depth of
foundation is unknown, or the type of foundation is known but the depth is unknown. Although soil borings and
other intrusive tests are capable of determining pile tip elevations, the time and cost of performing these tests on a
large number of bridges is prohibitive. Nondestructive tests are an effective alternative for assessing pile tip
elevations.
This study focuses on bridges which are supported by exposed pile foundations that have unknown lengths. The
purpose of this study is to evaluate the nondestructive use of flexural waves to determine the length of these bridge
pile foundations. Many nondestructive test methods rely on the use of longitudinal waves excited axially down a
pile. This type of testing is not feasible since the bridge superstructure prevents access to the top of a pile. Although
the theory behind flexural wave testing is more complicated than that of the traditional longitudinal wave testing,
flexural or bending waves can be excited laterally on the side of the pile with no physical interference from the
bridge superstructure. The pile in most cases can be modeled as a long slender member since its ratio of length to
section depth ratio is large. The propagation of the flexural waves within the beam is a function of its length, mass
density, moment of inertia, elastic modulus and end conditions. When dealing with bridge piles, the primary
unknown is the embedment length since all of the other variables can be assumed or measured.
The nondestructive test method developed in this study uses modal analysis techniques to interpret the pile length.
A three-step approach is used. First, the response model of the pile is determined by measuring a set of frequency
response functions for the pile. Second, the modal model consisting of the natural frequencies, modal damping
parameters, and mode shapes is calculated from the response model. Finally, the spatial model is determined from
which the embedded length of the pile can be obtained.
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Determination Of Unknown Depth Of Bridge Foundations Using Two Nondestructive Seismic Methods
Authors Farrokh Jalinoos, Larry D. Olson and Marwan F. AouadThis paper presents the test results of a research study on two nondestructive test (NDT) methods used for
determining the unknown depths of bridge foundations. Of the over 580,000 highway bridges in the National Bridge
Inventory, 106,000 (over-the-water) bridges have unknown foundation characteristics because no design plans are
available. Consequently, it is difficult for the state transportation departments (DOTS) to assess their susceptibility
to scour. Nondestructive and non-invasive test methods are needed to evaluate unknown foundation depth and
geometries.
This study documents the results for two acoustic NDT methods of Ultraseismic (a proposed new test
method), and the Parallel Seismic test method which were found to have the broadest application to the investigated
concrete, timber, and steel bridge substructures.
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Imaging And Characterizing Fractures Ahead Of Tunnel Face Using In-Tunnel Hsp Method
Authors Tomio Inazaki and Toshiyuki KurahashiA new method for imaging and characterizing fractures ahead of a tunnel face has been
developed and successfully applied to field measurements. The fundamental technique of the
method, we named in-tunnel HSP ( Horizontal Seismic Profiling ), is featured as an
combination of that being employed in VSP and reflection seismic method. The measuring
process of in-tunnel HSP is basically the same as refraction seismic survey. In-tunnel HSP can
provide 2-D sections for in-line and backward portions as well as the portion ahead of a tunnel
face, which enables us to verily the certainty of seismic events in the 2-D sections by
correlating them with actual fractures cropped out in a tunnel and characterize them
geotechnically prior to the prediction of undriven section.
We have applied in-tunnel HSP to 29 survey lines in 15 tunnels in Japan until now.
Field application of the method and engineering evaluation of the results has proved in-tunnel
HSP is quite practical to delineate and characterize fractures ahead of tunnel face and to
ensure the safety through the focused tunneling based on the prediction.
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Analysis Of Possible Relationships Between Seismic Properties And Some Of Rock Mass Geomechanic Properties Of The Outcropping Carbonate Rock Masses In Wad1 El-Kaffrien Dam Site (Jordan)
Authors Nasser Abu-Zeid and Fiorenzo VuillerminA seismic refraction and a geomechanic surveys were employed in the investigation of possible
relationships that may exist between elastic and geomechanic parameters of a number of outcropping carbonate
rock masses in the Wadi El-Kaffrein dam site, western Jordan. Seventeen seismic profiles were conducted
across the top of vertical exposures where the underlying geologic and structural conditions are quite visible to
be studied in detail. Therefore, the correlations could thus be correctly assessed.
The above mentioned prospections were completed on limestones, marly- and dolomitic limestones of the
Upper Cretaceous age. The seismic survey enabled the determination of the P- and S-wave velocities, the
dynamic elastic moduli, in addition, to a number of seismic ratios. While the geomechanic survey resulted in
the quantification of some of the geomechanic characteristics of the discontinuities present in the rock masses,
in addition, to the assessment of their overall qualities through the employment of two of the widely used
classification systems, namely, RMR and Q.
The obtained data were subjected to a bi-variate and multi-variate statistical analyses, so that possible
correlations could be evidenced. Such analyses considered eighteen seismic and geomechanic properties. The
analysis procedure started by investigating the seismic parameters themselves: Vp, Vs and the dynamic elastic
moduli, then these latter parameters were correlated to some of the geomechanical properties. Such
correlations led to the development of a preliminary classification system for the assessment of rock mass
quality, based upon seismically derived parameters as well as Jf and Jn. The system divides the rock mass in
five classes of quality.
Concerning the correlations conducted between the RMR- and Qquality indices and the geoseismic
parameters, it resulted that the RMR-indices correlate better to the seismic parameters.
Results of the multi-variate regression analysis show that the following geomechanic properties could be
estimated with a good approximation: Jn, Jf and RMR, while the prediction of RQD and Q indices can’t be
predicted from seismic parameters.
The study resulted in the individuation of zones or sectors that present a high grade of fracturing and/or
lithologic heterogeneity. More in detail, such sectors are located on the left abutment of the dam, where further
works should take place if the dam’s height will be increased. Moreover, the obtained bi-variate and multivariate
regression equations furnish a wealth of information upon which extrapolation of geomechanic
properties to other areas can be based.
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Shear Wave Velocities Of Gulf Coast Soils Determined From Crosshole And Seismic Cone Penetration
Authors Tracy Brettmann, Robert C. Gauer and Recep YilmazThis paper provides a summary of shear wave velocity data obtained from Texas and Louisiana Gulf
Coast soils using both crosshole test (CHT) and seismic cone penetration test (SCPT) methods. Both
methods are described and a side-by-side comparison of the results from several sites where both
methods were used is presented. The data collected were evaluated based on both soil type and geologic
formation. Best-fit equations are listed for each material type relating shear wave velocity to various
parameters.
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Optimization Of Geophysical Survey Design
More LessWe discuss the general problem of geophysical survey design, and present criteria that can be used to evaluate the
efficiency of a given survey. The survey design can be adjusted until an objective function defined by the criteria is
maximized. This process depends on the ability to compute the forward and inverse solutions. In addition, since the
objective function may be topologically complex, it is necessary to use an optimization technique capable of
locating the globally optimum solution. In this paper we show that the smallest singular value of the design matrix
constitutes a useful objective function, and we suggest that genetic algorithms are well-suited for discovering the
optimal solution.
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Robust Scheme For Ert Inverse Modeling
Authors Gianfranco Morelli and Douglas J. LaBrecqueImprovements in the automated interpretation of cross-borehole electrical resistivity measurements are presented
comparing results from both simulations and recent field surveys. Improvements were achieved by means of a
statistically robust inversion scheme and include faster convergence, availability of a stopping criterion, and
decreased dependence on data quality. The theoretical basis for the new method is explained concisely to concentrate
on its main features and capabilities that arise from the analysis of the results. Particularly, the effect of standard
noise and systematic errors present in the data on the reconstructed resistivity images are discussed. The aim is to
provide a simple guide for the correct choice of the parameters that characterize an inversion, as for a correct
understanding of the images obtained.
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Magaid: Pc-Based Target Characterization Software For Use With Total Field Magnetometer Survey Data
Authors Thomas Bell, Thomas Bell and Mark ProutyMagAID is a PC software application designed for use in interpreting the results of magnetic
surveys conducted for environmental cleanup and unexploded ordnance (UXO) clearance operations.
It employs a Windows-based graphical user interface, and accepts gridded and contoured data from
commercial mapping software. MagAID allows users to work with a contoured survey map,
interactively using the cursor to select magnetic anomalies visible in the survey map. Once an
anomaly has been selected, MagAID invokes a dipole-based Maximum Likelihood Estimation
algorithm to automatically determine the location (x, y, depth) and apparent size of the object
which caused the anomaly. This paper describes the target characterization procedure used in
MagAID and presents results of tests of the algorithm and the software’s performance using data
from controlled magnetic surveys at test sites containing buried inert ordnance and metal drums.
Typical estimation accuracies are of order fifteen to twenty percent in size and depth for compact
objects.
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Progress In Interpretation Of Transient Electromagnetic Data In Terms Of Subsurface Permeability
Authors M.E. Everett, C.N. Burden, K. Sananikone and B.E. HerbertThe relative importance of nonpoint-source (NPS) pollution in the degradation of water quality has
increased in the last two decades due to the control of pollutant releases from point sources (Sharpley and
Meyer 1994). The most important source of NPS pollutants are agriculture and urban areas, which impact
water quality in rivers, lakes, estuaries and groundwaters through the release of eroded sediments, fertilizers,
pesticides, and municipal sewage sludge. Because of this, NPS pollution is an important environmental
concern at state and national levels.
Several transport processes control the dispersal of NPS pollutants, including leaching to groundwater,
surface runoff (Pereira and Rostad 1990), and aerial transport and deposition (Glotfelty et al. 1984). Once
in groundwater, these contaminants can impact surface water during stream recharge. While the losses of
NPS pollutants from agricultural fields or urban areas can be small as a percentage of the total amount
released, the cumulative additions to river systems from large drainage areas can be significant.
The development of a reliable contaminant transport model for tracing the dispersal of NPS pollutants
through a heterogeneous aquifer requires knowledge of the spatial distribution of hydrologic parameters such
as hydraulic conductivity or permeability (Zheng and Bennett 1995). In the commonly used stochastic
approach (Dagan 1989), these distributions are treated as spatial random functions whose variance and
correlation length scale are determined from hydrological information collected by head measurements, pump
and tracer tests, and soil core analyses. Point estimates of the hydraulic conductivity or permeability are
determined by kriging, a geostatistical interpolation procedure which estimates unknown random functions
based on spatial correlations between point observations.
Noninvasive geophysical techniques are becoming an increasingly popular component of hydrogeological
studies since many geophysical data are sensitive to spatial variations in hydraulic conductivity and permeability.
In addition, the cost of surface exploration is only a fraction of the cost of drilling and a wide area1
coverage is readily obtained. Controlled source electromagnetic (CSEM) methods provide maps of electrical
conductivity and are an appropriate choice if the depth scale of investigation is on the order of 10 m-l km.
We are investigating the possibility of placing electrical constraints on the subsurface permeability as
part of a larger, integrated study to determine the fate of agricultural chemicals introduced at a research
site near the Brazos River. The research involves a joint analysis of the electrical conductivity structure, the
available soil cores, and other hydrologic data. A Bayesian approach is taken, following Copty et al. (1993),
in which geophysical data are used to update the variance and correlation length scale of a hydrologicallyderived,
random permeability field. In this paper we will describe the underlying theory and demonstrate an example using synthetic CSEM data that have been generated from a simulated geoelectrical section of our study site.
The incorporation of CSEM data into a determination of subsurface permeability is made more difficult
by the presence of man-made electrical conductors at the site. These include an aluminum pipeline and an
overhead power line. Such artifacts are characteristic of the human impact at many environmental sites. We
are presently applying the integral equation code of Qian and Boerner (1995) to account for the effect of the
aluminum pipeline on the CSEM data.
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Rapid 2D Inversion By Multichannel Deconvolution Of Geophysical Profile Data
More LessHigh resolution geophysical data are now often acquired in regular profile geometries, where pseudo
section display modes give very useful qualitative insight. For a more quantitative foundation on which to
base decisions we need a proper data inversion approach. These years the growth in airborne and surface
based acquisition power more than matches the growth in computer power. This has accentuated the need
for inversion methods that are rapid and yet safe and easy to use for non-specialists.
Approximate inversion methods based on the Born approximation are well suited for this purpose as
shown by Loke and Barker (1995) for the case of DC geoelectrical multi-offset profiling. Their approach is
adequate for relatively short, detailed profiles. In a similar study Li and Oldenburg (1992, 1994) show
how to solve the approximate inversion of DC data in the Fourier domain, thereby gaining an important
speed-up when data profiles are long and densely sampled. Meller et al. (1996a) test this method both
numerically and on densely sampled field data acquired with the Pulled Array Continuous Electrical
Profiling technology (Sorensen, 1995, 1996).
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Teaching The Practice Of Geophysics: A Prototype World Wide Web Environment For Conceptual Learning
Authors Thomas M. Boyd and Phillip R. RomigThe history of the United States has been driven by a surplus of resources (energy, materials, air,
water, open space). Today we are entering an era when there will no longer be more resources
than people. This transition from a resource-rich to a resource-limited environment is responsible
for many of the changes occurring within our society today, including the restructuring of industry
and government.
With regard to the earth sciences, one of the dominant issues of the 21st century will be how to
use a finite earth to support a burgeoning global population. We must continue to provide the raw
materials needed to feed, clothe, house, and provide a reasonable standard of living for all humans
on earth. We must learn to extract and use resources and dispose of the waste products of that
activity without poisoning the air we breathe and the water we drink. The quality of life of future
generations depends on the earth sciences being equal to this challenge.
A major impediment to the successful execution of these tasks is the fact that earth science has
evolved as a collection of loosely-related, poorly-coordinated specialties with little communication
or interaction between them. This has made it difficult for earth-scientists to provide the public
with a coherent education in the fundamentals of earth science so that they can make informed
choices. More importantly, it has discouraged the development of a common foundation for all
earth-science specialists, thus inhibiting our ability to work together to effectively solve the
important challenges facing us.
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A New Approach To Interpretation Of Airborne Magnetic And Electromagnetic Data.
Authors P. Traynin, M. Zhdanov, J. Nyquist, L. Beard and W. DollWe present a new technique for underground imaging based on the idea of space-frequency filtering and
downward continuation of the observed airborne magnetic and electromagnetic data. The technique
includes two major methods. The first method is related to the downward analytical continuation and
is based on the calculation of the total normalized gradient of the observed field. The second method
is based on Wiener filtering and takes into account a priori information about typical AEM anomaly
shape from a possible target.
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Regularization In 3-D Dc Resistivity Tomography
Authors Jie Zhang, William Rodi, Randall L. Mackie and Weiqun ShiWe demonstrate that the solution of 3-D DC resistivity tomography has strong numerical artifacts if the inverse
problem is not properly regularized. With only few data points but a large number of model parameters
(unknowns), the nonlinear inverse problem is ill-posed. Many studies have shown that some kind of model
correlation must be constructed to stabilize the inversion. Among those, Tikhonov regularization takes a
more explicit approach by damping spatial derivatives of the model function as opposed to applying ,ad hoc
smoothing. However, we show evidence that not all smoothness criteria in the class of Tikhonov methods
are well-posed for 3-D DC resistivity inversion. In fact, only under the second- or higher-order derivative
regularization, 3-D DC resistivity tomography can produce a physically meaningful solution which has no
dependence on the model discretization. In adopting effective smoothness criteria, the solution approximates
a continuous function with no more structure than is necessary to fit the data. Further, we demonstrate
that using Tikhonov method to regularize the model stepsize rather than the model itself does not improve
the ill-posedness of the inverse problem. As the result, only the data misfit has been minimized and model
correlation is not constrained. Finally, we apply our tomography approach to model real data collected at the
Mojave Generating Station in Laughlin, Nevada. For different model discretization, our approach prbduces
similar subsurface image.
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Magnetic Interpretation By Scanning Multiple Models For Best Least Squares Fit
Authors G. Hunter Ware and Hunter Andreas WareMagnetic interpretation is generally accomplished by comparing discrete anomalies in the
residual field with theoretical profiles of reasonable models. Simplified ‘rules of thumb”
based upon sets of model curves are also employed. Direct analytical inversion of field
data is impractical due to non-uniqueness and computational difficulties. The problem is
that even the simplest models may be adjusted in depth, horizontal position, and
orientation. More complicated models may also be varied in two- or three-dimensional
shape, and may involve multiple bodies. It is therefore very difficult to optimize model
parameters and measure the degree of fit or agreement with field data.
We have developed a procedure called MAGFIT for automatically comparing actual and
model profiles, and calculating a best least squares fit. With our algorithm, it is possible to
rapidly scan a simple set of models (such as dipoles) over many locations and orientations,
and determine the best fit as a function of position (and/or other model variables). “Best
fit” plots generally show closed contours around a best tit of 60% to 90%, identifying the
likely locations of magnetic bodies.
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Neural Network Classification Techniques For Uxo Applications
Authors Matthew Gifford and John E. FoleyA neural network was used to predict the mass of shallow subsurface conductive objects. The field data was from
an unexploded ordnance (UXO) survey conducted at Camp Simms, Washington DC. The survey instrument was the
Geonics EM-6 1 pulsed induction sensor. The purpose of the study was to develop a neural network architecture
that could be fielded at numerous sites under the US Army Corps of Engineer’s Ordnance and Explosives
Knowledgebase (OE-KB) program. The neural network was successful in predicting masses and depths. This
paper presents the results of a single training session and is also intended as a short tutorial in how to prepare and
present geophysical field data for analysis by a neural network.
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A New Interpretation Technique Of Surface Wave Measurements For Geotechnical Profiling
Authors Chaim J. Poran, Jorge A. Rodriguez-Ordonez and Takenori SatohNon-invasive shear wave velocity (SWV) profiling of soil deposits has long been
recognized as a cost-effective approach to obtain SWV data essential for seismic response
analysis including site amplification and liquefaction. The controlled-source
measurement of surface wave dispersion (CXW) is a relatively new non-invasive method
that is used for site characterization in the USA, Japan, and other countries. The CXW
method uses harmonic controlled sources to produce steady state Rayleigh waves in order
to measure the dispersion characteristics of soil and rock. Several interpretation
techniques may be used to obtain SWV structure from Rayleigh waves dispersion data
(in a process that is referred to as the Vf-Vs method). CXW measurement data from
more than 200 soil and rock sites were analyzed. These results were used to formulate
a new approach for the Vf-Vs interpretation method based on the new concept of
reference profiles that describes the average dispersion curve and SWV structure. The
reference profile is based on SWV dependency on confining pressure (depth) in soils.
Simple equations were developed for direct inversion of the reference profile. The new
procedure is completed by considering the effects of single layers with properties that are
different than the reference profile. This new interpretation procedure is rapid and
computationally efficient and its results are more representative of actual soil structure
than results obtained from other non-invasive methods. This paper shows that inversion
based on surface waves dispersion can provide a good estimate of average SWV
variation.
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3-D Electrical Resistivity Tomography For Environmental Monitoring
Authors Douglas J. LaBrecque, Gianfranco Morelli and Paul LundegardElectrical Resistance Tomography (ERT) images the electrical properties of the subsurface using crosshole
resistivity measurements. Interpretation of ERT data requires multi-dimensional inverse modeling. We discuss a 3-
D Occam’s inversion routine. The routine uses a finite-element forward solution and a conjugate gradient based
inverse routine. The algorithm finds the smoothest possible model that fits the data to a given a-priori level. In
general, the 3-D algorithm takes 10 to 20 iterations to converge to a final solution. However, the 3-D algorithm
requires only a single forward solution per iteration and does not require direct solution of a large system of
equations. Instead, it requires only the multiplication of a vector times the sensitivity matrix or its transpose. This is
accomplished without creating or storing the full sensitivity matrix.
A field example is shown in which ERT was used to monitor the injection of air from a vertical well at a shallow
petroleum remediation site in California. Data was interpreted using both 3-D and 2-D methods. Air injection
caused large changes in resistivity. At early times these were confined to an area near the injection point. Later the
changes were along a dipping, tabular region. At the latest times there is evidence of mixing of brackish water at
the depth of the injection point with fresh water in a shallower aquifer on the site. This mixing would have
decreased the resistivity and thus the apparent the size and magnitude of the zone of influence of sparging.
Results of 3-D and 2-D inversion agreed well when the regions of interest were approximately 2-D.
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Monitoring Air Sparging In Complex Aquifers
Authors Douglas J. LaBrecque, Gianfranco Morelli and Paul LundegardThe success of air sparging depends on the size and geometry of the region of air flow. The purpose of this study
was to use BRT surveys to observe changes in saturation, and thus air flow in a complex aquifer.
This paper describes the results of ERT surveys to monitor air flow in a heterogeneous acquifer in glacial till in
Washington State. Most of the air flow was confined a single, thin layer. Within this layer, the flow was not
uniform but moved preferentially in one direction. Some flow also occurred in a shallower layer but there is little
evidence of leakage into the upper part of the aquifer. After sparging there was a great deal of air trapped within the
layers.
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A Unique Data Acquisition System For Electrical Resistance Tomography
Authors William Daily, Abelardo Ramirez and Ken ZongeUnique capabilities are needed in instrumentation used
for acquiring data to do electrical resistance tomography (ERT) . A
data acquisition system is described which has a good combination of
the required capabilities and yet is rugged and user friendly. The
system has a multichannel detector for high data rates, can operate
over a wide range of load conditions, and will measure both inphase
and quadrature resistance at frequencies between 0.0007 Hz and 8
kHz. The system has been used in both the field and laboratory to
collect data with a typical accuracy between 1 and 10%.
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