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14th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 04 Mar 2001 - 07 Mar 2001
- Location: Denver, Colorado, USA
- Published: 04 March 2001
1 - 100 of 129 results
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Mapping Grain Size Facies For The Hydrogeologic Model Of The Middle Rio Grande Basin, New Mexico Using Airborne Time-Domain Electromagnetic Data
The Santa Fe Group aquifer in the Middle Rio Grande Basin is the main source of municipal water for
the greater Albuquerque metropolitan area. One objective of U.S. Geological Survey studies is to improve the
hydrogeologic models of the Middle Rio Grande Basin, so as to help land managers plan and develop water
supplies. Airborne time domain electromagnetic (TEM) data near the town of Rio Rancho, northwest of
Albuquerque, New Mexico, provided a three-dimensional depiction of the electrical resistivity distribution that
was used to infer the extent of geologic units within the Santa Fe Group aquifer. The correlation between
resistivity and grain size was based on lithologic and induction resistivity logs, which showed that the bulk
average resistivity in the saturated zone correlate with grain size as follows: Coarse (40-70 ohm-m), Medium
(20-40 ohm-m), Fine (10-20 ohm-m). These resistivities were used to infer likely grain sizes from the TEM
inversions. From lithologic well data, axial river-channel deposits are primarily composed of coarse grain size
facies, fluvial sand deposits are primarily composed of medium grain size facies, and fluvial silt and clay deposits
are primarily composed of fine grain size facies. Significant changes in the TEM response occur at several
mapped faults and at faults inferred from a high-resolution airborne magnetic survey. Coarse-grained sediments
that exist at 1500 m above sea level appear to be less than 50 m thick below the water table (apparently
pinched-out at the 1450 m level). At the 1350 m level, the lateral extent of axial-channel sand deposits appears
to be much reduced, suggesting that they are pinching out. The resistivity model provides a framework for
forecasting hydrologic conditions in areas less explored by drilling. Our interpretation of grain size facies
distribution provides direct input to ground-water flow models that are critical to water management agencies.
Introduction
The Albuquerque-Santa Fe region is rapidly growing. The Santa Fe Group aquifer in the Middle Rio
Grande Basin
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Identification Of Saltwater Intrusions And Coastal Aquifers Using The Bgr Helicopter-Borne Geophysical System
Authors Detlef G. Eberle and Bernhard SiemonAs part of a research programme, a helicopter-borne survey was conducted by the Federal
Institute for Geosciences and Natural Resources (BGR) of Germany covering the Coastal Aquifer Test
Field situated in NW-Germany between the estuaries of the Elbe and Weser rivers into the North Sea.
Electromagnetic data were collected in five frequencies and inverted to apparent resistivity/centroid
depth data sets for each frequency. These served as starting models to determine 1-D stratified n-layer
models for each data point using a Marquardt inversion. Results are presented as resistivity maps for
selected depth levels and as vertical sections depicting the variations of the true resistivity with depth
and distance along each flight line. Saltwater intrusions, seaside and inland freshwater aquifers and
glacial meltwater channels have been clearly identified and mapped. Airborne geophysics have proven a
fast and cost-effective tool for natural resource management.
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Using Airborne And Ground Electromagnetic Data To Map Hydrologic Features In Everglades National Park
Authors David V. Fitterman and Maryla Deszcz-PanGround-water flow requires the development of a three-dimensional model of aquifer properties
and boundaries. This task has been traditionally accomplished through drilling and water-quality
sampling in wells. While the data obtained by these means are highly accurate, they represent only a
very small fraction of the total model volume. Furthermore, in areas where drill sites are limited due to
difficult access, model geometry obtained by interpolating between widely spaced wells may be
somewhat inaccurate.
Helicopter electromagnetic (HEM) resistivity mapping provides high density data coverage over
large areas, including those where access is difficult. Interpretation of these data poses other problems
due to noise and errors in the HEM data. However, when combined with ground electromagnetic
soundings and limited well information, hydrologic features can be mapped with more certainty than
possible by interpolating between widely spaced wells.
As an example, we present a study from Everglades National Park, Florida. Data consist of an
HEM survey, time-domain electromagnetic soundings, well logs, and water quality samples. The
interpretation provides information on the depth to the base of the aquifer, the extent of saltwater
intrusion, and a three-dimensional picture of water quality in the aquifer.
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Tools And Techniques For The Application Of Highresolution Aeromagnetic Surveys To Engineering Problems
More LessModern high-resolution airborne magnetic surveys can be a practical tool for engineering and
environmental applications. Surveys conducted by fixed-wing aircraft over gentle topography can
detect magnetic point sources separated by 150 meters or linear magnetic sources that extend 150 meters
in at least one direction; examples include well casings, geologic unit boundaries, faults, and pipelines.
For smaller targets, and in areas of rugged topography, helicopter surveys are required.
Careful planning is the key to a successful high-resolution aeromagnetic survey. Flight-line
spacing should be no greater than the minimum terrain clearance in order to avoid undersampling of the
magnetic anomalies. Flight lines should be flown perpendicular to the expected strike of sources of
interest.
Even after standard data corrections are applied, variations in terrain clearance can result in
uneven resolution of features across the survey area. Equivalent source continuation of the data can be
used to evaluate the magnetic field on a surface that is more reasonable for interpretation than the
surface on which the data were collected. This is demonstrated on helicopter magnetic data for a
hazardous waste site in Tennessee. Phase-shifts of magnetic anomalies due to the local direction of the
geomagnetic field vector can be corrected using a reduction-to-the-pole filtering operation. In many
cases, this has the effect of moving the anomaly peaks directly over the sources, as is demonstrated
using aeromagnetic data over an oil field in Oklahoma.
At the interpretation stage, various techniques and software tools are available for extracting
geologic and cultural information from the data. The magnetic fields of shallow sources can be
separated from those of deeper sources using a process known as matched bandpass filtering. Three
methods for locating magnetic sources in three dimensions and identifying properties of the sources are
demonstrated on aeromagnetic data for Tucson, Arizona.
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Aeromagnetic Mapping Of Hydrologically Important Faults, Albuquerque Basin, New Mexico
More LessData from high-resolution aeromagnetic surveys over the Albuquerque basin show expressions
of numerous hydrologically important faults that offset basin fill or volcanic rocks, many of which were
previously unknown. The faults generally strike northerly and are commonly sinuous and linearly
extensive, up to 50 km in length.
Aeromagnetic profiles across the faults reveal a range of signatures, from symmetric curves with
one inflection point to asymmetric curves with multiple inflection points. All the fault signatures can be
explained by the juxtaposition of lithologic layers having different magnetic properties. The asymmetric
signatures are distinctive and indicate a thin magnetic layer on the upthrown side and a thick magnetic
layer on the downthrown side of the fault, called the "thin-thick layers" model. The thicker,
downthrown layer may have resulted from sedimentation related to growth faulting. The model may
indicate a concentration of possibly high-permeability, coarse-grained material on the downthrown side
of the fault. This is corroborated by a comparison of water-level data to aeromagnetic signatures over
two splays of the hydrologically important Hubbell Springs fault system. Therefore, aeromagnetic data
have potential for providing information on the causes of subtle changes in water level that are important
for siting individual water wells.
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Aerogeophysical Measurements Reveal Collapseprone Hydrothermally Altered Zones At Mount Rainier Volcano, Washington
Authors Carol A. Finn, Thomas W. Sisson and Maryla Deszcz-PanNew helicopter-borne electromagnetic and magnetic data, combined with detailed
geologic mapping, reveal that appreciable thicknesses of mostly buried altered rock lie
mainly in the upper west flank of Mount Rainier identifying this as the most likely source
for future large debris flows capable of reaching now densely-populated areas. Most of the
summit, central core and upper east side of the volcano are free of significant volumes of
weak, highly-altered rock and are at relatively lower risk of collapse. In addition, the lack
of highly altered rock lies in the volcano's core may impede collapse retrogression and limit
volumes and inundation areas of future debris flows. High-resolution geophysical and
geologic observations yield an unprecedented view of the 3-D distribution of collapse-prone
altered rock at Mount Rainier, and have potential application to hazardous volcanoes worldwide.
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Developing Geophysical Signatures To Constrain Geologic Mapping
Authors Jeff Wynn, Sue Karl, Bruce Smith and Anne McCaffertyThe Greens Creek ore deposit is one of the largest producing volcanogenic massive sulfide
(VMS) deposits in southeast Alaska, and is found on north-central Admiralty Island in southeastern
Alaska. This deposit and all other known VMS prospects in the region are found only in Triassic Hyd
Group volcanic rocks. Future success of mineral exploration in the region is thus highly dependent on
accurate geologic maps that accurately portray where these Hyd Group rocks are located in a region
that is largely covered by dense vegetation, water, and tidal mud-flats. Because of this extensive cover,
we have used ground and airborne geophysical information to constrain the geologic mapping. A large
airborne geophysical survey, incorporating both electromagnetic (EM) and magnetic sensors was flown,
supplemented by ground magnetic and VLF-EM resistivity profiles over key areas where geology
could be unequivocally mapped. From these, we developed a matrix of geophysical signatures of key
rock units that was then used in predictive geophysical models (essentially pseudo-geology maps)
derived from the airborne data. Geologic ground control was then used to convert these models into a
substantially revised geologic map of the region that now shows more than a 30% increase in Triassic
Hyd Group rocks.
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Spectral Induced Polarization Studies Of Mine Waste Piles In Colorado And New Mexico
Authors David L. Campbell and Robert J. HortonTo help interpret spectral induced polarization (SIP) data collected in the field on waste piles
from sulfide mineral-bearing mines in Colorado and New Mexico, the piles were extensively sampled
and the SIP properties of the samples were measured in the laboratory. The laboratory spectra were fit
using 2-term Cole-Cole relaxations. Simple IP indices which can readily be compared with field data
were also defined and calculated from the laboratory results.
Samples containing unaltered sulfide minerals had strong relaxation features in the frequency
range from about 0.01-100 Hz. These relaxation features consist of resistivity values that start out low
and that continue to drop with increasing frequency, and phase values that are generally negative and
large and whose curves are concave down, over that frequency range. In most western U.S. mine waste
piles, however, the original sulfide minerals have been weathered and chemically altered to an extent,
changing to sulfate and oxide minerals such as jarosite, anglesite, and goethite. As this happens the
sulfide relaxation features on the corresponding spectra fade and disappear. The resistivity curves rise
and flatten over that frequency range, whereas the phase curves drop in absolute value and flatten or
even become concave up.
As water is added to dried samples, they become more conductive and their spectra change
markedly, typically changing from spectra like those of weathered mine waste to spectra that
superficially resemble those of material containing unaltered sulfide minerals.
Unfortunately, laboratory spectra generally fail to resemble spectra measured in the field. This
might be because of varying water content, or because of disturbances resulting from the sampling
process. We suspect, however, that inherent scaling factors are responsible for the differences between
spectra measured on cm-size laboratory samples and those measured on m-size field blocks.
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Using Resistivity To Map Acidic Waters At The May Day Mine Dump, Silverton, Colorado
Authors Bruce D. Smith, David L. Campbell and Winfield G. WrightThe May Day Mine dump is located on a mountain side above Cement Creek, north of Silverton,
Colorado. As Cement Creek flows past the May Day Mine, dissolved metal loads increase in the
stream, although there are no tributaries entering the stream reach. It had been thought that these metals
were being leached from the dump by snowmelt and rainwater. Induced polarization studies show local
pockets in the upper bench of the May Day Mine dump with acidic pore waters. The pore water appears
to get less acidic as it migrates through the dump. Nevertheless, water samples from a well located at
the toe of the dump are highly acidic, and they contain large concentrations of dissolved metals. An
airborne EM survey was recently flown over the area, which shows a linear conducting feature that
enters Cement Creek just south of the May Day Mine dump. The airborne survey was followed up with
ground geoelectrical surveys, which confirm the existence of the conducting feature and suggest that it
dips steeply and extends to a depth of at least a few tens of meters. We speculate that this conductor
may reflect a fissure zone that carries water. Such a fissure zone could be the source of some of the
water that enters Cement Creek near the May Day Mine. The metals dissolved in that water might
originate through natural processes, rather than human-caused ones.
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Using A Vernacular Language To Present Unfamiliar Lexicons In Mining And Reclamation
More LessThere are different contexts to describe man-made and natural elements in mining and its
associated landforms. All of them are important in perceiving the process of surface mining and in
making land-use decisions. The study of mining and reclamation must bridge the chasm between
science and art; one way is to use a common language. Too often, one discipline undervalues the other’s
expertise. Scientists, designers, and citizens can be aware of a wider mutual understanding in their
different perspectives of mining and landscape through the use of a vernacular language (i.e. the
normal/common spoken form) rather than filling sentences with their unique subject vocabulary
(lexicon). The public, governmental agencies, and industry can then more effectively communicate with
one another.
In sharing data and presenting site information at public meetings, it behooves industry and
consultants to speak with citizens and each other in easily understood forms or to explain specialized
expressions indigenous to a particular field. We speak of and view mining from four fundamentally
different disciplines:
1) Architectural—spoken by the “designer” (including land planner or landscape architect)
2) Natural context—the “science speak” of the geologist, hydrologist, or engineer
3) Social—regulated by health, safety, or transportation departments
4) Cultural—relating to aesthetics and customary beliefs
The author presents a spreadsheet of the four perspectives and includes citizen reaction to
permitting through newspaper articles. Regulatory consideration is taken from a State and Federal law
standpoint. People are more willing to accept the presence of mining if they understand the science and
if the industry is responsive to aesthetic and sociocultural factors.
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Integrating Electrical Geophysical Surveys And Subsurface Probing To Locate Karst Aquifer Recharge Features, Northern Illinois
Authors Shawkat Ahmed and Philip J. CarpenterSinkholes, solutionally enlarged bedrock fractures, soil pipes, and swallow holes are important
recharge features for shallow karst aquifers. These features also may transmit significant quantities of soil
water, sediment and waste material within the vadose zone. In many cases, however, sinkholes are filled
with sediment or debris and have no topographic expression. In this study, geophysical methods are
employed to noninvasively identify and characterize buried sinkholes associated with soil pipes, enlarged
bedrock fractures and conduits. Geophysical surveys suggest a 500-m long soil pipe, or networks of pipes
underlie a portion of the Perry Farm Park in Bourbonnais, Illinois. Boring and electrical resistivity
soundings suggest Silurian dolomite bedrock lies at a depth of approximately 4 m across most of the site,
and soil pipes are probably related to hydraulically active fractures within the bedrock. Electrical resistivity
pseudosections suggest an undulating bedrock surface. Buried sinkholes appear in inverted pseudosections
as 4-8 m wide low conductive zones. Electromagnetic conductivity profiling identified high conductivity
anomalies over the locations of buried sinkholes, over soil pipes and topographically low areas. Values as
high as 33 mS/m were recorded over the sinkholes and over portions of the pipe system, whereas the average
background conductivity is about 22 mS/m. A linear zone of elevated conductivity trends WSW from the
sinkholes, and may indicate a soil pipe linked to other sinkholes in a ravine. Some reaches of the pipe
system also exhibit small sand "volcanoes", suggesting subsurface liquefaction and upward movement of
sediment. Probing data indicate drill stem drops of 0.3 m close to a known buried sinkhole. These drops
may be due to cavities in the soil or weathered dolomite bedrock.
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Mapping A Paleochannel System Controlling Contaminant Migration At A Wood-Treating Facility Using Electromagnetics
Authors Stewart K. Sandberg, William Corso, Jessica R. Levine, Gary Newhart and Greg PowellA wood-treating facility, located near Sheridan, Oregon, has been the focus of a groundwater
contamination investigation. A geophysical survey was conducted in April, 2000. Objectives of this
geophysical survey included detection and delineation of coarse-grained channel-type deposits in the
unconsolidated section above bedrock. These deposits constitute preferential pathways for groundwater
flow, and hence contaminant migration through the subsurface. Contaminants consist of petroleumbased
creosote and pentachlorophenol (PCP) solutions. Dissolved phases of these contaminants
comprise the groundwater contamination expected, and the delineation of the extent of this
contamination was the overall objective of the investigation. In addition, dense non-aqueous phase
liquid (DNAPL) contamination was expected in accumulations in topographic lows of the bedrock
surface near the source area. Therefore, another objective of the geophysical survey was to investigate
any topography on the bedrock surface.
In this preliminary phase of the investigation, geophysical methods used consisted of transient
electromagnetic (TEM) soundings, a resistivity sounding, and an extensive terrain conductivity (EM-31)
survey. Due to the extensive sources of cultural interference at the site (buildings, fences, railroad
tracks, etc.), the geophysical survey was limited. Three profiles of 20-m central-loop TEM soundings
were obtained, along with some isolated soundings where it was thought that cultural interference could
be avoided. TEM approximate depth sections clearly identify the paleochannel system in cross-section.
Correlation with terrain conductivity response provides confidence in the terrain conductivity
interpretation of the paleochannel system in areas where only that type of data were obtained. One
resistivity sounding, and an adjacent TEM sounding, were modeled simultaneously to the same layeredearth
in order to test the idea of improving the resolution of the interpretation. This proved valuable in
delineating an electrical equivalent of the working model of the hydrogeological section based on nearby
drilling information.
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Supervised Classification Of An Arid Groundwater Flow System
Authors Alberto Barud-Zubillaga and Dirk Schulze-MakuchThis investigation addresses the hydrogeology of the White Sands National Monument (WSNM)
to determine the characteristics of an arid groundwater flow system using remote sensing methods. A
supervised classification using Landsat-7 data sets were used to identify the different terrains leading to
a better understanding of the White Sands, where depth to water table is closer in areas with lower
elevations and resulting in wetter or moister regions. Results showed that the ancient Lake Otero floor
has been carved deeper and deeper in the western margin of the White Sands area by water and wind
since the Pleistocene. The Alkali Flat is now exposing the ancient deposits of the Lake Otero floor. The
entire White Sands Dune Field today is increasing in size as the Alkali Flats and Lake Lucero shrink due
to dryer conditions as time goes on. The active dune field is growing on top of the ancient Lake Otero
floor to the east. Groundwater flow is to the west and discharges in the hydraulic sink of Lake Lucero, a
highly saline playa ephemeral lake.
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Detection Of Shallow Water Table Fluctuation Using The Spectral Analysis Of Surface Waves (Sasw) Technique
Authors Jaime O. Hincapie, Diane I. Doser, Deren Yuan and Mark R. BakerThe spectral analysis of surface waves (SASW) technique uses measurements of surface wave
dispersion to construct shear velocity versus depth profiles. Although the method is commonly used in
geotechnical investigations, applications to environmental studies have been limited. We have evaluated
the potential of SASW to monitor water table fluctuation at shallow depths (< 2 m) at two sites located
near the Rio Grande northwest of El Paso, Texas. The shallow depth of the water table and presence of
low velocity zones make use of the seismic refraction method difficult at these sites. The
inhomogeneity of sediment grain size and seasonal variations in sediment moisture and salinity also
make use of electrical techniques problematic. The first study site was located on an earth-fill levee ~20
m west of the Rio Grande. An irrigation ditch was located immediately west of the levee. We
successfully detected water infiltration from the ditch into the levee by comparing shear velocity profiles
from surveys taken before and after irrigation. The second site was located on homogeneous soils of a
historical channel of the Rio Grande (~10 m east of the present Rio Grande). Seasonal fluctuations of
the water table related to changes in river level were detected at this site and confirmed with borehole
information. Our results suggest the SASW method is effective for detecting the depth and fluctuation of
the water table and provides additional valuable information (e.g. shear velocity, shear modulus) when
coupled with other types of seismic surveys.
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Studies Of Grain Size Variations In Fluvial Sediments Of The Rio Grande Using Magnetic Techniques
Authors Diane I. Doser, Richard P. Langford, Mark R. Baker and Oscar Dena-OrnelasCharacterizing the heterogeneity of fluvial sediments in the Rio Grande Valley near El Paso,
Texas is important to understanding shallow groundwater flow, groundwater/river exchange and the
build-up of soil salinity. We have found the magnetic technique an effective way to rapidly evaluate
grain size changes within the river soils. This is in contrast to electrical techniques (resistivity,
conductivity) that are strongly affected by seasonal variations in soil moisture and salinity. Medium to
coarse-grained sands of the ancestral Rio Grande contain significant (up to 10% by weight) amounts of
magnetite, and thus appear as magnetic highs. We have conducted magnetic surveys at four study areas
within the Mesilla Valley northwest of El Paso. The sites have a range of sizes (10’s to 100’s of meters
in dimension) and geologic complexity (homogeneous channel sands to complex interfingered crevasse
splay and flood plain deposits). Borehole grain size analysis, as well as surface geophysical studies (e.g.
conductivity, resistivity, seismic, GPR) were conducted at the sites. Our results indicate we can detect
small crevasse splay (~2-3 m wide) channels buried at depths of 1-2 m, as well as larger channels (~20
m) at depths of ~20-30 m, through simple analysis of magnetic contour maps and magnetic gradients.
We believe the technique has the potential for rapidly evaluating the location of channels within the Rio
Grande valley, for corroborating surface soil maps and determining potential contaminant flow
pathways.
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Estimating Petrophysical Data From Borehole Geophysics
Authors M. Pokar, L.J. West, P. Winship and A.M. BinleyA study of the ability of ground penetrating radar and natural gamma logging to indicate
porosity, lithology and permeability in the unsaturated zone of a sandstone aquifer is described. The
aquifer (Sherwood Sandstone, UK) consists of a series of fluvially derived sequences fining upwards
from medium to fine sandstone and siltstone. Laboratory measurements on core samples of porosity,
grain size, mineralogy and hydraulic conductivity are reported. Vertical hydraulic conductivity is lower
than horizontal hydraulic conductivity, probably as a result of preferential sand grain orientation and
fine/medium sand lamination. Clay is the primary influence on hydraulic conductivity, although porosity
also has some effect.
Data for hydraulic conductivity, porosity and clay content are compared to dielectric constant
values from zero-offset 50 MHz cross borehole radar profiles and natural gamma activity. Natural
gamma activity increases with increasing clay content and reducing hydraulic conductivity.
Dielectric constant averaged over six months also correlates with clay content, despite scatter due to
variations in the moisture content during this period. Dielectric constant (unlike gamma activity) is very
strongly influenced by moisture content because of the very high dielectric constant of water compared
with those of mineral solids and air. However its correlation with clay content is preserved in the
unsaturated zone because clay rich layers also have high moisture retention. Dielectric constant
increases with reducing hydraulic conductivity when the profile is relatively wet, but not when it is drier.
However low hydraulic conductivity layers may be detectable using ground penetrating radar because
they show large seasonal fluctuations in dielectric constant.
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Application And Comparison Of Methods For Horizontal Flow Measurements In Fractured Bedrock
Authors William H. Pedler and Wayne MandellThe ability to measure ambient groundwater flow conditions is a valuable tool in the evaluation
of hydrologic properties in environmental and geotechnical studies. Vertical flow characterization
methods (spinner flow meter, USGS heat pulse, EM flow meter and Hydrophysical logging) are
commonly applied and readily accepted for hydrogeologic and hydraulic evaluation in both fractured
and porous mediums. The ability to detect and measure ambient horizontal flow in a wellbore provides
a means with which to further define the ambient flow conditions in an aquifer. A family of downhole
methods may provide the ability to highly resolve the location of horizontally flowing cells; measure the
velocity and direction; and possibly, evaluate the degree to which a fractured flow system behaves in a
porous and/or plate/channel flow manner. This enhanced definition could present a means to
dramatically improve our understanding of the subsurface flow system and remediation efforts.
To evaluate their appropriateness and accuracy, the US Army Environmental Center is presently
sponsoring a comparative study of methods to characterize horizontal flow in boreholes. As part of this
study, a preliminary field comparison was conducted at two US Army sites (Fort Campbell and
Jefferson Proving Grounds) both sited in fractured karst hydrogeologic settings. The methods applied
were;
1) Hydrophysical logging (HPL)
2) Colloidal Borescope Flow Meter (CBFM)
3) Acoustic Doppler Velocimeter (ADV)
4) K-V heat pulse flow meter (KVFM)
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Condition Risk Assessment Of Underground Utilities With Sewreel Seismic Imaging
More LessAmerica has a major investment in underground utilities beneath its cities, principally sewers and
pipelines, and there is widespread concern that this urban infrastructure has not been adequately
maintained to meet needs. Managers of these utilities rely on condition assessments based on internal
inspection and limited geotechnical information. These assessments are inadequate as they fail to
effectively sample, test and account for external ground conditions. Unexpected catastrophic ground
collapse due to pipe collapse is a relatively common occurrence. Excessive expenditure on new
construction, rehabilitation and foreshortened repair life are also consequences of the failure fully
understand and evaluate the influence of ground conditions above and around these utilities.
Recently, a specially developed seismic imaging technology called SEWREEL has demonstrated
the ability to locate significant voids or weak ground around underground utilities and to prevent
collapses. SEWREEL is similar to crosshole seismic tomography and provides detailed information
between the pipe and the ground surface. This method represents a significant improvement over
surface seismic methods. Its use to prevent collapses around a gravity main is illustrated with a recent
field example from Orlando, Florida.
The application of SEWREEL within a condition assessment program provides the means for
establishing the subsurface condition of the soils around the pipe in a reproducible, timely, and efficient
manner. Any comprehensive program of condition risk assessments for underground utilities should
include seismic imaging as a key component in a preventive engineering and risk assessment and
reduction strategy.
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The Use Of A Giant Magneto-Resistance (Gmr) Based Magnetometer For Differentiation Of Subsurface Electrical And Non-Electrical Materials
Authors T. David McGlone and Doria L. KutrubesGround penetrating radar (GPR) has been used with moderate success to locate electrical
conduits buried within reinforced concrete slabs. However, when conduits are located very near walls or
directly beneath rebar of similar size, GPR as a methodology of investigation alone has been inadequate.
X-ray techniques are also not useful as they emit dangerous gamma radiation, and both X-ray and EM
induction techniques penetrate only a few inches and cannot differentiate between rebar and conduits
cased in steel or aluminum. Therefore, the ability of a magnetometer with a wide frequency response
bandwidth is of interest for these situations.
This experiment uses Smoke Creek Instrument's IMAG01 magnetometer to explore the
frequency responses of several common construction materials such as pipes, conduits, and spikes. The
material samples are placed beneath 0.15 m of a natural, decomposed granitic soil with a consistency of
coarse sand similar to that used in concrete. The magnetometer is used to measure each situation and the
results presented herein. Measurements are first made with no artificial external field applied, and then
with a controlled source magnetic field.
The IMAG01 is a single-axis magnetometer based on the giant magneto-resistance (GMR) effect
and detects magnetic field strength of both static and electromagnetic fields. This version of the
instrument has a frequency response of DC – 100 kHz with a sensitivity of approximately 0.02 nT. As
such, it is expected to respond to the potential magnetic field of the ferrous material and the magnetic
component of the 60 Hz electromagnetic field of electrically-active wire without the controlled source
transmitter. With the transmitter, the instrument is expected to respond to the induced magnetic field of
the metallic materials as well as the potential and 60 Hz fields of the previous situation. Of particular
interest is the ability to differentiate between metallic, non-electrical materials and electrically-active
conduits when placed in close proximity.
The use of wide bandwidth receivers in electromagnetic geophysical surveys can provide
information that limited bandwidth instruments can not. A magnetic field receiver with a small physical
size allows array measurements of electromagnetic fields in a manner similar to arrays used for seismic
measurements over relatively small surface areas. This makes feasible 2D and 3D electromagnetic
imaging, although for purposes of this experiment, only 1D measurements were made.
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Ground-Penetrating Radar Amplitude Analysis For Archaeological Applications
More LessThe primary goal of most GPR surveys is to identify the size, shape, depth and
location of buried remains and related stratigraphy. The most straightforward way to
accomplish this is by identifying and correlating important reflections within twodimensional
reflection profiles. These reflections can often be correlated from profile to
profile throughout a grid, which can be very time consuming. Another more sophisticated
type of GPR data manipulation is amplitude slice-map analysis that creates maps of
reflected wave amplitude differences within a grid. The result can be a series of maps that
illustrate the three-dimensional location of reflection anomalies derived from a computer
analysis of the two-dimensional profiles. This method of data processing can only be
accomplished with a computer using GPR data that are stored digitally.
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Integrated Use Of Geophysics, Gis And Archeological Records To Locate Seneca Village
Authors Roelof Versteeg, Nan Rothschild, Diana Wall, Cynthia Copeland, Herbert Seignoret and Koray ErgunSeneca Village was an African American - Irish village located in what is now New York City's
Central Park. As part of the construction of Central Park the village was razed in the 1850s and no
surficial remnants remain. Increasing interest in the history of Seneca Village led to an effort to use a
combination of geophysics, archeology and GIS to locate possible remnants of the village. This effort is
complicated due to possible intense (but unknown) site modification during the construction of the park
and post park infrastructure construction. Using a range of geophysical tools (GPR, resistivity and
EM31) a suite of layers of geophysical data was collected. These were integrated in a GIS with a high
quality basemap (which contains the current park layout), the subsurface infrastructure layout and
orthorectified and digitized historical maps. By interpreting these results jointly a number of more and
less promising areas were identified which possibly can contain undisturbed remnants of Seneca Village.
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Numerical Inversion Of Borehole Flow Logs
More LessBorehole flow logs are most effectively interpreted in conjunction with other borehole data.
Other geophysical logs are needed to help identify the hydraulically active zones within the scatter of
flow measurement data produced by borehole conditions. Ambient hydraulic-head conditions also need
to be taken into account in flow-log analysis. Hydraulic-head differences between zones intersecting the
borehole can be eliminated from the interpretation by subtracting a flow profile obtained under ambient
conditions from another flow profile obtained under stressed (pumping or injection) conditions. Fitting
of hydraulic flow model predictions to the flowmeter data set insures that flow interpretations are
physically possible and yields quantitative estimates of both zone transmissivity and zone hydraulic
head. Applications of flowmeter log interpretation methods are illustrated using data obtained in
fractured sedimentary and crystalline bedrock aquifers in Arizona, Wisconsin, Kentucky, and New York.
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Borehole-Surface And Cross-Borehole Mise-Á-La-Masse Delineation Of A Radial Injection Tracer Experiment In Partially Saturated Fractured Basalt
Authors Robin E. Nimmer, James L. Osiensky and Kenneth F. SprenkeBorehole-surface and cross-borehole mise-à-la-masse (MALM) measurements were taken over
time during an eight-well, radial injection, tracer experiment in partially saturated, fractured, Columbia
River basalt. In this experiment, an enhanced conductivity tracer stream was energized directly through
a current electrode placed in the bottom of the injection well. A constant concentration tracer solution of
potassium chloride was injected continuously above a perched water table at an average rate of 10
liters/day under a constant hydraulic head for 34 days. An asymmetrical ground water mound developed
over time during which borehole-surface and cross-borehole MALM measurements were taken to
delineate migration of the tracer. A 15 x 15 array of porous pot electrodes (copper sulfate), located
symmetrically about the centrally located injection well, was used for the borehole-surface MALM. The
cross-borehole MALM utilized 66 downhole electrodes in the eight wells combined with seven transect
arrays each comprised of six land surface, porous pot electrodes. Changes in the electrical potential
distributions over time were contoured to delineate anomalies caused by the presence of tracer solution
in the fractured basalt. Borehole-surface measurements delineated the lateral migration of tracer over
time. The cross-borehole measurements helped delineate the depth of a possible preferential pathway in
the fractured basalt.
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A High-Resolution, Short-Range, Directional Borehole Radar
Authors David L. Wright, Jared D. Abraham, David VonG. Smith and S. Raymond HuttonThe U.S. Geological Survey (USGS) has recently developed a prototype directional
borehole radar system designed to complement a commercial system that the USGS has used for
several years for fracture mapping in crystalline rock. The new prototype is designed for higher
resolution, shorter range operation. We have numerically modeled several types of antennas
using finite difference time domain (FDTD) code. Based on the FDTD numerical modeling, we
designed, fabricated, and physically tested a pair of cavity-backed monopole antennas. The
antennas were then incorporated into a pulsed time-domain radar system. Tests of the system in
a laboratory sand pack and an outdoor pit have demonstrated antenna directionality, good
detection of small metal and dielectric targets, and radial resolution of a few centimeters. Such a
radar system, with some modifications, might find use in utility and other obstacle avoidance in
horizontal drilling operations.
An important part of the development of the directional radar is data processing and
visualization. We can process our data in a number of ways, including average waveform
removal, filtering, and range-gain compensation. After processing, radar data can be displayed
in three dimensions using a graphical user interface (GUI). Received waveforms are mapped
into a cylindrical volume, where each waveform fills a cylindrical wedge. The angle subtended
by each wedge depends on the total number of waveforms acquired azimuthally in the borehole
at each depth. The length of the image cylinder is a function of the number of stations occupied
by the radar in the borehole. The GUI allows the operator to visualize the data using a variety of
tools, including: rotate, zoom, annulus view, slicing, and control of the color table into which
data values are mapped.
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Airborne And Ground Geophysical Surveys For Locating And Mapping Underground Storage Tanks At Bellows Air Force Station, Hawaii: An Integrated Approach
Authors Sandra A. Takata, Jeffery B. Hackworth and Doug McConnellAirborne and ground based geophysical surveys were conducted at Bellows Air Force
Station (BAFS), Oahu, Hawaii to locate buried metal potentially representing underground
storage tanks (USTs) and other buried debris of environmental concern. The airborne survey
was used for reconnaissance to screen approximately 3,000 acres of land and shallow offshore
areas at BAFS for buried debris. Once the airborne anomalies were evaluated for their potential
to have a UST source, focussed ground surveys were conducted to confirm the location and to
more accurately characterize anomaly source objects.
BAFS was established in 1917 as the Waimanalo Military Reservation and was
deactivated in 1970. Throughout BAFS’ history, USTs along with their associated pipelines
were installed to maintain base activities. Over the years, structures associated with the USTs
have been demolished resulting in the uncertainty of the UST locations.
Over 650 geophysical anomalies were identified in the airborne survey (both
electromagnetic and magnetic data). Following review of the flight path videos and further
scrutinizing of the data, most of the anomalies were attributed to surface cultural features or
geology and were eliminated from further investigation. The remaining 73 anomalies were
located on the ground and visual field reconnaissance at these locations eliminated some
anomalies from further investigation based on the terrain being unsuitable for construction and
surface metal hidden under thick canopy. Eighteen of the 73 anomalies were selected for
detailed ground surveys. The follow-up ground-based geophysical surveys were conducted
using frequency- and time-domain EM, magnetics and ground penetrating radar. Select
anomalies identified in the ground geophysical data were trenched and the sources excavated.
The technical approach of integrating site knowledge with airborne geophysical targets to
select ground geophysical survey locations proved to be efficient and successful in locating and
characterizing buried debris at Bellows Air Force Station.
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Sh-Wave Seismic Reflection Imaging Of Anomalous Foundation Conditions—Mississinewa Dam, Indiana
Authors Edward W. Woolery, Ron L. Street and Stephen T. HornbeckThe Mississinewa Dam in northern Indiana is a 2.4-km-long, 43-m-high, compacted earth-fill
embankment founded on approximately 25 m of glacio-alluvial sediment that overlies Mississippian
limestone bedrock. Differential settlement at the crest manifested in 1988; instrumentation indicates that
the increased rate of vertical deformation initiated in 1982, and has been approximately constant since
that time. As part of an effort to understand the cause for the irregular settlement, a noninvasive
seismic-reflection survey was performed in the general area of the crest deformation. The objectives
were to acquire a high-resolution image of the bedrock surface beneath the structure, and to identify
signal anomalies in the data that may be indicative of severely weathered rock or karst morphology. The
SH-wave reflection profile successfully imaged a coherent, relatively continuous signal consistent with
known points of top-of-rock elevation. Depth to bedrock along the profile varied from 20 m near the
right abutment, to 52 m near the outlet works at the western terminus. A sharp amplitude anomaly in the
bedrock reflector directly below the settlement area was imaged, and interpreted as a karst structure.
Subsequent geotechnical drilling of the anomaly found a 6.5 m discrepancy in the expected top-of-rock
elevation, as well as soft foundation fill, thus confirming the geophysical interpretation.
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Use Of Seismic Reflection Amplitude Versus Offset (Avo) Techniques To Image Dense Nonaqueous Phase Liquids (Dnapl)
Authors Michael G. Waddell, William J. Domoracki and Tom J. TemplesField experiments were conducted to determine the location and distribution of subsurface DNAPL
contamination at two DOE sites by use of two-dimensional, high-resolution seismic reflection surveys and
borehole geophysical data. These studies make use of seismic reflection amplitude versus offset (AVO)
technology to directly detect the presence of subsurface free-phase DNAPL.
The research was conducted at Savannah River Site, SC and Hanford Site, WA. At each of these sites
research consisted of site evaluation, seismic model studies, seismic acquisition, processing, and interpretation.
The seismic model studies were undertaken prior to field acquisition to determine the likelihood of an AVO
response from the DNAPL. The full Zoeppritz equations were used to create the model responses.
At M-Area, Savannah River Site the model data show that at the base of the DNAPL plume the
seismic amplitude should flip polarity on the far offsets (Class 2 AVO). The 2D seismic reflection field data
processed into near and far offset stacks indicate such anomalies occur at the depth where the known DNAPL
occurs. At the 200 West Area, Hanford Site the DNAPL is suspected to pool at two different stratigraphic
intervals. The model data show that at the upper interval, the Hanford Fine/Plio-Pleistocene boundary,
amplitude should decrease along the boundary if DNAPL is present (Class 1 AVO anomaly dim-out). At the
lower boundary the models show that, if DNAPL is present, the seismic amplitude should become very negative
(Class 3 AVO bright spot). A series of 2D seismic lines were acquired and processed. In the areas where
DNAPL was suspected to occur (high concentrations measured in wells) seismic amplitude was found to
decrease dramatically along the Hanford Fine/Plio-Pleistocene contact and along the Plio-Pleistocene/caliche
contact the seismic amplitudes became even more negative consistent with the model study.
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Investigation Of Seepage Through An Earthen Dam Using Ground Penetrating Radar
Authors Mark J. Howell and Scott J. LudlowA GPR survey was conducted during a seepage investigation of an earth dam. The survey
successfully imaged areas where perched water breached a geotextile drain fabric, delineated mounded
groundwater, and revealed structural features within the dam. This information, in conjunction with an
array of detailed geotechnical data assimilated by others, was used to evaluate seepage conditions and
develop remedial procedures. This paper presents images of selected features within the dam and
documents the methodologies used to for data acquisition and processing.
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A Gpr Mapping Survey On Jacui River
Authors Jandyr. M. Travassos and Adekunle. A. AdepelumiA combined marine and land GPR survey was conducted on the Jacui river, southern Brazil,
where it flows close to an abrupt end of the Serra Geral Cretaceous magmatic cover. The extruded basalt
overlays sandstone of the Triassic/Jurassic period. The survey covered an area on the river and on its
bank opposite to the basalt. The land survey was conducted on medium to coarser sand with occurrences
of gravel. About half of the area was forested displaying variable topography. The field configuration
allowed depths of penetration in excess of 15 m on land. Most lines averaged 200 m in length covering a
total area of 90,000 m2. Landlines successfully mapped the top of the sandstone unit below the soil
cover. A previously unknown landfill was also mapped. River lines revealed a rocky bottom with strong
and chaotic reflections reaching the end of the data window.
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Integrating Geophysical Methods In A Hydrogeological Pilot Study In The Northern Coastal Plain Of Rio De Janeiro State - Brazil
Authors Abel Carrasquilla, Luiz Geraldo Loures and Sergio L. FontesON/MCT and LENEP/UENF undertook a geophysical pilot study in the Northern
Coastal Plain of Rio de Janeiro State, near Rio das Ostras Town, 200 km NE from Rio de Janeiro
City. This region is characterized by both scarcity of drinking water for human consumption
and existence of saline intrusions, this last caused mainly by the presence of a complex
quaternary geology and excessive exploitation of aquifers. The research was performed by
employing magnetic and electromagnetic methods (time (TEM) and frequency (FEM) domains),
in order to evaluate the effectiveness of these methods in this particular geological
condition to allow the formulation of a more comprehensive hydrogeological research in the
region in near future. FEM profiles and one-dimensional interpretation of TEM data were capable
of depicting different geological formations, to locate the main aquifer and to reveal the
fresh-saline water contact. On the other hand, the magnetic method showed the presence of
a NE-SW direction geological fault, which is located just in the geological contact between
sand and clay, coincidentally with the fresh-saline water contact. Our results evince a promising
future for using electromagnetic geophysical methods in this region in small scale surveys,
as well as it confirmed that a precursory regional survey, chiefly using the magnetic method,
is essential in the indication of most suitable places to locate productive fresh-water wells.
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An Empirical Approach To Interpretation Of A Challenging Helicopter Electromagnetic Dataset
Authors Louise Pellerin, Victor F. Labson and Victor F. LabsonA helicopter electromagnetic survey acquired at the U.S. Deptartment of Energy Idaho National
Engineering and Environmental Laboratory used a traditional mining airborne method flown at low
levels for detailed characterization of shallow waste sites. The low sensor height, used to increase
resolution, invalidates most assumptions traditionally used in processing HEM data. Although the survey
design strategy was sound, interpretation techniques routinely used in industry, proved ineffective.
Apparent resistivity maps were severely distorted, and hence unusable, due to low flight height effects,
high magnetic permeability of the basalt host, and the conductive, highly three-dimensional nature of the
waste site targets.
To accommodate these interpretational challenges we modified a one-dimensional inversion
routine to include a linear term in the objective function to allow for the magnetic and three-dimensional
responses in the in-phase data. Although somewhat of an ad hoc approach the use of this term in the
inverse routine, referred to as the shift factor, was successful in defining the waste sites and reducing
problems due to the low flight height and magnetic characteristics of the host rock. Data from three areas
processed with this scheme highlight different interpretational aspects of the survey. Wastes sites were
delineated with the shift terms in two of the areas, allowing for separation of the anthropomorphic
targets from the natural one-dimensional host. In the other area, the estimated resistivity and the shift
factor were used for geological mapping. This was particularly interesting in that the disturbance of the
soil in the near surface was mapped with the shift term, because of the high magnetic content of the
native soil.
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Monitoring Of An In-Situ Air Sparging Experiment Using Electrical Resistivity Tomography
Electrical resistivity tomography (ERT) was successfully used for monitoring the air distribution
in an in-situ air sparging (IAS) experiment at the Oregon Graduate Institute of Science and Technology.
The experiment evaluated the removal of residual gasoline from the source zone of a simulated gasoline
spill in a tank filled with water-saturated sands The percent difference in resistivity provided a
quantitative view of air saturation in the sand tank. The initial background resistivity image suggests the
tank filled with water-saturated sands is geoelectrically non-homogenous though the sands in tank are
homogeneous in the sense of grain sizes. The background resistivity images also showed the evidence of
trapped residual gasoline around the water table.
The in-situ air sparging (IAS) in three different modes was carried out. The IAS at a flow rate of
20 cubic feet per minute (cfm) resulted in a larger radius of influence than the IAS at 5 cfm. At the same
air flow rate, the IAS in the pulse mode created a larger affected zone than the IAS in the continuous
mode. Two weeks was not long enough for the trapped air to escape from the tank. Unlike the
continuous IAS operation, the pulse operation produced a more complex air distribution in the tank.
Skewed airflow at both 5 cfm and 20 cfm flow rates indicated that there are preferential pathways in the
tank.
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Conductivity And Susceptibility Mapping Using Broadband Electromagnetic Sensors
Authors Haoping Huang and I.J. WonThe apparent conductivity and apparent magnetic permeability are derived from broadband
electromagnetic data using a conductive and magnetic half-space model. The apparent permeability is
first estimated from the inphase and quadrature (or amplitude and phase) data at a low frequency. This is
followed by the computation of the apparent conductivity at each frequency, using the pre-computed
apparent permeability. The apparent conductivity may be computed from broadband data using five
different algorithms. In general, three of the five methods yield a unique solution; the three yielding a
unique solution are the apparent conductivity defined from inphase, phase, or phase-amplitude.
A suite of synthetic data based on two-layer models is presented and it is concluded that the
phase-amplitude approach is preferred method. The apparent permeability is virtually independent of the
conductivity contrast at low induction numbers. However, the permeability contrast affects the apparent
conductivity, especially when a resistive layer overlays a conductive basement. The field data examples
show the usefulness of the broadband electromagnetic data and the resultant frequency-dependent
conductivity-permeability maps for characterizing complex environmental sites.
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Transient Controlled-Source Electromagnetic Induction In A Uniaxial Half-Space
Authors Mansour A. Al-Garni and Mark E. EverettAnisotropy in Earth materials often leads to unexpected geophysical responses that can not be
anticipated from simple isotropic models. In this paper, we investigate the effects of lateral anisotropy on
the time-domain, controlled-source electromagnetic (CSEM) response of a uniaxial conducting half-space
excited by a horizontal loop. In the theoretical results, a “paradox of anisotropy” is observed in which the
response is more pronounced along the strike of relatively conductive fractures. This is opposite to what
one might expect based on the isotropic theory. The results presented here have applications to fracture
mapping in bedrock.
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Inversion Of Multifrequency Electromagnetic Data To A Conductive And Permeable Sphere
Authors Haoping Huang and I.J. WonWe have developed an algorithm for inverting broadband electromagnetic (EM) data to
determine its electrical conductivity, magnetic permeability, radius, and depth using a sphere model. The
algorithm is based on traditional non-linear inverse methods with singular value decomposition. Studies
using synthetic GEM-3 broadband EM data indicate that the technique is dependable and produces fastconverging
solutions.
The geometrical parameters obtained through this technique are very close to true values, and the
electric parameters for noise-free data are correct to within a few percent. When a 10 percent noise is
added to the data, the geometric parameters are still well determined, but the electric parameters are
poorly resolved, particularly for a sphere that is both conductive and permeable. We also present an
analysis of the relative importance and reliability of the model parameters based on their partial
derivatives with respect to EM response.
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Quantitative Geoelectric Mapping Of The Hydraulic Conductivity Field
More LessThis paper describes in a stochastic framework how geoelectrics can be used to quantitatively
map the hydraulic conductivity field. This is achieved by describing both a microscale and a macroscale
model of aquifer electrical and hydraulic flow. The microscale model reveals that electrical and
hydraulic conductivities are a function of two common microscopic parameters. They are connected
pore volumes and connected pore surface areas, both assumed to be spatial random variables. Assuming
either a predominating pore-volume or pore-surface electrical flow environment, the self-similar scaling
of pore sizes, resulting in skewed pore size distributions, produces power-law relationships between
electrical and hydraulic conductivities and porosity and specific surface area. In turn these power-law
relationships imply a log-log linear relationship between electrical and hydraulic conductivities. The
coefficients of this log-log linear relationship are a function of several unique aquifer properties.
However, they can be field calibrated by linearly regressing coincident and equal-scale field
measurements of electrical and hydraulic conductivity. This electrical-hydraulic conductivity
relationship offers the quantitative physical rationale for converting inexpensive field measurements of
electrical conductivity into hydraulic conductivity.
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Constrained Time-Lapse Resistivity Imaging Inversion
By M.H. LokeChanges in the subsurface resistivity with time are frequently measured by conducting 2D or 3D
resistivity imaging surveys over the same area at different times. Examples of such studies include
mapping the flow of water through the unsaturated zone, changes in the aquifer saturation due to water
extraction, subsurface flow of chemical pollutants and leakage from dams. One common technique to
interpret the data from such surveys is carry the inversion of each data set individually and then
determine the relative changes in the subsurface resistivity from the differences in the resulting models.
This approach can lead to artifacts in the sections showing the relative changes in the subsurface
resistivity. In order to reduce such artifacts, a joint inversion technique that uses the results from the
inversion of the first data set as a reference model to constrain the inversion of the later time data sets is
used. Several types of cross-model constrains are examined. They are the simple damped or Marquardt
constrain, the l2 norm least-squares smoothness constrain and the l1 norm or robust constrain. By using
the appropriate cross-model constrain, artifacts in the sections showing the relative changes in the
subsurface resistivity can be significantly reduced. Where the changes in the model resistivity values are
expected to vary spatially in a gradual manner, the l2 norm constrain is more appropriate. When the
changes have sharp boundaries, the l1 norm constrain gives better results. In cases where the relationship
between the resistivity of the subsurface material and water content follows Archie’s Law, the change in
the water saturation can be estimated from the change in the model resistivity.
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Use Of Electromagnetic Induction Tools In Salinity Assessment/Appraisals In Eastern Colorado
More LessElectromagnetic induction (EMI) is a relatively low-cost and rapid method for measuring,
assessing spatially soil salinity. The two studies were conducted to evaluate data obtained with a
single-frequency (EM38 meter) and multi-frequency (GEM300 sensor) EMI instruments and to relate
apparent conductivity measured by these instruments with the more conventional conductivity of the
saturated soil extract. These two studies were geo-referenced for soil correlation sampling, salinity
mapping and future monitoring of salinization and/or degradation. The one study area (54 hectares)
utilized a subset sample area for the comparison procedure; the other site (323 hectares) was conducted
solely with the EM38 instrument. Data was incorporated into modeling programs and further used into
mapping software to develop two-dimensional maps.
Correlation coefficients between the two instruments ranged from 0.8039 to 0.8617. Although
the GEM300 sensor predicted somewhat less accurately the conductivity of the soil samples collected
and also produced higher apparent conductivity measurements, spatial patterns of apparent and
electrical conductivity produced by the two instruments were similar, reasonable and practical for the
end user, the agricultural producer.
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A New Fast Method To Measure Azimuthal Resistivity Anisotropy
More LessApparent electric anisotropy can be estimated with an electrode configuration formed as the letter
X. One electrode is placed at the end of each branch of the X and one is placed at about 1/3 of the
distance from the center. Together with one electrode at the center, by the instrument, this makes a total
of nine electrodes. By combining different electrodes it is now possible to measure the potential
difference to current ratio in a large number of ways. Wenner arrays can e.g. be formed along the two
diagonals of the X and square array measurements can be made with the four outermost electrodes. It is
convenient to use multi-channel equipment since many electrode combinations can be measured quickly.
For single reading methods we usually normalize the data by calculating an apparent resistivity.
The X-configuration measurements are also normalized, but in this case we use a homogeneous
anisotropic half-space for the normalization. There is of course no explicit formula for this conversion.
The data are fitted to the model in a least-squares sense by Monte-Carlo inversion. The output of the
inversion is apparent resistivity, apparent anisotropy direction and apparent anisotropy coefficient. The
apparent resistivity is defined as the geometric mean of the horizontal resistivities and it is usually a
good estimate of the bulk resistivity of the investigated rock volume. The influence of any local
heterogeneity is small due to the large number of measurements with different geometry and the rather
large electrode separations.
Examples are shown where X-configuration measurements have been performed on Precambrian
rocks in Sweden. The apparent anisotropy direction correlates well with direction of bedding in folded
supracrustal rocks and with the direction of foliation in metamorphic rocks.
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Problem Of Inductive Interference Between Current And Measuring Lines At Electrical Sounding
Authors Abbas Mohammed Abbas, Alexey Bobachev, Alexandr Karinski and Vladimir ShevninVertical electrical sounding (VES) is rather simple and very popular geophysical method with
wide spectrum of application for the decision of shallow depth hydrogeological, engineering geological
and ecological problems. During the last few years VES method has been transformed to a very
powerful technology, called electrical imaging or electrical resistivity tomography (ERT). At the field
registration of electrical signals in measuring line, some problems can arise and produce distortions,
which sometimes can spoil the data obtained. For realization of great amount of measurements at ERT,
pulsed current with short impulses (0.5-2 sec) is used. Such current is very close to AC. An example of
field data, highly distorted at large distances was the main cause of this investigation. In this report the
origin of induction is considered. This inductive interference is more dangerous in the case of low
ground resistivity. Calculations for Schlumberger array and dipole axial array were made. Criteria for
checking inductive interference are formulated. Knowledge of this phenomenon can help in avoiding
distortions and receiving high quality field data. Some other sources of distortions are also discussed.
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Anomalies From Horizontal Metal Pipes In Resistivity And Ip Fields
Authors Albert Ryjov and Vladimir ShevninIn urban and industrial areas geophysical methods are frequently applied for investigation of
metal pipes in ground or for other problems decision in their vicinity. Study of such pipes’ influence can
include: 1 - detection of a pipe (its position, orientation, depth), 2 - estimation of pipe technical
condition (corrosion, quality of isolation), 3 - distorting influence of a pipe on the fields of electrical and
EM methods at the decision of various geological problems. I.e. the pipes can be both object of study
and noise. For resistivity and IP methods pipes more often appear as noise, though from these methods
some pipes' parameters can be received.
It is possible to apply numerical methods to model pipes influence on apparent resistivity and IP
fields, though for a simple problem - rectilinear indefinitely long pipe - the application of analytical
method based on the strict account of boundary and starting conditions at the decision of the Laplace
equation is possible. The analytical decision allows estimating influence of a pipe in a wide range of
pipe and environment parameters. The problem is decided for horizontal cylinder with a covering in a
field of a point current source.
Influence of several pipe's parameters was investigated. Among these are depth and orientation
of pipe, pipe and covering resistivity, influence of array's and current electrodes' removal from the pipe
axis. In all cases IP values are more sensitive to pipe's parameters changes in comparison with apparent
resistivity. Pipe influence grows with current electrode approaching to the pipe axis and with lowering
the resistivity of covering.
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Transmission Line Approximation Of Pipelines With Cathodic Protection
Authors A. Mousatov and E. NakamuraThe effective and reliable pipeline exploitation requires regular control of their technical
condition. Now electromagnetic geophysical methods are widely applied to determine a position and
depth of the pipeline and to estimate a state of its electrical insulation and cathodic protection operation.
One of most effective methods for the solution of the specified problems is the non-contact
measurement of magnetic field created by the current, flowing in the pipeline. The cathodic protection
station or external generator connected directly to the pipeline and grounded in infinity can be used as
such current source.
The forward problem solution for the pipeline with variable insulation resistance and the pipe
conductivity requires application of numerical mathematical methods for 3D medium. Even in the
simplified 2D statement, assuming axial symmetry, the problem remains enough difficult, taking into
account specific geometry of model and very high contrast of cylindrical layers’ resistivities.
In this paper the approximation method to calculate the magnetic field above the pipeline is
considered. The transmission line approximation is used to substitute the pipeline with variable electrical
resistance of insulation. In this case the current distribution along the pipeline is obtained. Then the
magnetic field from the current variating along the pipeline is calculated. The dimension of a problem is
lowered and the analytical solution can be received with the considered approach.
On the basis of the described approximation the models of pipelines with zones of a various
extent of electrical insulation damages were accounted. The derivative of a magnetic field along an axis
of the pipeline corresponds to a current outflow, which is, in this case, quantitative characteristic of the
insulation condition.
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Time Domain Em Profiling Over A Shallow Conducting Target
By J.B. MerriamAn em 37-47 pro le over a known target at one to three m depth is described here. The response
of the highly conductive target is easily separated from the background response of the moderately
conductive till and is traced from the earliest gate of the em-47 ultrahigh range (7 s after turn o ),
to the latest gates of the em-37 medium range (28ms after turn o ). Three separate responses are
identi ed on the basis of their decay characteristics and spatial variation. They are interpreted to
be: a galvanic response, a vortex response, and a viscous magnetic remanence (VRM) response.
In time and frequency domain electromagnetics the vortex response is often expected to be the
dominant response, indeed, the galvanic response has only recently been added to many modeling
packages. In this environment - a very good conductor, with high permeability, at shallow depth in
a moderately conducting host, the galvanic response is the largest, followed by the viscous magnetic
remanence. The vortex, or inductive, response is only briefly above the noise level.
Many other em surveys have been performed over this target, including VLF, em-31 and
maxmin. These show either no response, or a response that is barely above the noise level. The
frequencies used in these techniques (less than 56 kHz) are probably too low to strongly excite the
galvanic or VRM modes, and it is possible that they are only exciting the relatively weak vortex
mode. Thus, time domain em is not only e ective in this environment, it is the only one of the
common em techniques that is e ective.
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Joint Influence Of Resistivity Anisotropy And Inhomogeneity For A Single Dipping Interface Between Isotropic Overburden And Anisotropic Basement
Authors Evgueni Pervago, Aleksandr Mousatov and Vladimir ShevninThe media with joint influence of anisotropy and inhomogeneity have large practical interest.
The difficulty of practical anisotropy studying with collinear arrays results from the fact, that anisotropy
exhibits itself weaker, than inhomogeneity at equal resistivity contrasts. The relative anisotropy and
inhomogeneity influence is considered with the help of mathematical modeling for gentle dipping
interface with anisotropic basement. The algorithm is based on the integral equations’ method for 3D
models with anisotropy.
This model is considered for three different directions of strike of dipping interface and strike of
anisotropic basement, in comparison with isotropic model of dipping interface and horizontally - layered
model with the anisotropic basement. The modeling data are submitted as azimuthal diagrams and
results of their spectral analysis. Spectral analysis helps to receive some diagnostic parameters for
anisotropic - inhomogeneous media.
All resistivity arrays on sensitivity to anisotropy are divided into two groups: collinear arrays
(Schlumberger, pole-pole, pole-dipole, dipole axial) with the axes ratio equal λ, and non-collinear arrays
(dipole equatorial, T-array, etc.) with sensitivity up to λ5. Most sensitive to inclined contact is dipole
axial array, and it is the least sensitive to anisotropy.
The inhomogeneity influence is displayed in the first harmonic of azimuthal diagrams’s
spectrum, and anisotropy - in the second harmonic. The inhomogeneity also influences on the second
harmonic. The absence of the first harmonic (and following odd ones) testifies to absence of
inhomogeneity influence. For comparison of relative influence of anisotropy and inhomogeneity, it is
necessary to consider the ratio of the sum odd to the sum of even harmonics. When O/E ratio is >1, the
inhomogeneity influence prevails, and when <1 - the anisotropy influence prevails. At joint influence of
basement anisotropy (λ = 2) and dipping interface (dip is 5°), the anisotropy influence prevails only for
non-collinear arrays.
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Application Of Geophysical Methods To Delineate Contamination In Fractured Rock At The University Of Connecticut Landfill, Storrs, Connecticut
Authors Carole D. Johnson, John W. Lane Jr., John H. Williams and F.P. HaeniAn integrated suite of geophysical methods was used to characterize the hydrogeology of a
fractured-rock aquifer to identify contamination or pathways for contaminant migration near a former
landfill at the University of Connecticut, Storrs, Connecticut. Surface-geophysical methods were used to
identify the dominant direction of fracture orientation and to locate potential leachate plumes. Two
shallow, electrically conductive anomalies near surface-water discharge areas north and south of the
landfill were interpreted as leachate plumes. Two other sheet-like electromagnetic (EM) anomalies were
identified and targeted for drilling and borehole-geophysical investigation. These methods were used to
determine the location, orientation, and lateral continuity of fractures and to quantify the hydraulic
properties of the transmissive fractures.
One borehole was located to intersect an anomaly observed at a depth of about 18 meters. The
EM-conductivity log measured a high electrical conductivity anomaly at a depth of 21 meters, which
coincides with a layer observed to contain sulfide mineralization. The lack of high conductivity fluids in
the borehole supports the interpretation that this anomaly is caused by a lithologic change rather than by
fractures that contain conductive leachate.
The second borehole was positioned to intersect a conductive feature at a depth of about 18
meters. A fracture with similar strike and dip was observed at a depth of about 22 meters in optical- and
acoustic-televiewer images, in borehole-radar surveys, and was determined to be hydraulically
conductive during heat-pulse flowmeter tests. This feature was also characterized by a high-conductivity
spike in the EM log. Although the specific conductance of the fluid in this depth zone was high, it could
not account for the spike in the EM log. The anomaly is interpreted to be caused by a combination of
conductive fluids in the fracture and of conductive minerals in the rock. Water-quality samples from a
discretely isolated zone near a depth of 22 meters indicated the presence of landfill leachate. This
investigation illustrates the effectiveness of the use of combined geophysical methods for identification
and evaluation of electrically conductive contaminant plumes.
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Deterministic Deconvolution Of Ground-Penetrating-Radar Data At A Limestone Quarry
Authors Jianghai Xia, Tom Weis, Evan Franseen and Richard MillerA 30 m by 30 m two-dimensional grid was designed on a flat bench behind a quarry face of
predominantly limestone with thin shale layers located at a Bonner Springs, Kansas site to test the utility
of ground-penetrating radar (GPR) for stratigraphic studies. GPR data were collected along seven lines
parallel to the quarry face and seven lines perpendicular to the quarry face, each separated by 5 m, using
50 MHz, 100MHz, 200 MHz, and 400 MHz antennas. As a part of the project, confirmation of reflection
events, ringing suppression, and velocities of electromagnetic (EM) wave propagation in the limestone
were studied. GPR instrument wavelets were successfully collected in the air. With a known GPR
instrument wavelet, ringing in GPR data was significantly suppressed by a deterministic deconvolution.
The validity of using a wavelet acquired in air as the operator for deterministic deconvolution was
shown in the real-world application of a radar system with 400 MHz antennas to a quarry site consisting
of interbedded limestones and shale partings. A total of 78 horizontal holes were drilled in key locations
on three exposed quarry faces where data were acquired before and after conductive steel rods, 1.5 m in
length, were placed in the holes. Diagnostic GPR responses from the horizontal steel rods serve as
known reflectors. The steel rods provide critical information for: 1) confirmation and nature of specific
geologic reflection events in the GPR data, 2) GPR resolution limits, 3) accuracy of velocities calculated
from common-midpoint data, and 4) identification of multiples. The effectiveness of suppressing ringing
waveforms suggests that the deterministic deconvolution of GPR data with the GPR instrument wavelet
in the air should be included as a standard GPR data processing step.
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Application Of A Geophysical “Tool-Box” Approach To Characterization Of Fractured-Rock Aquifers: A Case Study From Norwalk, Connecticut
The U.S. Geological Survey conducted a geophysical investigation at a site in Norwalk,
Connecticut where solvents have contaminated a fractured-rock aquifer. Borehole, borehole-toborehole,
and surface geophysical methods were used to characterize the bedrock fractures, lithologic
structure, and transmissive zone hydraulic properties in 11 boreholes and their vicinity. The
geophysical methods included conventional logs, borehole imagery, borehole radar, flowmeter, and
azimuthal square-array dc resistivity soundings.
Integrated interpretation of geophysical logs at borehole and borehole-to-borehole scales
indicates that the bedrock foliation strikes northwest, dips northeast, and strikes north-northeast to
northeast, dips both southeast and northwest. Although steeply dipping fractures that cross-cut foliation
are observed, most fractures are parallel or sub-parallel to foliation. Steeply dipping reflectors
observed in the radar reflection data from three boreholes near the main facility building delineate a
north-northeast trending feature. Results of radar tomography conducted close to a suspected
contaminant source area indicate that a zone of low velocity and high attenuation exists above 50 feet
in depth - the region containing the highest density of fractures. Flowmeter logging was used to
estimate hydraulic properties in each of the boreholes. Thirty-three transmissive zones were identified
in 10 of the boreholes. The vertical separation of the transmissive zones in a borehole typically is 10 to
20 feet.
Open-hole and discrete-zone transmissivity was estimated from flowmeter data acquired under
ambient and pumping conditions. The open-hole transmissivity ranges from 2 to 86 feet squared per
day (ft2/d). The estimated transmissivity of individual transmissive zones ranges from 0.5 to 70 ft2/d.
Draw down monitoring in nearby boreholes under pumping conditions identified hydraulic connections
along a northeast-southwest trend between boreholes as far as 560 feet apart. The vertical distributions
of open fractures can be described by power law functions, which suggest that the fracture network
contains transmissive zones consisting of closely spaced fractures surrounded by a less fractured and
much less permeable rock mass.
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Lateral And Vertical Delineation Of Water Producing Fractures And Zone Specific Water Quality Data With Csamt And Hydrophysical Logging
Authors Martin J. Miele, Tony Morgan and Greg BauerFracture zones and faults are typical targets for groundwater production in areas characterized by
hard bedrock. Groundwater production and water quality may vary within those aquifers. The vertical
and lateral interconnection between fracture sets is a major consideration for the suitability of a
sustained water producer. The site for this investigation is a tree nursery facility in Southern California,
in San Diego County. The operation needs added water supply from additional well(s). The area
consists of rolling hills that abut the San Luis Rey River. The geology in the area is characterized by
granitic bedrock reported to be tonalite.
One of the existing and favorable wells on the facility (favorable well) was drilled to a depth of
approximately 1,000 feet. The well had an artesian flow of 55 gpm. A widely spaced CSAMT (MT)
survey was conducted along a traverse located adjacent to the well and across the local valley. The
geologic or geoelectric strike of the area was known, therefore, the MT data was processed in transverse
magnetic mode. The CSAMT traverse indicated that the well was drilled into a system with two
anomalous zones interpreted to be major fracture zones. One extended down to a depth slightly greater
than 1,000 feet. A second anomalous zone interpreted to be fractures existed at depths greater than
2,000 feet. The two systems are apparently unconnected in the MT data. The shallow fracture zone was
logged with HydroPhysicalä testing which indicated four major water bearing fracture systems with
varying water quality conditions. The deeper fracture system correlates with another local well reported
to be completed at depths greater than 2,000 feet. Water quality varied between the wells.
Three additional CSAMT traverses were conducted in the valley and local hills. A shallow
fracture system and deeper fracture system occurred in the data. The fracture systems correlate between
traverses.
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3-D Gpr Imaging Of The Neodani Fault, Central Japan
Authors Ernest C. Hauser, Daiei Inoue, Shintaro Abe, K. Kusunoki and Yoshida OdaGPR data collected across a segment of the Neodani Fault in the Tokai region of central Japan
represents the first 3-D GPR data successfully collected across a major seismogenic fault in Japan.
Despite the inherent difficulty of GPR to significantly penetrate wet, clay-rich soils, a 3 meter bedrock
offset across the fault was imaged through 3-6 meters of saturated unconsolidated material. The GPR
data were collected astride the known trace of the fault in a 12.5 x 12.5 m area adjacent to an earlier
excavated trench, which provided constraints on depth and overburden stratigraphy.
These GPR data were collected using a stepped-frequency (1-45 MHz) GPR system in bistatic
mode using a constant antenna center point separation of 2.5 m. The data comprise a series of 23
parallel 2-D profiles spaced 0.5 m apart, with GPR soundings along each profile collected at a 0.5 m
spacing.
On October 28, 1891, the 80 km long Neodani Fault and surrounding region experienced one of
the largest historic earthquakes in Japan, the Nobi earthquake, which registered an estimated magnitude
8.0. This study is near the epicenter of that earthquake, and not far from the Earthquake Fault
Observation Museum, which is designated a “National Monument of Special Interest.” Significant
variation of vertical offset occurred along the largely strike-slip Neodani Fault and this museum
preserves a spectacular excavation of the fault exhibiting 6 m of vertical offset.
These results indicate that 3-D GPR profiling is a viable method for locating or observing
bedrock fault offsets concealed beneath unconsolidated material, even in the generally GPR-challenging
ground conditions found in Japan, and could be used to nondestructively identify or locate optimal sites
for trenching.
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Use Of Ground Penetrating Radar For The Determination Of Soil Profiles In Slopes Of Rio De Janeiro
Authors Patricia L. Grazinoli, Euripedes A. Vargas Jr. and Franklin AntunesThe present paper describes the experience in the use of the Ground Penetrating Radar (GPR) in
the determination of soil-rock interfaces and weathering profiles in natural slopes in the City of
Rio de Janeiro, Brazil. A number of slides and other stability problems occurred in natural slopes
during the summer of 1996, when heavy rains affected the degree of saturation of the soils above
the rock, mostly colluvial and residual soils of gnaissic origin. In two such slopes, described in
the present paper, a slide occurred at the soil-rock interface.
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Gpr As A Cost Effective Bedrock Mapping Tool For Large Areas
Authors Jutta L. Hager and Mario CarnevaleHager GeoScience, Inc. conducted a geophysical investigation at the site of a former
manufacturing plant in Massachusetts at which contamination had been detected in fractured bedrock.
The area of interest covered approximately 48 acres of mixed-use and residential neighborhoods,
including a park and cemetery. There was concern about the location and movement of contaminants
off the site toward a nearby river. The client had already installed a number of bedrock wells around the
contaminant source in order to track the migration direction of the contaminants, as well as an air
sparging system to treat contaminated groundwater at the source. After noting anomalous data during
pumping of bedrock recovery wells, the client hired HGI to help determine the best locations for
additional deep rock pumping wells, as well as to track groundwater (and thus contaminant) movement
off the site. HGIs investigation integrated GPR and seismic refraction surveys with data from geologic
and aeromagnetic maps, outcrop data, and fracture trace analysis.
The study area is located along the northeast margin of the Narragansett Basin. Geological
research prior to starting the geophysical fieldwork identified a network of lineaments/fractures,
predominantly trending northwest. The geophysical surveys were designed to relate the lineaments to
possible fractures, as well as to map the stratigraphy and bedrock surface to determine the direction of
groundwater flow. To this end, approximately 3,000 linear feet of seismic refraction data and 10,000
linear feet of GPR data were collected, mostly along roads and sidewalks.
The seismic refraction survey established the correlation between lineaments and possible
bedrock fractures on the basis of low seismic velocities. Sufficient GPR data were collected to produce
a bedrock contour map that helped resolve the apparent contradictions between predicted and actual
hydraulic gradients under pumping conditions, as well as give the client additional pumping well
locations.
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An Automated Facility To Study Processes Using 4D Gpr
Authors Roelof Versteeg and Ralf BirkenOne of the key innovations in geophysics is the use of so called time-lapse or 4D geophysics.
This technology has been applied in all ranges of geophysics from whole earth to near surface. While it
is easy enough to collect two almost identical surveys on approximately the same location, and it is
fairly common to observe differences between these datasets, a detailed quantitative interpretation of the
differences in terms of processes has up to now not been possible. There are two reasons for this. One,
the temporal data density of the data is most often very low (comprising only two or three datasets), and
consequently we will temporally alias our observational data on high frequency processes. Second, we
lack a fundamental understanding on how to interpret our data in terms of processes. To understand
the possible consequences of this (and to resolve these issues) we need to have high quality 4D data
with high temporal (as well as spatial) densities preferably collected in a controlled environment so that
we can understand how we to image and study the processes.
This was the motivation behind the creation of Columbia's subsurface imaging laboratory. This
laboratory allows for the controlled creation of medium scale models consisting of unconsolidated
materials. It consists of a large (6 x 6 x 8 feet) aboveground tank, a completely automated data
acquisition facility and an infrastructure for model creation and disassembly.
As there was no model on which to base this facility its construction and implementation
involved a trial and error approach. However, after a two year construction and testing phase the facility
is now fully operational, and is able to generate super high density 4D datasets of superior quality. In
creating this facility we have learned a significant amount of things about building medium scale
physical models and automated data acquisition and processing. The essential elements in creating this
and similar facilities are discussed, as well as potential pitfalls that one can face with the hope that this
paper will provide valuable guides for future efforts in this direction.
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1D Inversion Of 4D Radar Data To Image Fluid Flow
Authors Roelof Versteeg and Shan WeiOne of the essential steps in geophysics is the conversion of geophysical data to a map of
physical properties - something which can only truly be achieved through inversion. While full
wavefield inverse methods are widely applied in acoustical imaging, their application to radar has been
limited for a number of reasons. These include the complexity of antenna – soil coupling, the fact that
antenna radiation patterns are unknown and the underdetermined nature of radar data (in that radar data
is in general single rather than multi offset) as well as the large number of parameters we have to invert
for. Thus, the majority of current radar inversion efforts are actually kinematic inversions. We
investigate the feasibility of a 1D full waveform radar inversion effort. We demonstrate that while we
can do 1D radar inversion using a mixed simplex/Powell optimization scheme, the results show a
fundamental ambiguity between parameters. From theoretical grounds the extension of the inversion to
higher spatial dimensions would seem not to be sufficient to resolve this ambiguity. However, an
alternative to 1D inversion is 1D timelapse inversion which is more feasible as it adds a whole new level
of information. We implement a first simple form of 1D timelapse inversion on a data set collected in
Columbia’s subsurface imaging lab which demonstrates both the success and the potential of this
method.
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Imaging Fluid Flow And Relative Hydraulic Conductivity Using Ground Penetrating Radar In A Controlled Setting
Authors Roelof Versteeg and Ralf BirkenOne of the core goals in near surface geophysics is the imaging of fluid flow and the associated
determination of hydrological parameters. While the direct determination of hydrological parameters has
been pursued by a number of scientists the inherent uncertainty in a direct estimation from static data
seems to make the success of such an approach very doubtful. However, while it is nearly impossible to
estimate flow and hydrological parameters from static data it is significantly easier to estimate flow and
hydrological parameters from dynamic data. The implementation of this effort is demonstrated on a 162
fold 4D GPR dataset over an oil injection experiment which was collected in the subsurface imaging lab
at Columbia University.
Analysis of this dataset shows that we can isolate fluid flow in four dimensions in near real-time
using geophysical imaging. This in turn allows us to deduce relative hydraulic conductivity. While the
dataset at hand is an ideal dataset the applications of this method (using a range of other geophysical
methods) to subsurface remediation and process imaging are extremely promising.
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Application Of Borehole-Radar Methods To Image Two Permeable Reactive-Iron Walls At The Massachusetts Military Reservation, Cape Cod, Massachusetts
Authors Peter K. Joesten, John W. Lane, Jennifer G. Savoie and Roelof J. VersteegA pilot-scale study was conducted at the Massachusetts Military Reservation, Cape Cod,
Massachusetts, to assess the use of a hydraulic-fracturing method to create vertical permeable walls of
zero-valent iron to remediate ground water contaminated with chlorinated solvents at depths exceeding
the range of conventional iron-wall installation methods. At the test site, ground-water contamination
extends from 24 to 37 meters (m) below land surface. A treatment zone consisting of two parallel
reactive-iron walls 12 m long, 13 m high, and 0.15 m thick, separated by about 6 m, was designed to
intersect and remediate a portion of the CS-10 plume. The U.S. Geological Survey used a cross-hole,
common-depth radar scanning method to test the continuity and estimate the lateral and vertical extent
of the two reactive-iron walls. The cross-hole radar surveys were conducted in boreholes on opposite
sides of the iron injection zones. Significant decreases in the amplitude of the radar pulse observed in
scans traversing the injection zones were interpreted by comparing radar field data to results of twodimensional,
finite-difference, time-domain models and laboratory-scale physical models developed to
predict the effects of wall edges and discontinuities on common-depth cross-hole radar measurements.
As part of a feasibility study, single-hole radar reflection data was used successfully to image the walls.
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Crosshole Georadar Measurements On An Alpine Rock Glacier
Authors Martin Musil, Hansruedi Maurer, Klaus Holliger and Alan G. GreenTo investigate the internal structure of a rock glacier in the eastern Swiss Alps, traveltime and
amplitude tomographic inversions have been applied to borehole georadar data. Pronounced variations
of electromagnetic velocity at the study site required a nonlinear inversion algorithm to be applied.
Traveltimes and amplitudes provided complementary subsurface information, such that a joint analysis
of the resulting velocity and attenuation tomograms allowed an enhanced interpretation of the data.
Features resolved included two distinct zones of enriched ice content, a sharp transition between frozen
and unfrozen material, and a highly attenuating zone within the bedrock.
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Ground-Penetrating Radar And Swept-Frequency Seismic Imaging Of Shallow Water Sediments In The Hudson River
Authors Roelof Versteeg, Eric A. White and Karl RittgerGround-penetrating radar and swept-frequency seismic sub-bottom data were collected on the
Hudson River between Kingston and Saugerties, New York, in April, 1999, as part of a pilot project to
create a comprehensive benthic map of the Hudson River. The radar and seismic data were collected
simultaneously to evaluate the usefulness of each method for shallow-water stratigraphic mapping. The
data were used in preparation of a benthic map and for creation of a facies distribution map.
The results show that in shallow water (less than 20-feet deep) in the Hudson River, the radar
method obtains better penetration and resolution than the seismic method. Virtually all radar data
collected in shallow water shows detailed sub-bottom structure, whereas 65 percent of the seismic data
does not show any sub-bottom penetration, due to the presence of methane gas in the sub-surface and
(or) a hard water bottom.
The majority of the interpreted facies show sub-bottom deposition that formed in a relatively low
energy environment. Significant changes do occur over relatively short distances however. This allows
a GIS-based interpretation of the mapping of the spatial distribution of the facies and the recognition and
differentiation of sedimentary regimes in the river.
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A Comparison Of Vertical And Horizontal Gpr Velocity Estimates In Alluvial Sediments
Authors William P. Clement and Michael D. KnollWe have measured the horizontal and vertical electromagnetic velocity through an alluvial, sandand-
cobble aquifer near Boise, Idaho. To measure the horizontal velocity, we deployed antennas in two
wells 6.9 m apart. To measure the vertical velocity, we placed one antenna on the surface adjacent to a
well and lowered the other antenna down that well. We collected data at three wells; two wells at either
end and a well located in between the two end wells. We thus have one measure of the horizontal velocity
and three measures of the vertical velocity. The horizontal and vertical velocity are essentially the same
below the water table at 2 m depth. In the vadose zone, the velocities differ, but we think that refracted
waves cause the velocity differences. The directional independence of the velocities indicates that we can
assume isotropic models in our interpretation of the aquifer.
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Field Test Results Of A Beam-Steered Ground Penetrating Radar Array
Authors Neil. F. Chamberlain and William. M. RoggentThis paper presents the results of field tests of a beam-steered ground penetrating radar (GPR)
system. The radar comprises an array of transmitting antenna elements and a conventional GPR receiver
system. The radar operates in bistatic mode. Each transmit antenna element is equipped with its own
transmitter electronics and timing circuitry. Beam-steering is achieved by appropriately delaying the
trigger pulse to each transmitter module. The transmit signal has a narrower and more powerful beam
when compared to convention single-antenna GPR systems, resulting in deeper penetration and enabling
better rejection of clutter through signal processing.
Results of field tests are presented for the radar operating on a medium of dry granite. The tests
verify that a narrow-beam GPR signal can be successfully steered in this particular medium.
Additionally, the results are discussed in relation to finite difference time domain analyses, with regard
to the broader implications for radar design and operation.
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Measurement Of Radiation Impedance Using A Portable Vibrator To Evaluate Mechanical Properties Of The Subsurface
Authors Yasuhiro Kaida, Dai Nobuoka, Jan Brouwer and Vincent NijhofIn civil engineering and environmental applications, it is important to evaluate the mechanical properties of
soils and rocks. Reijnders et al., (1999) introduced a method to measure the radiation impedance of the very shallow
subsurface using a portable vibrator. The method is based on the determination of the frequency-dependent integral
elastic response of the soils below the vibrator baseplate using reference signals measured at the source.
Radiation impedance is defined as the ratio of the stress acting on a particle and the resulting particle
velocity. A vibratory source equipped with accelerometers on baseplate and reaction mass allows the determination
of the force that acts on the surface as well as the baseplate velocity. If one makes the assumption that the velocity of
the baseplate is in phase with the velocity of the material directly beneath it, both the stress and the velocity, and thus
the radiation impedance, can be determined from measurements of the accelerations at the source.
Several field experiments were conducted to evaluate this method and are introduced in this paper. One of
these experiments was performed to evaluate the variation of radiation impedance over an area of re-filled and
compacted ground. High impedance was observed for the re-filled area. Obviously, this variation of radiation
impedance reveals the effect of compaction. This observation indicates that measurement of radiation impedance can
be applied to evaluate the mechanical properties of very shallow subsurface materials.
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Sonic Pulse Velocity Testing To Assess Condition Of A Concrete Dam
Authors Edward D. Billington, Dennis A. Sack and Larry D. OlsonA velocity analysis was performed to assess the condition of a concrete arch dam. Sonic pulse
velocity (SPV) measurements were made at 28 locations along the dam by impacting the downstream
face and recording arrival times on the upstream face. The sonic energy was generating using a remotely
operated impact source specifically designed for the project that could be lowered from the dam crest.
The compressional wave energy was recorded using a string of hydrophones on the upstream face of the
dam. A tomographic analysis of the arrival time data was performed to generate cross sections at each
station to show the velocity distribution through the dam. An underwater inspection of the upstream face
provided the opportunity to correlate velocity anomalies with diver observations. The results of this
study will serve as a baseline for future assessments.
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High Resolution Seismic Reflection Survey At Keechelus Dam
Authors Richard Miller and Richard MarkiewiczReplacement of the existing Bureau of Reclamation dam (Keechelus) located at the headwaters
of the Yakima River in Washington became necessary after the incidental encounter of a void during
trenching along the dam crest. Subsequent geophysical surveys discovered several other voids along the
structure, mandating the replacement of this high-risk dam. High resolution seismic reflection surveys
conducted along the downstream toe provided a continuous image of till, swamp bog, alluvial, outwash,
and lacustrine sediments which make up the Quaternary unconsolidated material that lay on Tertiary
rhyolite bedrock. Each of the three seismic lines possessed uniquely different signal-to-noise and resolution
characteristics. One of the profiles was acquired perpendicular to the long axis of the dam and
through a swamp area. This profile provided an excellent image of the till/lacustrine sediment contact
with indications of very steeply dipping contacts between the lacustrine and outwash sediments. Delineation
of these features, as shallow as 20 ft, is significant to the engineering of a new structure, which
must provide a watertight seal through the very permeable outwash and alluvial fan sediments. Resolution
potential of this 24-fold, 150+ Hz data set is around 10 ft (~½ wavelength). A dramatic channel type
feature separating a thin sequence of outwash and lacustrine sediments from a thickening wedge of
lacustrine type sediments is interpretable directly beneath the trace of the proposed new structure. Based
on the seismic reflection data set it should be possible to more precisely fit the preconstruction engineering
designs to the actual geology, which will be encountered during construction.
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The Use Of Non-Invasive Geophysics To Assess Damage By Burrowing Animals To Earthen Levees Near Dayton, Ohio
Authors Matthew Barner, Ernest Hauser and Paul WolfeGround-penetrating radar (GPR), electrical resistivity, and streaming potential methods were
employed at southwestern Ohio, flood-control levee sites to test their effectiveness for determining the
amount of damage by animal burrows. Three 36-m long sections of earthen levee were tested, each
containing visible groundhog burrows. Overall levee dimensions and burrow locations dictated the
extent of the field surveys at each site. The GPR data were gathered with 80 and 300 MHz antennas.
2D electrical resistivity imaging was performed using a system with a dipole-dipole electrode
configuration. Electrical resistivity data were collected under dry conditions and while saline water was
pumped into selected visible burrows. The intent was to increase the resistivity contrast of the burrows.
Non-polarizing electrodes were used to gather streaming potential data. One core sample taken at each
site helped provide information on levee composition.
The GPR data provided the most reliable information on levee damage at the West Carrollton
site. Depth of penetration and resolution were good enough to map potential burrows to depths of about
2.4 meters and show a potentially interconnected network of burrows throughout the upper half of the
levee. GPR data for the other levees yielded results only to depths of around 1.5 meters and with lower
resolution compared to the West Carrollton data. Electrical resistivity imaging obtained adequate depth
penetration at all three sites, although resolution was too poor to effectively mark any burrows and
estimate any internal damage caused by the burrows. The streaming potential data lacked signal above
the noise range and thus provided little useful information. Based on the GPR data, significant
groundhog damage may exist in the West Carrollton levee. In addition, this levee may contain an
extensive, interconnected network of burrows. Not enough information is available for the Dayton and
Middletown sites to make similar conclusions.
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Dependence Of Sandstone Dielectric Behaviour On Moisture Content And Lithology
Authors L.J. West, Y. Huang and K. HandleyAn investigation of the radar frequency dielectric behaviour of Sherwood Sandstone from
Yorkshire, England is reported. The equipment used, a large one port co-axial cell, and the method of
data interpretation are described. The co-axial cell measures the frequency dependent complex
reflection co-efficient of drill core samples. The dielectric properties are extracted from the complex
reflection co-efficient, the cell calibration data, and specimen geometry. Proof testing of the dielectric
cell is described. The cell works well over the frequency range of 75 to 1000MHz for saturated
specimens, and over the frequency range of 200 to 1000MHz for dry specimens.
The frequency dependent dielectric behaviour of sandstone specimens with mean grainsize
ranging from 300 to 125 microns over the full range of saturation levels is described (samples were
progressively wetted by addition of de-ionised water, saturated by soaking, and then dried by
evaporation). The results of the study show that the real component of the dielectric constant, Kr, is
independent of frequency and lithology for between 350MHz and 1000MHz. Below 350MHz, dielectric
behaviour is lithology dependent, with finer grained specimens showing higher Kr than medium grained
specimens.
The behaviour of Kr with moisture content is compared with that predicted by the Topp equation
and the Complex Refractive Index Method (CRIM). The Topp equation, and CRIM with Ks between
3.5 and 5 provide a good match to the data for all lithologies at 500MHz. They are also likely to provide
a good match at 100MHz for the medium grained sandstone lithology. The results indicate that
determination of moisture content/porosity is best done at higher frequencies (e.g. using Time Domain
Reflectometry) where a unique relationship exists. In contrast, characterisation of lithology from
dielectric behaviour is best carried out at lower frequencies (i.e. in 50 or 100MHz radar surveys), where
the dielectric constant is lithology dependent.
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Small-Strain Elastic Properties Of Compacted Clayey Soils During Drying
Authors Gokhan Inci, Takaaki Kagawa and Nazli YesillerIn this study ultrasonic methods were used for the determination of small-strain elastic properties
of compacted clayey soils. Three clayey soils compacted at different moisture contents and compactive
efforts were tested while drying. Piezoelectric transducers were used to generate and receive P- and Swaves
on cubical soil samples with oblique corner cuts. Five elastic parameters of cross-anisotropy
were calculated using three P-wave and two S-wave velocity measurements. Constrained, Young's, and
shear moduli in the vertical and horizontal directions increased during drying. Young's and shear
moduli increased monotonically, while constrained modulus exhibited a peak for wet of optimum and
optimum water content samples. It was observed that Poisson's ratios decreased during drying and that
the soils compacted with high compactive effort had lower Poisson's ratios compared to the soils
compacted with low compactive effort at the same saturation level. Anisotropy value, n, increased for
wet of optimum and optimum water content samples during drying, which was attributed to particle
reorientation during shrinkage process. On the other hand, n did not change significantly for soils
compacted at dry of optimum water content.
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Mapping Karst Conditions Using 2D And 3D Resistivity Imaging Methods
Authors Tomasz Z. Labuda and Andrew C BaxterAuthors of this document have performed several geophysical investigations within the eastern
United States. Most of this experience has involved subsurface investigations in carbonate rock units.
As compared with different geophysical techniques including micro gravity, electromagnetics, ground
penetrating radar, spontaneous potential, and other methods, the best results in characterizing subsurface
conditions in karst terrain were accomplished using surface resistivity imaging.
The following discussion is based on two case studies, both involving two-dimensional and theedimensional
surface resistivity surveys performed over Paleozoic age carbonate rocks in northern
Virginia and West Virginia. The purpose of this document is to show the benefits and pitfalls of twodimensional
and three-dimensional resistivity imaging used to characterize subsurface conditions in the
karst terrain. Both methods are compared based on different site geologies, array geometries, and target
size and depth. A short discussion of dipole-dipole method commonly used in two-dimensional arrays,
versus pole-pole used in three-dimensional arrays is presented to indicate differences in expected data
resolution.
The intention of this document was also to highlight the importance of volumetric visualization
of three-dimensional data sets for analysis and interpretations. Creating volumetric three-dimensional
models of resistivity is useful in evaluating the location, orientation and character of potential fracture
systems within relatively uniform rock masses.
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Seismic Characterization Of The Karst Bedrock Surface In The Southeast Industrial Area At Anniston Army Depot, Alabama
Authors M.D. Thompson, S.F. Miller, M.A. Glennon, J.M. Cooper and W.A. MandellEnvironmental concerns at the Southeast Industrial Area (SIA) of Anniston Army Depot
(ANAD) have been the impetus for geotechnical investigations over the last 20 years. These previous
investigations have all failed in one respect, to map the bedrock topography of the underlying Knox
Group dolomites. The bedrock surface is an important target: a) lows points provide a greater column
of overburden in which to trap contaminants; b) bedrock pinnacles may act as rapid access points into
the karst conduit systems; and c) hydraulically transmissive zones are often found within the weathered
portion (epikarst) of the bedrock. The approach taken by Argonne was to construct a bedrock surface
map integrating existing borehole data with selective seismic-refraction profiling. Initial inspection
revealed that depth-to-bedrock measurements based solely on auger refusal were not acceptable in
many cases because refusal occurred on top of either large boulders or chert horizons.
Seismic refraction profiling within the SIA required overcoming several technical challenges.
First, numerous sumps, fans, and blowers produced strong background noise, as did the constant
vehicular traffic and sporadic railroad activity. Secondly, construction design for the SIA was not
conducive to seismic profiling as compacted gravel-fill, reinforced concrete, and asphalt paving
comprised the ground surface. One solution was simple brute force with upwards of 30 stacks using a
weight-drop source to overcome the background din. Other solutions included collecting data during
off-hour periods, mounting geophones in gravel, grassy, and asphalt areas adjacent to the major
roadways, and paying careful attention to where the shot point was located to avoid impacting on the
hardened concrete surfaces. In the latter case, the concrete roadbed acts as a wave-guide, primarily
transmitting the seismic energy horizontally and obscuring the underlying geology.
Seismic profiling and analysis of borehole data indicate that at least 4 major bedrock
depressions, averaging 20-to-30 m in local relief, occur along the southeast and northeast boundaries.
Drilling records indicate more frequent occurrences of clay-filled cavities and fractured and weathered
rock along the southeast boundary, suggesting a causal relationship for the location of the bedrock
lows. Eight confirmatory boreholes were used to ground truth the seismic models. Four of these
boreholes confirmed the presence of the bedrock lows, with one low located where previous
investigations had indicated shallower bedrock. The ability to acquire seismic data in an active
industrial area where utility lines and sewer systems preclude using electrical or electromagnetic
methods helps ANAD to intelligently guide drilling, regulatory, and remediation strategies.
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Investigation Of An Earth Fissure Using Electrical And Seismic Methods
Authors Bjorn E. Sundquist and Barbara A. LukeEarth fissures present a hazard in the alluvial desert soils of the Las Vegas valley
(Nevada), particularly when their presence is masked by cemented surficial materials.
The authors performed electromagnetic conductivity, electrical resistivity, and surfacebased
seismic measurements over a known earth fissure lacking surface expression.
Preliminary reconnaissance using electromagnetic conductivity measurements indicated
the presence of a buried pipeline traversing the site. The presence of the fissure was
clearly indicated by the electrical resistivity measurements, which were performed in a
28-electrode dipole-dipole array, although the geometry of the fissure was not resolved.
Results were compared with a synthetic dataset. An experimental frequency-domain
surface-based seismic cavity detection investigation was also conducted over the fissure.
The method was moderately successful for this application.
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The Freshwater Resource Of Andros Island, Bahamas, Determined By Resistivity
Authors Angela L. Adams, Paul J. Wolfe, Cindy K. Carney and Mark R. BoardmanThe freshwater lens on Andros Island, Bahamas, is an important source of potable water for local
use and for export to the city of Nassau on New Providence Island. The production of large quantities of
water for export raises concern for the long-term sustainability of the resource. Resistivity soundings
provide a relatively easy way to evaluate and monitor the thickness of the freshwater lens. This
technique works well because there is a large contrast in resistivity between the freshwater and saltwater
saturated carbonate rocks that make up the island. Over the last 8 years we, along with students from
our universities, have conducted a number of resistivity surveys across the northern part of the island. A
number of problems, which complicate the acquisition of resistivity soundings on the island, will be
presented. Working around these problems we have developed a profile of the freshwater lens across
the island.
Our results show:
a single lens spans the island from Red Bays on the west to Conch Sound on the east,
the lens averages about 15 m thick over much of the island,
the lens thins to about 8 m under the well field areas, and
the lens thins to about 10 m in the vicinity of Charlies Blue Hole.
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Nuclear Magnetic Resonance Imaging Of Water Content In The Subsurface
More LessThe technique of nuclear magnetic resonance (NMR) imaging of subsurface water
content is described and critically evaluated from both experimental and theoretical
points of view. We describe data from soundings conducted in different parts of Colorado
and New Mexico, and describe constraints on the technique imposed both by
the present state of the instrument technology and by field conditions, such as cultural
noise, the subsurface geolectric section, pore saturation and pore size distribution, and
magnetic impurities in the soil. We describe also a newly derived general formula for
the NMR response voltage, valid for arbitrary transmitter and receiver loop geometry
and arbitrary conductivity structure of the medium in which the nuclear spins reside.
It is shown that in cases where the conductivity is large enough such that the electromagnetic
skin depth at the Larmor frequency is of the same order or smaller than
the measurement depth, there are diffusive retardation time effects that significantly
alter the standard NMR response formula used in the literature. Large quantitative
differences are found between conducting and insulating cases in physically relevant
situations.
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A Regional Tem Survey To Map Saline Water In The Cambrian-Ordovician Sandstone Aquifer Of Southeastern Wisconsin
Authors John Jansen, Robert W. Taylor and Ted PowellThe Cambrian-Ordovician aquifer is the major source of ground water for municipal and
industrial use in southeastern Wisconsin. Decades of over pumpage have created a regional cone of
depression centered in the western suburbs of Milwaukee. Total Dissolved solids levels in several
sandstone aquifer wells have more than doubled over the last ten to fifteen years. A time-domain
electromagnetic induction (TEM) survey was conducted to determine the cause of the rising TDS levels.
A total of 69 soundings were performed using a Geonics EM-57. The modeled soundings were
used to construct three geo-electrical cross sections across the survey area. The TEM survey indicated
that the rising salinity levels in the wells is related to vertical migration of saline water from the lower
aquifer in response to decades of over pumping. The findings are being used to evaluate future well
sites and rehabilitation options to reduce salinity levels in existing wells.
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The Effect Of A Conductive Surface Layer On Tem Central-Loop Soundings In Groundwater Exploration
More LessOne of the most popular geophysical methods of groundwater exploration is electromagnetic
sounding. In particular, central-loop configuration transient electromagnetic (TEM) soundings have
been shown to be quite effective in mapping horizontal strata of hydrogeological significance.
However, conductive surficial layers can interfere with what seems to be a straightforward application
of this method. Three effects of surficial conductive material can be addressed. First, the two-valued
apparent resistivity function becomes significant when surficial conductive layers are present,
necessitating a thorough understanding of this phenomena. Second, a breakdown in the definition of
apparent resistivity can occur in which there is no equivalent halfspace resistivity which would produce
the observed voltage in the measurement coil. Third, when collecting soundings near a lateral change in
surficial conductivity, migration of induced currents toward the conductor results in a sounding curve in
which transient voltages can drop very steeply, and can even change sign. This effect has been
explained in previous work in two ways. First, there can be a paramagnetic effect within soils near the
transmitter wire (mostly for the coincident loop configuration, but also for roving receiver locations near
the transmitter wire when using a large transmitting loop), and second, this can be due to an induced
polarization effect. A third explanation is developed here in which multiple current density rings are
induced in the subsurface. One ring corresponds to induction in material directly below the transmitter
loop, and a second ring can be developed non-symmetric with the transmitter loop, resulting from a
lateral surficial conductive layer.
These effects in TEM central-loop soundings are illustrated by analyzing data obtained at two
tidal flat environments, near Union Beach, and on the oceanside of Cape May Peninsula, New Jersey,
and from the surface of a lava flow resulting from the August 1999 eruption of Cerro Negro volcano,
Nicaragua.
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Electromagnetic Mapping (Em Map) Of Perched Water To Improve Remedial Activities At Hill Air Force Base
Authors T. Phillips and M. CoxThis report contains a description of how a proprietary geophysical technique was used to assist
groundwater investigations, in and around Operable Unit 1, OU1, located on the eastern side of Hill Air
Force Base, (HAFB), Utah. The primary objective was to define the sources of a groundwater plume
contaminated with Light Non Aqueous Phase Liquids, LNAPLs, which originated within OU1. An
additional objective was to locate subsurface channels for the plume that extended into off-base areas.
Although investigations have defined 7 individual groundwater bearing zones, most of these
investigations focused on the near-surface water, i.e., about 20 ft below the surface, groundwater
flowing horizontally on top of the clays of the Alpine Formation and within the sands and gravels of the
Provo Formation.
Results of the geophysical surveys defined water channels feeding seeps along the east and north
sides of two landfills within OU1. An additional survey defined sources for groundwater recharge.
Channels contributing to the off-Base groundwater were mapped in a separate survey. The technique
also proved useful in determining groundwater sources for off-Base landslides. A final survey was able
to confirm that water in the lower water-bearing zones of the Alpine Formation was not connected to the
near-surface groundwater plume.
The paper defines how this proprietary geophysical technique, based on U. S. Patent #5,825,188,
is a cost-effective means of defining groundwater flows. When coupled with the other investigative
tools used at OU1, including monitoring well/piezometer installations, Cone Penetrometer Tests (CPT),
and soil borings, the technique improves the overall understanding of groundwater flows at the site and
provides for a higher degree of confidence in the siting of other investigative tools. Examples are
provided showing where a groundwater channel feeding a seep, missed in a large drilling/CPT program,
was identified. In addition, the technique also provided essential data for the design of a groundwater
interception trench to connect contaminated groundwater and LNAPLs.
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Geophysical Surveys For Groundwater In The Gaza Strip
Authors W. Ed Wightman, James Hild and Steffen HodgesThe Gaza Strip (Gaza) is an area 40 km long and 8 km wide along the eastern shore of
the Mediterranean Sea. The primary sources of groundwater in Gaza are shallow aquifers
within the Kurkar formation, a transgressive-regressive sedimentary section consisting
primarily of interbedded sands and clays. The Kurkar rests on a marine clay called the Saqiye,
which forms the base of the aquifer. The quantity of fresh water that can be withdrawn
depends on a number of factors, such as recharge, geology, and hydraulic properties.
Moreover, since the aquifer is in contact with seawater, interfaces between fresh and saline
water are in dynamic equilibrium, and the distribution of the interface is a critical parameter
defining the resource. The water table in Gaza is at an elevation of about sea level.
Groundwater characterization studies of the Kurkar aquifer have been an ongoing
project in Gaza, funded by USAID. Recently, geophysical surveys were used to better define
the shallow stratigraphy and groundwater salinity distribution within the Kurkar aquifer.
Various non-intrusive geophysical methods were tested for this purpose, however the best
results were derived from an integrated interpretation of time domain electromagnetic
soundings (TDEM) and seismic reflection data. The scope of work consisted of acquiring,
processing, and interpreting 24.7 km of seismic reflection data, and 84 TDEM soundings. A
small amount of Induced Polarization, resistivity and seismic refraction data was also acquired
but did not aid significantly in the interpretation.
The salinity of ground water within a geologic unit can often be inferred from
resistivity, since it is strongly influenced by salt concentration that lowers groundwater
resistivity. However, clay horizons also often have low resistivities, and can be misinterpreted
as saline water. On the other hand, seismic reflection data images the lithologic boundaries
between sand and clay layers, and is insensitive to groundwater salinity. Thus if a lithologic
boundary is coincident with a resistivity boundary it is more likely to correspond to a clay layer.
On the other hand a low resistivity region which is not defined by a seismic reflector may
simply reflect a change in the resistivity of the pore water. Therefore, by combining the two
methods it is possible to better interpret the cause of low resistivity values. Several wells close
to seismic lines were available with geologic logs allowing the correlation of the geophysical
data with the geologic logs.
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Improving Understanding Of Peatland Hydrogeology Using Electrical Geophysics
Authors Lee D. Slater, Andrew Reeve and Danney GlaserA geophysical survey was completed in Caribou Bog, a large peatland in Maine, to evaluate
peatland stratigraphy and hydrology. Geophysical measurements were integrated with direct
measurements of peat stratigraphy from probing and with measurements of fluid chemistry. Consistent
with previous field studies, GPR was an excellent method for delineating peatland stratigraphy.
Prominent reflectors from the peat-lake sediment and lake sediment-mineral soil contacts were precisely
recorded up to 8 m deep. However, GPR provided no information below the mineral soil contact. 2D
resistivity and induced polarization (IP) imaging was used to further investigate the stratigraphy of this
peat basin. We observe that the peat is chargeable and that IP imaging is an alternative method for
defining peat thickness. This chargeability is attributed to the high surface charge density on partially
decomposed organic matter. The conductivity imaging resolved glaciomarine sediment thickness and its
variability across the basin. Terrain conductivity measured with a Geonics EM31 correlated with
glaciomarine sediment thickness and was effective in characterizing variability in layer thickness over
approximately 18 km2. The electrical imaging indicates that variations in glaciomarine sediment
thickness may exert a key control on the hydrogeology and vegetation distribution within this peatland.
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Applications Of Shallow Geophysics In A Regional Geological And Hydrogeological Investigation, Oak Ridges Moraine, Southern Ontario
Authors S.E. Pullan, A. Pugin, J.A. Hunter, S.D. Robinson, M.A. Annecchione and G.E. LeblancGeophysics has played a critical role in a multi-disciplinary research program in the Oak Ridges
Moraine area, southern Ontario, designed to improve understanding of the regional geological and
hydrogeological framework of a significant Quaternary aquifer complex. Geophysics was particularly
important because of the large study area, complex glacial stratigraphy, and thickness of overburden
sediments (up to 200 m). Borehole, surface and airborne geophysical surveys have provided information on
the subsurface at scales of investigation varying from sub-metre to kilometre. A suite of geophysical logs
were obtained in eleven, deep (90-190 m) stratigraphic boreholes. These data were particularly important
in identifying downhole stratigraphic relationships and effecting regional correlation. Ground probing radar
was used extensively in glaciofluvial sand and gravel deposits of the ORM to obtain depth to water table and
very-near-surface structural and sedimentological information. Electromagnetic techniques were tested, but
not used extensively during the project. Over 50 line-km of land-based, shallow seismic reflection profiles
provided a means of investigating the subsurface architecture and stratigraphic relationships of the complete
sequence of unconsolidated sediments. These surveys were instrumental in identifying the regionally
extensive and eroded nature of the Newmarket Till beneath the ORM deposits. Data from a regional gravity
survey consisting of over 5500 stations were inverted to obtain an interpretation of bedrock topography.
Though as yet unproven, high-resolution airborne magnetic data have identified anomalies which may be
related to channel features within 200 m of the surface. Overall, the combined geophysical data have
provided high-quality control for a large set of archival data, and have allowed an assessment of the lateral
continuity of major hydrostratigraphic units. This paper outlines the objectives and applications of these
geophysical surveys in the Oak Ridges Moraine study, and assesses their potential applications in other
regional hydrogeological investigations.
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High-Resolution Seismic Survey To Evaluate Groundwater Potential At A Proposed Electric Power Generation Facility
Authors William Black, Richard Miller and Richard PalmHigh resolution seismic reflection profiling was effective in delineating geologic units within the
upper 200 ft at a proposed power plant site in southern Minnesota. Glacial activity is responsible for the
topography of the bedrock surface evident on seismic reflection data across this site. Gravel and sands
deposited during glacial retreat into bedrock lows can provide a significant source of water for a variety
of residential, commercial, and municipal applications. Selection of power generation plant sites is
routinely based on availability of fuel, proximity to existing power grid, and a ready source of water. In
this case the first two criteria were met; the purpose of the seismic survey was to evaluate and direct a
drilling program that would establish the third. Dominant frequencies of the nominal 24-fold data
acquired along two lines are, on the average, in excess of 150 Hz, which provides practical vertical bed
resolutions (~½ wavelength) on the order of 15 ft. Bedding geometries interpreted from CMP stacked
sections were consistent with several boreholes located around this 80-acre site. Several apparent structural
features interpreted at bedrock depth on the stacked sections were mapped as potential water
sources. The bedrock low interpreted from drill data was observed on the stacked seismic section but
was confined to a much smaller area than mapped using the few scattered drill holes. A complex bedding
geometry associated with the advance and retreat of glaciers is evident on the seismic data above
the bedrock surface. Drilling confirmed the seismic interpretation and provided additional information
about the nature of the material within the bedrock lows. This proposed site was abandoned due to insufficient
quantities of gravel and coarse sands within the glacially sculpted bedrock lows.
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Geophysical Mapping Of Groundwater Potential In A Rural Water Supply Project: Malawi, Africa
Authors Richard L. Kellett, Jim R. Anscombe, Paul D. Bauman, Philip Hankin and Leon EngelbrechtA total of 250 boreholes were sited in the East Mangochi District of Malawi, using a unique
combination of borehole geophysics, terrain conductivity, frequency domain electromagnetics, and
electrial resistivity tomography. The project represents one of the most intense applications of
geophysical surveys to rural water supply in deeply weathered crystalline basement terrains. A total of
388 line kilometers of EM, 22 line kilometers of ERT, and 23 borehole logs were collected in a threemonth
program in 1999. These data have been processed and interpreted in conjunction with previous
hydrogeological reports and satellite and airphoto interpretations, to produce over 480 targets for lowyield
village hand pumps. Such a detailed geophysical survey was considered necessary in the region
because of the highly irregular distribution of aquifers and the low success rate of past drilling programs.
The EM surveys proved to be the most efficient in terms of rapid surveying, but the ERT sections were
essential for resolving ambiguities in the EM response of lateral and vertical resistivity changes. Water
saturated deep weathering zones, and vertical fracture sets were the most commonly mapped targets
having strong responses in both the EM34 and the Apex MaxMin systems. Resistivity sections across
these features accurately mapped the depth to unweathered bedrock, the presence of a saprock layer, and
the thickness of the shallow laterite. Data management and interpretation was aided by the use of a
Geographical Information System (GIS) that allowed continual monitoring of the survey, and rapid
access to the results. Drilling of the boreholes commenced in September 2000 using the Global
Positional System (GPS) locations, village maps, and physical markers on the ground to locate the
targets. To date, of the 35 wells drilled, 27 have been completed as successful water wells.
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Natural Aggregate -- Geophysical Opportunities
Authors William H. Langer and Karl J. EllefsenDuring the early part of the 20th Century, the increased demand for surfaced roads prompted a
number of reports in the geologic literature on using aggregate for highway construction. Since that
time many other excellent reports concerning research, development, testing, and evaluation of natural
aggregate - crushed stone, sand, and gravel - have been written. However, most professional geologists
and geophysicists, even those who work in the mineral-resource field, commonly ignore natural
aggregate in favor of other seemingly more glamorous aspects of geology.
Today, at the beginning of the 21st Century, we recognize the indisputable need for a continuing
and uninterrupted supply of high quality aggregate. The United States annually produces over 2.5
billion tons of aggregate, which represents approximately 9 tons per person per year. During the next
decade, the restoration and rehabilitation of an aging infrastructure will require enormous amounts of
aggregate, much of which will need to meet strict specifications for new high-performance construction
materials. In addition, stringent environmental laws, land-use conflicts, zoning restrictions, and the "Not
in my back yard" syndrome make it increasingly difficult, time-consuming, and costly to obtain permits
for extraction of aggregate.
Geophysicists have the opportunity to provide critical information concerning the identification,
characterization, and extraction of suitable aggregate, and concerning protection of the environment
from adverse impacts of aggregate extraction. Geophysical surveys of aggregate deposits also provide
opportunities for conducting traditional research on geophysical methods. Research is needed on
techniques to efficiently acquire and process ground-based geophysical data, airborne geophysical data,
and remotely sensed data. Such data are needed to detect and delineate potential sources of crushed
stone and deposits of sand and gravel. Better techniques are also needed to determine deposit thickness,
characterize variations within a deposit, characterize physical properties of aggregate resources,
characterize mine sites, and characterize areas of potential environmental impacts.
It is time to shift paradigms, to implement new ideas and methods, to get involved in aggregate
research. In doing so, the inquisitive geophysicist will soon recognize numerous challenging
opportunities that make natural aggregate a subject as fascinating and rewarding as any other aspect of
geophysics.
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Mapping Of Bedrock Aquifers In The Denver Basin Using Borehole Geophysical Logs
Authors L.R. Arnold and S.G. RobsonGround water is a natural resource that is present in bedrock aquifers that underlie much of the
Front Range Urban Corridor in Colorado. In 1996, mapping of bedrock-aquifer outcrop and subcrop
areas along the western margin of the Denver Basin was undertaken by the U.S. Geological Survey in
cooperation with the Colorado Department of Natural Resources, Division of Water Resources, and
the Colorado Water Conservation Board. Detailed mapping of outcrop and subcrop areas of the Denver
Basin bedrock aquifers was done to better define areas where the aquifers receive recharge
directly from precipitation at the ground surface and where they are in direct hydraulic connection
with alluvial aquifers. Geophysical logs from oil, gas, and water wells and lithologic logs from
exploration borings were used to map the subsurface structure and thickness of the bedrock aquifers.
Structural contour maps of the bedrock aquifers then were used in combination with topographic
maps of the land surface, information from field observations, and published geologic maps to determine
where bedrock-aquifer outcrop and subcrop areas occur. The results of the study indicate that 1)
the bedrock aquifers along the western margin of the Denver Basin generally dip gently eastward into
the basin and have steepening dips in the vicinity of the mountains of the Front Range and 2) the
width of bedrock-aquifer recharge areas varies greatly as a function of dip. The presence of deltaplain
paleochannels was inferred from a thickness map of one of the bedrock aquifers. The study
demonstrates that geophysical logs can be a useful tool in mapping the subsurface structure of bedrock
aquifers, especially in areas where it is difficult to distinguish between formations on the basis of
lithology and in areas where bedrock is covered by a thick mantle of unconsolidated sediments.
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An Investigation Of Alluvial Sediments Using S-Wave Refraction: A Case Study
Authors Karl J. Ellefsen and Adrienne BarnettIn the Big Thompson River valley of north-central Colorado, an S-wave refraction survey was
conducted to determine the thickness and the gross stratigraphy of the alluvial sediments, because this
information is needed to evaluate resources like ground water and aggregate. The refraction data were
collected along three roads that cross the valley; the data were processed to estimate velocities and
thicknesses for a layered-earth model; and from these models, three cross sections of the river valley
were constructed. The river valley is covered by a layer of soil, which is 0.2 to 1.5 m thick. Beneath the
soil, there is one layer of alluvium at some locations and two layers at other locations. For the two
westernmost cross sections, the total thickness of the alluvium ranges from about 6 to 10 m near the
center of the valley and from about 2 to 6 m near the sides of the valley. The easternmost cross section is
somewhat more complex than the other two, because it is near a large tributary. In this cross section, the
thickness of the alluvium ranges from about 8 to 10 m in the southern half of the valley and from about
3 to 13 m in the northern half. In all three cross sections, the alluvium overlies bedrock, which is the
upper transition member of the Pierre Shale.
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Detection Of The Oil, Natural Gas, And Coal Production Infrastructure By Magnetic Surveys, Front Range Of Colorado
Aeromagnetic surveys flown over areas of past and current production of oil, natural gas, and
coal in the northern Front Range of Colorado have proven useful in identifying natural gas wells and
magnetic clinker associated with abandoned coal mines. Magnetic anomalies spatially associated with
wells that produce natural gas in the greater Wattenberg area, north of Denver, are commonly of short
wavelength (<150 m) and variable amplitude (15-90 nT), depending on the location of the well with
respect to the flight line of the aircraft performing the survey. Steel pipe, which serves as casing in the
wells, is the likely source of the magnetization detected by the airborne magnetometer. Magnetic clinker
is associated with “baked” rock overlying the coal that has burned since mine abandonment. Anomalies
associated with clinker are commonly of longer wavelength (>250 m) than those associated with gas
wells and are variable in amplitude (17-40 nT). Because producing or abandoned gas wells and
abandoned coal mines can present challenges to future land development, geophysical surveys that help
locate such features can greatly benefit land use planners in making informed decisions.
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Seismic Refraction And Dozer Trenching To Estimate Borrow Quantities At Casitas Dam Borrow Site, Ventura California
Authors Richard A. Hopkins, Richard D. Markiewicz, Jeanne L. Major and Curtis P. CainFrom May 5 to May 12, 1998, personnel from Reclamation’s Technical Service Center
(TSC), Denver Federal Center completed approximately 5,280 line feet of seismic refraction
surveys at the Casitas Dam Borrow Area, near Ventura, California (Figure 1). The seismic
refraction surveys were concentrated in an area identified as potentially having 3.3 million cubic
yards (CY) of borrow material required for the proposed remediation. The seismic refraction
surveys were conducted to provide information about the configuration of the top of bedrock for
both the Tertiary age Sespe and Vaqueros Formations (limestone, siltstone and sandstone),
bedrock velocities, and possible weathered zones above the bedrock interface. These parameters
affect borrow material volume calculations. Initial exploration drilling provided inconsistent or
irregular refusal depths.
Rippability test sites were selected to evaluate conditions for both the Sespe and
Vaqueros Formations. A single site was selected within the Sespe Formation. Vaqueros site
locations were selected to determine the quality of the rock exposed at the surface and to
evaluate the lower and higher bedrock velocities interpreted from the seismic refraction data.
Subsurface condition information gained from the rippability testing would then be extrapolated
to quantify the exploration drilling and seismic refraction interpretations in areas not groundtruthed.
Seismic refraction surveys conducted at Casitas Dam Borrow Area are interpreted to
have three subsurface seismic velocity layers. There was general agreement between the "bucket
auger” refusal depths and calculated depth to the surface of seismic layer 3. Rippability results
indicate the Sespe Formation is relatively uniform laterally and vertically to the depth associated
with the top of seismic layer 3 and thus support the geophysical interpretation. Seismic layer 3
within the Sespe Formation appears to be a high velocity mudstone/claystone material that is
both tight and moist rather than an unweathered sandstone or conglomerate unit. The results of
the rippability testing support lateral variability within the Vaqueros Formation as interpreted
from the seismic refraction data. The results also infer that the seismic velocity is more
responsive to clay/moisture content.
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Buried Landfill Delineation With Induced Polarization: Progress And Problems
Authors Norman R. Carlson, Jennifer L. Hare and Kenneth L. ZongeIn recent years, the use of induced polarization (IP) data for delineating buried landfills has
increased significantly, due to technological advances that have made this method faster and less
expensive, and therefore more applicable to the environmental industry. As the database of IP
information grows to include information over waste of differing composition and in differing
environments, interpretation has improved significantly. Problems and limitations still exist, of course,
but the IP method has become an efficient and economic tool in evaluating waste sites. We discuss here
progress in interpretation, including methods to process large amounts of data rapidly in order to
decrease costs, and we discuss problems that still exist, such as electrode stability, which still limit the
field survey speed.
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Joint Application Of Radiomagnetotellurics (Rmt) And Induced Polarization (Ip) To I Waste Site Exploration
Authors Biilent Tezkan, Stephan Recher, Mark Hsnig and Fritz M. NeubauerOn a waste site near Cologne/Germany RMT and time domain IP measurements were
carried out on the same profiles. At 320 stations RMT apparent resistivity and phase data were
measured for eight frequencies between 16 and 240 kHz. Civilian and military radio stations were
used as transmitters broadcasting in the RMT frequency range. The lateral borders of the waste
site are characterized by strongly decreasing apparent resistivities compared to the surrounding
host. A clear phase difference outside and inside the waste site is also observed.
The RMT data were interpreted by a 2D inversion algorithm quantitatively. The derived
2D conductivity models indicate the waste site as good conductive anomaly and show lateral and
vertical extension of it. A transient electromagnetic system was used for the IP measurements at
the RMT profiles where the complete transient curve was recorded (anolog to seismic roll along
procedure). For the interpretation of the observed data, a modified electromagnetic algorithm is
used to calculate the response of layered polarizable ground. The transient signals are then inverted
using the Marquardt method to derive the Cole-Cole parameters of each layer. Sensitivity
studies with synthetic data sets for 1D polarizable media show a significant dependence of the
parameter resolution on the magnitude of the IP-effect. Application of the algorithm to the field
data is used to derive pseudosections of the Cole-Cole parameters for near surface structures.
At several locations at the waste site, where the models of RMT and IP data show
anomalous features in conductivity and Cole-Cole parameters, drillings were caried out yielding
soil examples which will be examined in the laboratory.
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A Survey Of Waste Pits At The Idaho National Engineering And Environmental Laboratory Using The New Usgs Prototype Atv Towed Vetem Platform
Authors Jared D. Abraham, David VonG. Smith and David L. WrightThe Very Early Time Electromagnetic (VETEM) system is a prototype EM system that can be
configured in a number of ways for different applications. The main aim for VETEM is to improve
state-of-the-art shallow subsurface electromagnetic imaging in environments where the electrical
conductivity is too high for ground-penetrating radar (GPR) to be effective. The VETEM system also is
unusual in that it records both the transmitted as well as the transient decay part of the signal, and
operates within a much higher frequency spectrum than traditional Time-domain Electromagnetic
(TDEM) systems. The VETEM system, as a research prototype, is constantly being improved to make it
more effective for a wider range of shallow earth imaging problems of interest to the Department of
Energy and to other agencies with similar needs.
The USGS prototype VETEM system was successfully deployed over Pit 4 and Pit 10 in the
Subsurface Disposal Area (SDA) of the Radioactive Waste Management Complex (RWMC) at the
Idaho National Engineering and Environmental Laboratory (INEEL). This field deployment was the
first use of the new VETEM all-terrain-vehicle (ATV)-towed platform, which has an integrated realtime-
kinematic global positioning system (RTK-GPS) system that can provide positioning at a decimeter
level of accuracy. Coverage of these large areas would have been impractical without our very recent
development of the ATV-towed version of VETEM.
The uniformly high quality data obtained over Pit 4 and Pit 10 have yielded high-resolution timeslice
images of subsurface conductivity variations. These time-slice images illustrate the high-resolution
time-dependent response of the VETEM system to the buried waste. These data can be directly
inspected for the lateral position of objects and relative conductivity variations within the subsurface.
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Evaluation Of New Geophysical Tools For Investigation Of A Landfill, Camp Roberts, California
Authors W.E. Doll, T.J. Gamey, J.E. Nyquist, W. Mandell, D. Groom and S. RohdewaldCharacterization of material changes with depth (profiles) in many landfill sites can be
problematic for some conventional geophysical methods. Localized anomalies within the
landfill can complicate mapping of underlying layers, and layered-model techniques are
inappropriate for imaging laterally discontinuous landfills.
Recently-developed geophysical hardware and software tools provide the opportunity to
image the vertical structure of a landfill and its geologic setting. In May, 2000 a sequence of
geophysical data sets were acquired at a landfill site at Camp Roberts, CA to test the benefits of
new hardware and software for characterizing the three-dimensional boundaries of the landfill
and the geologic setting. Conventional magnetic and electromagnetic measurements provided a
backdrop for these new methods. A Geometrics G-858 magnetic gradiometer equipped with a
real-time GPS positioning system was used to map the areal extent of the landfill.
Resistivity, seismic refraction, and electromagnetic data were acquired along profile lines
to characterize the vertical extent of the landfill and geology. Seismic refraction data were
processed with conventional time-delay methods, and with newer tomographic methods. The
multielectrode resistivity data were compared with data acquired with the capacitively-coupled
OhmMapper system
The landfill boundaries that are defined in map view by the magnetic data are supported
in profile by the seismic refraction data and multielectrode resistivity data. The seismic data are
most effective in identifying trench locations when a tomographic inversion is used, instead of a
conventional delay-time approach to interpretation. This shows a localized high-velocity zone
that coincides with the trench boundaries that are defined by the magnetic data. The
multielectrode resistivity data show a disruption of layering where trenching has occurred.
Both the seismic data and the multielectrode resistivity data provide evidence that the
shallow geology is laterally discontinuous and heterogeneous. The high electrical conductivity
of the near surface imposed limitations on the penetration depth of both the OhmMapper and
multielectrode resistivity systems. The multielectrode system was better suited for penetrating
this zone than was the OhmMapper.
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Characterisation Of Hazardous Waste Sites Using The Bgr Helicopter Geophysical System
The Federal Institute for Geosciences and Natural Resources of Germany (BGR) completed a
research and development project aiming at optimising its helicopter-borne geophysical system for
circumstances relevant for high resolution site characterisation. The overall objective was to adapt the
existing helicopter-borne geophysical system previously used for groundwater and mineral exploration
to survey conditions where the anomalies to be recorded are much smaller in shape and size.
The BGR helicopter-borne system permits simultaneous electromagnetic (AEM), magnetic, and
natural gamma-ray surveying. At the suggestion of the BGR, the AEM system manufactured by
Geoterrex-Dighem, Toronto, includes several improvements
increased dipole moment
only horizontal coplanar coils
minimum interference by overlapping coils
operation at five frequencies
new calibration concept: amplitude and phase calibration during flight.
These instrumental modifications were tested running various surveys over two former military
training areas south of Berlin, Germany. Special detection surveys to locate typical waste, such as
individual steel drums, scrap metal, steel pipes, petrol tanks, ordnance, buried at depths varying from
0.3 – 1.5 m were carried out with a nominal flight line spacing of 50 m. AEM and magnetic anomalies
produced by these materials are extremely weak and in the order of only 1 ppm or 0.1 nT at a height of
app. 25 m a.g.l. Therefore, suitable detection algorithms were developed and tested using an airborne
data set collected over an area where thousands of individual anomalies were identified. Lots of them
were subsequently verified on the ground. More than 95% of the anomalies selected for verification
could be confirmed either by visual inspection of the ground surface or ground geophysical surveying or
excavation. The GPS/GLONASS positioning error was less than 10 m in most cases.
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Changes In Geoelectrical Properties Accompanying Microbial Degradation Of Lnapl
Field geophysical studies have identified anomalously high conductivities within and
below the free product zone in soils at sites with "aged" contamination by light, nonaqueous
phase liquids (LNAPL). Laboratory experiments and simple numerical modeling
studies were conducted to test the hypothesis that these anomalously high conductivities
result from products of LNAPL biodegradation. These experiments consisted of 20-L
glass vessel reactors with 18 L of inoculated sand and 6 L of pore water. Diesel fuel was
added to the top of the sand. These experiments simulated a smear zone with a top layer
of LNAPL. Duplicate reactors of the following types were maintained for 120 days;
nutrients added (at levels observed in the field), no nutrients added, killed (autoclaved)
control with nutrients. The killed control showed no signs of diesel fuel biodegradation.
The biologically active reactors showed evidence of diesel fuel biodegradation (e.g.,
reduced dissolved oxygen, increasing numbers of oil-degrading microbes). Diesel fuel
biodegradation was accompanied by increases in the concentrations of volatile organic
acids, calcium ion, pore water conductivity, total dissolved solids, surfactant production,
and diesel fuel emulsification. These results are complemented by the numerical
modeling results, which also showed potential enhancements in aqueous phase
conductivity of 3700 mS/cm when complete mineralization is assumed. However, the
more realistic equilibrium models predict enhancements in the 500-1500 mS/cm range.
From this study, two important observations are made that have significant ramifications
on the measured geoelectrical properties at aged LNAPL sites. First, microbial
degradation of LNAPLs produces a variety of acids that enhance chemical weathering of
the aquifer materials, resulting in high TDS content and thereby increasing the
conductivities of the pore waters. Second, emulsification of the LNAPL by surfactant
production has the potential to change the wetting phase from LNAPL wetted to water
wetted, providing electrically conductive paths within LNAPL saturated zones. Both of
these observations are consistent with our field investigations where we have reported
conductivity values 2-5 times background values from contaminated zones. Finally, our
field studies have also shown that the LNAPL saturated zone is conductive and not
resistive. Thus, the above laboratory experiment and numerical modeling results
demonstrate that LNAPL biodegradation readily explains the temporal changes in
conductivity observed in geophysical investigations of impacted aquifers.
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Dielectric Permittivity Of Clay-Water-Organic Compound: Frequency Dependence
By Birsen CananThrough non-invasive or other geophysical techniques, the ultimate goal of a geophysical survey
is the identification of subsurface materials and their spatial distributions. Reaching this goal will only
be possible through a complete understanding of the physical properties of different materials. A
detailed petrophysical work is essential to gain insight of what kind of changes can be observed through
geophysical techniques.
In this research, the effects of organic contaminants in a clay-water mixture were investigated via
dielectric permittivity under controlled laboratory conditions. For this investigation four organic
contaminants, trichloroethylene (TCE), tetrachloroethylene (PCE), ethylene glycol, and phenol, were
added to previously prepared samples of clay-water mixtures, and their dielectric permittivities were
measured. The dielectric permittivities of contaminated versus uncontaminated samples were compared
to determine if there was any pattern to help to identify a particular organic contaminant via dielectric
permittivity values of the mixtures.
Depending on the nature of the organic material, the changes observed are: 1) A decrease of the
magnitude of dielectric permittivity and shift of the relaxation frequency to lower frequencies for
ethylene glycol and phenol and, 2) An increase in the magnitude of dielectric permittivity and shift of
the relaxation toward higher frequencies for TCE and PCE contaminated samples between 100 KHz and
350 MHz frequencies.
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Laboratory Complex Resistivity Investigation Of Organic Contaminants: Denver Federal Center Soils
More LessThe Denver Federal Center is a DNAPL contamination site that lends itself to the use of complex
resistivity in monitoring the migration and concentrations of contamination. The physical properties of
DNAPLs often render monitoring-well data misleading by underestimating actual concentrations present
in the liquid and vapor phases. Smectite-rich soil at the site and chemical degradation of volatiles
through dechlorination, however, suggests the use of complex resistivity as a monitoring tool sensitive to
clay-organic reactions. To aid the interpretation of complex resistivity field data, this study involves
controlled contamination of site soils in the laboratory for comparison.
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The Role Of Advanced Monitoring In Steam Stripping For Insitu Remediation Of Dnapl
More LessThe use of advanced subsurface monitoring was an integral part of a 1999 pilot study of Steam
Stripping/Hydrous Pyrolysis Oxidation at a Trichloroethene (TCE) contaminated site at the Portsmouth
Gaseous Diffusion Plant in Ohio. The Pilot Study was conducted to determine costs, strategies, and
design parameters for the use of Steam Stripping to remove TCE from a moderately permeable sand/silt
aquifer (Gallia Member) and the upper two feet of the underlying, organic-rich Sunbury Shale.
Two complimentary systems were used to monitor temperature and steam movement. Point
measurements of temperature were made using an array of 314 thermocouples. Measurements between
boreholes were made using electrical resistivity tomography (ERT). Without ERT, even the relatively
large number of in-situ thermocouples gave a sparse and often misleading view of the temperature
distribution and steam front location within the aquifer.
Preferential flow paths exist within the aquifer. Previous studies on the same site had concluded
that “channels” existed within the aquifer and these preferential flow paths had adversely impacted the
remediation of the area. Although extensive characterization had been conducted on the site during this
study and previous studies the locations and extent of these areas were not well defined.
The use of ERT/thermocouples to provide high resolution, near-real-time monitoring of the
temperature increases due to steam injection, showed that portions of the aquifer were not being heated.
The areas not being heated appeared to correlate with depressions in bedrock elevation but the bedrock
topography is complex the correlation was not always obvious. Using monitoring results, areas not
heated during the few weeks were targeted for new injectors. Using this adaptive approach, the aquifer
was adequately heated and the bulk of the TCE removed from the study area.
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Nondestructive Monitoring Of Sub-Asphalt Water Content Using Surface Ground Penetrating Radar Techniques
Authors Katherine R. Grote, Susan S. Hubbard, John Harvey and Yoram RubinThe strength of sub-asphalt aggregate layers is crucial to understanding the relationship between
traffic loading and pavement damage. Because these strengths are greatly influenced by water content,
and because there are currently no techniques available for obtaining reliable, non-destructive subasphalt
water content estimates, pavement damage assessment is often inaccurate. Although Ground
Penetrating Radar (GPR) methods have the potential to provide high-resolution, non-destructive water
content estimates, the accuracy of this technique is largely unknown. Based on the successful results of
a pilot study conducted under ideal conditions to estimate water content using GPR techniques, we
applied this concept to estimate the sub-asphalt water content in two differently layered pavement
sections. Both pavement sections were subjected to infiltration over several weeks, and GPR data were
collected periodically throughout the infiltration period. By relating the changes in GPR response during
infiltration to the water content of the aggregate layers, we can observe the movement of water in the
sub-asphalt layers. Analysis of the GPR data indicates that the water content of the aggregate layers
varies with depth, lateral location, and time. These results suggest that GPR techniques may provide an
efficient and high-resolution method for obtaining non-invasive water content estimates.
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Ground Penetrating Radar Survey To Evaluate Roadway Collapse In Northern Ohio
Authors Ernest C. Hauser and Mark J. HowellA Ground Penetrating Radar survey was conducted to evaluate roadway collapse in a residential
neighborhood in northern Ohio. The primary targets of the survey were voids or void-forming
conditions in the underlying fine-grained soils. Boring logs indicated that the subsurface materials
consist of sandy and silty fill overlying natural soils. The thickness of the fill varied from 2 to 13.5 feet
(0.5-4 m) below the pavement surface. Natural soils included silty sand, sandy silt, sand and silty clay
containing variable amounts of rock fragments.
Three roads near the Lake Erie shoreline were selected for the investigation due to a history of
pavement and structural subsidence. Large concrete repairs to the roadway were observed in several
locations, one of which exhibited a large void as seen through a borehole that had lost part of its
patching material. This observed void measured 7 feet (2 m) deep and 3 to 4 feet (1 m) in minimum
extent.
GPR data were collected with a GSSI SIR-2® system. The 300 MHz antenna used gave less
resolution than an 800 MHz antenna also tested, but the 300 MHz penetrated to at least 15 feet (~4 m)
below the surface and was selected for the survey.
GPR data were collected along parallel profiles on each of the three roadways. The profiles were
located near the centerline (crown) of the road, and approximately 4 feet off of the unoccupied curb. The
known 7-foot deep void gave an anomalous, high-amplitude response in the GPR profiles; moreover,
other locations showed similar high-amplitude anomalies. Following the review of the main GPR
profiles, suspect areas were surveyed to determine the lateral extent of the radar anomalies. A good
spatial correlation of the mapped GPR anomalies and subsidence related features observed on the
surface suggest that significant voids likely exist and could pose a potential for roadway subsidence and
collapse. Subsequent borings are reported to have confirmed the presence of voids or loosely compacted
substrate at the anomalous areas mapped by GPR.
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Void Detection Underneath Rigid Pavements: Numerical Simulation Of The Impulse Response Method
Authors Nenad Gucunski and Hudson JacksonThe Impulse response (IR) technique is a seismic method used in evaluation and
condition monitoring of rigid pavements. It enables nondestructive evaluation of in situ
pavement material properties and detection of several defects within the pavement
structure. Voids or loss of support underneath rigid pavements can be detected due to
changes in the flexibility spectrum and damping characteristics of the pavement system.
Numerical simulation of the IR technique for void detection underneath concrete slabs in
rigid pavements is presented. The simulations were conducted using the finite element
method. Effects of several pavement parameters on the pavement response and the ability
to detect voids are presented and discussed.
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Development Of Ground Penetrating Radar For Railway Infrastructure Condition Detection
More LessRailway track engineers are focused on cost-effective track maintenance to support revenue train
traffic. Therefore, effective detection of railway track maintenance needs and planning of maintenance
activities are required to improve efficiency. One area requiring improvement to ensure safe train
operations and efficient maintenance is identification and diagnosis of track support problems. This
paper presents some of the research that was conducted to investigate the ability of ground penetrating
radar (GPR) data to accurately measure railway track condition in an effort to help guide railway
engineering and maintenance activities. The research effort included evaluation of data collection
techniques, data interpretation methods, and comparison of GPR data to available track condition
information.
The main indicators of poor track condition detectable using GPR are increased moisture or
changes in the ballast layer below the tie, both resulting in dielectric permittivity changes. Poor roadbed
ballast condition generally is due to fouling, which indicates ballast contamination, typically due to
particle abrasion, but potentially from other sources. Increased moisture is a strong indicator or poor
track condition since trapped water can dramatically accelerate the ballast fouling process and fouled
ballast tends to hold water in the track structure. The initial research indicated that GPR could be used
to identify poor track conditions such as water trapped in the track structure and fouled ballast due to
strong dielectric contrast between the tie and the supporting layers depending on ballast moisture and
degree of fouling. Additional testing demonstrated the ability to identify obstacles to mechanized
maintenance using GPR, although improvements are required to improve accuracy.
Further research on the application of GPR to railway track engineering is required to further
define the expected dielectric permittivity of common track materials and better target the transmitted
energy from the GPR antenna to ensure the strongest signal from the track substructure layers is
available for analysis.
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High Frequency Masw For Non-Destructive Testing Of Pavements—Accelerometer Approach
Authors Nils Rydén, Peter Ulriksen, Choon B. Park, Richard D. Miller, Jianhai Xia and Julian IvanovThe dispersive nature of surface waves in pavement systems is imaged through a multichannel approach using one accelerometer as receiver and multiple shot points. The image obtained from a wavefield transformation method shows multimodal dispersion curves up to 10 kHz.
We present results from a simplified MASW data acquisition method applied to a pavement
surface. The method can simulate an arbitrary number of channels. The sensor separation can be
chosen arbitrarily small. In these experiments, the upper frequency limit is 10 kHz, which can be
increased by the exchange of one sensor. The method is tested by one manual and one automated
procedure. Both rely on source-receiver reciprocity. The automated procedure is regarded as
necessary when a large number of channels is combined with a small sensor separation. The
manual method will not provide the necessary accuracy and endurance for that kind of
measurement, but is promising for less complicated setups. We present recommendations for
high frequency measurements on pavements.
In the subsequent data processing, we follow the procedure of multichannel analysis of
surface waves - MASW. It has recently been developed as a geophysical method for near-surface
investigation. We demonstrate that the MASW technique can identify detailed aspects of the
high frequency total wave-field of both surface and body-wave events.
Results of dispersion curve extraction indicate that higher modes of surface waves are
dominating at depths associated with the transition between the asphalt and the base layer.
However, the deviation from the fundamental mode is not large because all modes are
converging in an asymptotic manner with increasing frequency. The study indicates that the MASW method is a fast and cost efficient method for measuring pavement stiffness parameters.
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Seismic Investigation Of Pavements By Masw Method — Geophone Approach
Authors Choon B. Park, Julian Ivanov, Richard D. Miller, Jianghai Xia and Nils RydenA feasibility test of the multichannel approach to seismic investigation of a pavement system is
described. This test followed the procedure normally taken in the multichannel analysis of surface waves
(MASW) method by using geophones and a light (8-oz) hammer source. A wavefield transformation of
recorded multichannel data shows a strong fundamental-mode dispersion curve image in the frequency
range of 30-600 Hz with normal (30-50 Hz) and reverse (50-600 Hz) trends. However, the transformation
shows that this fundamental mode disappears quite abruptly and higher modes start to dominate in
the higher frequencies up to 2000 Hz. Simultaneous recording of both vertical and horizontal components
of seismic wavefields facilitates identification of seismic events. In order to record the horizontally
travelling direct (or possibly guided) P-wave event in the uppermost layer, it seems critical to use horizontal
phones with longitudinal orientation. Results of test indicate that for an investigation focused into
the uppermost layers of a pavement system it is essential to use a different acquisition system that can
deal with much higher (> 2000 Hz) frequencies. In addition, complicated and unique elastic properties
of pavement systems call for an inter-disciplinary study to develop an effective multichannel seismic
method for this area of application.
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Electrical Density Gauge Replacement For Nuclear Density Gauge
Authors Dennis M. Anderson, William J. Ehni and John LundstromThe paper will present the mathematics as well as the form and function of applying new
geophysical engineering equipment, the Electrical Density Gauge (EDG), for geotechnical
engineering application, such as building and highway foundation analysis.
In 1992 the Dennis M. Anderson and William J. Ehni, theorized that the relationship
between soil electrical resistivity and geotechnical engineering could be developed for applications
in soil engineering. The paper provides a systematic approach to investigating soil electrical
properties, setting geo-electric constants for a test specimen, and then using the observed
electrical properties of an undisturbed, in-situ soil specimen to calculate the in-situ density of the
soil that is under test.
The EDG's primary advantage over existing geophysical technology is that the equipment
does not use a nuclear source, but still offers a high level of efficiency for quality control/quality
assurance (QC/QA) work and testing related to soil in-situ density. EDG technology will eclipse
nuclear density gauge technology within three to eight years. The electrical density technology
represents the next generation in foundation analysis for civil construction.
The EDG equipment and electronics have been designed and built to meet the
geotechnical industry needs. Mr. John Lundstrom, Electrical Engineer is the primary electronic
designer and manufacturer of the EDG. The EDG uses solid state electronics on a printed circuit
board so that the instrument will be field durable for the construction industry. The electronics of
the first generation EDG is contained a durable high-density plastic case that is 14 inches by 10
inches by 7 inches. The EDG is powered by rechargeable batteries. It has a 386 micro-chipprocessor,
an alphanumeric keypad, and a liquid crystal display (LCD) read out. The EDG comes
equipped with a set of standard tools for setting the soil material electrical constant and a standard
probe for field investigations. The first generation commercial instrument will be capable of
measuring in-situ density and moisture. The EDG operator makes the field measurements. Then
internal math processors will perform the analytical analysis, and present the operator with the insitu
density data for the test site. The lightweight, compact size, durability, and non-nuclear
character of the EDG will allow the user to easily transport and use the instrument on
construction sites domestically and worldwide.
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Seismoelectric Investigations At Clean And Contaminated Sites
Authors Elizabeth Strachan and Paul J. WolfeWe have conducted studies of the seismoelectric effect at a site with a clean aquifer and a site
with fuel leaking from a storage tank farm. The main problem with applying the seismoelectric effect is
background electrical noise from power lines. At the clean site there was a shallow water table, about
2 m deep, and no man-made obstacles. We conducted detailed studies to determine the physical
characteristics of the site. We used this site to develop appropriate data acquisition techniques and the
resulting data to test data processing techniques. From the arrival time of the seismoelectric signal we
were able to determine the depth of the water table which was verified by drilling.
On the basis of these results we designed a survey to attempt to determine the extent of the fuel
layer on the water table at an inactive military fuel site along the Ohio River. Monitoring wells had
previously recorded a fuel layer up to 50cm thick. The water table in the contaminated zone is about
15m below the ground surface. We made seismoelectric measurements at 8 stations expected to cross
the fuel plume. We had a control site closer to the river where no contamination had been observed and
the water table was 9m deep.
The control site showed a clear seismoelectric response at a time corresponding to the water
table. The records from the contaminated zone did not show a clear response. We expected the stations
at the ends of the profile to be out of the fuel zone but there were no monitoring wells near our end
stations. Our conclusion is that fuel suppressed the seismoelectric response and that the extent of the fuel
plume was larger than expected. This conclusion is tentative but the results suggest the need for further study.
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