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12th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 14 Mar 1999 - 18 Mar 1999
- Location: Oakland, California, USA
- Published: 14 March 1999
1 - 100 of 108 results
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In Situ Stress Measurements Can Help Define Local Variations In Fracture Hydraulic Conductivity At Shallow Depth
Authors C.A. Baiton and D. MoosIn most fractured reservoirs the natural fractures and faults provide the primary pathways
for fluid flow. We have used comprehensive in situ stress, fracture and flow information from a
number of fractured reservoirs and found that critically stressed faults (the subset of preexisting
faults in a reservoir which are active in today’s stress field) systematically control
formation permeability (Barton, Zoback, and Moos, 1995). Thus, while we agree with the many
researchers who have suggested that the state of stress may influence fracture transmissivity, it
is the critically stressed faults (not mode I tensile fractures, as is generally thought) that are the
most permeable fracture planes in situ. We demonstrate how this new, predictable relationship
between in situ stress and permeability can be used to optimize production from fractured and
faulted geothermal reservoirs. In environmental and geotechnical applications, the
identification (and prediction) of permeable fracture systems along which contaminants or
groundwater can flow is a primary target in most site characterization studies. We demonstrate
how the technologies we have developed to analyze and characterize fracture systems and
determine the state of in situ stress can be directly applied to shallow fractured aquifers as well
as reservoirs at depth.
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Geotechnical Applications Of Vsp Surveys Using The Seismic Cone Penetrometer
Authors Kevin Jarvis, Rosemary Knight and John HowieWe present a number of new ways to use the seismic cone penetrometer (SCPTU). These
applications include the prediction of stratigraphic boundaries in advance of the cone as well as
the refinement of the first break shear wave velocities. These applications depend upon the
acquisition of vertical seismic profile (VSP) data using the cone. The VSP acquisition is
achieved using standard SCPTU accelerometers and is an extension of SCPTU shear wave
velocity surveys. The data consist of both an upgoing and downgoing seismic wavefield. These
wavefields are easily identifiable and can be separated with a series of simple processing
techniques. VSP data are most commonly associated with common depth point (CDP) reflection
seismic surveys. We have demonstrated how the data can also be used effectively to obtain
geotechnical information.
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Use Of Square Array Resistivity To Assess The Style Of Jointing On The Palisades Cliffs, Northern New Jersey
Authors Patricia M. Kenyon and Lyndon A. BrownThe square electrode array has been shown to be particularly sensitive to anisotropy in
the subsurface (Habberjam, 1972); however, little work has been published which uses it (Lane,
et al., 1995). We discuss a study which has used both the square array and the Wenner array for
determining joint patterns on the Palisades Cliffs, which overlook the Hudson River on its west
side between Hoboken, New Jersey, and Nyack, New York. These joint patterns are of interest,
because rockfalls from the cliffs are common and pose a significant hazard to a road which runs
along their base.
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Roadside Application Of Seismic Surface Waves Over Abandoned Mines
Authors B. Burcin Avar and Barbara A. LukeRoadway collapses have occurred along stretches of the Appalachian Highway in
southern Ohio due to ground subsidence associated with abandoned coal mine workings. The
authors joined a team led by faculty from Wright State University to investigate efficient
methods for detection of potential trouble areas before collapses occur. Seismic surface wave
measurements were made in both “sounding” and “profiling” modes. In the sounding mode, the
Spectral-Analysis-of Surface-Waves (SASW) method is employed to establish shear wave
velocity profiles at discrete locations. This approach is intended to delineate areas of reduced
overall stiffness, which might be caused by raveling of earth above mine workings, or, if not,
imply susceptibility for future instabilities. In the profiling mode, measurements are made at
constant offset and constant spacing in order to observe magnitudes and spectral content of body
wave interference. This approach is geared toward detection of discrete cavities or major fracture
zones caused by cavity collapse. Data were collected along two stretches of highway, one of
which has been drilled to establish ground truth. Results of the applications of sounding and
profiling methods for rapid assessment of sites with discrete subsurface anomalies are
encouraging. Additional analyses are planned to expand the capabilities of surface wave
applications through enhanced understanding of reflection and conversion phenomena caused by
subsurface anomalies.
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Self-Potential Methods To Investigate The Water Seepage Flow Through Earthfill Embankment Dams
Authors Kevin M. Payne and Robert F. CorwinSelf Potential (SP) methods are being utilized to detect changes in the seepage flows through
embankment dams. SP refers to the natural voltages generated by the earth, as opposed to those
which can be induced in the earth by applying an electric current. A streaming potential is the
measurable voltage potential that is caused by groundwater flow. An SP survey designed to
monitor groundwater flow must be carefully planned and conducted in order to isolate the
changes in streaming potentials from other sources of SP. By repeating an SP survey over an
embankment dam when the water reservoir elevation was at its seasonal minimum and
maximum, the changes in streaming potentials due to the different flow paths and volumes could
be identified. Further monitoring of SP levels could forewarn of deteriorating conditions within
the dam.
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Seismic Survey Across An Environment With Very High Poisson’S Ratio
Authors Michael Roth, Roman Spitzer and Frank NitscheDuring the course of a seismic survey across a region characterized by extremely high
Poisson’s ratios we observed an unusual phase. This phase arrives between the direct
P-wave and ground roll and has a significant amplitude at short offsets. Numerical
modelling indicates that it is a non-geometric shear wave generated by the interaction of
the explosive seismic source with the free surface. For the processing and interpretation
of very shallow seismic data, the presence of such non-geometric waves causes problems,
because the additional complexity in the seismic record may be interpreted in terms of
structural features in the subsurface. Another interesting phenomenon in high Poisson’s
ratio environments is the dramatic rotation of the P-wave polarization at the free surface.
This effect has to be taken into account when analysing multi-component recordings.
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Non-Destructive Testing Of Ground Strength Using The Sasw-Method
More LessThe shear modulus of soils and rock is an important parameter for estimating the mechanical
behaviour of these materials under many different types of loading. A relatively little used but
promising geophysical method for assessing variation in shear wave velocity and shear modulus
with depth is Spectral Analysis of Surface Waves (SASW). The result of an SASW-measurement
is a dispersion curve of Rayleigh wave velocity versus wavelength. A profile of shear wave
velocity and shear modulus with depth can then be derived from the dispersion curve using
simple approximate techniques or forward modelling. Contrary to seismic refraction methods,
SASW can be used to detect low-velocity layers.
Two case studies that show the capability of the SASW-technique for non-destructive evaluation
of ground strength are presented here. The first concerns a railway bridge in southern Sweden.
The SASW-method was used for investigating the ground strength under the bridge pillars
before constructing the bridge. The second case study concerns assessment of ground stability
improvement at a construction site outside Copenhagen. At the latter study it was found that
forward modelling was necessary in order to determine an accurate profile of shear modulus with
depth.
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Non-Intrusive Evaluation Of Submarine Tunnel Foundation Using Dynamic High-Frequency Surface Wave Prospecting
Authors Tieshuan Guo and Lanbo LiuThe dynamic surface wave prospecting technique was used in the non-intrusive evaluation
of the foundation quality of the Yongjiang River submarine tunnel. The prospecting results
show that the overall strength of the foundation is high enough to meet the designed
standard, however, there do exist low velocity heterogeneities. One possible reason for the
occurrence of low velocity zones is the disturbance of the river water and mud during the
process of mortar consolidation. The interface between the mortar layer and the
underlying gravel layer can not be well resolved. The results from the surface wave
technology have been confirmed by other methods such as the physical modeling
simulation and prospecting using body waves.
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Frequency-Wavenumber Analysis Of Passive Surface Waves
Authors Daren J. Zywicki and Glenn J. RixThe Spectral Analysis of Surface Waves (SASW) test is a non-invasive field technique which uses
the dispersion properties of surface waves to determine the shear wave velocity profile at a site.
Traditional SASW methods typically use an active source to generate surface (Rayleigh) waves
that are measured by a linear array of geophones. The depth of investigation is usually limited by
the inability of the source to produce low frequency, long wavelength surface waves. This
limitation can be overcome by using passive measurements of surface waves arising from
microtremors and/or cultural noise such as traffic. Surface wave dispersion relationships are
determined using frequency-wavenumber analyses. Ground vibrations are recorded by a twodimensional
array of sensors deployed on the ground surface. The frequency-wavenumber
spectrum is determined with Capon’s Minimum Variance Distortionless Look method, which
adapts the array’s sensor weights to the observed signal and noise characteristics. The sensor
weights produce a spatial filter that passes undistorted any monochromatic plane wave traveling
at a velocity corresponding to a selected wavenumber ka and suppresses waves traveling at
velocities corresponding to wavenumbers other than ko. Once the dispersion curve is determined
using frequency-wavenumber analysis, a non-linear, smoothed inversion algorithm is employed to
determine the shear wave velocity profile. The approach is described and illustrated using
measurements performed on the campus of the Georgia Institute of Technology.
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Surface-Wave Group-Velocity Tomography For Shallow Structures
Authors Leland Timothy Long and A.H. KocaogluIn order to identify shallow structures using fundamental-mode Rayleigh waves,
we use the multiple filter technique and measure group velocity across a study area.
First, we identify the mean velocity by stacking the power densities along constant
velocities for the complete set of filtered traces. Second, we identify peaks in the power
density consistent with the fundamental mode and eliminate those associated with higher
modes. Third, we perform a constrained inversion for optimal arrival times using
minimal deviations in travel time between stations corrected for differences in distance.
The resulting travel times are used in a tomographic inversion for group velocities in the
study area. The tomographic inversion is repeated for many frequencies giving a
dispersion curve for each point in the study area. Then, the vertical velocity structure at
any point in the study area can be interpreted from its dispersion curve. Velocity
anomalies of 10 percent are easily mapped by this technique. Rayleigh waves in the
frequency range of 2 to 50 Hz are sensitive to soil velocities at depths of 1 to 10 meters.
This depth range contains the near-surface structures that control dispersion of
underground contamination or which were introduced by the burial of wastes. The use of
tomography eliminates the need to make measurements directly above contamination.
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Configuration Of Near-Surface Shear-Wave Velocity By Inverting Surface Wave
Authors Jianghai Xia, Richard D. Miller and Choon B. ParkThe shear (S)-wave velocity of near-surface materials (such as soil, rocks, and
pavement) and its effect on seismic wave propagation are of fundamental interest in many
groundwater, engineering, and environmental studies. Ground roll is a Rayleigh-type
surface wave that travels along or near the surface of the ground. Rayleigh wave phase
velocity of a layered earth model is a function of frequency and four earth parameters: Swave
velocity, P-wave velocity, density, and thickness of layers. Analysis of the Jacobian
matrix in a high frequency range (5-30 Hz) provides a measure of sensitivity of dispersion
curves to earth model parameters. S-wave velocities are the dominant influence of
the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to Pwave
velocities and densities, estimations of near-surface S-wave velocities can be made
from high frequency Rayleigh wave for a layered earth model. An iterative technique
applied to a weighted equation proved very effective when using the Levenberg-
Marquardt method and singular value decomposition techniques. The convergence of the
weighted damping solution is guaranteed through selection of the damping factor of the
Levenberg-Marquardt method. Three real world examples are presented in this paper. The
first and second examples demonstrate the sensitivity of inverted S-wave velocities to
their initial values, the stability of the inversion procedure, and/or accuracy of the inverted
results. The third example illustrates the combination of a standard CDP (common
depth point) roll-along acquisition format with inverting surface waves one shot gather by
one shot gather to generate a cross section of S-wave velocity. The inverted S-wave
velocities are confirmed by borehole data.
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A Study On The Characterristics Of Geo-Technically Different Sites By Surface Wave Inversion Method
By Heeok JungThis study aims at the characterization of two geotechnically different sites by an inversion
analysis of surface wave dispersion. Site #l was a construction site where loose soil layer
overlays weathered gneiss zone. Site #2 was reclaimed land where dredged ocean sediments
is underlain by original ocean sediments. The phase velocities of Rayleigh waves were
obtained by FFT of the Rayleigh waves of the sites. An initial velocity model was assumed
from the phase velocities. The final shear wave velocity profiles as well as the spatial
resolution of the profiles were obtained using the generalized inverse method. The shear wave
‘velocity of site #l increases gradually to a depth of 10 meter while that of site #2 shows a low
velocity zone at a depth of 1 meter. Then, it increases rapidly to a depth of 3 meter, and stays
almost constant up to a depth of 10 meter. The spatial resolution of the shear wave profiles is
better in the shallower layers. This study indicates that the inverse method of surface wave
dispersion can be used effectively to characterize geotechnical sites with reasonable spatial
resolution; lm of averaging thickness in the upper 3m, 2 m averaging thickness in 3-6 meter
depth, 3-4 m averaging thickness below 6 meter depth.
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Multimodal Analysis Of High Frequency Surface Waves
Authors Choon Byong Park, Richard D. Miller and Jianghai XiaSurface waves on a multi-channel record are converted directly into images of multimode
dispersion curves through a simple wavefield transformation method. Pre-existing multichannel
processing methods require preparation of a shot gather with exceptionally large number
of traces that cover wide range of source-to-receiver offsets for a reliable separation of different
modes. The method described here constructs high-resolution images of dispersion curves with
relatively small number of traces. This method is best suited for near-surface engineering
project where surface coverage of a shot gather is often limited to near-source locations and
higher-mode surface waves can be often generated with significant amount of energy.
Performance of the method is illustrated through tests using both real and synthetic data.
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The Combination Of The Sasw Method And Dcresistivity In Characterization Of Old Landfills
Authors Mats Svensson, Christian Bernstone and Torleif DahlinTwo landfills were investigated aiming at finding out what advantages could come out of
using two geophysical methods providing each geometrical information or mechanical
information in combination. An old landfill previously characterized by 2D-resistivity
surveying was reinvestigated using surface wave seismics, SASW. Four points were chosen
for SASW tests located along the resistivity profiles. On a modern landfill an area used as
reference site, a specific cell with a volume of 40 x 40 x 9 m, a second survey was carried out
using true 3D-resistivity and SASW in five points. The objective of the project was mainly to
find out if a change in resistivity corresponded to a change in shear wave velocity. It was
concluded that the combination of the SASW method and resistivity surveying is an efficient
use of geophysical methods when the aim is to geotechnically characterize the subsurface.
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The Resistivity Piezocone Penetration Test (Rcptu) For Quality Control Of Geotechnical Ground Densification
More LessSoil densification by vibro-replacement is commonly prescribed to reduce the potential for large
strains and instability associated with pore pressure generation in granular soil during
earthquakes. Penetration tests such as the standard penetration test (SPT) and piezocone
penetration test (CPTU) are used to assess the need for such ground treatment and to monitor the
effect of the work. Contractors are required to increase the measured penetration resistance by
an amount specified by the consulting engineer. Bulk soil electrical resistivity data recorded
with a resistivity module situated behind a standard piezocone (called an RCPTU) provides an
essentially independent quality control index at little additional cost. This extra data may be very
useful in silty sands, which are often the cause of disputes between ground improvement
contractors and geotechnical engineering consultants. RCPTU data recorded before and after
vibro-replacement at two sites are analyzed in the empirical framework proposed by Archie in
1942, taking into account measured or assumed changes in pore fluid resistivity and anticipated
lateral variability based on experience at a similar research site. The results suggest that changes
in resistivity due to densification work can be measured using a resistivity module and are
reasonably repeatable.
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Geophysical Investigation Of An Abandoned Landfill
Authors Andrew P. de Wet and Rob SternbergWe have investigated the structure of an abandoned landfill in Lancaster, PA, with
near-surface geophysical techniques, including magnetics, conductivity and magnetic
susceptibility. Our previous study used a wide range of data, including field observations,
historical aerial photographs and magnetic and conductivity measurements along 10
transects across the site to delineate the general boundaries of two landfills (de Wet et al., in
press). That low resolution data suggested that one of the landfills was heterogeneous. In
this study we investigated the compositional variations in this landfill by 1) collecting
higher resolution magnetic and susceptibility data along several of the transects, and 2)
completing several high resolution grids over selected areas of the landfill.
The total field magnetic intensity data and the magnetic susceptibility data showed
large anomalies over the landfill compared to background areas and clearly defined the
boundaries of the landfill. The magnetic susceptibility data for the landfill (collected using a
Bartington MS2D instrument) indicated numerous anomalies in the upper 10 to 15 cm of
the landfill. Specific magnetic susceptibility anomalies were unrelated to the large magnetic
intensity anomalies measured in this study, or the anomalies in the quadrature or in-phase
conductivity data collected in the earlier study. Data collected at different sampling intervals
along widely spaced transects were successful in delineating the landfill and identifying the
general heterogeneity of the landfill itself. Detailed grid maps provided additional
information about the subsurface that was not evident from the transect data. Sampling
scale is an important consideration in geophysical investigations of this type. High
resolution grid maps showing a variety of geophysical results provide maximum
information about subsurface conditions.
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A Sea-Bottom Seismic Energy Source For Shallow Water Engineering Applications
Authors R.L. Good, R.A. Bums and J.A. HunterA sea-bottom seismic source, patterned after the so-called “Buffalo Gun” land source, has been
developed and tested for shallow water (river, lake, sea) applications. The sea-bottom source is
designed to be lowered on a tether, and to penetrate into soft bottom sediments to a selectable depth
(O-O.5 m). The device uses various strengths of water-proofed 8-gauge blank loads, and is fired
remotely from the surface. Zero time is measured by a transducer electrically connected to the
surface vessel. The source is relatively lightweight (14.4 kg) and can be operated from a small boat
or through ice. It uses environmentally friendly blank loads detonated within the bottom sediments
(i.e. not in direct hydraulic contact with fish). The loads are a fraction of the cost of seismocaps and
do not have the same handling/shipping restrictions.
Tests have been conducted in both fresh and salt water to water depths of 30 m. Comparative tests
with a seismocap in contact with the water bottom, fired beneath a 10 kg steel plate, showed that the
sea-bottom source produces significantly more energy, particularly in the low frequency range (30-
150 Hz). Based on these results, this device has potential applications in a variety of problems in
exploration, engineering, and environmental seismology in shallow water areas, such as:
i) seismic refraction surveys beneath lakes and rivers using water-bottom or surface hydrophone
arrays (mapping bedrock surface, velocity variations);
ii) seismic reflection surveys using water-bottom geophones (or hydrophones) to map gelogical
structure of surficial sediments/bedrock for through-ice applications;
iii) water-bottom Spectral Analysis of Surface Waves (SASW), utilizing the Scholte wave, whereby
the sub-bottom, shear-wave, velocity-depth profile (hence dynamic shear moduli) can be determined.
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A Step Frequency Georadar Module For An Environmental Cone Penetrometer
Authors Fan-Nian Kong, James M. Strout and John H. LoevholtNGI has developed a module for the cone penetration test implementing the NGI step
frequency radar system. The instrument will allow nearly continuous measurements of the
dielectric constant and the bulk soil resisitivity throughout a vertical soil profile. The module
will be used in environmental investigations, where contaminants may alter these parameters
from the natural value in uncontaminated soils. This paper presents the technical details of the
design and some preliminary laboratory test results.
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Geophysics Comes Of Age In Oil Sands Development
Authors Paul Bauman, Richard Kellett, Eric Gilson, Russ Pagulayan and Anil SharmaMost energy forecasters predict that the development of shallow heavy oil reserves in Canada’s
enormous oil sand deposits will play a vital role in bridging the gap between North America’s
reliance on conventional oil in this century, and the full integration of alternative energy supplies
in the next century. Over the last year, a number of surface geophysical techniques have been
successfully applied to oil sands exploration and development. These innovations have come
after over fifteen years of near stagnation in the area of innovative applications of geophysics to
the oil sands. These applications include the direct exploration and detection of oil sands, the
calculation of bitumen saturation from surface, the exploration for water supplies beneath the oil
sands, geological mapping, mapping and imaging of thick clays and shales for geotechnical
purposes, and non-intrusive monitoring of leachate plumes. Geophysical techniques successfully
applied to these problems include 2-D electrical resistivity imaging, transient EM, ground
penetrating radar, and high resolution seismic reflection. Other techniques which will probably
be used in the very near future include induced polarization, surface nuclear magnetic resonance,
and various push-probe sensing techniques. This paper will review present applications of the
above techniques in the surface mineable ore reserves of the Athabasca deposit.
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Use Of Engineering Geophysics In The Design Of Highway Passing Lanes
By P. MichaelsThe addition of passing lanes has been a long standing need on Idaho Route 55. This
narrow, winding road is one of the few routes that links Boise, Idaho, with destinations to the
north. The roadway is bounded by a steep rising slope on the east, and a precipitous drop to the
Payette River on the west. Additionally, the area is known for landslide hazards. Given these
hazards and uncertainties in the subsurface geology, Idaho Transportation Department (ITD)
commissioned an engineering geophysical survey to aid in the design of the passing lanes just
south of Banks, Idaho.
The engineering design required that several issues be addressed. These included the
following:
1. Slope Stability
2. Amount of blasting required.
3. Need for retaining walls.
4. Volumes of soil and rock to be excavated.
To help design engineers address these questions, P-wave refraction mapping was used to
produce cross-sections perpendicular to the roadway. A novel approach was needed, since traditional
in-line shooting would have required offsets greater than were available. The solution was
to perform a 3-D, ground consistent, delay-time inversion. Soil and rock properties, and variations
in the thickness of overburden were determined. These results were found to be in
agreement with track-hoe trenching done at selected locations along the roadway.
The geophysical work revealed that the site differed significantly from assumptions used
in the initial design stages of the project. The potential for landslides was found to be significant.
Given the updated geologic picture and projected costs, ITD decided to abandon this site for the
passing lanes. The project was terminated, thus avoiding excessive costs that would have been _
experienced due to the differing site conditions.
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Integrated Geophysical Investigation Helps Improve A Landfill Closure Design
More LessThis paper describes the geophysical investigation conducted at the Fort Hunter Liggett
(FIX) Landfill in Monterey County, California. The geophysical data were obtained using
a custom survey system that facilitated the simultaneous acquisition of ground penetrating
radar (GPR), electromagnetic (EM), and magnetic (MAG) data. This paper also describes
how the investigation results helped to improve a landfill closure design.
The preliminary closure design concept for the landfill incorporated a standard regulatory
cover constructed over 54 acres - the estimated landfill extent as determined from aerial
photographs. The preliminary design was problematic for a number of reasons, including
its high cost and its impact on plant species protected by county ordinance.
To mitigate these drawbacks, an alternative design concept was developed. Included in
the design work was a geophysical investigation to verify the overall extent of the landfill
and accurately delineate individual refuse trenches within the landfill. GPR EM and MAG
data were obtained simultaneously by towing geophysical sensors from a custom-built
non-metallic trailer.
The investigation determined that the landfill encompassed approximately 36 acres, as
opposed to the 54 acres estimated from the aerial photographs. Eleven large refuse
trenches were delineated, as were several small outlying pockets of debris, including a
cache of buried 55-gallon drums. As a result of the geophysical investigation, the area to
receive cover was significantly decreased, thereby reducing the amount of cover material
required (hence the overall cost), and sparing many of the protected native trees. Potential
on-site borrow sources were also identified.
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Routine Application Of Radar In Underground Mining Applications
Authors Hylton White, Alten du Plessis, Keith Noble and Mike TreloaR & D into in-mine geophysics was initiated by the South African mining industry in the 1980’s when
a need was identified for techniques to routinely delineate geological structures from underground
excavations. Since then ground penetrating radar (GPR) has been established as a routine tool in
underground mining in South Africa for geological structure delineation and roof integrity studies.
Routine underground application of GPR requires reliable ruggedized instrumentation and innovative
survey methodologies. To this end the RockRadar system was developed, which provides excellent
data quality in the harsh confined conditions encountered underground. The system allows for rapid
data acquisition and in-mine data processing and interpretation. GPR is best used as a complementary
tool to in-mine exploration drilling, enabling more complete characterization of sites. GPR is seldom
used as a “blind’ technique where little information is available about the geology ahead of mining.
This strategy has enabled the successful routine underground use of radar while other in-mine
techniques such as radio tomography and in-mine seismics have as yet developed limited routine
implementation. Applications in metamorphic, igneous and sedimentary environments have been
developed. These include ore-body delineation, mapping of faults and intrusives, and delineation of
roof discontinuities. Data will be presented from the gold mines (delineation of ore-body and
faulting), platinum mines (delineation of roof discontinuities and ore-body disruptions), and basemetal
mines (delineation of roof discontinuities).
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Land Streamer: A New System For Highresolution S-Wave Shallow Reflection Surveys
More LessA new field system named “Land Streamer” has been developed and successfully
applied for high-resolution S-wave shallow reflection surveys at the paved areas. The land
streamer is a multi-channel receiver system with a CDP cable and a pair of non-stretchable
textile belts. A total of 48 geophone units are clamped on each belt with 50 cm spacing.
The streamer is 30 m in total length with 23.5 m long active section. It is easily towed by a
vehicle that carries the seismograph or by a few crew members. The system provides us
considerably clean records although each geophone unit of the streamer is indirectly
coupled with paved ground surface through a small baseplate of aluminum 8 cm wide and
11 cm long. Because of its easiness to move the spread, we can accomplish a high
performance of the field work.
A field survey carried out on the sidewalk along a heavy traffic roadway reveals the tool
is quite hardy to the traffic noise. We successfully obtained high-resolution profiles for the
near surface to about 60 m in depth using the land streamer.
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Physical Ultrasonic Modeling Of Rayleigh Wave Field In The Near Vicinity Of An Anisotropic Wave Barrier.
Authors Elizabeth Its, Alexei Anisimov and Vitalii KashkevichSince composite wave barriers were suggested by Biagi et al. (1990), Leung et al. (199(l), Its
and Lee (1993a) to increase the screening efficiency for protection of buildings from groundtransmitted
vibration, several theoretical and experimental studies were conducted to analyze
scattering of surface waves at various models of the barriers. In particular, 3D physical ultrasonic
experiments were carried out (Its, Anisimov, and Lee; 1998) to study Rayleigh wave
scattering at a narrow vertical anisotropic slab embedded between two isotropic blocks. These
experiments were designed to analyze the surface wave in the far-field zone. The experimental
scattering coefficients were compared with the theoretical results obtained by the Green’s
function method (Its and Lee; 1993b, 1994). In this paper we report the results of ultrasonic
experiments which were designed for the near-field zone. The same model of the anisotropic
barrier as in (Its, Anisimov, and Lee; 1998) was used, but the profiles were shot in a wavelength
vicinity as well as inside the slab.
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A Shallow Marine Seismic Reflection Survey In Suisun Bay
Authors Zhuping Liu, James W. Rector and Patrick Williams23 shallow marine seismic reflection profiles were collected in Suisun Bay, CA, to help
characterize the shallow faults associated with the earthquakes occurred in California during the
past 40 years. We analyzed the noises in these shallow marine seismic reflection data, and applied a
series of traditional seismic data processing techniques to improve the S/N ratio. Due to the low
fold of the reflection data, we constructed analogue single-channel marine reflection profiles,
sorted them to CMP domain and stacked the data, which proves to be an effective way in showing
shallow structures.
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Seismic Refraction Case Studies At Environmental Sites
Authors Craig Murray, Dean Keiswetter and Edward RostoskySeismic refraction was used to investigate two sites with groundwater contamination.
Although seismic refraction is not a viable technique for directly locating the
contaminants, it can provide very useful geologic information. The ground water at both
of these sites is contaminated with dense, non-aqueous phase liquids (DNAPL) that sink
through the groundwater until reaching an aquitard, a geologic layer that is relatively
impermeable. Seismic refraction was used on both of these sites to map the surface of
the aquitard and locate low areas in which DNAPLs might accumulate. Tricloroethene
contamination is present in the groundwater of a military installation located in a karst,
limestone setting. Fractures and solution cavities in the limestone dominate the
hydrogeology. The surface of the limestone is quite uneven as a result of dissolution and
glacial action. We collected detailed seismic refraction data in the region surrounding the
source to map the elevation of the limestone and locate low areas where the DNAPLs
might collect. The velocities of the limestone were also useful for indicating where the
most intense dissolution had created fractures and cavities. A mixture of several
DNAPLs contaminated the groundwater at the second site. There are no consolidated
sediments within 100 feet of the surface, but a very dense unconsolidated aquitard
separates the contaminated sand and gravel aquifer and the lower, clean aquifer used for
groundwater supply. Several techniques, ground probing radar, resistivity and seismic
refraction were used on this site, but seismic refraction was the most effective at locating
low spots in the clay layer. The seismic refraction data also revealed several areas with
very coarse-grained gravel that could serve as pathways for contamination to leave the
site.
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Recovering Of Scattering Objects By Means Of Focusing Transformation Of Seismic Multioffset Data
More LessFocusing transformations of multi-offset data is a powerful tool to study complex
media (trapp intrusion, complicated upper part of a cross-section, thin interior layers
and so on). Its main advantage is a possibility to perform focusing simultaneously
with respect to sources and geophones and on this base to strength spatial structures
with specific disposition. But the choice of their optimal parameters (number of
sources and geophones, distance between centers of their bases, a degree of nonsymmetry
and so on) for a specific geological situation is rather bulky procedure.
The method to perform this choice by means of synthetic data gained with the
help of finite difference simulation of wave propagation for realistic models is proposed
and tested. The necessary condition to provide efficiency of the method is to
take into account all specific features of the specific process of wave propagation.
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High-Resolution, Multichannel, Marine Seismic Surveying Using A Small Airgun Source
Authors A. Pugin, S.E. Pullan, R.A. Burns, M. Douma and R.L. GoodA high-resolution, multichannel, CMP, marine seismic reflection survey was carried out in Lake
Simcoe, southern Ontario, Canada, in June 1998, to examine large channel features within the glacial
sediments beneath the lake bottom. Over 300 line-kilometers were obtained at close line spacings
with an oil-filled hydrophone receiver array and a 1 cu. in. airgun fired accurately at 5 meter
horizontal intervals. Firing on distance was accomplished by integrating a real-time DGPS
(Differential Global Positioning System) and a computer-driven triggering system which adjusts the
firing rate depending on the ship’s velocity. The survey was conducted with an average boat speed
of approximately 4 knots (7 km/hour), and multichannel records were recorded on a Geometrics R-
24 seismograph at 2.2-3 second intervals.
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Tests Of Shallow-Penetrating Em Systems Over An Underground Bunker
Authors David L. Campbell and J. Christopher EckhartWe compared three shallow-penetrating EM systems by making surveys along identical
grids over a 1940s-era, abandoned, buried bunker (?) at the Denver Federal Center. These
systems were the Geophex GEM-2, GSSI GEM-300, and Geonics EM3 1. All three systems
detected certain objects, including likely foundations for large tanks, possible underground
openings, and present-day electric light poles. A feature of particular interest was a long, narrow
anomaly that we think may reflect a buried cement wall. This wall shows up on the quadrature
(or quadrature-phase, q-p) component as a relative high at low frequencies and as a relative low
at high frequencies. At intermediate frequencies it was invisible on the q-p component. In
particular, this includes 9.8 kHz, the operating frequency of the EM3 1. Although the EM3 1 did
pick up indications of the wall on its in-phase (i-p) component, the strong signatures of the many
bunker features drove its i-p values off scale over much of the survey area, so that they were
relatively less useful than the i-p maps from the other two systems. An advantage of the GEM-2
and GEM-300 instruments, therefore, is that they can be used to sweep a range of frequencies
and more clearly detect features such as the (cement block without rebar?) bunker wall. A
disadvantage, however, is that neither GEM-2 nor GEM-300 appear to be well calibrated.
Conductivity values from interpretations of their multifrequency data sets, made using a standard
EM modeling program, seemed unreasonable. We conclude that the programmable
multifrequency systems, such as GEM-2 and GEM-300, have an advantage for reconnaissance of
certain archaeological sites, but that better-calibrated systems like the EM3 1 should be used
where it is important to know true conductivity values, as in studies of geology, water resources,
or ore deposits.
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Gravity In Cave Exploration
Authors Mary Krasovec and Frank Dale MorganA new method for the inversion of gravity data is introduced which is applicable to
regions with a limited number of allowable density values, such as uniform density bodies
set in a contrasting background. The method relies on geometrical constraints, such as
compactness, size, or shape of the anomalous body or bodies. A gridded forward model
is used to separate the geometry of the model from the density by dividing the earth into
regular prisms. Each prism is allowed only a limited number of density values: in this
case the density can be the background rock density or the cave density of 0. The prisms
are chosen to increase in size with depth in order to realistically model structures which
can be resolved with gravity data.
In this application, a genetic algorithm is used to find the best configuration of zero
density prisms to fit the data and is subject to geometrical constraints as mentioned
above. The method is described and tested on 2D synthetic models, then applied to
gravity data collected at the Indian Echo Caverns in Hershey, Pennsylvania.
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Detection Of Near-Surface Voids Using Surface Wave
Authors Choon Byong Park, Richard D. Miller and Jianghai XiaGround roll is displayed, on an uncorrelated field record obtained using a monotonic
sweep, in increasing or decreasing order of frequency with each frequency well separated from
all others. Phase velocity and attenuation characteristics of each frequency contain the average
elastic property of near-surface materials down to approximately half the wavelength.
Uncorrelated field record, therefore, by itself can be associated with a two-dimensional display
of the change in near-surface elastic property. Through the redundancy in data acquisition and
a simple data processing step, the uncorrelated field records can be transformed into a stacked
section that can be correlated directly to image of the change in elastic property of near-surface
materials with respect to a certain reference location. This method can be effectively used to
detect near-surface anomalies of various kinds.
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Cave Detection Using Gpr
Authors Philip M. Reppert, Yervant Vichabian, John Sogade and Frank Dale MorganThis presentation demonstrates a new method for the detection of caves. The method is
based on using instantaneous frequency and instantaneous amplitude. The results
indicate that the new methodology can accurately detect caves at great depth and with
varying geometry.
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A Case Study Of The Use Of Multi-Electrode Earth Resistivity In Thinly Mantled Karst
Authors M.J.S. Roth, J.R. Mackey and J.E. NyquistSinkhole formation in many areas of the northeast United States occurs in clay soils over
relatively shallow (methods of site investigation have a low probability of encountering subsurface karst features
that are associated with future sinkhole formation. Non-intrusive methods such as ground
penetrating radar and electromagnetic methods have only limited applicability, and there are
difficulties associated with their interpretation. This paper details a case study of the use of earth
resistivity equipment with multiple electrodes to investigate a site in eastern Pennsylvania. Site
geology, selection of resistivity line orientation, data interpretation, and comparison of results
with intrusive borings are discussed. Recommendations for site investigations in thinly mantled
karst are given.
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Resistivity In Cave Exploration
Authors Frank Dale Morgan, Weiqun Shi, Yervant Vichabian, John Sogade and William RodiSimple d.c. resistivity and profiling have been used in the past for cave mapping. One can
also attempt to obtain an inversion tomogram using a pseudo-section type gatherer. In
this presentation we will demonstrate that the pseudo-section method can be inadequate
and leads to very poor imaging accuracy, unless the desired target is in the center region
of the survey.
A new, more comprehensive method of obtaining the data will be presented. The method
is tested using examples from Harrison’s Caves, Barbados, West Indies and Indian Echo
Caverns, Pennsylvania. The technique can also be applied to accurate imaging of tunnels
and underground structures.
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Electromagnetic Cave-To-Surface Mapping System
Authors John Sogade, Yervant Vichabian, Amy Vandiver, Philip Reppert and Frank Dale MorganThe principle, design, and construction of an instrument for remote location of position
and depth of a transmitter in a cave will be presented. Two people can use the system to
map a cave location on the ground surface. The instrument consists of an EM dipole
transmitter/receiver configuration. The vertical component of the transmitted field reaches
a maximum at the point where the transmitter and receiver are vertically collinear. At this
position the horizontal component of the field reaches a minimum. A survey procedure is
developed and tested to efficiently locate this position.
Results of calibration work done at MIT and a field test of the system at Indian Echo
Caverns, in Hershey, Pennsylvania are presented. We were able to verify theoretical
results with the calibrations. The system was subsequently able to successfully map the
cave system at Indian Echo Caverns.
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Induced Polarization In The Detection Of Cave Systems
Authors John Sogade, Yervant Vichabian and Frank Dale Morgan2D Induced Polarization (IP) surveys were carried out across a protile extending over the
North Canyon of the Indian Echo Caverns in Hershey, Pennsylvania . In this
presentation, we show the resulting images from inverted IP data. We expect zero
chargeabilities in the areas which correspond to the cavity/void (air) and relatively higher
chargeabilities in the areas which correspond to the surrounding water saturated rocks.
The resulting contrast provides the mechanism for detection of cave systems. The data
was inverted using a robust 2D IP modeling/inversion code.
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Self Potentials In Cave Detection
Authors Yervant Vichabian and Frank Dale MorganIn this presentation, we show an example of the use of self potential geophysics to locate
caves or voids. We develop a simple model of water flow where a cavity is a preferential
drainage path generating streaming potentials above the local groundwater flow. Two
temporal data sets with the same positions were collected at the Indian Echo Caverns
located in Hershey, Pennsylvania. One was in dry conditions and the second, one year
later, with the ground saturated with rainfall. The data was gathered using the gradient
method and later processed by integration. The data set in the dry conditions shows a
single anomaly located over a known cave location and the data set in the wet conditions
has several anomalies. Both sets show groundwater flow feeding into a nearby river.
The differences of the two data sets are due to the change in resistivity of the soil caused
by rainwater and the enhancement of very near surface infiltration paths of rainwater.
We show that SP can be used for cave detection under the proper conditions.
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Defining The Bedrock/Overburden Boundary In Covered Karst Terranes Using Dipole-Dipole Electrical Resistivity Tomography
Authors Wanfang Zhou, Barry F. Beck and J. Brad StephensonSinkhole collapse is one of the main limitations on the development of karst areas, especially
where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown
thatsinkholes are likely to develop in cutter zones as a result of subterranean erosion by flowing
groundwater. Because of the irregular distribution of pinnacles and cutters on the bedrock
surface, uncertainties arise when “hit-or-miss” borehole drilling is used to locate potential
collapse sites. A high-resolution geophysical technique capable of depicting the details of the
bedrock surface is essential for guiding the drilling program.
Dipole-dipole electrical resistivity tomography (ERT) was used to map the bedrock surface at a
site in southern Indiana where limestone is covered by about 9 m (meters) of clayey soils.
Forty-nine transects were conducted over an area of approximately 42,037 m2. The electrode
spacing was 3 m. The length of the transects varied from 81 to 249 m. The tomographs were
interpreted with the aid of soil borings. The repeatability of ERT was evaluated by comparing the
rock surface elevations interpreted from pairs of transects where they crossed each other. The
average difference was 2.4 m, with a maximum of 10 m.
The discrepancy between interpreted bedrock-surface elevations for a transect intersection may
be caused by variations in the subsurface geology normal to the transect. Averaging the elevation
data interpreted from different transects improved the ERT results. A bedrock surface map was
generated using only the averaged elevation data at the transect junctions. The accuracy of the
map was further evaluated using data from four exploratory boreholes. The average difference
between interpreted and actual bedrock surface elevations was less than 0.4 m. The map shows
two large troughs in the limestone surface: one coinciding with an existing sinkhole basin, while
the other is in alignment with a small topographic valley. Because sinkholes were observed at
the same elevation interval in similar valleys in the vicinity, the delineated trough may have
implications for future land use at the site.
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Electrical Resistance Tomography Of Unsaturated Flow And Transport In Yucca Mountain
Authors Michael Buettner, William Daily, Abe Ramirez and Gilles BussodElectrical Resistance Tomography (ERT), a new geophysical imaging technique, was used to study
the movement of a tracer through the test block at the Unsaturated Zone Transport Test (UZTT) at
Busted Butte, Nevada. Data were collected four times starting in July and ending in early
September, 1998. ERT baseline images show a resistivity structure which is consistent with the
known lithology in the rear part of the test block. There appears to be a low resistivity region in
the front half of the block, particularly near the bottom. Difference images from August 19 and
September 9 show clear and consistent resistivity decreases in the region near injection holes 18,
20, and 21 which can be associated with the injection of conductive water. The images show very
little effect in the region around the other injection holes, 23, and 24 through 27 where far less
water was injected. Difference images from August 19 and September 9 show resistivity decreases
which could be interpreted as water moving down into the block. This is the same region which
has an anomalously low resistivity in the baseline image. These results should be considered
preliminary, and are subject to further interpretation.
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Variability Of Electrical Resistivity At The Rice Creek Field Station, Oswego, New York: Implications For The Distribution Of Groundwater
Authors Samuel T. Peavy and David W. ValentinoElectrical resistivity measurements were made to determine the variability of surficial
deposits, the depth to bedrock and to characterize the distribution of groundwater at the Rice
Creek Field Station near Oswego, New York. The field station is underlain by drumlin deposits
and ablation till associated with Pleistocene glaciation. These deposits reside on Ordovician
quartz sandstone of the Oswego Formation that outcrops within 1500 m of the study site.
Locally the Oswego Formation contains subvertical fractures with an average spacing of less than
0.5 m. Twenty offset Wenner electrical resistivity surveys were conducted in June and August of
1998 along trails and across an open field within the field station grounds. Analysis of
pseudosections and simple 1-D modeling and 2-D least squares inversion indicate the following:
1) low resistivity zones associated with perched water tables within the chaotic drumlin deposits;
2) highly variable and resistive near-surface measurements along Rice Creek indicative of large
(>l m diameter) glacial erratics as observed in the creek bed; 3) a transitional zone below -250 ft
elevation of subcircular highs separated by relatively low resistivities that continue into the
deepest portions of the data, which is coincident with the projected depth to bedrock beneath the
field station and is interpreted to be an undersaturated zone within the fractured Oswego
Sandstone; and 4) low resistivities below an elevation of - 190 ft are interpreted to be the top of
the saturated domain within the fractured bedrock.
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Integrated Use Of Continuous Seismic-Reflection Profiling And Ground-Penetrating Radar Methods At John’S Pond, Cape Cod, Massachusetts
Authors C.J. Powers, F.P. Haeni and Spence SmithContinuous seismic-reflection profiling (CSP) and ground-penetrating radar (GPR) surveys
were conducted by the U.S. Geological Survey in April 1998 over the northern part of
John’s Pond, a glacial kettle pond southeast of Otis Air National Guard Base, Cape Cod, Massachusetts.
The surveys were conducted to delineate the types and thickness of sedimentary
units that may control the infiltration of contaminated groundwater into John’s Pond.
Sand-and-gravel deposits, collapse features and recent organic sediments were imaged
with the CSP and GPR methods. Hummocky to chaotic reflections were interpreted as sandand-
gravel deposits. Slightly wavy, parallel reflections located in depressions in the sand-andgravel
deposits were interpreted as filled collapse features. Lower amplitude, horizontal, laminar
reflections were interpreted as organic sediments. Entrapped methane gas within some of
the organic sediments created a reflection zone that obscured deeper reflections in the CSP
records.
The CSP and GPR methods provide complementary information over most of the surveyed
part of the pond. The methods detect similar interfaces, but a particular interface may
produce a stronger reflection in one record than in the other. For example, regions of the pond
containing organic sediments with entrapped methane gas, which prevent penetration of the
acoustic signal, were penetrated and imaged by GPR. Conversely, regions of the pond containing
electrically conductive sediments or deep water, which attenuate the GPR signal, were imaged
using CSP. The CSP and GPR data were interpreted to generate a bathymetric map and a
map of sediment type and thickness beneath John’s Pond.
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Temporal Saltwateref’F’Ects On A Porous Sand Derived From Geophysical Monitoring Through A Tidal Cycle At Crescent Beach State Park, Maine
Authors S.K. Sandberg and L.D. SlaterThis paper presents results of an experiment in which data from a combination of geophysical
methods (resistivity, induced polarization, electromagnetic profiling, and magnetic profiling)
were used to identify shallow geologic and hydrogeologic structure, and to provide a model for
groundwater flow in a dynamic beach environment. Based on this study, the beach appears to be
underlain by a thin beach sand, overlying bedrock. Within the overlying sediments, a patch of
glacial till was mapped toward the ocean side of the beach. Bedrock has lithologic variability, as
evidenced by a non-uniform magnetic response. In particular, a northeast-southwest trending
magnetic body, believed to be a basalt dyke, was detected by its magnetic response.
During the transgressive phase of the tidal cycle, saline groundwater flow appeared to be quite
rapid, extending ahead of the surfical water by at least 4 m, as evidenced by the electromagnetic
response. During the regressive phase, a slug of saline water remained up beach from the high
tide mark for several hours prior to draining. Also during this regression, we observed
electromagnetic evidence for two fracture zones, or other preferential groundwater flow
pathways. These fracture zones appear to contain a relatively constant concentration of saltwater,
while surficial sediments above and to the side are flushed with fresh groundwater during tidal
regression.
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Time-Domain Electromagnetic Techniques To Study South Florida Aquifers
Authors Paul J. Wolfe, Benjamin H. Richard, Erin E. Sesslar and Steven EdlavitchTime-domain electromagnetic soundings were acquired at 38 locations along profiles in
the Big Cypress National Preserve in southwest Florida to interpolate the thickness of the fresh
water aquifer between widely spaced boreholes. A Geonics EM-47 system with a 160m
transmitter wire was used for data acquisition. Since larger loops provide larger dipole moments
we used the largest practical configuration. A 20m by 20m loop was used at most sites, although
at a few sites we were restricted to a 12m by 28m loop. To determine the consistency of the data
obtained with the various loop sizes, we performed soundings using different loop configurations
at several locations. Soundings were taken with loop sizes ranging from 40m to 5m squares.
Inversion of the sounding data gave similar models for the various configurations. As
anticipated the larger loops gave better deep data. Another concern was the effect of overhead
power lines in some of the sounding areas. To establish the effects of the wires a set of
walkaway soundings were conducted. These showed no measurable effect due to the wires. The
interpreted data indicate that the subsurface is generally well represented by a two or three-layer
model of saturated fresh water over brackish water carbonates. Borehole information indicates
that a low permeability zone acts as an aquitard between the two zones. The results found a
fairly constant depth to the interface along a 24km north-south transect and a 15km east-west
transect. Two areas showed anomalous depths that may indicate leakage through the aquitard.
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Boise Hydrogeophysical Research Site (Bhrs): Objectives, Design, Initial Geostatistical Results
Authors Warren Barrash, Tom Clemo and Michael D. KnollThe Boise Hydrogeophysical Research Site (BHRS) is a wellfield developed in a shallow,
coarse (cobble-and-sand), alluvial aquifer with the goal of developing cost-effective methods for
quantitatively characterizing the distribution of permeability in heterogeneous aquifers using
hydrologic and geophysical techniques. Responses to surface geophysical techniques (e.g., seismic,
radar, transient electromagnetics) will be calibrated against a highly characterized control volume
(the wellfield) with 3-D distributions of geologic, hydrologic, and geophysical properties determined
from extensive field measurements. Also, these data sets will be used to investigate relationships
between properties and to test petrophysical models. Well coring and construction methods, and the
well arrangement in the field, are designed to provide detailed control on lithology and to support
a variety of single-well, crosshole, and multiwell geophysical and hydrologic tests. Wells are
screened through the cobble-and-sand aquifer to a clay that underlies the BHRS at about 20 m depth.
In addition, the wellfield design optimizes well-pair distances and azimuths for determination of
short-range geostatistical structure. Initial geostatistical analysis of porosity data derived from
borehole geophysical logs indicates that the omnidirectional horizontal experimental variogram for
porosity (possible proxy for log permeability) is best fit with a nested periodic model structure.
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Geophysical Surveys Across The Boise Hydrogeophysical Research Site To Determine Geophysical Parameters Of A Shallow, Alluvial Aquifer
At the Boise Hydrogeophysical Research Site (BHRS), we are characterizing the
hydrogeophysical parameters of a cobble-and-sand, unconfined aquifer using a wide variety of
geophysical methods. Our goal is to develop methods for mapping variations in permeability by
combining non-invasive geophysical data with hydrologic measurements. We are using seismic,
ground penetrating radar, and electrical methods in a variety of configurations to provide images
of and parameter distributions at the BHRS. Issues such as resolution, depth of penetration, and
the ability to image the desired parameters will help determine the most effective methods.
Supporting data sets from the BHRS include core analyses and geophysical logs from 18 wells at
the site. We will use these data to verify our geophysical interpretations. The various geophysical
methods and acquisition geometries, combined with the well control, will provide an outstanding
data set to characterize the heterogeneity of the subsurface beneath this alluvial aquifer, and find
ways to map permeability with geophysical information.
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Permeability Estimation Using Spectral Induced Polarization Measurements
Authors John T. Sturrock, David P. Lesmes and F. Dale MorganMany different models have been developed for the hydraulic permeability of
sedimentary rocks and soils. Generally, the models fall into two basic categories: fluid flow
through equivalent tubes and fluid flow around grains. Although different in approach, both tube
and grain models have been observed to produce reasonable results. The major obstacle to in-situ
permeability prediction is accurately estimating the relevant model parameters such as porosity,
specific surface area, tortuosity, and grain/pore size distribution. Spectral Induced Polarization
(SIP) measurements are sensitive to many of the same micro-geometricalparameters which are
required for permeability prediction. The objective of this paper is to test a new methodology
for inverting broadband SIP measurements for micro-geometricalparameters and then to use the
derived model parameters to estimate the sample permeabilities. A series of laboratory
experiments were performed using five different sandstone samples, for which the hydraulic
permeability, porosity, specific surface area, and SIP response was measured over the frequency
range of 10” to lo6 Hz. The SIP data were then used to independently estimate the formation
factor, porosity, specific surface area, surface fractal dimension and the grain size distribution of
each sample. As has been previously demonstrated, we observe that the IP response at a fixed
frequency increases with the specific surface area of the sample. However, permeability
estimates based on surface area alone are inaccurate. Much more robust permeability estimates
are obtained when using the grain size distribution obtained in the inversion of the broadband SIP
data.
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Determining Elastic Soil Properties At Small Strains
More LessGeotechnical and geophysical techniques have been combined to determine the elastic
parameters of a series of lacustrine clays at a location where series of highway underpasses
are to be constructed. Piezocone penetrometer and triaxial tests on borehole core samples
allowed variations of the soil structure and the shear modulus /J over a range of larger
strains to be measured. The small-strain behavior of p was established from seismic
measurements. Extremely low ,U values of less than 50 MPa and Poisson’s ratios near 0.5
for small strains were found. Together with experience gained from the first construction
phase, these results will be used to optimize the design of construction process.
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Predicting Clay Content And Poroszty From Gamma-Ray And Conductivity Logs
Authors Wendy W. Corona and Gary MavkoAs the first step in predicting flow properties from geophysical logs, a gamma-ray
log was used to predict clay volume fraction (& 0.021), and the conductivity log was used
to predict porosity fraction (& 0.029) in the Oak Ridges Moraine (ORM) complex,
Ontario. First, empirical relations between the gamma-ray count and lab-measured clay
volume were derived and used to predict clay volume throughout the entire well for five
different lithologies. Then, Archie’s Law was used with a clay correction to calculate
formation factor. Finally, relations between lab-measured porosity and predicted
formation factor were derived and used to predict porosity throughout the entire well for
each lithology.
In the Aurora well, we reasonably predicted the volume fraction of clay and
porosity, and were able to identify known aquifer and aquitard units using the computed
formation resistivity factor. The porosity and formation factor pseudo-logs may be used
to model permeability at the well. The derived cementation factors were: m = 1.6 in the
sands, silts and gravels; m = 1.8 in the silt/clay rythmite series; and m = 1.3 in the poorly
sorted, clayey silt tills.
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Accounting For Saturation Heterogeneity In Obtaining Estimate Of Water Content From Dielectric Data
Authors Christina Y. Chan and Rosemary J. KnightMeasurements of dielectric constant can be used to determine water content or saturation in
the vadose zone; estimates of these parameters are needed in modeling contaminant transport. If a
sedimentary system is a homogeneous mixture, the dielectric constant-water content or saturation
relationship can be described by the time propagation (TP) model. If the sedimentary system
consists of thin layers, effective medium theory (EMT) describes the system; whereas if the system
consists of thick layers, ray theory describes the system.
In this paper, we present a flow chart for determining water content and saturation from
field measurements. In this procedure we look only at binary (e.g. clay-sand) sedimentary
systems and assume that if layers are present they are parallel to the surface. Using dielectric
measurements taken perpendicular to the surface of the earth and measurements taken parallel, we
can determine whether the system is homogeneous or layered. We then use the dielectric
measurements and lithologic information determined from direct sampling to calculate the water
content and saturation of the system. This method not only gives the global water content and
saturation of the whole system, but also gives the water contents and saturations of the two
different sedimentary layers if they are present. We also present examples of the error that can be
incurred if this method is not followed and an incorrect relationship is used to interpret to dielectric
data.
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Influence Of Microstructural Properties On Geophysical Measurements In Sand-Clay Mixtures
Authors Dorthe Wildenschild, Jeffery J. Roberts and Eric D. CarlbergWe have performed a series of laboratory experiments on saturated sand-clay
mixtures. Measurements include frequency-dependent electrical properties using the
four-electrode technique (10 mHz to 1 MHZ), permeability, porosity, and acoustic
velocities. We mixed clean Ottawa (quartz) sand with Na-montmorillonite (Wyoming
bentonite) in a number of different configurations containing 0 to 10% clay: as a
dispersed mixture, as discrete clay clusters, and arranged in distinct layers. Solutions of
CaC.& ranging from 0.0005 N to 0.75 N (0.05 to 64 mS/cm) and deionized water were
used as saturating fluids. We found the electrical properties to be dependent on clay
content, fluid conductivity, and microstrudure in a complex fashion. Increasing fluid
conductivity and increasing clay content generally resulted in higher electrical
conductivity. For an individual sample, two main regions of conduction exist: a region
dominated by surface conduction and a region where the ionic strength of the
saturating fluid controlled conduction. The sample geometry (dispersed, nondispersed,
or layered clay configuration) was found to greatly affect the magnitude of the surface
conductance in the range of low fhrid conductivity.
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Ultrasonic Characterization Of Synthetic Soils For Application To Near Surface Geophysics
Authors B.P. Bonner, P. Berge, C. Aracne-Ruddle, C. Bore, E. Hardy and C. TrombinoEffective seismic interrogation of the near subsurface requires that measured parameters, such as
compressional and shear velocities and attenuation, be related to important soil properties. Porosity,
composition (clay content), and fluid content and identification are of particular interest. The
ultrasonic (100-500 kHz) pulse transmission technique was used to collect data for highly attenuating
materials appropriate to the vadose zone. Up to several meters of overburden was simulated by
applying low uniaxial stress of 0.1 MPa to the sample. The approach has been to make baseline
measurements for pure quartz sand, because the elastic properties are relatively well known except at
the lowest pressures. Second phases are then added to modify the sample microstructure and
ultrasonic measurements are made to characterize the effect of the admixed second phase. Samples are
fabricated from Ottawa sand mixed with a swelling clay (Wyoming bentonite), and with a common
organic soil amendment, peat. Compressional (P) velocities are low, ranging from 169 to 360 m/s for
the mixtures at low stress. Shear (S) velocities are about half of the compressional velocity, but show
different sensitivity to microstructure. Adding clay increases the shear amplitude dramatically with
respect to P, and also changes the sensitivity of the velocities to load. These experiments demonstrate
that P and S velocities are sensitive to the amount and type of admixed second phase, even at low
concentrations. Other properties of the transmitted signals which include the ratio of S and P
amplitudes, velocity gradient with depth, and the frequency content of transmitted pulses, provide
additional information and are not now used in near-surface surveys.
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Comparing Geophysical Measurements To Theoretical Estimates For Soil Mixtures At Low Pressures
Authors P.A. Berge, J.G. Berryman, B.P. Bonner, J.J. Roberts and D. WildenschildRelationships between geophysical and hydrogeological properties are needed for
improving characterization of the subsurface at contaminated sites using geophysical
methods. As part of an on-going project with the goal of improving underground imaging,
we are developing rock physics theories for relating elastic wave velocities to soil
composition and fluid-flow parameters. Here we use effective medium theories to estimate
velocities of unconsolidated sand-peat moss mixtures and compare the estimates to
measured compressional and shear velocities from low-pressure laboratory experiments.
We found that a self-consistent effective medium theory model using sand and porous peat
as the constituents provides estimated velocities that are in good agreement with measured
velocities. Understanding the microstructural assumptions inherent in various mixture
theories guides the selection of appropriate theories for modeling velocities in soils.
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Saturation And Contamination Dependence Of Electrical Properties In The Mhz Range
Authors Andreas Weller, Claus Urban and Joachim SchonartA Dielectric Measurement System is used to investigate the influence of pore filling and
saturation on electrical permittivity and conductivity in a frequency range from 2 MHz to 1.3
GHz. The small size of the sample holder requires a special preparation method of the samples.
The influence of the geometry of the sample holder was investigated with synthetic samples of
known permittivity.
Only the experiments with glass beads and pure sand verified the validity of known mixing laws.
Samples with varying clay content reveal the dominant influence of clay minerals on electrical
permittivity. The large specific internal surface of clay minerals requires the consideration of
surface effects. Mixing laws considering only the volume fraction of the components fail to
describe the electrical properties of samples with strong surface effects. Temporal variation of
electrical properties are observed which are caused by the imbibition process after the injection.
The intrusion of hydrocarbon contaminants into the pore space resulted only in small changes of
electrical permittivity compared to significant changes due to varying water and clay content.
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Large-Scale Geophysical Mapping And Its Application For Ground Water Protection In Urban Areas.
Authors Kurt I. Sørensen and Verner H. SøndergaardThe water supply in Denmark strongly relies on ground water, but pure water may soon be in
short supply. The Danish Parliament has therefore decided, that protection of ground water
against polution should be intensified by means of protection zones and area1 planning activities,
which means that for instance urban expansion will be under stronger control.
In order to act on a proper basis the knowledge about the geological and hydrogeological
situation has to be considerably improved. Over the next 10 years some 140 million US dollars
will be spent to obtain a better geological mapping of recharge areas with great drinking
water interests. These areas add up to a total of some 15.000 km2 in Denmark.
Geophysical methods, especially electrical and electromagnetic methods, are expected to
play an important role, accounting for more than half of the 140 million dollar mapping
expenses.
A large scale mapping project around the City of Aarhus has provided good experiences
about using high density geophysical measurements combined with drillings covering large
areas.
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Geophysical Imaging Of A Former Landfill And Its Associated Leachate Plume
An integrated geophysical investigation was conducted at a closed landfill site
(LF-30) at the decommissioned Wurtsmith Air Force Base (WAFB), Oscoda, MI in May
and June of 1998. Frequency domain horizontal loop electromagnetic (HLEM),
magnetic, self-potential (SP), electrical resistivity (ER), and ground penetrating radar
(GPR) methods were used. The site is nearly flat topographically, with less than two
meters of local relief, and is covered by dense tree growth approximately 23 years old.
Previous hydrogeologic investigations show PCE, TCE, DCE, vinyl chloride, and BTEX
are the main constituents of a leachate plume emanating from the landfill. Additionally,
the landfill plume contains elevated levels of total dissloved solids(TDS) leached from
the fill material, causing groundwater plume conductivities three times background
values. Groundwater flow in the area is toward the northeast and discharges into Van
Etten Lake.
All of the methods utilized in this study were successful in delineating the landfill.
The electromagnetic and magnetic data suggest the existence of eight separate clustered
burial cells (and one outlier) containing significant amounts of metal within the landfill.
Additionally, areas of fill are characterized by attenuated GPR signals and very low
apparent resistivities compared to background. Furthermore, the EM and SP surveys
were most effective in delineating an anomalous zone approximately 1600 R wide, where
boundaries are coincident with the lateral boundaries of the BTEX plume as determined
by scattered monitor wells. The difference in size between the small LF-30 landfill and
the much larger associated plume width, suggest the possibility of an additional source.
Therefore, it is likely that another landfill (LF-3 l), located up gradient from LF-30, is a
major contributor to this contaminant plume.
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The Temporal Variation Of A Saltwater Contaminant Plume As Evidenced Through Long-Term Resistivity Monitoring
Authors Gail Lipfert, Stewart Sandberg, Lee Slater, Andrew Reeve and Marc LoiselleThe site of a formerly uncovered sand/salt pile in Verona, Maine was used to investigate
the changes in resistivity over time of a fractured bedrock ground-water system. Terrain
conductivity of the area immediately down-slope from the site of the former sand/salt pile was
measured using a small horizontal loop electromagnetic (EM) profiling technique. High terrain
conductivity measurements delineate the contaminant plume. In areas where overburden is
shallow, anomalies in terrain conductivity along traverses indicated fracture locations. The angular
shape of the plume of high terrain conductivity represents the orientations of regional and local
bedrock fractures and indicates fracture control of the ground-water flow. A permanent array of
electrodes was established along six of the EM traverses and electrical resistivity measurements
were taken every one to two weeks over a five-month period. A correlation of rain events to
apparent resistivity changes are attributed to the migration of the plume.
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Applications Of Some New Techniques To Detect Groundwater Contamination At Mine Tailings Dams
Authors G. Buselli and K. LuWe have recently applied electrical and electromagnetic methods to two
environmental problems at minesites: firstly, to an acid mine drainage problem at the
tailings dam of a disused pyrite mine at Brukunga, South Australia, and secondly, to
the detection of any seepage from a tailings dam at the operating Ranger minesite,
Northern Territory, Australia. Using these examples, we stress the importance of the
use of combined methods for both measurement and interpretation of data.
At the first minesite, we used a combination of direct current (DC), transient
electromagnetic (TEM) and self-potential (SP) surveys to locate the source of
groundwater entering the tailings dam and to map the depth to water-table over the site.
At the second minesite, more extensive experimental surveys have been made with
these and the induced polarization (IP) method. The surveys include the use of a 2D
array of electrodes and measurements with a multichannel data acquisition system.
Results of a previous airborne electromagnetic survey have been interpreted using
newly-developed conductivity-depth imaging software.
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A Mise A La Masse Survey To Track Ground Water Cokduits In Weathered Bedrock Near A Salvage Yard
Authors John Jansen, Phillip Osborn and Michael WilsonA series of monitoring wells were installed as part of a ground water investigation around
a salvage yard. Most of the wells encountered relatively dense silty clay deposits and
shale. A few wells hit voids related to a paleochannel system filled with high TDS
ground water. A mise a la masse survey was conducted to map the trend of the
paleochanneis by placing a current electrode inside a monitoring well completed in a
channel feature. The survey identified the area of highest hydraulic conductivity and the
trend of the major channel system. The survey also appears to have mapped individual
minor channel features. The survey results were used to predict the trend of the
paleochannel system to the east of the site. The predicted location of the paleochannels
was confirmed by a test drilling program.
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An Integrated Approach To Monitoring A Field Test Of In Situ Contaminant Destruction
The development of in situ thermal remediation techniques requires parallel development
of techniques capable of monitoring the physical and chemical changes for purposes of
process control. Recent research indicates that many common contaminants can be
destroyed in situ by hydrous pyrolysis/oxidation (HPO), eliminating the need for costly
surface treatment and disposal. Steam injection, combined with supplemental air, can
create the conditions in which HP0 occurs. Field testing of this process, conducted in the
summer of 1997, indicates rapid destruction of polycyclic aromatic hydrocarbons (PAHs).
Previous work established a suite of underground geophysical imaging techniques capable
of providing sufficient knowledge of the physical changes in the subsurface during thermal
treatment at sufficient frequencies to be used to monitor and guide the heating and
extraction processes. In this field test, electrical resistance tomography (ERT) and
temperature measurements provided the primary information regarding the temporal and
spatial distribution of the heated zones.
Verifying the in situ chemical destruction posed new challenges. We developed field
methods for sampling and analyzing hot water for contaminants, oxygen, intermediates
and products of reaction. Since the addition of air or oxygen to the contaminated region is
a critical aspect of HPO, noble gas tracers were used to identify fluids from different
sources. The combination of physical monitoring with noble gas identification of the native
and injected fluids and accurate fluid sampling resulted in an excellent temporal and spatial
evaluation of the subsurface processes, from which the amount of in situ destruction
occurring in the treated region could be quantified. The experimental field results
constrain the destruction rates throughout the site, and enable site management to make
accurate estimates of total in situ destruction based on the recovered carbon. As of
October, 1998, over 400,000 kg (900,000 lb) of contaminant have been removed from the
site; about 18% of this has been destroyed in situ.
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Induced Polariza Tion Imaging Of A Jet Fuel Plume
Time-domain induced polarization data has been collected and analyzed for a site at the
Otis ANGB, Massachusetts Military Reservation, Cape Cod, Massachusetts. The
specific plume is designated FS- 12 and is due to break in the subsurface fuel transport
pipeline. Approximately 70,000 gallons of jet fuel were spilled in the early 1970’s. The
predominant contaminants are benzene and ethyl dibromide (EDB).
The data have been inverted to give resistivity and chargeability tomograms to depths of
300 feet and 1400 feet in extent. We also processed the IP time domain signals for short
and long times separately and produced a time domain gross spectral chargeability
difference. Both the chargeability and spectral chargeability difference show good
agreement with the known location of the plume from sampling wells.
The IP mapping provides detailed information that cannot otherwise be obtained from the
sparse borehole distribution.
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Self Potential Monitoring Of Jet Fuel Air Sparging
Authors Yervant Vichabian and Frank Dale MorganIn recent years, several remediation methods have been developed based on in-situ
oxidation. However, there are not efficient ways to monitor the effectiveness of these
methods in enhancing oxygen levels or delineating their zone of influence. In this
presentation we study the use of self potential (SP) geophysics to monitor the air sparging
and soil vapor extraction system at FS-12 on the Massachusetts Military Reservation,
Cape Cod, Massachusetts. Data has been collected over a time period of two years using
the gradient SP method with 130 permanent measuring stations. We have observed a
large 800 to 1000 mV SP anomaly in the region where the remediation system is in
operation. Using the Nemst equation, we convert the SP anomaly to the partial pressure
of oxygen, from which a contour plot is generated showing the effective area of sparging.
In this paper, we show that SP can be used to monitor remediation methods, which rely
on increasing oxygen levels in the subsurface.
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3-D Gpr Imaging Of Complex Fluvial Stratigraphy At The Boise Hydrogeophysical Research Site
Authors William R. Peretti, Michael D. Knoll, William P. Clement and Warren BarrashA series of three-dimensional (3-D) ground-penetrating radar (GPR) data sets were acquired
over the central wellfield area at the Boise Hydrogeophysical Research Site (BHRS). The survey
region is 30 m x 18 m and encompasses 13 wells. The goal of the surveys is to image the
complex fluvial (cobble-and-sand) stratigraphy around the wellfield. These images will be used
to construct 3-D models of the sedimentary architecture and to help constrain fine-scale models
of hydrologic and geophysical parameters at the site. The data sets were acquired using 25 MHz,
50 MHz, 100 MHz and 200 MHz antennas. Depth of penetration ranges from -9.6 m for the 200
MHz data to -22 m for the 25 MHz data. Processing significantly improves the reliability and
interpretability of the images. The images suggest that the deposit can be subdivided laterally
and vertically into several distinct units or radar architectural elements; these elements are
typically separated by erosional bounding surfaces. Horizontal bedding, cross-bedding and
channel structures are clearly evident in the 100 MHz and 200 MHz data, and a clay layer that
underlies the cobble-and-sand aquifer at -20 m depth is successfully imaged in the 25 MHz and
50 MHz data. The water table, at a depth of l-2 m, is imaged in the 100 MHz and 200 MHz data.
Time slices and vertical cuts through the data volumes are used to identify the shape and
orientation of the different architectural elements, and to accurately locate important
hydrostratigraphic boundaries. These data are being used to construct a 3-D model of the
hydrogeologic zonation of the aquifer. Hydrologic and geophysical parameter values associated
with each zone will be determined from additional field measurements (e.g., hydraulic tests in
wells, crosshole radar and seismic tomography, transient electromagnetics, and well logs). The
3-D GPR surveys provide valuable information about the location, scale and geometry of
different stratigraphic units at the BHRS.
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An Imaging Algorithm For Gpr Data
Authors Yoshinori Sanada and Yuzuru AshidaThe finite difference time domain (FDTD) method is calculated by the explicit
finite approximation scheme of Maxwell’s curl equations. Since it allows arbitrary
electrical conductivity and permittivity variations within a model, the FDTD method
has become one of the powerful forward modeling methods for electromagnetic (EM)
phenomena. On the other hand, the reverse time migration is the imaging algorithms,
which is performed by inserting the recorded data as boundary conditions at each
recorder position in reverse time order. Due to the advantage of simple calculation
algorithm and applicability to arbitrary model, both methods are powerful tools for GPR.
In the present paper, the synthetic GPR data are calculated by using of the FDTD
method with the exploding reflector method. The reverse time migration is formulated
using FDTD scheme and applied to model with lossless and lossy media. In lossless
media, the forward and reverse time schemes are expressed by the completely same
scheme. The proposed method is successfully applied to synthetic data for two models,
these are steeply dipping structure and point diffractors model. In lossy media, the
forward scheme includes diffusion term, while the reverse time scheme includes
divergence term. In such a case, we show the condition that the EM wave velocity is
regarded as constant, then this methodology is applicable successively. We also discuss
the reverse time migration for the lossy media after the amplitude recovery under the
lossless media condition.
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Terrain- Permittivity Mapping: Gpr Measurements Of Near-Surface Soil Moisture
Authors David P. Lesmes, Robert J. Herbstzuber and David WertzGPR can be used to rapidly and non-intrusively measure soil moisture content
over spatial scales that are intermediate between the point measurement techniques (e.g.
TDR and neutron probes) and remotely sensed soil moisture measurements which are
made at the regional scale (e.g. airborne SAR measurements). In bi-static GPR surveys
the direct ground wave passes from the transmitter to the receiver through the nearsurface
soil. The GPR ground wave velocity can therefore be used to determine the
permittivity (or dielectric constant) of the near-surface soil. Estimates of soil moisture
content can be obtained using dielectric mixing equations. Variance in the velocity of the
ground wave can largely be attributed to variance in the level of soil moisture. We are
using GPR ground wave velocity measurements, with a constant antenna off-set, to map
lateral variations in terrain permittivity and to estimate near-surface soil moisture content.
Experiments have been conducted to determine the effects of frequency, antenna spacing,
and soil type on the soil moisture estimates and the effective depth of penetration.
Furthermore, we have used this methodology to monitor changes in soil moisture during
controlled spill experiments. Ongoing experiments include the monitoring of seasonal
fluctuations in soil moisture over a grass covered field at the Weston Geophysical
Observatory in Weston, Massachusetts.
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An Investigation Of Cross-Borehole Ground Penetrating Radar Measurements For Characterizing The 2D Moisture Content Distribution In The Vadose Zone
Authors Lee Paprocki and David AlumbaughThe use of cross-borehole ground penetrating radar (GPR) imaging for determining the two dimensional (2D) in situ moisture content distribution within the vadose zone is being investigated. The ultimate goal is to use the GPR images as input to a 2D hydrologic inversion scheme for recovering the van Genuchten parameters governing unsaturated hydraulic flow. Initial experiments conducted on synthetic data have shown that at least in theory, cross-borehole GPR measurements can provide realistic estimates of the spatial variation in moisture content that are needed for this type of hydrologic inversion scheme. However, the method can not recover exact values of moisture content due to 1) the break down of the empirical expression often employed to convert GPR velocity images to moisture content, and 2) the smearing nature of the imaging algorithm. To test the applicability of this method in a real world environment, crossborehole GPR measurements were made at a hydrologic/ geophysical vadose zone test site in Socorro, New Mexico. Results show that the GPR images compare well with the
uncalibrated borehole neutron log data. GPR data acquisition will continue once an infiltration test has started, and the results from these measurements will be employed in a 2D hydrologic inverse scheme.
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Ground Penetrating Radar & Avo
Authors Philip M. Reppert, Frank Dale Morgan and M. Nafi ToksozThis presentation concerns the use of Amplitude Versus Offset (AVO) with ground
penetrating radar (GPR) data. We demonstrate that AVO may have possible uses with
GPR. Theory is presented in this paper on the principle of AVO as applied to
electromagnetic (EM) waves. The theory is demonstrated on field data obtained over a
drain field in Massachusetts.
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Conversion Of Hydraulic Conductivity To Synthetic Radar Traces
More LessIn the interest of interpreting radar data and establishing a empirical correspondence between
hydraulic conductivity and ground penetrating radar data, hydraulic conductivity logs have
been converted to synthetic radar traces. The assumption in the computation is that hydraulic
conductivity is closely related to porosity and thus water content and dielectric constant.
The synthetic radar traces have been used to interpret the radar cross section in terms of
zones of hydraulic conductivity.
In the absence of a deterministic model between radar waveforms and hydraulic conductivity
empirical relations are sought. Radar data have been acquired at the MADE site near Columbus,
Mississippi. Hydraulic conductivities measured in wells have been converted to dielectric
constant using a simple empirical relationship and used to compute the radar traces that
would be obtained at the borehole. The comparisons with the field radar traces are encouraging.
Key words: radar, hydraulic conductivity, synthetic radargram
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Investigation Of Mountainous Rock Destruction: A New Physicothematical Conception
Authors A.L. Aleinikov, V.T. Belikov and L.V. EppelbaumInvestigation of mountainous rock destruction under different thermodynamic conditions
is one of the most important problems of the engineering hazard. It was earlier considered
that this destruction is taking place when at least one component of the stress tensor
reaches some critical value called as a material strength. However, conducted analysis
allowed us to conclude that such a notion is correct only for a relatively short-term action
and is unsuitable by a prolonged stress to target. On the basis of the detail theoretical
analysis we propose a new physico-mathematical conception describing the destruction
process as a phase transition. From this viewpoint, generation of microfault is an initiation
of a new phase. Such a generation causes emission of a definite number of seismo-acoustical
impulses. Quantity of the impulses during the time unit shows an intensity of process
destruction and frequency of oscillation indicates dimension of the microfaults forming.
Thus, the suggested conception allows to develop procedures for monitoring of durability
of different artificial underground constructions: mines, reservoirs and military objects as
well as other artificial and natural structures.
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Use Of Vsp Techniques To View, In 3D, Partially Saturated Layers In The Shallow Subsurface At Lawrence Livermore National Laboratory, California
Authors Paul A. Milligan, James W. Rector III and Robert BainerA multi-offset VSP (vertical seismic profiling) method was developed to help define aquifer boundaries and below
water table partially saturated zones at the Lawrence Livermore National Laboratory, Livermore Site, California.
The VSP method consisted of using a multilevel hydrophone array giving 48 channels of recorded seismic data per
shot. Hydrophone spacing was 0.5 m, with anti-tubewave baffling devices inserted between each phone, and at each
end of the array to slow down and attenuate tube wave noise contaminating the desired data signals. The seismic
source consisted of an impact weight-drop machine.
The VSP data was processed, and inverted to produce 2D p-wave interval velocity sections with up to 0.5 m of vertical
resolution in the unconsolidated sediments below the water table. Multiple 2D velocity sections were created,
corresponding to radial arms of walk-away shot point records from multiple wells. These 2D velocity sections were
then visually projected into the 3D survey volume, together with well locations, predefined hydrostratigraphic unit
(HSU) layer boundaries, and the interpreted isopach map of a major HSU of interest. The interactive graphical display
of all these data in 3D allowed correlations to be made between the VSP velocity images and the pre-existing data.
The discovery of several low velocity zones below the water table was attributed to partially saturated pore spaces.
Two thin (less than 1 m thick) layers with p-wave velocities as low as 500 m/s appeared to be perched on top of major
aquifers undergoing pump extraction and treatment. Gas analysis from water samples taken from one of the aquifers
indicated normal atmospheric air as a probable gas source. High pumping flow rates (20 to 30 gal/min) may
help to explain how venturi action could be responsible for the intake of air from the vadose zone into porous but
less permeable silty-sand units overlying sand-gravel aquifers. Possible uses for this VSP technique would be to delineate
the zone of pumping influence, and help in the choice of suitable new well locations for either pumping or
monitoring.
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Hydrogeological Property Estimation Using Tomographic Data At The Boise Hydrogeophysical Research Site
Authors John E. Peterson, Ernest L. Majer and Michael D. KnollCrosswell seismic and radar data were acquired at the Boise Hydrogeophysical Research
Site (BHRS) as part of an effort to characterize the hydrological property distribution in a
heterogeneous alluvial aquifer. The dielectric constant and amplitude attenuation values
obtained from inversion of the radar data, as well as the seismic P-wave velocity values
obtained from inversion of the seismic data, reveal similar spatial patterns. Comparison of
this tomographic information with coincident wellbore logs suggest that the geophysical
attributes are sensitive to hydrogeological variations. Information obtained from the radar
tomography data were used to produce 2-D, high-resolution images of porosity and electrical
conductivity. Comparison of these images with coincident log data reveal that these estimates
are reasonable and suggests that the tomography data will be useful at this site for delineating
variations in hydrological parameters.
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Dielectric Dependence On Salinity And Probe Length In Tdr-Measurements
Authors Sari Penttinen, Pekka Hanninen and Raimo SutinenTime domain reflectometry (TDR) is widely used in soil sciences and forestry to determine soil
dielectric properties and water content. The effects of TDR probe length, geometry and water
electrical conductivity (%) on dielectric values were studied. The three-prong TRD probes, 0.06
m in diameter and lenght varying from 0.035 to 0.15 m with varying the spacing of the probes
tested in water, whose electrical conductivity (%) varied from 0.6 mS/m to 700 mS/m. The
scattering in dielectric values increased significantly, when the probe lengt was 0.035 m or
shorter. However, the scatter level was minor with probes bend outward or inward. Probe
spacing greater than 0.06 m resulted in unreliable readings. The increase of electrical
conductivity of water with %> 200 mS/m resulted in scattered data, particularly with the 0.035-
m-long probes. Dielectric data for all probe lenghts were scattered with water electrical
conductivity with oW> 500 mS/m.
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Field Investigation Of Salt Transport Processes Using .Resistivity/Ip Imaging
Authors Lee Slater and Stewart K. SandbergTwo examples of the use of combined resistivity/induced polarization (IP) monitoring of salt
transport under natural hydraulic loads are presented. Previous workers have performed electrical
monitoring of the transport of a tracer artificially injected into the subsurface. The detection of
tracer transport due to natural hydraulic processes is a more difficult goal as neither the hydraulic
load nor the resistivity contrast can be controlled. Tide-induced salt transport at the saltwaterfreshwater
interface was investigated in the first study. 2D resistivity and IP models resolved the
structure of the interface. The IP modeling provided greater clarity of the subsurface
hydrology/geology than the resistivity modeling alone. Modeling of electrical changes over a
tidal cycle revealed resistivity increases in the near surface and at depth, 5-l 5 m up-beach of the
high tide mark. The results indicate a phase lag between the subsurface hydrological response
and the tidal cycle. In the second study, salt transport from the site of an old road-salt storage pile
was investigated. The hydraulic mechanism was natural recharge events that occurred over a
four-month period. 2D conductivity and IP models resolved the subsurface distribution of salt,
initially identified from an EM3 1 survey. Modeling of resistivity change on lines spaced 33.5 m
apart revealed changes at the location of salt contamination. The changes on each line appear
closely coupled and correlate with the recharge pattern over the monitoring period. The studies
indicate that salt transport occurring under natural hydraulic conditions can be detected with
resistivity, and to some extent, IP.
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Self Potential Mapping Of Contaminants
Authors Yervant Vichabian, Philip Reppert and Frank Dale MorganVarious geophysical techniques are useful for detection and mapping of subsurface
contamination plumes. Self potential seems to be a particularly promising technique due
to its cost effectiveness and speed of acquisition and processing. The electrochemical
phenomena of organic contaminants and their interactions with microorganisms in the
subsurface provide a detection method for self potential (SP) geophysics. In this
presentation, we couple biodegradation with several examples of SP detecting
contaminant plumes.
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An Electromagnetic Induction Tomography Field Experiment At Lost Hills, Ca
Authors H. Michael Buettner and James G. BerrymanWe have collected borehole to surface electromagnetic induction field data for a shallow steam
injection that is underway at Mobil Oil’s Lost Hills-3 field in San Joaquin Valley. Earlier
work had been done at the same site by Wilt et al. (1996). This site is an interesting test
for techniques under development for environmental engineering, because it can be viewed as
an excellent analog of a shallow environmental remediation using steam injection. Surface
magnetic field data (vertical and radial fields, magnitude and phase) were collected using 18
receiver stations along two profiles which ran radially from the EM transmitter well from 5
m to 120 m. The data at each surface station were collected while the EM transmitter was
raised slowly from a depth of 120 m to a final depth of 20 m. As part of this experiment, a
calibration of the EM transmitter was also performed. Magnetic field data from Lost Hills were
successfully collected, including both vertical and horizontal (surface radial) magnitude and
phase data along a northerly profile and along a westerly profile. We have observed that the
radial receiver data appear to be better behaved than the vertical receiver data, suggesting that
these data may be less sensitive to environmental clutter (numerous metallic pipes crisscrossing
the site at the surface) than are the vertical data. Some simple 1-D modeling has been done to
confirm that the expected conductivity change in the steam zone should produce an observable
anomaly in the measured data when comparing the pre-steam to the post-steam conditions.
Results of this test were positive. Further analyses of these data making use of a new code
developed in a companion paper are in progress and will presented separately.
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High-Performance Computational And Geostatistical Experiments For Testing The Capabilities Of 3-D Electrical Resis’Iiance Tomography
Authors Steven Carle, Abelardo Rarnirez, William Daily, Robin Newmark and Andrew TompsonThis project explores the feasibility of combining geologic insight, geostatistics, and high-performance
computing to analyze the capabilities of 3-D electrical resistance tomography (ERT), Geostatistical
methods are used to characterize the spatial variability of geologic facies that control subsurface
variability of permeability and electrical resistivity. Synthetic ERT data sets are generated
from geostatistical realizations of alluvial facies architecture. The synthetic data sets enable comparison
of the “truth” to inversion results, quantification of the ability to detect particular facies at
particular locations, and sensitivity studies on inversion parameters.
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Electrical Imaging Of Engineered Hydraulic Barriers
Authors William Daily and Abelardo RamirezElectrical resistance tomography (ERT) was used to image the full scale test
emplacement of a thin-wall grout barrier installed by high pressure jetting. ERT was also
used to monitor the waterflood of a thin-wall grout barrier to vertfy its hydraulic integrety.
Both case studies were done by comparing images of electrical resistivity before and after a
change was induced in the subsurhace. Barrier materials or flood water were imaged as
anomalies which were more electrically conducting than the native sandy soils at the test
sites. Although the spatial resolution of the ERT was not sufficient to resolve flaws in the
barrier under construction smaller than a reconstruction voxel(50 cm on a side), the images
did show the spatial extent of the barrier materials and therefore the general shape of the
structure.
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Laboratory Scale Tests Of Electrical Impedance Tomography
Authors Abelardo Ramirez, William Daily, Andrew Binley and Douglas LaBrecqueElectrical impedance tomographs (magnitude and phase) of known, laboratory-scale
targets are reported. Three methods are used to invert electrical impedance data and their
tomographs compared. The first method uses an electrical resistance tomography (ERT)
algorithm (designed for DC resistivity inversion) to perform impedance magnitude
inversion and a linearized perturbation approach (PA) to invert the imaginary part. The
second approximate method compares ERT magnitude inversions at two frequencies and
uses the frequency effect (FE) to compute phase tomographs. The third approach,
electrical impedance tomography (EIT), employs fully complex algebra to account for
the real and imaginary components of electrical impedance data. The EIT approach
provided useful magnitude and phase images for the frequency range of 0.0625 to 64 Hz;
images for higher frequencies were not reliable. Comparisons of the ERT and EIT
magnitude images show that both methods provided equivalent results for the water
blank, copper rod and PVC rod targets. The EIT magnitude images showed better spatial
resolution for a sand-lead mixture target. Phase images located anomalies of both high
and low contrast IP and provided better spatial resolution than the magnitude images.
When IP was absent from the data, the EIT algorithm reconstructed phase values
consistent with the data noise levels.
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Simultaneous Acquisition Of P- And S-Wave Crosswell Seismic Profiles In A Contaminated Basalt Aquifer
Authors Thomas M. Daley, John E. Peterson and Ernest L. MajerMultiple seismic crosswell surveys have been acquired by LBNL in a fractured basalt aquifer
at Idaho National Engineering and Environmental Laboratory (INEEL). Most of these surveys
used a high frequency piezoelectric seismic source to obtain P-wave velocity tomograms.
Additionally, we deployed a new type of borehole seismic source, know as an orbital vibrator, in
a crosswell seismic survey as part of the subsurface characterization program at this
contaminated groundwater site. This source, know as an orbital vibrator, allows simultaneous
acquisition of P- and S-waves. The orbital vibrator was developed for oil field applications and,
to our knowledge, this is the first environmental scale crosswell application. Both velocity and
attenuation tomograms have been calculated. The velocity tomograms show a relationship to
contaminant propagation in the groundwater (as measured by logs and cores). Zones of high
contaminant flow are consistent with zones of low velocity (both P- and S-wave) and high
attenuation. We believe horizontal fracture zones at the boundaries of basalt flows are
controlling the contaminant flow and the seismic properties. Additionally, high amplitude guided
waves are observed and we believe these waves delineate thin (l-2 m) zones of velocity contrast
which relate to fracturing and, in some instances, coincide with contaminate flow.
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Reverse Vsps And Crosshole Seismic Tomography While Coring
Authors William P. Clement, Lee M. Liberty and Michael D. KnollAt the Boise Hydrogeophysical Research Site, numerous 20 m deep wells were continuously
cored with a split spoon sampler to detail geologic changes beneath the site. During the coring of
one of these wells, we recorded the energy radiating from the drill stem using a combination of
downhole hydrophones and surface geophones. The hydrophone strings were located in a pair of
adjacent wells and the geophones were located along the surface between these wells and the
well being cored. The experiment’s geometry provides a series of reverse VSPs and crosshole
tomographic data sets that can help distinguish P-wave velocity changes in the subsurface.
Energy from the core tip is not necessarily the first arrival. However, the near continuous
recording of the source as it penetrates the ground enables us to locate the energy from the core
tip. We also record energy when the source is in the vadose zone. Comparing the seismic while
coring results with a similar experiment using a borehole sparker source confirms the reliability
of recording seismic energy generated by the split spoon sampler. Core tip recording is an
inexpensive seismic method that has great potential for borehole-to-surface and crosshole
tomographic imaging at relatively large offsets in near-surface environments.
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Surface And Borehole Seismic Characterization Of The Boise Hydrogeophysical Research Site
Authors Lee M. Liberty, William P. Clement and Michael D. KnollWe conducted borehole to borehole, borehole to surface, and surface seismic experiments
to optimize data acquisition parameters, obtain a seismic velocity model, and to characterize seismic
stratigraphic units in a shallow aquifer at the Boise Hydrogeophysical Research Site. The
aquifer consists of coarse (cobble-and-sand) fluvial deposits underlain by clay at 18-2 1.5 m depth.
We acquired data with a downhole seismic source (sparker), a sledge hammer source, a hydrophone
string, a borehole geophone, and a surface geophone array to compare seismic signal quality
and to place limitations on each seismic method. To fully characterize seismic reflections, the
source-receiver geometry is an important parameter for both borehole and surface experiments.
Direct arrivals and the presence of seismic reflections suggest a correlation between porosity
changes and seismic velocities at the site. We have defined four seismic stratigraphic horizons
that correlate with surface and borehole ground penetrating radar results, geophysical logs, and
lithologic logs. These results provide an initial framework for hydrologic modeling.
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Characterization Of Shear-Wave Velocity Anisotropy In The Upper 33 M Of A Dipping, Consolidated Sandstone
Authors Gilein J. Steensma, Paul E. Murray, Michael L. Batzle and Thomas M. BoydThe extraction of groundwater or hydrocarbons from the subsurface and the effective remediation
of contaminated sites depend upon knowledge of preferential fluid migration. Shear-waves (s-waves)
have proven to be an effective tool to ascertain fracture orientation and the stress regimes responsible for
fractures. Grain and void alignment due to stratigraphy, rock texture and intergranular pore space can
cause s-waves velocity anisotropy, or variations in velocity with direction. S-waves with particle motion
parallel to the stiffer direction of the medium propagate faster than those parallel to the more compliant
direction of the medium.
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Object Identification Using Multifrequency Emi Data
Authors Dean Keiswetter, I.J. Won, Bruce Barrow and Tom BellUnexploded ordnance (UXO) cleanup is the number one priority Army Cleanup Problem
requirement and is identified as a major problem throughout the Department of Defense (DOD).
A recent SERDP technical report summarizes the status of capability for buried UXO detection,
discrimination, and identification as follows: (a) can detect UXO, within definable limits; (b)
cannot effectively discriminate UXO anomalies from ‘false alarm’ anomalies; and (c) cannot
identify UXO. False alarm anomalies are defined here as geophysical anomalies caused by
buried UXO debris, other metallic objects, gravel and cobbles, soil heterogeneities, tree roots,
and other natural and cultural features. False alarm anomalies significantly contribute to the cost
of UXO remediation due to the large number of unnecessary excavations, A major initiative in
the research and development community, therefore, is to develop discrimination (target
identification) capabilities. One potential methodology for target identification involves utilizing
the broadband scattered electromagnetic induction response. This technique, which is known as
Electromagnetic Induction Spectroscopy (EMIS), h as recently become feasible due to the
development of the GEM-3 sensor. The GEM-3 is an efficient, broadband, handheld EMI sensor
than employs a unique monostatic coil design. Analyzing the EMI spectral content for target
identification is not new. In fact, elementary EM theory states that an object must exhibit
different responses at different frequencies. All fundamental EM equations involving a timevarying
source testify as such. By fully characterizing and identifying an object without
excavation, we should be able to significantly reduce the number of false targets. EMIS should
be fully applicable to many other problems where target identification and recognition (without
intrusive search) are important.
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Geophysical Qa/Qc For A Uxo Eeka, Fort Ritchie Army Garrison, Md
Authors Kent Boler, Mike Ervine, Alex Smith, John Breznick and Chris EvansICF Kaiser, under contract to the US Army Corps of Engineers, Baltimore District
(CENAB) performed an engineering evaluation/cost analysis (EE/CA) in support of Base
Realignment and Closure (BRAC) to assess potential hazards due to unexploded
ordnance (UXO) in former and suspected range areas at Fort Ritchie Army Garrison, MD
(Fort Ritchie). EM61 geophysical surveys were conducted over portions of the now
developed cantonment area as part of a statistically based sampling plan to locate and dig
metallic anomalies potentially representing UXO. Select QA/QC aspects of the EM61
based ordnance location and sampling program are presented. Performance testing
documents a significant decrease in non-UXO anomalies vs mag-and-flag techniques at
this site. EM61 calibration pit results for simulated UXO items and analysis of 10%
replicate grid data show acceptable data accuracy, precision, and comparability.
Tracking of calibration pit test results show negligible variation of peak amplitudes
between instruments, or with variations in temperature and precipitation.
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Locating And Determining Dimensionality Of Uxos Using Time Domain Electromagnetic Induction
Authors Leonard R. Pasion and Douglas W. OldenburgIn this paper we assume that the Time Domain Electromagnetic (TEM) response of a buried
axisymmetric metallic object can be modelled as the sum of two dipoles centered at the midpoint of
the body. The strength of the dipoles depends upon the relative orientation between the object and
the source field, and also upon the shape and physical properties of the body. Upon termination of
the source field, each dipole is assumed to decay as k (t + a)-’ e-‘/y. The parameters k, LY, p and y
depend upon the conductivity, permeability, size and shape of the object, and these can be extracted
from field or laboratory measurements by using a nonlinear parametric inversion algorithm. Our
preliminary work, carried out using an analytic solution for a sphere and laboratory measurements
of steel plates, suggest the following methodology. The value of p might be used as a diagnostic
to assess whether the metallic object is non-magnetic or magnetic. If the object is thought to be
magnetic, then the ratios of ICI/k2 and pi//3 2 are diagnostic indicators as to whether the geometry
is plate-like (uninteresting) or rod-like (a high candidate for being a UXO).
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A Field Evaluation Of Airborne Techniques For Detection Of Unexploded Ordnance
Authors W.E. Doll, P. Hamlett, J. Smyre, D. Bell, T.J. Gamey and J.S. HolladayUnited States Defense Department estimates indicate that as many as 11 million acres of
government land in the U. S. may contain unexploded ordnance (UXO), with the cost of
identifying and disposing of this material estimated at nearly $500 billion. The size and
character of the ordnance, types of interference, vegetation, geology, and topography
vary from site to site. Because of size or composition, some ordnance is difficult to
detect with any geophysical method, even under favorable soil and cultural interference
conditions. For some sites, airborne methods may provide the most time and cost
effective means for detection of UXO. Airborne methods offer lower risk to field crews
from proximity to unstable ordnance, and less disturbance of sites that may be
environmentally sensitive.
Data were acquired over a test site at Edwards AFB, CA using airborne magnetic,
electromagnetic, multispectral and thermal sensors. Survey areas included sites where
trenches might occur, and a test site in which we placed deactivated ordnance, ranging in
size from small “bomblets” to large bombs. Magnetic data were then acquired with the
Aerodat HM-3 system, which consists of three cesium magnetometers within booms
extending to the front and sides of the helicopter, and mounted such that the helicopter
can be flown within 3m of the surface. Electromagnetic data were acquired with an
Aerodat 5 frequency coplanar induction system, deployed as a sling load from a
helicopter, with a sensor altitude of 15m. Surface data, acquired at selected sites,
provide a comparison with airborne data. Multispectral and thermal data were acquired
with a Daedelus AADS 1268 system. Preliminary analysis of the test data demonstrate the
value of airborne systems for UXO detection and provide insight into improvements that
might make the systems even more effective.
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A Comparison Of Towed And Mounted Helicopter Magnetometer Systems For Uxo Detection
Authors T. Jeffrey Gamey and Reinhold MahlerTraditional helicopter magnetic surveys have always utilised a towed platform, or bird, to
remove the need to compensate for the magnetic effects of the helicopter. Such platforms have
evolved to include total field, vertical gradient and triaxial gradient configurations. For detection
of small objects in the near surface, the principle limitation of towed systems is positional control
in three dimensions. In particular, the sensor height above the ground (Z) is critical for small
objects, and consistency as much as the actual altitude is an important consideration for
interpretation.
For environmental surveys with towed systems, nominal survey heights are reduced from 30 to 5
or 10 metres above ground level, tree cover permitting. In order to achieve these altitudes safely,
air speed is reduced which subsequently reduces aerodynamic stability. Speed changes also alter
the distance between helicopter and platform making control more difficult. Higher speeds drag
the bird back and upwards, and usually into a nose down orientation. Platform altitude is
monitored by laser altimeter and displayed to the pilot, but most pilots find this insufficient
warning, preferring direct visual observation. Depth perception from the top of a 30m cable is
still marginal, and minimum survey heights are limited by caution to 5m clearance under ideal
conditions (+/-3m) with 7-10m being more typical altitudes. For positioning in X&Y, lines can
theoretically be flown as close as the budget will allow. In practice, X&Y control is limited to
10m line spacing due to +/-5m of bird swing.
One solution to this problem is to mount sensors directly onto the helicopter in the tips of
stingers or booms. This provides a significant improvement in the pilot’s control, but requires a
more sophisticated data acquisition and processing system. When the mounting platform is a
direct extension of the helicopter and is placed at eye level for perspective, positional control can
be improved to +/-lm in any direction.
IABG in Germany has supported several years of airborne platform development through
European field projects. In 1997/98, the first phase of a research project consisted of comparison
surveys of Aerodat’s towed triaxial gradiometer and their boom mounted system. Tests were
conducted over a controlled test site, which included targets such as UXO, barrels, UST’s, pipes
and debris in varying amounts and configurations. This paper presents some of the results of
these comparative tests. Other tests by IABG at this site included multi-spectral and
photogrammetric surveys, but are not discussed within the scope of this paper.
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The Application Of An Integrated Magnetometer/Global Positioning System For An Unexploded Ordnance Investigation
More LessGeophysical techniques have been used for many years to detect unexploded
ordnance (UXO) buried in the subsurface. UXO detection can be performed by using
either “mag and flag” methods where anomalous metallic items are located without data
collection and flagged for excavation or dug immediately, or using a data collection
system where anomalous areas are selected from the data record and the positions are
reacquired for excavation. In recent years, recorded data surveys have begun to take
precedence over unrecorded surveys.
There are two elements to a successful recorded data geophysical survey:
1. Data collection (magnetic/electromagnetic field)
2. Location tracking.
Having only one of the above elements without the other yields nearly useless
information and certainly makes for an unsuccessful survey. Traditional methods of
tracking the location of data collected by the geophysical survey instrument have been to
use either a time/distance method or a mechanical measurement tool (i.e. wheel
odometer). Technological developments in survey equipment and the Global Positioning
System (GPS) have added a new way of tracking position during surveys in select
environments.
A geophysical investigation was conducted at a former military range in Mississippi
using an integrated magnetometer/GPS system. The use of the integrated system yielded
extremely good results in open areas but had limited use in areas under heavy tree
canopy. Results of the surveys performed indicate that the integration of GPS with
geophysical instruments is a viable tool and may become the standard operating
procedure in certain environments. The limitations of the integrated system primarily
stem from the ability of the GPS system to “speak” with satellites through tree cover.
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Mapping Buried Metallic Objects And Titaniferous Placers In The Mississippi Sound, Gulf Of Mexico
Authors Jeff Wynn and Kevin LaurentRecent experiments with a marine induced polarization (IP) streamer system have shown
that it can find and map titaniferous placers on and beneath the sea floor (Wynn and Laurent,
1998) and facies changes in the substrate beneath sandy shoals (Wymr and others, 1998). Close
examination of profile data acquired in the Mississippi Sound, however, shows that the system
can also detect certain conductive, polarizing objects that are buried beneath the sea floor and not
discernible with conventional bathymetry. The streamer system is designed to detect
polarizeable materials down to at least 6 meters below the sediment-water interface. The recent
data show broad phase-shift (IP) anomalies along the east side of Cat Island, off the coast of
Biloxi. MS. Dark, titaniferous sands can be seen in dissected benches and berms on the island’s
coastline, reinforcing the conclusion that the IP system is mapping the ilmenite-rich (FeTiO,)
sediments reported in shoals there (Foxworth, 1962). Interspersed in these data, however, are
several narrow electrical anomalies of two kinds: resistivity, and resistivity-IP anomalies. Both
kinds show pronounced drops in the already-low resistivity, and several of these also have
coincident phase-shift anomalies. The polarizing objects observed are -10 to 20 meters across,
but the IP anomalies can be seen - 20 to 40 meters laterally from their centers. The sources of
these anomalies are probably man-made in origin, buried under a veneer of modern sediments.
We speculate that these are shipwrecks, sunken buoys or parts thereof, lost and later buried (in
water now about 3 meters deep) by Hurricane Camille, which passed through the area in 1969.
We cannot, however, preclude emplacement and burial at an earlier time.
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New Field And Modeling Results From A Simulated Waste Pit Using The Enhanced Very Early Time Electromagnetic (Vetem) Prototype System
Tests in July, 1998, of an improved version of the prototype VETEM system demonstrated
improved depth of investigation at the Cold Test Pit (CTP) at the Idaho National Environmental
and Engineering Laboratory (INEEL). The improved depth of investigation is due primarily to
the development of larger loop antennas and a new transmitter capable of driving up to 30 times
more current than the original transmitter into the larger loop. An overlapped antenna
configuration was tried and proved effective in detecting buried objects. New display software
allows us, for the first time, to generate area1 time-slice displays of our data in the field for fast
qualitative evaluation. Comparisons of results using two different antenna configurations over
the Large Object Pit portion of the CTP show generally good agreement, but show an apparent
position offset of the two data sets from each other. The test over the Calibration Cell portion of
the CTP using the overlapped antenna configuration successfully located most of the indicated
targets in the cell. Results over both portions of the CTP suggest that the indicated locations of
some of the buried objects may not be completely accurate. New antenna and 3D forward
modeling codes run using an input waveform, antenna size and configuration, and system
bandwidth matching those of the VETEM system produce waveforms and show sensitivities
similar to those seen in recorded field data.
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Vertical Radar Profiling To Determine Dielectric Constant, Water Content And Porosity Values At Well Locations
Authors Michael D. Knoll and William P. ClementA vertical radar profiling (VRP) experiment was conducted at the Boise Hydrogeophysical
Research Site to determine if direct arrivals and reflections can be recorded using the surface-toborehole
survey geometry. The receiving antenna was deployed downhole to insulate it from
cultural noise. The transmitting antenna was located on the surface near the wellhead and
oriented radially to the well axis. Although the antenna polarizations are perpendicular, we were
able to record radar direct-arrivals and reflections. Picked first-arrival traveltimes were inverted
to determine layer interval velocities. The VRP-derived velocity estimates fall in the same range
as velocity estimates determined from crosshole radar tomography at the site; average velocities
for the unsaturated and saturated zones are 0.140 rn/ns and 0.080 m/ns, respectively. Resolution
of the VRP-derived velocity estimates is significantly better than the resolution of CMP-derived
velocity estimates. Dielectric constant and porosity estimates were also derived from the VRP
data using a simple petrophysical model, and compared to porosity estimates derived from
neutron logging. Correlation between the two porosity estimates is encouraging. While the VRP
method has not been widely used in site investigations to date, the results of this study suggest
that VRPs provide an accurate, high-resolution, and cost-effective means of obtaining dielectric
constant, EM velocity, volumetric moisture content and porosity values at well locations.
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Borehole Geophysics Applied To The Study Of Landfill Sites In Fractured Bedrock Terrains
Authors Horatio Ferriz and William PedlerGeotechnical and hydrogeologic studies for landfill sites in the fractured bedrock environment can
be a technically challenging prospect. Many of the problems involved in landfill construction in
fractured bedrock can be solved by detailed surface mapping of fracture density and orientation,
but ultimately data has to be collected from the subsurface via exploratory wellbores. Fracture
density and orientation can be obtained through acoustic televiewer logs or digital borehole
imaging logs, but identifying the fracture or fracture group that is water-bearing (hydraulically
conductive) is critical in characterizing the flow of water at the study site.
Hydrophysical logging and digital borehole imaging were used to log 13 wells to evaluate a
prospective canyon landfill site in weathered crystalline rocks. Hydrophysical cross-hole testing
was employed to evaluate hydraulic connections between wells and estimate medium scale
hydraulic conductivity. The results of this study found that: (1) For most wells, the more highly
transmissive intervals were within 30 feet of the piezometric level. This behavior is consistent
with the deeply weathered nature of the bedrock, which behaved in a manner hydraulically similar
to a silty sand. The hydraulically conductive intervals ranged in thickness between 2 and 8 feet,
and represented between 1% and 5% of the total length of the bedrock section, (2) In all but one
instance, the water bearing intervals were within 60 feet of the piezometric surface. In other
words, groundwater flow was largely concentrated in shallow fracture zones. (3) Based on the
hydrophysical logging results for ambient horizontal flow, a typical range of specific discharge
values for a discrete fractured interval between 0.3 and 0.02 A/day was observed at the study site.
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A Wireline-Operated Downhole Packer For Eiydraulic Measurements In Boreeoles
Authors F.L. Paillet, J.H. Williams and A.E. HessA wireline-operated packer was designed for reconnaissance logging in open boreholes and
field tested at a site in Belvidere, Illinois. Although the wireline packer uses a single downholeinflated
packer, analysis converts the wireline-packer data to estimates of hydraulic properties of
zones between adjacent packer measurement stations. A synthetic data example shows how the
single packer can be used to efficiently and effectively identify the depth intervals where
hydraulically active fractures or bedding planes intersect a borehole. Data analysis subsequently
provides estimates of the hydraulic head, transmissivity, and water quality for each of these zones.
Field tests of the wireline packer in Illinois produced estimates of zone hydraulic head and relative
transmissivity that agreed with values given by conventional straddle-packer hydraulic tests.
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Use, Comparison, Presentation, And Interpretation Of Borehole Image And Acoustic Televiewer Technology
By T.M. StaatzBorehole televiewers and image systems are used to accurately show sub surface
structural features. The borehole image system can show features not evident with the
acoustic televiewer under ideal conditions. Depending on the hole conditions, and the
data desired, either the image system or the televiewer may be the preferred instrument.
This paper will address:
1. General - The operation, calibration, survey setup, and complimentary logs to run
with each of these instrument types.
2. Comnarisons - The strengths, weaknesses, and similarities of the image system and
acoustic televiewer.
3. Data acauisition - Data gleaned from surveys can include: fractures and fracture
orientation; lithology and lithlogic changes; strike and dips; bedding planes, joint
sets, and foliation; compressional, extensional and horizontal stress determination;
information on how the borehole was drilled; and orientation of core. Interpretation
of this data will be discussed.
4. Presentation and annlication - Real examples will be used to show how a geoprofessional
would apply this data to varying situations. Data from a range of diverse
environments will be included, and various data formats will be presented.
Applications for when and type of surveys to run will be investigated.
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Field Evaluation Of A New Borehole Resistivity Probe Using Capacitive Electrodes.
Authors C.J. Mwenifumbo and Q. BristowA prototype resistivity probe using capacitive electrodes has been built under a collaborative
project funded jointly by the Geological Survey of Canada (GSC) and IFG Corporation, a
Canadian manufacturer of borehole geophysical instrumentation. The GSC carried out field tests
at its borehole test facilities in Ottawa and at the New Calumet massive sulphide deposit in
Quebec. Results indicate that the capacitive technique is sensitive in the resistivity range 100-
10,000 ohm-m and compares well with galvanically and inductively measured resistivities. Field
tests were also carried out for several transmitter-receiver spacings and in plastic-cased fluidfilled
boreholes. Capacitive resistivity logs acquired in a plastic-cased borehole are almost
identical to those acquired in the same but uncased borehole. Therefore, capacitive resistivity
logs promise to be valuable for use in environmental applications where boreholes drilled in
unconsolidated sediments are often plastic-cased.
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Assessing Exploratory Borehole Seals With Electrical Geophysical Techniques
Authors Stanford A. Gibson, Craig H. Benson and Tuncer B. EdilThis paper describes how time domain reflectometry (TDR) and electrical resistivity
logging were used to examine the integrity of seals placed in exploratory boreholes. Laboratory
tests were conducted in PVC columns containing layers simulating sealants and defect materials.
TDR or resistivity probes were installed in the columns and used to log the materials in the
column. Column testing showed that the TDR method could be used to identify defect types and
sizes. However, conduction losses limited the useful length of the TDR probe to less than 3-m.
Resistivity logging proved more suitable and thus field tests were conducted with the resistivity
probes. Resistivity logs clearly delineated portions of a borehole seal that contained either
sealant or defect materials.
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Characterization Of Well Seals Using An Ultrasonic Method
Authors Martin C. Christman, Tuncer B. Edil and Craig H. BensonA downhole ultrasonic method was modified and used to test water well seals. The
method analyzes reflections of ultrasonic waves off interfaces to determine the presence of
different materials between the casing and formation. The method of waveform analysis was
modified for thicker well casings. Indices for characterizing waveforms were compared, and
delineation between water and bentonite grout was improved. Laboratory tests using sand as an
annular material also led to changes in the method of interpretation. Test results from two water
wells are presented. Bentonite grout and sand were detected in the armulus of both wells.
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A Three-Dimensional Interpretation Of An Aem Survey In The Upper San Pedro Basin, Arizona
Authors Mark W. Bultman, Mark E. Gettings and Jeff WynnIn March 1997, a time-domain Airborne ElectroMagnetic (AEM) survey was flown over the
upper San Pedro basin, Arizona, by Geoterrex-Dighem Ltd. of Ottawa, Canada. This survey was
contracted and analyzed by the U.S. Geological Survey Geologic Division. A mathematical
transformation of the AEM data, referred to as a conductivity depth transform or CDT, was used
to build a three-dimensional model of resistivity in the basin. Comparisons of the CDTs and
resistivity measured in electric well logs from nine Fort Huachuca test wells show that the CDTs
are a good representation of the electrical conductivity of basin sediments for the upper 150 m.
Below 150 m trends in conductivity systematically decrease with depth, and therefore conductors
deeper than 150 m represented on the CDTs are underestimated. The depth to the water table
generally corresponds to the uppermost maximum of conductivity, but there are many
exceptions. It is possible that the relationship is coincidental with those areas where the water
table happens to fall in a zone of (more conductive) silts and clays. The eastern half of the basin
near the Tombstone Hills contains an increasing proportion of silts and clays with depth. We
infer that this is due to weathering of the volcanic rocks off of the Tombstone Hills into a low
energy environment during the filling of the basin. This resulted in a southwestward extending
wedge of silt and clay which is most extensive at depth and least at the surface. The lower part of
this wedge may include outflow tuff from the Tombstone caldera. This material limits the
permeability of much of the aquifer and thus is hydrologically significant.
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A World Of Applications For Helicopter Electromagnetics To Environmental And Engineering Problems
By Greg HodgesThe variety of environmental and engineering applications for helicopter-borne
electromagnetic surveys has increased in recent years. Helicopter EM surveys
undertaken by Geoterrex-Dighem have been used for:
l measuring soil conductivity near oil pipelines to assess corrosion risk,
l estimating soil type and depth for pipeline planning,
l mapping sand and gravel deposits for aggregate resource exploration,
l measuring the extent of salt water invasion into the Florida Everglades,
l mapping soil salinity in Australian farmland, assessing groundwater resources,
l locating brine leaks from abandoned Texas oil wells,
l site characterization and detection of leaks in mine tailings dams,
l assessing land slumping hazards in Japan,
l measuring ocean bathymetry
l locating igneous dikes in coal mines.
Magnetic and radiometric data can be collected simultaneously, and have been used to
find buried metal structures, and illegally abandoned radioactive sources.
The major advantages of helicopter EM over ground EM surveys are the reduced cost for
surveys over large areas, the speed of surveying coverage (approx 1000 hectares/hour),
and the multiple frequency/multiple depth sample nature of the data, which render a 3-
dimensional conductivity image of the ground conditions.
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A Case History Of The Scintrex Helicopterborne Pds3M Pipeline Detection System To Aid In The Safety And Planning Of 3D Seismic Surveys
Authors Terry McConnell and Bob LoThe Scintrex PDS-3M Pipeline Detection System was engineered to detect steel pipelines as
small as 2 inches in diameter under favourable conditions. Using three Scintrex CS-2 cesium
vapour magnetometer sensors arranged in a triangular fashion in a helicopter towed bird, the
PDS-3 system measures all three components of the magnetic gradient and the total magnetic
intensity. The data is interpreted to yield the location, likely layout of the magnetic responses.
The combination of high sensitivity gradient measurements and data collection from an airborne
platform allows for rapid surveying over areas which are difficult to access, such as over
swamps. The PDS-3M survey yields a good infrastructure maps for the 3D seismic acquisition
companies to plan their survey. The presented case history is taken from a PDS-3M survey over
an oil field in Mexico. It shows that the existing infrastructure maps are often grossly simplified
- creating uncertainty in the “safe areas” for seismic shot point locations. With the increased
confidence of the pipeline locations due to a PDS-3M survey, the safe areas for shotpoints were
identified and gaps in the seismic coverage were tilled in.
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Gem-2A Broadband Airborne Em System
Authors Bill SanFilipo and I.J. WonA helicopter towed bistatic EM system capableofsimultaneously transmitting and receiving at
several user-selected frequencies has been developed, and is currently operational in mineral
exploration programs in Australia. This system employs advanced digitally controlled
transmitter electronics providing flexible user selections for current waveforms, as well as
advanced built-in digital signal acquisition and processing including real-time sine/cosine
correlation for spectral analysis of the data. A reference, or “bucking” coil is incorporated,
which is wired in reverse polarity to the primary receiver coil, thus removing the direct system
response as observed iq the absence of a conductive target (i.e. at high altitudes). This feature
is critical in maximizing the observability ofthe weaker response from currents induced in the
earth. A case study is presented with presentations of apparent conductivity maps for real
survey data.
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