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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 - 50 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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