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14th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 04 Mar 2001 - 07 Mar 2001
- Location: Denver, Colorado, USA
- Published: 04 March 2001
41 - 60 of 129 results
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Anomalies From Horizontal Metal Pipes In Resistivity And Ip Fields
Authors Albert Ryjov and Vladimir ShevninIn urban and industrial areas geophysical methods are frequently applied for investigation of
metal pipes in ground or for other problems decision in their vicinity. Study of such pipes’ influence can
include: 1 - detection of a pipe (its position, orientation, depth), 2 - estimation of pipe technical
condition (corrosion, quality of isolation), 3 - distorting influence of a pipe on the fields of electrical and
EM methods at the decision of various geological problems. I.e. the pipes can be both object of study
and noise. For resistivity and IP methods pipes more often appear as noise, though from these methods
some pipes' parameters can be received.
It is possible to apply numerical methods to model pipes influence on apparent resistivity and IP
fields, though for a simple problem - rectilinear indefinitely long pipe - the application of analytical
method based on the strict account of boundary and starting conditions at the decision of the Laplace
equation is possible. The analytical decision allows estimating influence of a pipe in a wide range of
pipe and environment parameters. The problem is decided for horizontal cylinder with a covering in a
field of a point current source.
Influence of several pipe's parameters was investigated. Among these are depth and orientation
of pipe, pipe and covering resistivity, influence of array's and current electrodes' removal from the pipe
axis. In all cases IP values are more sensitive to pipe's parameters changes in comparison with apparent
resistivity. Pipe influence grows with current electrode approaching to the pipe axis and with lowering
the resistivity of covering.
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Transmission Line Approximation Of Pipelines With Cathodic Protection
Authors A. Mousatov and E. NakamuraThe effective and reliable pipeline exploitation requires regular control of their technical
condition. Now electromagnetic geophysical methods are widely applied to determine a position and
depth of the pipeline and to estimate a state of its electrical insulation and cathodic protection operation.
One of most effective methods for the solution of the specified problems is the non-contact
measurement of magnetic field created by the current, flowing in the pipeline. The cathodic protection
station or external generator connected directly to the pipeline and grounded in infinity can be used as
such current source.
The forward problem solution for the pipeline with variable insulation resistance and the pipe
conductivity requires application of numerical mathematical methods for 3D medium. Even in the
simplified 2D statement, assuming axial symmetry, the problem remains enough difficult, taking into
account specific geometry of model and very high contrast of cylindrical layers’ resistivities.
In this paper the approximation method to calculate the magnetic field above the pipeline is
considered. The transmission line approximation is used to substitute the pipeline with variable electrical
resistance of insulation. In this case the current distribution along the pipeline is obtained. Then the
magnetic field from the current variating along the pipeline is calculated. The dimension of a problem is
lowered and the analytical solution can be received with the considered approach.
On the basis of the described approximation the models of pipelines with zones of a various
extent of electrical insulation damages were accounted. The derivative of a magnetic field along an axis
of the pipeline corresponds to a current outflow, which is, in this case, quantitative characteristic of the
insulation condition.
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Time Domain Em Profiling Over A Shallow Conducting Target
By J.B. MerriamAn em 37-47 pro le over a known target at one to three m depth is described here. The response
of the highly conductive target is easily separated from the background response of the moderately
conductive till and is traced from the earliest gate of the em-47 ultrahigh range (7 s after turn o ),
to the latest gates of the em-37 medium range (28ms after turn o ). Three separate responses are
identi ed on the basis of their decay characteristics and spatial variation. They are interpreted to
be: a galvanic response, a vortex response, and a viscous magnetic remanence (VRM) response.
In time and frequency domain electromagnetics the vortex response is often expected to be the
dominant response, indeed, the galvanic response has only recently been added to many modeling
packages. In this environment - a very good conductor, with high permeability, at shallow depth in
a moderately conducting host, the galvanic response is the largest, followed by the viscous magnetic
remanence. The vortex, or inductive, response is only briefly above the noise level.
Many other em surveys have been performed over this target, including VLF, em-31 and
maxmin. These show either no response, or a response that is barely above the noise level. The
frequencies used in these techniques (less than 56 kHz) are probably too low to strongly excite the
galvanic or VRM modes, and it is possible that they are only exciting the relatively weak vortex
mode. Thus, time domain em is not only e ective in this environment, it is the only one of the
common em techniques that is e ective.
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Joint Influence Of Resistivity Anisotropy And Inhomogeneity For A Single Dipping Interface Between Isotropic Overburden And Anisotropic Basement
Authors Evgueni Pervago, Aleksandr Mousatov and Vladimir ShevninThe media with joint influence of anisotropy and inhomogeneity have large practical interest.
The difficulty of practical anisotropy studying with collinear arrays results from the fact, that anisotropy
exhibits itself weaker, than inhomogeneity at equal resistivity contrasts. The relative anisotropy and
inhomogeneity influence is considered with the help of mathematical modeling for gentle dipping
interface with anisotropic basement. The algorithm is based on the integral equations’ method for 3D
models with anisotropy.
This model is considered for three different directions of strike of dipping interface and strike of
anisotropic basement, in comparison with isotropic model of dipping interface and horizontally - layered
model with the anisotropic basement. The modeling data are submitted as azimuthal diagrams and
results of their spectral analysis. Spectral analysis helps to receive some diagnostic parameters for
anisotropic - inhomogeneous media.
All resistivity arrays on sensitivity to anisotropy are divided into two groups: collinear arrays
(Schlumberger, pole-pole, pole-dipole, dipole axial) with the axes ratio equal λ, and non-collinear arrays
(dipole equatorial, T-array, etc.) with sensitivity up to λ5. Most sensitive to inclined contact is dipole
axial array, and it is the least sensitive to anisotropy.
The inhomogeneity influence is displayed in the first harmonic of azimuthal diagrams’s
spectrum, and anisotropy - in the second harmonic. The inhomogeneity also influences on the second
harmonic. The absence of the first harmonic (and following odd ones) testifies to absence of
inhomogeneity influence. For comparison of relative influence of anisotropy and inhomogeneity, it is
necessary to consider the ratio of the sum odd to the sum of even harmonics. When O/E ratio is >1, the
inhomogeneity influence prevails, and when <1 - the anisotropy influence prevails. At joint influence of
basement anisotropy (λ = 2) and dipping interface (dip is 5°), the anisotropy influence prevails only for
non-collinear arrays.
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Application Of Geophysical Methods To Delineate Contamination In Fractured Rock At The University Of Connecticut Landfill, Storrs, Connecticut
Authors Carole D. Johnson, John W. Lane Jr., John H. Williams and F.P. HaeniAn integrated suite of geophysical methods was used to characterize the hydrogeology of a
fractured-rock aquifer to identify contamination or pathways for contaminant migration near a former
landfill at the University of Connecticut, Storrs, Connecticut. Surface-geophysical methods were used to
identify the dominant direction of fracture orientation and to locate potential leachate plumes. Two
shallow, electrically conductive anomalies near surface-water discharge areas north and south of the
landfill were interpreted as leachate plumes. Two other sheet-like electromagnetic (EM) anomalies were
identified and targeted for drilling and borehole-geophysical investigation. These methods were used to
determine the location, orientation, and lateral continuity of fractures and to quantify the hydraulic
properties of the transmissive fractures.
One borehole was located to intersect an anomaly observed at a depth of about 18 meters. The
EM-conductivity log measured a high electrical conductivity anomaly at a depth of 21 meters, which
coincides with a layer observed to contain sulfide mineralization. The lack of high conductivity fluids in
the borehole supports the interpretation that this anomaly is caused by a lithologic change rather than by
fractures that contain conductive leachate.
The second borehole was positioned to intersect a conductive feature at a depth of about 18
meters. A fracture with similar strike and dip was observed at a depth of about 22 meters in optical- and
acoustic-televiewer images, in borehole-radar surveys, and was determined to be hydraulically
conductive during heat-pulse flowmeter tests. This feature was also characterized by a high-conductivity
spike in the EM log. Although the specific conductance of the fluid in this depth zone was high, it could
not account for the spike in the EM log. The anomaly is interpreted to be caused by a combination of
conductive fluids in the fracture and of conductive minerals in the rock. Water-quality samples from a
discretely isolated zone near a depth of 22 meters indicated the presence of landfill leachate. This
investigation illustrates the effectiveness of the use of combined geophysical methods for identification
and evaluation of electrically conductive contaminant plumes.
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Deterministic Deconvolution Of Ground-Penetrating-Radar Data At A Limestone Quarry
Authors Jianghai Xia, Tom Weis, Evan Franseen and Richard MillerA 30 m by 30 m two-dimensional grid was designed on a flat bench behind a quarry face of
predominantly limestone with thin shale layers located at a Bonner Springs, Kansas site to test the utility
of ground-penetrating radar (GPR) for stratigraphic studies. GPR data were collected along seven lines
parallel to the quarry face and seven lines perpendicular to the quarry face, each separated by 5 m, using
50 MHz, 100MHz, 200 MHz, and 400 MHz antennas. As a part of the project, confirmation of reflection
events, ringing suppression, and velocities of electromagnetic (EM) wave propagation in the limestone
were studied. GPR instrument wavelets were successfully collected in the air. With a known GPR
instrument wavelet, ringing in GPR data was significantly suppressed by a deterministic deconvolution.
The validity of using a wavelet acquired in air as the operator for deterministic deconvolution was
shown in the real-world application of a radar system with 400 MHz antennas to a quarry site consisting
of interbedded limestones and shale partings. A total of 78 horizontal holes were drilled in key locations
on three exposed quarry faces where data were acquired before and after conductive steel rods, 1.5 m in
length, were placed in the holes. Diagnostic GPR responses from the horizontal steel rods serve as
known reflectors. The steel rods provide critical information for: 1) confirmation and nature of specific
geologic reflection events in the GPR data, 2) GPR resolution limits, 3) accuracy of velocities calculated
from common-midpoint data, and 4) identification of multiples. The effectiveness of suppressing ringing
waveforms suggests that the deterministic deconvolution of GPR data with the GPR instrument wavelet
in the air should be included as a standard GPR data processing step.
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Application Of A Geophysical “Tool-Box” Approach To Characterization Of Fractured-Rock Aquifers: A Case Study From Norwalk, Connecticut
The U.S. Geological Survey conducted a geophysical investigation at a site in Norwalk,
Connecticut where solvents have contaminated a fractured-rock aquifer. Borehole, borehole-toborehole,
and surface geophysical methods were used to characterize the bedrock fractures, lithologic
structure, and transmissive zone hydraulic properties in 11 boreholes and their vicinity. The
geophysical methods included conventional logs, borehole imagery, borehole radar, flowmeter, and
azimuthal square-array dc resistivity soundings.
Integrated interpretation of geophysical logs at borehole and borehole-to-borehole scales
indicates that the bedrock foliation strikes northwest, dips northeast, and strikes north-northeast to
northeast, dips both southeast and northwest. Although steeply dipping fractures that cross-cut foliation
are observed, most fractures are parallel or sub-parallel to foliation. Steeply dipping reflectors
observed in the radar reflection data from three boreholes near the main facility building delineate a
north-northeast trending feature. Results of radar tomography conducted close to a suspected
contaminant source area indicate that a zone of low velocity and high attenuation exists above 50 feet
in depth - the region containing the highest density of fractures. Flowmeter logging was used to
estimate hydraulic properties in each of the boreholes. Thirty-three transmissive zones were identified
in 10 of the boreholes. The vertical separation of the transmissive zones in a borehole typically is 10 to
20 feet.
Open-hole and discrete-zone transmissivity was estimated from flowmeter data acquired under
ambient and pumping conditions. The open-hole transmissivity ranges from 2 to 86 feet squared per
day (ft2/d). The estimated transmissivity of individual transmissive zones ranges from 0.5 to 70 ft2/d.
Draw down monitoring in nearby boreholes under pumping conditions identified hydraulic connections
along a northeast-southwest trend between boreholes as far as 560 feet apart. The vertical distributions
of open fractures can be described by power law functions, which suggest that the fracture network
contains transmissive zones consisting of closely spaced fractures surrounded by a less fractured and
much less permeable rock mass.
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Lateral And Vertical Delineation Of Water Producing Fractures And Zone Specific Water Quality Data With Csamt And Hydrophysical Logging
Authors Martin J. Miele, Tony Morgan and Greg BauerFracture zones and faults are typical targets for groundwater production in areas characterized by
hard bedrock. Groundwater production and water quality may vary within those aquifers. The vertical
and lateral interconnection between fracture sets is a major consideration for the suitability of a
sustained water producer. The site for this investigation is a tree nursery facility in Southern California,
in San Diego County. The operation needs added water supply from additional well(s). The area
consists of rolling hills that abut the San Luis Rey River. The geology in the area is characterized by
granitic bedrock reported to be tonalite.
One of the existing and favorable wells on the facility (favorable well) was drilled to a depth of
approximately 1,000 feet. The well had an artesian flow of 55 gpm. A widely spaced CSAMT (MT)
survey was conducted along a traverse located adjacent to the well and across the local valley. The
geologic or geoelectric strike of the area was known, therefore, the MT data was processed in transverse
magnetic mode. The CSAMT traverse indicated that the well was drilled into a system with two
anomalous zones interpreted to be major fracture zones. One extended down to a depth slightly greater
than 1,000 feet. A second anomalous zone interpreted to be fractures existed at depths greater than
2,000 feet. The two systems are apparently unconnected in the MT data. The shallow fracture zone was
logged with HydroPhysicalä testing which indicated four major water bearing fracture systems with
varying water quality conditions. The deeper fracture system correlates with another local well reported
to be completed at depths greater than 2,000 feet. Water quality varied between the wells.
Three additional CSAMT traverses were conducted in the valley and local hills. A shallow
fracture system and deeper fracture system occurred in the data. The fracture systems correlate between
traverses.
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3-D Gpr Imaging Of The Neodani Fault, Central Japan
Authors Ernest C. Hauser, Daiei Inoue, Shintaro Abe, K. Kusunoki and Yoshida OdaGPR data collected across a segment of the Neodani Fault in the Tokai region of central Japan
represents the first 3-D GPR data successfully collected across a major seismogenic fault in Japan.
Despite the inherent difficulty of GPR to significantly penetrate wet, clay-rich soils, a 3 meter bedrock
offset across the fault was imaged through 3-6 meters of saturated unconsolidated material. The GPR
data were collected astride the known trace of the fault in a 12.5 x 12.5 m area adjacent to an earlier
excavated trench, which provided constraints on depth and overburden stratigraphy.
These GPR data were collected using a stepped-frequency (1-45 MHz) GPR system in bistatic
mode using a constant antenna center point separation of 2.5 m. The data comprise a series of 23
parallel 2-D profiles spaced 0.5 m apart, with GPR soundings along each profile collected at a 0.5 m
spacing.
On October 28, 1891, the 80 km long Neodani Fault and surrounding region experienced one of
the largest historic earthquakes in Japan, the Nobi earthquake, which registered an estimated magnitude
8.0. This study is near the epicenter of that earthquake, and not far from the Earthquake Fault
Observation Museum, which is designated a “National Monument of Special Interest.” Significant
variation of vertical offset occurred along the largely strike-slip Neodani Fault and this museum
preserves a spectacular excavation of the fault exhibiting 6 m of vertical offset.
These results indicate that 3-D GPR profiling is a viable method for locating or observing
bedrock fault offsets concealed beneath unconsolidated material, even in the generally GPR-challenging
ground conditions found in Japan, and could be used to nondestructively identify or locate optimal sites
for trenching.
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Use Of Ground Penetrating Radar For The Determination Of Soil Profiles In Slopes Of Rio De Janeiro
Authors Patricia L. Grazinoli, Euripedes A. Vargas Jr. and Franklin AntunesThe present paper describes the experience in the use of the Ground Penetrating Radar (GPR) in
the determination of soil-rock interfaces and weathering profiles in natural slopes in the City of
Rio de Janeiro, Brazil. A number of slides and other stability problems occurred in natural slopes
during the summer of 1996, when heavy rains affected the degree of saturation of the soils above
the rock, mostly colluvial and residual soils of gnaissic origin. In two such slopes, described in
the present paper, a slide occurred at the soil-rock interface.
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Gpr As A Cost Effective Bedrock Mapping Tool For Large Areas
Authors Jutta L. Hager and Mario CarnevaleHager GeoScience, Inc. conducted a geophysical investigation at the site of a former
manufacturing plant in Massachusetts at which contamination had been detected in fractured bedrock.
The area of interest covered approximately 48 acres of mixed-use and residential neighborhoods,
including a park and cemetery. There was concern about the location and movement of contaminants
off the site toward a nearby river. The client had already installed a number of bedrock wells around the
contaminant source in order to track the migration direction of the contaminants, as well as an air
sparging system to treat contaminated groundwater at the source. After noting anomalous data during
pumping of bedrock recovery wells, the client hired HGI to help determine the best locations for
additional deep rock pumping wells, as well as to track groundwater (and thus contaminant) movement
off the site. HGIs investigation integrated GPR and seismic refraction surveys with data from geologic
and aeromagnetic maps, outcrop data, and fracture trace analysis.
The study area is located along the northeast margin of the Narragansett Basin. Geological
research prior to starting the geophysical fieldwork identified a network of lineaments/fractures,
predominantly trending northwest. The geophysical surveys were designed to relate the lineaments to
possible fractures, as well as to map the stratigraphy and bedrock surface to determine the direction of
groundwater flow. To this end, approximately 3,000 linear feet of seismic refraction data and 10,000
linear feet of GPR data were collected, mostly along roads and sidewalks.
The seismic refraction survey established the correlation between lineaments and possible
bedrock fractures on the basis of low seismic velocities. Sufficient GPR data were collected to produce
a bedrock contour map that helped resolve the apparent contradictions between predicted and actual
hydraulic gradients under pumping conditions, as well as give the client additional pumping well
locations.
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An Automated Facility To Study Processes Using 4D Gpr
Authors Roelof Versteeg and Ralf BirkenOne of the key innovations in geophysics is the use of so called time-lapse or 4D geophysics.
This technology has been applied in all ranges of geophysics from whole earth to near surface. While it
is easy enough to collect two almost identical surveys on approximately the same location, and it is
fairly common to observe differences between these datasets, a detailed quantitative interpretation of the
differences in terms of processes has up to now not been possible. There are two reasons for this. One,
the temporal data density of the data is most often very low (comprising only two or three datasets), and
consequently we will temporally alias our observational data on high frequency processes. Second, we
lack a fundamental understanding on how to interpret our data in terms of processes. To understand
the possible consequences of this (and to resolve these issues) we need to have high quality 4D data
with high temporal (as well as spatial) densities preferably collected in a controlled environment so that
we can understand how we to image and study the processes.
This was the motivation behind the creation of Columbia's subsurface imaging laboratory. This
laboratory allows for the controlled creation of medium scale models consisting of unconsolidated
materials. It consists of a large (6 x 6 x 8 feet) aboveground tank, a completely automated data
acquisition facility and an infrastructure for model creation and disassembly.
As there was no model on which to base this facility its construction and implementation
involved a trial and error approach. However, after a two year construction and testing phase the facility
is now fully operational, and is able to generate super high density 4D datasets of superior quality. In
creating this facility we have learned a significant amount of things about building medium scale
physical models and automated data acquisition and processing. The essential elements in creating this
and similar facilities are discussed, as well as potential pitfalls that one can face with the hope that this
paper will provide valuable guides for future efforts in this direction.
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1D Inversion Of 4D Radar Data To Image Fluid Flow
Authors Roelof Versteeg and Shan WeiOne of the essential steps in geophysics is the conversion of geophysical data to a map of
physical properties - something which can only truly be achieved through inversion. While full
wavefield inverse methods are widely applied in acoustical imaging, their application to radar has been
limited for a number of reasons. These include the complexity of antenna – soil coupling, the fact that
antenna radiation patterns are unknown and the underdetermined nature of radar data (in that radar data
is in general single rather than multi offset) as well as the large number of parameters we have to invert
for. Thus, the majority of current radar inversion efforts are actually kinematic inversions. We
investigate the feasibility of a 1D full waveform radar inversion effort. We demonstrate that while we
can do 1D radar inversion using a mixed simplex/Powell optimization scheme, the results show a
fundamental ambiguity between parameters. From theoretical grounds the extension of the inversion to
higher spatial dimensions would seem not to be sufficient to resolve this ambiguity. However, an
alternative to 1D inversion is 1D timelapse inversion which is more feasible as it adds a whole new level
of information. We implement a first simple form of 1D timelapse inversion on a data set collected in
Columbia’s subsurface imaging lab which demonstrates both the success and the potential of this
method.
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Imaging Fluid Flow And Relative Hydraulic Conductivity Using Ground Penetrating Radar In A Controlled Setting
Authors Roelof Versteeg and Ralf BirkenOne of the core goals in near surface geophysics is the imaging of fluid flow and the associated
determination of hydrological parameters. While the direct determination of hydrological parameters has
been pursued by a number of scientists the inherent uncertainty in a direct estimation from static data
seems to make the success of such an approach very doubtful. However, while it is nearly impossible to
estimate flow and hydrological parameters from static data it is significantly easier to estimate flow and
hydrological parameters from dynamic data. The implementation of this effort is demonstrated on a 162
fold 4D GPR dataset over an oil injection experiment which was collected in the subsurface imaging lab
at Columbia University.
Analysis of this dataset shows that we can isolate fluid flow in four dimensions in near real-time
using geophysical imaging. This in turn allows us to deduce relative hydraulic conductivity. While the
dataset at hand is an ideal dataset the applications of this method (using a range of other geophysical
methods) to subsurface remediation and process imaging are extremely promising.
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Application Of Borehole-Radar Methods To Image Two Permeable Reactive-Iron Walls At The Massachusetts Military Reservation, Cape Cod, Massachusetts
Authors Peter K. Joesten, John W. Lane, Jennifer G. Savoie and Roelof J. VersteegA pilot-scale study was conducted at the Massachusetts Military Reservation, Cape Cod,
Massachusetts, to assess the use of a hydraulic-fracturing method to create vertical permeable walls of
zero-valent iron to remediate ground water contaminated with chlorinated solvents at depths exceeding
the range of conventional iron-wall installation methods. At the test site, ground-water contamination
extends from 24 to 37 meters (m) below land surface. A treatment zone consisting of two parallel
reactive-iron walls 12 m long, 13 m high, and 0.15 m thick, separated by about 6 m, was designed to
intersect and remediate a portion of the CS-10 plume. The U.S. Geological Survey used a cross-hole,
common-depth radar scanning method to test the continuity and estimate the lateral and vertical extent
of the two reactive-iron walls. The cross-hole radar surveys were conducted in boreholes on opposite
sides of the iron injection zones. Significant decreases in the amplitude of the radar pulse observed in
scans traversing the injection zones were interpreted by comparing radar field data to results of twodimensional,
finite-difference, time-domain models and laboratory-scale physical models developed to
predict the effects of wall edges and discontinuities on common-depth cross-hole radar measurements.
As part of a feasibility study, single-hole radar reflection data was used successfully to image the walls.
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Crosshole Georadar Measurements On An Alpine Rock Glacier
Authors Martin Musil, Hansruedi Maurer, Klaus Holliger and Alan G. GreenTo investigate the internal structure of a rock glacier in the eastern Swiss Alps, traveltime and
amplitude tomographic inversions have been applied to borehole georadar data. Pronounced variations
of electromagnetic velocity at the study site required a nonlinear inversion algorithm to be applied.
Traveltimes and amplitudes provided complementary subsurface information, such that a joint analysis
of the resulting velocity and attenuation tomograms allowed an enhanced interpretation of the data.
Features resolved included two distinct zones of enriched ice content, a sharp transition between frozen
and unfrozen material, and a highly attenuating zone within the bedrock.
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Ground-Penetrating Radar And Swept-Frequency Seismic Imaging Of Shallow Water Sediments In The Hudson River
Authors Roelof Versteeg, Eric A. White and Karl RittgerGround-penetrating radar and swept-frequency seismic sub-bottom data were collected on the
Hudson River between Kingston and Saugerties, New York, in April, 1999, as part of a pilot project to
create a comprehensive benthic map of the Hudson River. The radar and seismic data were collected
simultaneously to evaluate the usefulness of each method for shallow-water stratigraphic mapping. The
data were used in preparation of a benthic map and for creation of a facies distribution map.
The results show that in shallow water (less than 20-feet deep) in the Hudson River, the radar
method obtains better penetration and resolution than the seismic method. Virtually all radar data
collected in shallow water shows detailed sub-bottom structure, whereas 65 percent of the seismic data
does not show any sub-bottom penetration, due to the presence of methane gas in the sub-surface and
(or) a hard water bottom.
The majority of the interpreted facies show sub-bottom deposition that formed in a relatively low
energy environment. Significant changes do occur over relatively short distances however. This allows
a GIS-based interpretation of the mapping of the spatial distribution of the facies and the recognition and
differentiation of sedimentary regimes in the river.
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A Comparison Of Vertical And Horizontal Gpr Velocity Estimates In Alluvial Sediments
Authors William P. Clement and Michael D. KnollWe have measured the horizontal and vertical electromagnetic velocity through an alluvial, sandand-
cobble aquifer near Boise, Idaho. To measure the horizontal velocity, we deployed antennas in two
wells 6.9 m apart. To measure the vertical velocity, we placed one antenna on the surface adjacent to a
well and lowered the other antenna down that well. We collected data at three wells; two wells at either
end and a well located in between the two end wells. We thus have one measure of the horizontal velocity
and three measures of the vertical velocity. The horizontal and vertical velocity are essentially the same
below the water table at 2 m depth. In the vadose zone, the velocities differ, but we think that refracted
waves cause the velocity differences. The directional independence of the velocities indicates that we can
assume isotropic models in our interpretation of the aquifer.
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Field Test Results Of A Beam-Steered Ground Penetrating Radar Array
Authors Neil. F. Chamberlain and William. M. RoggentThis paper presents the results of field tests of a beam-steered ground penetrating radar (GPR)
system. The radar comprises an array of transmitting antenna elements and a conventional GPR receiver
system. The radar operates in bistatic mode. Each transmit antenna element is equipped with its own
transmitter electronics and timing circuitry. Beam-steering is achieved by appropriately delaying the
trigger pulse to each transmitter module. The transmit signal has a narrower and more powerful beam
when compared to convention single-antenna GPR systems, resulting in deeper penetration and enabling
better rejection of clutter through signal processing.
Results of field tests are presented for the radar operating on a medium of dry granite. The tests
verify that a narrow-beam GPR signal can be successfully steered in this particular medium.
Additionally, the results are discussed in relation to finite difference time domain analyses, with regard
to the broader implications for radar design and operation.
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Measurement Of Radiation Impedance Using A Portable Vibrator To Evaluate Mechanical Properties Of The Subsurface
Authors Yasuhiro Kaida, Dai Nobuoka, Jan Brouwer and Vincent NijhofIn civil engineering and environmental applications, it is important to evaluate the mechanical properties of
soils and rocks. Reijnders et al., (1999) introduced a method to measure the radiation impedance of the very shallow
subsurface using a portable vibrator. The method is based on the determination of the frequency-dependent integral
elastic response of the soils below the vibrator baseplate using reference signals measured at the source.
Radiation impedance is defined as the ratio of the stress acting on a particle and the resulting particle
velocity. A vibratory source equipped with accelerometers on baseplate and reaction mass allows the determination
of the force that acts on the surface as well as the baseplate velocity. If one makes the assumption that the velocity of
the baseplate is in phase with the velocity of the material directly beneath it, both the stress and the velocity, and thus
the radiation impedance, can be determined from measurements of the accelerations at the source.
Several field experiments were conducted to evaluate this method and are introduced in this paper. One of
these experiments was performed to evaluate the variation of radiation impedance over an area of re-filled and
compacted ground. High impedance was observed for the re-filled area. Obviously, this variation of radiation
impedance reveals the effect of compaction. This observation indicates that measurement of radiation impedance can
be applied to evaluate the mechanical properties of very shallow subsurface materials.
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