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18th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 03 Apr 2005 - 07 Apr 2005
- Location: Atlanta, Georgia, USA
- Published: 03 April 2005
1 - 20 of 146 results
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A Ground Penetrating Radar System for High Loss Environments
Ground penetrating radar (GPR) is most successful when the electrical conductivity, σ, of the ground is low and scattering losses are low. In that case it is assumed that radiated electromagnetic pulses maintain their shape as they propagate. If σ ~ ωε, where ω is angular frequency and ε is the dielectric permittivity of the ground, this assumption fails and radiated pulses both attenuate rapidly and broaden, resulting in indistinct images of the subsurface. We aim to extend the effective depth of investigation, enhance the clarity of subsurface images, and improve the accuracy of inversion for subsurface electromagnetic properties in high loss earth. We have built a new GPR that achieves high dynamic range by means of a unique phase-preserving linear/logarithmic receiver and real-time signal digitizing and averaging. Back-shielded antennas minimize radiation into the air. The first experimental results in high loss conditions show improvements over previous GPR surveys.
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Improved Parallel Seimic Technique for Foundation Assessment
Authors Ernst Niederleithinger, Alexander Taffe and Thomas FechnerNondestructive and geophysical techniques play an increasing role in civil engineering. Among
many others the investigation of pile foundations with unknown length is a major field of application.
There are many techniques as pile integrity testing, mise a la masse and parallel seismic, each with
special advantages. The parallel seismic technique (hammer impact on a pile head, registration of elastic
waves in a nearby borehole) benefits from the fact that it does not need calibration. First results using an
improved interpretation methods suggest that an accuracy up to 1% could be achieved. In addition,
information of the surrounding soil is acquired. The work described was done in the frame of the joint European project RUFUS (Butcher,
2003).
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Seismic Refraction Interpretation Using Finite Difference Method
More LessCorrect interpretations of seismic refraction profiles are contingent upon accurate processing,
especially in problematic sites where hidden layers may be present. It is found that the Generalized
Reciprocal Method (GRM), which is used to solve hidden layer problems, does not give accurate results
in some areas without additional modification to the processing flow. To improve the final GRMderived
velocity-depth (V-D) model, the Finite Difference (FD) method is employed to calculate the
first arrival’s direct and refracted times for any velocity medium. This newly-developed FD technique is
then used to improve the initial GRM-derived V-D model. Three synthetic seismic data sets are used to
validate this technique, each processed by GRM-only and FD modified GRM. In each case, the FD
modifications are found to provide more accurate results in comparison to those from GRM-only
processing flows.
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Estimating Vibration Response of East Canyon Dam, Utah, From P-, S-, and Surface-Wave Measurements
Authors Richard D. Miller, Julian Ivanov, Richard D. Markiewicz and Daniel O’ConnellCompressional and shear reflection, compressional and shear VSP/check shot, compressional
refraction tomography, and multi-channel analysis of surface waves (MASW) techniques were evaluated
and determined effective and accurate in defining and delineating the seismic wave velocity structure of
rocks supporting a thin-arch cement dam in north-central Utah. A reliable measure of seismic properties
as a function of depth is important to the comprehensive and accurate appraisal of site response and
vibration modes in concrete dams. Models used to predict dam performance during earthquakes are only
as realistic as the material attributes incorporated into those simulations. Proven correlation between
seismic properties and stiffness/rigidity is the basis for highly detailed measurements of the seismic
wavefield at this dam site. Optimal 30-fold CMP seismic reflection profiles provided images from
within the massive conglomerate supporting the dam. The conglomerate possessed bedding plains
dipping upward of 20 degrees and visible fractures both along bedding plains and at right angles to
dominant bedding surfaces. Of particular interest was the right abutment of the dam, which was not only
most accessible but the strongest influence on the dam performance during ground shaking events. VSP
profiles through both the cement dam and downstream toe provided excellent velocity control and
identified changes in rock types within the first 200 ft below ground surface. Calculations of Poisson’s
ratio based on continuous, detailed, coincident measurements of compressional and shear-wave
velocities at each significant geologic contact and within each major geologic unit were critical to
realizations of site response at this site where failure potential is known to exist.
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Geophysical Investigation at Archeological Sites In Peninsular Thailand
Authors Warawutti Lohawijarn and Pongdhan SampaongeonGeophysical investigation successfully located remnants at archeological sites in Nakhon Si Thammarat Province in Peninsular Thailand. The vertical gradient of geo-magnetic field, electrical ground resistance mapping and ground-penetrating radar (GPR) were employed in the present investigation. Anomalous zones of magnetic gradient, highly resistive patches of shallow ground and discontinuities in the subsurface layering on GPR images were observed. They were indicators of buried archeological remnants, shortly after the survey verified by archeological pitting. These results show that geophysical techniques are a powerfully non-destructive tool in locating promising positions of buried archeological remnants before archeological pitting is commenced. This can save time and budget for archeological investigations elsewhere.
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Differential Surface Wave Detection of Changes In Fluid Saturation In Soils
Authors L.T. Long and Tatiana TotevaDifferential surface-wave analysis is a new approach to the interpretation of temporal variations
in surface wave dispersion. The analysis technique utilizes the difference between traces recorded
before and after a change in shear-wave velocity. A multiple filter technique is used in order to measure
perturbations in dispersion as a function of frequency for each point along a refraction line. The traces
are normalized at the time of arrival of the phase of interest. The amplitude of the difference between
the normalized traces is a direct indicator of the phase difference and, hence, travel-time difference. The
differential surface wave technique allows determination of perturbations in shear-wave velocities with
much greater precision than is possible for the direct determination of structure. The perturbed structure
can then be computed relative to a reference structure that need only approximate the actual structure.
We tested this technique by recording traces along a refraction line near a shallow injection well. We
detected a 2% change in velocity over a distance of 3 meters for water injected at a depth of 0.5 meters.
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Buried Channel Delineation Using A Passive Surface-Wave Method In Urban Area
Authors Koichi Hayashi, Tomio Inazaki and Haruhiko SuzukiA passive surface wave method has been applied to delineate buried channels in urban area of
Japan. S-wave velocity structure down to 100m is very important in the local site effect of strong
ground motion caused by earthquake. Especially, the buried channels filled with alluvial deposits
intensify seismic waves and cause strong ground shaking. We have tried to delineate threedimensional
S-wave velocity structure down to 100m on the basis of the passive surface wave method. The test site is in Soka city, Saitama prefecture, Japan and the size of the site is about 3 km
square. Sixty-two passive surface wave methods were carried out to delineate buried channels filled
with alluvial deposits, which is embedded about 50m beneath surface of this area. Array size is
about 50 to 100m and triangular or L shaped arrays with 10 or 11 receivers were deployed. A spatial
auto correlation method was applied to the approximately ten minutes vertical component of microtremors
data. Phase velocity curves were calculated in the frequency range of between 2 and 10 Hz.
Fundamental mode of phase-velocity curves are clearly obtained in all observation points. A one
dimensional inversion using a non-linear least square method has been applied to the phase-velocity
curves and one-dimensional S-wave velocity structures were obtained. The resultant onedimensional
structures were interpolated into a three-dimensional structure. We succeeded to map
the shape of buried channel and the depths of the channel agree very well with the borehole data.
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Masw and GPR Survey To Delineate Depth-Tobedrock and Crystal Cavities for Mineral Exploration, Hiddenite, North Carolina
Authors Mario Carnevale, Jutta Hager, Jonathan W. Brinkmann and Brian R. JonesMASW and GPR methods were used as exploration techniques to locate potential
mineral deposits within a geologic setting of highly deformed metamorphic rocks
overlain by lateritic soil horizons. A 24-channel land-streamer system was used to profile
overlying lateritic horizons and bedrock up to depths of 100 feet. MASW survey
geometry was optimized for depths up to 60 feet and high horizontal resolution. Highpowered
100-MHz bi-static antennas were used to collect data along the seismic lines as
well as in areas unsuitable for the MASW method.
The two exploration methods provided independently derived constraints to the
bedrock depth and structural model. The GPR method was effective in identifying
shallow targets in the laterite, determining depth to competent bedrock, and identifying
targets within the shallow bedrock zone. The MASW 2-D shear wave velocity (Vs)
profiles were useful for delineating the laterite/rock interface and identifying anomalies
near the top of and within the competent bedrock zones. The data were integrated to
develop a bedrock structure map and spatially delineate exploration targets.
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Towards The Standardization of Multi-Station Surface Wave Method for Site Investigation
Authors Chih-Ping Lin, Tzong-Sheng Chang and Chun-Hung LinThe application of surface wave method for site investigation becomes more and more popular in
practical uses due to its non-intrusive tests and convenient operations. In particular, multi-station
recording permits a single survey of a broad depth range and high levels of redundancy with a single field
configuration. However the field testing configuration plays a major role to ensure good data quality for
constructing a dispersion curve with a wide range of frequencies. The adjustments of field configuration
usually need experienced testers’ “educational guess” after referring to the results from preliminary tests
for ensuring the data quality. This guessing process makes the surface wave method an unfriendly
technique for general field engineers. The effects of survey line parameters, such as near offset, receiver
spacing, and offset range, are investigated and respective criteria for deciding the proper configuration are
illustrated. Furthermore, a new concept called “pseudo-section” is introduced to synthesize a series of
data for selecting optimum offset range for each frequency and increasing the lateral spatial resolution.
This study will lead to further standardization of the surface wave testing.
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Use of Numerical Simulations To Explain Sasw Field Measurements
Authors A. Nasseri-Moghaddam, G. Cascante, C. Phillips and J. HutchinsonThis paper uses numerical models to explain the results of two SASW field tests in the presence
of a void. The Fourier spectra of the field data contain a region with high energy concentration, in the
proximity of the void. Numerical models are constructed and the responses at the surface of the medium
and around voids of different sizes and embedded depths are monitored. The numerical results show that
part of the incident energy is trapped in the void region. The trapped energy bounces back and forth
between the boundaries of the void until it is attenuated by radiation. The effect of the trapped energy is
seen as a concentration of energy over the void region in the frequency domain. The amount of trapped
energy is a function of the size and embedment of the void as well as the frequency content of the
source. Moreover, the void absorbs part of the energy and radiates it as body waves. The numerical
observations conform closely with the field data. Therefore the recorded responses at the surface carry
valuable information about the void. The characteristics of the void can be extracted from the surface
responses by analyzing the responses in time, frequency and spatial domains.
Keywords: Rayleigh waves, finite differences method, void detection, geophysical testing
method, frequency domain analysis
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Discussion On Some Practical Equations With Implications To High-Frequency Surface-Wave Techniques
Authors Jianghai Xia and Yixian XuWe discuss five useful equations related to high-frequency surface-wave techniques and
their implications in practice. These equations are theoretical results from published literature
regarding source selection, data-acquisition parameters, resolution of a dispersion curve image in
the frequency-velocity domain, and the cut-off frequency of high modes. The first equation
suggests Rayleigh waves appear in the shortest offset when a source is located on the ground
surface, which supports our observations that surface impact sources are the best source for
surface-wave techniques. The second and third equations, based on the layered earth model,
reveal a relationship between the optimal nearest offset in Rayleigh-wave data acquisition and
seismic setting—the observed maximum and minimum phase velocities, and the maximum
wavelength. Comparison among data acquired with different offsets at one test site confirms the
better data were acquired with the suggested optimal nearest offset. The fourth equation
illustrates that resolution of a dispersion curve image at a given frequency is directly proportional
to the product of a length of a geophone array and the frequency. We used real-world data to
verify the fourth equation. The last equation shows that the cut-off frequency of high modes of
Love waves for a two-layer model is determined by shear-wave velocities and the thickness of
the top layer. We applied this equation to Rayleigh waves and multi-layer models with the
average velocity and obtained encouraging results. This equation not only endows with a
criterion to distinguish high modes from numerical artifacts but also provides a straightforward
means to resolve the depth to the half space of a layered earth model.
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Multi-Component Ground Penetrating Radar for Improved Imaging and Target Discrimination
Authors Douglas S. Sassen and Mark E. EverettGround penetrating radar (GPR) is routinely used to image subsurface targets that have high degrees of directionality including faults, pipes, rebar, and unexploded ordnance. The ability to image and discriminate these subsurface targets depends strongly on the antenna configuration used and the orientation of the radar antennas with respect to the orientation of targets. However, designing optimal surveys is complicated by the fact that the dimensions and orientation of targets are often unknown. This dilemma is addressed by using a multi-component GPR survey technique that is sensitive to the vector nature of the received signal. The received instantaneous amplitude signals from this survey technique are represented by a second-rank tensor. Using tensor manipulations it is possible to determine the amplitude signal for any antenna orientation from two co-polar and one cross-polar antenna configuration. This is used to produce images of the subsurface for any antenna orientation, eliminating the need for prior knowledge of subsurface targets to design surveys. Also, the eigenvalues and eigenvectors of the amplitude tensor could be used to discriminate targets on the basis of polarization, and to determine their orientation. Field studies using buried pipes provide experimental support for this technique.
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Estimating Dipping Angle With Cross-Borehole Radar: No Tomography Required
The effects of a single dipping layer on ray path inversion for zero-offset and fixed-offset crossborehole radar is investigated theoretically and experimentally. The theoretical investigation includes deriving the forward solution to the first arrival travel time profile as the borehole antennae are lowered across the dipping layer. The first arrival travel time profile will consist of direct, critically refracted, or cross-dip refracted arrivals. The slope of the travel time profile with depth can be used to distinguish cross-dip from critically refracted arrivals. The inversion model is used to calculate the dip angle and depth using the zero-offset and one fixed-offset travel time profile. For this geometry, full tomography is unnecessary. The experimental investigation includes measuring zero-offset and a fixed-offset first arrival time profile across a dip, which has been previously characterized by borehole logging of the electrical conductivity and neutron count. The inversion of the two borehole radar travel time profiles provides consistent information with the borehole logging in regards to a dipping clay layer located approximately 9 m below ground surface.
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Identifying Shallow Expansive Clays Prior To Site Development Using Capacitively Coupled Resist
Authors K. Michael Garman and Scott F. PurcellCapacitively coupled resistivity (CCR) has proven useful for mapping shallow soil conditions
over large sites prior to development. Because the CCR unit can be towed at a rate 1 to 5 kilometers per
hour, it allows large sites to be surveyed quickly. The availability of a multi-channel CCR instrument
allows resistivity profiles to be generated from a single pass along a transect line. The CCR profiles
provide useful information to the geotechnical engineers evaluating the site, in particular:
1. The ability to delineate areas with shallow expansive clays, which are a concern for shrinking
and swelling beneath a foundation; and
2. The ability to delineate areas of deeper surficial sands, which are suitable for fill.
As part of a recent study, resistivity values were compared to the plasticity index and grain-size
analyses of the soils. The CCR data were then used to delineate areas of high plasticity clay, low
plasticity clay and clean sands.
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Resolving Fine -Scale Hydrological Features In Electrical Resistivity Tomography Images
Authors Douglas LaBrecque, Roger Sharpe, James Brainard, Mehdi Eliassi and David AlumbaughThe resolution of electrical resistivity tomography surveys is limited by the data quantity, data
quality, and the survey configuration (electrode count, spacing, borehole separation). Inevitably, we are
forced to interpret data from regions containing substantial variability at a scale much finer than the
resolution of the method. To understand the effects of these fine-scale structures on interpretation, data
were created by converting hydrological parameters from high-resolution flow simulations to electrical
conductivity. The data were then forward modeled using fine, high-resolution meshes and inverted using
the same coarse meshes that were used for normal inversion of field data. The modeling simulated
earlier experiments from the Sandia-Tech Vadose Zone Facility. In those experiments, geophysical
imaging techniques were employed to monitor the unsaturated flow of potable water and transport of a
salt tracer through fluvial sediments.
Comparing images to the original hydrological models shows that the images are smoother and
much of the fine detail is lost but major features such as the center of a zone of infiltrating water are
correctly represented. However, comparing images of synthetic data with images from previous field
studies shows that the hydrological models themselves poorly represent the original field conditions.
The synthetic models show narrower zones of infiltration with much higher saturation than the field
case.
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Exploring Directional Differences In Resistivity Results In Karst
Authors Ronald Manney, Mary J.S. Roth and Jonathan E. NyquistWe are involved in a multi-year study to evaluate electrical resistivity as a tool to characterize
shallow karst. As part of this study, we have conducted a 56-line multielectrode resistivity survey (28 x
28 grid of tests) and a 12-line azimuthal survey on a site with a known cave and other karst features.
(The cave is located at the approximate center of the grid and at the center of the azimuthal tests.) In the
inversion results for these lines, the air-filled cave appears to be a high resistivity feature when the tests
are run perpendicular to geologic strike and a low resistivity feature when the tests are conducted
parallel to strike. We have used data from borings taken at the site and an understanding of karst
geology to develop a subsurface model that results in similar anomalous results when used as data for a
resistivity forward modeling program. We conclude that the fracture patterns in the rock and water in
the vadose zone create the conditions causing the anomalous results observed and propose an
investigation approach that would help identify such conditions in the field.
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The Effect of Changing The Salinity of The Groundwater On Sip of Sandstones
Authors Andreas Weller, Julian B.T. Scott, Norbert Schleifer and Ron D. BarkerPrevious work between the University of Birmingham and Technical University Clausthal
examined the basic shape of the complex conductivity spectra of sandstones saturated with a synthetic
groundwater solution and compared the two different laboratory measurement procedures. It is known
that the Spectral Induced Polarisation (SIP) is also dependent upon the pore fluid so we examined the
effect of two different salinity groundwaters on the SIP response. The changes caused by decreasing the
salinity of the groundwater varied widely for different sandstones with both increasing and decreasing
phase angles recorded along with some changes in the shape of the spectra. We are now continuing to
make multiple salinity measurements with a variety of cations in order to better quantify these changes.
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Integrated Geophysical Characterization at A Contaminated Site
Authors Mario Carnevale, Jutta Hager and Brian R. JonesA multidisciplinary geophysical study was performed at Naval Air Station, Brunswick, Maine.
The goal of the study was to provide a better understanding of the possible migration pathways of the
contaminants in bedrock fractures and deeper stratigraphic zones whose geometries might be controlled
by bedrock morphology. The scope of work consisted of mapping the bedrock surface, identifying and
locating fracture zones, and mapping the continuity and extent of key stratigraphic horizons. Seismic
refraction and reflection, GPR, and resistivity were used to meet the study goals.
Geophysical data were combined with existing borehole and cone penetrometer information to
produce an integrated database that was used to create models of the bedrock and key stratigraphic
surfaces. Both raw data and model-slice profiles were prepared to illustrate the morphology of the
surfaces and identify possible bedrock fractures.
The results of the study included delineation of the post-glacial bedrock valley terrain,
identification of glacial depositional features, and suggestion of a pattern for fracture corridors in the
study area. Low-velocity zones identified in the refraction surveys and fractures interpreted from the
resistivity profiles correlated well with some previously identified regional and local lineaments.
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Airborne Magnetometer Data Reduction and Evaluation at The former Eod Area (Naad 01) Camp Navajo, Northern Arizona
Authors Raye M. Lahti, Joseph W. Dauchy, Joe B. Davis, William Myer and Randall WilkinsonThis paper discusses the results of the field activities conducted to evaluate airborne
magnetometer data collected over the Former Explosive Ordnance Disposal (EOD) Area (NAAD
01) at Camp Navajo, located near Flagstaff, Arizona. Airborne magnetometer data, collected by Oak
Ridge National Laboratory (ORNL), was ground validated at selected anomaly locations. The
anomalies evaluated were selected based upon sensor height above ground surface (altitude),
geology, signal strength, and the ORNL “UXO likeness” model.
A land surveyor reacquired the reported location of the selected anomalies in the field. Then,
an 8- by 8-meter investigation area was delineated around each selected airborne anomaly location.
The grid was re-surveyed with ground-based cesium vapor magnetometer (CVM) and time-domain
electromagnetic sensors (EM61). Surface materials were documented and removed and a second
survey with both sensors was conducted. Anomalies identified from the ground-based surveys that
were considered the most likely cause of the airborne magnetometer anomaly were excavated. Data
documented included material density, size, and distance from the declared airborne magnetometer
anomaly location.
In areas of optimal altitude (below 3.5 meters) and favorable geology (minimal basaltic
interferences), there was excellent correlation (>95 percent) between the airborne and the groundbased
surveys. In most cases the airborne magnetometer platform was able to detect heavy
fragmentation items as small as 10 by 20 centimeters with a positional error of less than 2-meters.
Additional evaluations of the ground based CVM and EM61 data, shows similar geophysical
response from the regionally predominant basaltic formations.
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Highly Contaminated Uxo Sites: Combination of GPR and Emi for Discrimination of Clustered Scatterers
The most fundamental electromagnetic limitation on discrimination of subsurface unexploded
ordnance (UXO) during cleanup operations is that one must use quite low frequencies to penetrate the
ground. Operating between some 10’s of Hz and some 100’s kHz, electromagnetic induction (EMI)
sensor signals are sensitive to many aspects of target shape and composition. In that band, the signals do
not suffer the scattering and absorption loss problems that challenge ground penetrating radar (GPR).
However, EMI transmitted wavelengths are many, many orders of magnitude greater than the size of
targets of interest. This means that distinct targets cannot readily be picked out by timing the arrival of
echoes or by noting the direction they are coming from, as for wave phenomena. Clustered targets will
respond simultaneously and their signals overlap. This is a particularly important problem because most
UXO cleanup sites contain much metallic clutter. The number of targets and their locations are hard to
tell from EMI data only.
Our full-polarimetric UWB GPR operates between some 10’s of MHz and about 800 MHz, i.e. at
a low enough frequency to penetrate the soil, minimize scattering losses, and elicit essential target
resonances, but necessarily too low to form precise target images. What GPR can often do, however, is
time the arrival of target echoes from distinct targets, even when they are clustered, and feed into EMI
processing some crucial information on number of targets, approximate locations, and other geometrical
data. Altogether, EMI signal optimization constrained by GPR data produces separate EMI signature
patterns for each item, indicating whether the object is UXO-like or not.
Traditional fast EMI forward modeling contains too many free parameters, which is a serious
challenge to inversion algorithms, especially for multiple targets. In this paper we propose a three step
approach for UXO discrimination: (1) preliminary screening with GPR information to identify or rule
out obvious UXO candidates; (2) Analyze EMI data with simple dipole model, using GPR information
as prior information. The results are again used to identify or rule out obvious UXO candidates; (3) For
cases where final decisions can not be made in step one and two, a pattern matching approach is
employed to identify each candidate UXO, using the first two step results as prior information. Study on
examples illustrates how this three step approach may help improve UXO discrimination and
identification.
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