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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 - 100 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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Broadband Electromagnetic Detection and Discrimination of Underwater Uxo
Authors Bill SanFilipo, Steve Norton and I.J. WonThe electromagnetic induction (EMI) response to metal objects in the marine environment,
including UXO and clutter, is significantly altered by the conductive seawater. Two effects can be
observed – propagation effects associated with the seawater skin depth that attenuate and phase rotate
the response, and the addition of the response associated with the perturbation of the flow of currents
induced in the surrounding water, referred to as the current channeling response (CCR). These effects
distort the multi-frequency spectral response of the target from the normal free-air eddy current response
(ECR, including magnetic polarization for ferrous targets).
If the spectral character is used to classify or identify the target, these effects must either be
incorporated into the model or library used as the reference, or they must be minimized so that they can
be ignored. Since the relative amplitude of the CCR to ECR depends on variable conditions, including
sensor-target geometry, seawater salinity, and the surface condition of the target (paint or corrosion can
result in a negative CCR by forcing background currents around the target), explicitly accounting for
CCR is difficult and subject to knowledge of these variables. Accounting for propagation effects also
depends on knowledge of sensor-target distance. Thus, restricting operational parameters to minimize
the effects of the seawater may be the most straightforward and reliable approach.
In order to quantify the CCR and propagation effects over the full range of operational
parameters, we have developed some analytical models for special cases and we have performed
controlled underwater experiments with spheres to compare with the sphere models, and UXO
surrogates (pipes). Another algorithm that we developed is the perfectly conducting or insulating
spheroid in a conductive medium, which helps quantify the effect of aspect ratio and orientation on CCR
and ECR at the inductive limit and for insulated targets.
As a preliminary test on the potential application of EMI based discrimination, we have
performed our simple single-point spectral matching algorithm on underwater data using a free-air
library. By limiting the frequency range to 12 kHz, and target-sensor geometries to ranges with high
signal/noise (inherently limits lateral offset distances), the ECR dominates over CCR, and propagation
effects are not too great (wave induced noise also is less at lower frequencies). Under these constraints,
the matching algorithm performance is reasonably effective at identifying the target for a small suite (5)
of items in the library. Identifying clutter, based on goodness-of-fit, is more difficult, but will work for
many clutter items.
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Detection of Uxo and Landmines Using 2-D Modeling of Geoelectrical Resistivity Data
Authors Gad El-Qady and Keisuke UshijimaBuried landmines pose serious hazard in more than 60 countries across the globe. Recent
estimations suggest as many as 110 million such objects lay worldwide. Detecting these mines with
standard metal detectors is difficult, as many are comprised primarily of plastic, with often only a firing
pin as the sole metallic component. The development of geophysical methods for unexploded Ordnance
(UXO) detection has been underway for almost ten years, and arose in the international context from the
need to rapidly characterizing large areas of potentially UXO-contamination.
In this work, the geoelectrical resistivity could be one of the most useful parameters to
discriminate ordnance types, shapes and locations especially near the surface. This study represents a
successful trial to adopt the geoelectrical resistivity technique for detecting buried UXO and other clutter
environmental noise. A forward and inversion routine had been adopted and applied to synthetic
resistivity models for different UXO and landmines. In addition, models of non metal ordnance had been
used, and were successfully detected. The obtained results show high efficiency of the technique to
image those models. Now we are trying to adopt the technique to work in the filed, hence apply it for
test site.
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Automatic Detection of Uxo Anomalies Using Extended Euler Deconvolution
Authors Kris Davis, Yaoguo Li and Misac NabighianThe paper presents an algorithm for automatic detection of UXO anomalies in total-field
magnetic data based on the concept of structural index (SI) of a magnetic anomaly. The magnetic
field produced by a dipole-like, source such as a UXO, varies with inverse distance cubed and
therefore has an SI of 3. Identifying magnetic anomalies having structural indices of 3 enables direct
detection of potential UXO targets. The recently developed Hilbert based extended Euler
deconvolution method (Nabighian and Hansen, 2001) provides an efficient means for calculating the
SI by utilizing the Euler equations for the two components of 3D Hilbert transform. We first perform
extended Euler deconvolution to compute SI as a function of position and window size. At each
location, we examine the Euler solutions from different window sizes and identify the ones whose SI
is within a small tolerance of 3. Existence of such a solution indicates the presence of a potential
UXO target.
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Processing and Inversion of Magnetic Gradient Tensor Data for Uxo Applications
Authors Vinicio Sanchez, David Sinex, Yaoguo Li, Misac Nabighian, David Wright and David vonG SmithWe present a set of techniques for processing and inverting multi-component magnetic gradient
data for UXO detection and discrimination. A magnetic gradiometer for the UXO application, developed
jointly by US Geological Survey and Colorado School of Mines, measures the full gradient tensor of a
magnetic field. In contrast to total-field intensity measurements, gradient data have the advantage that
they are less affected by common-mode noise and may carry more useful information about subsurface
targets. Multiple tensor components also enable the extraction of a coherent common signal to further
increase the signal-to-noise ratio. The increased data quality opens new avenues for developing novel
approaches to UXO detection and discrimination. We have examined two different processing methods
based on the inherent relationship among the multiple tensor components, and we have developed an
inversion algorithm for recovering dipole and quadrupole source parameters from tensor data. We
outline these algorithms and illustrate the advantages of gradient data over total-field measurements.
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Travel Time Tomography Using Frequency Responses Collected By Cwr Experimentation
Authors Mohammad Farid, Akram N. Alshawabkeh and Carey M. RappaportDetecting and imaging dense non-aqueous phase liquids (DNAPLs) in the subsurface is a challenging problem that is of interest to the Department of Energy, Department of Defense, and several local and state agencies. Current DNAPL detection techniques, such as Direct Push Probe Technologies (DPT) and In-Situ Tracers (IST) have risks and limitations. Cross-well radar (CWR) is a radar-based geophysical technique with low invasiveness for real time monitoring of DNAPLs. This technique uses electromagnetic waves transmitted and received through antennas in the subsurface. The computerized tomography is applied to a pilot-scale experimental facility constructed by the authors (referred as SoilBED) in this research. Cross-tomography data at multiple depths and locations were collected to simulate a 1/100 scaled contaminated soil problem. The experimental results are converted to the time domain signals. The resulting signal travel times are compared with the simulated results by FDTD. The transformed signals are used to prepare signal intensity and travel time tomograms of the SoilBED cross-sectional slices, for background and scattered field to study the signature of the scatterers. The results are able to visualize the dielectric objects in the saturated soil.
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Soil /Metal Discrimination Analysis of EM 61 Mk Ii Time Decay Response at former Camp Croft, Sc
Electromagnetic (EM) data recently acquired for a munitions detection project, processed using
standard Geosoft Oasis Montaj methods and analyzed in profile, produced more than 1000 targets per
acre, far more than the number predicted by prior clearance in adjacent areas.
Standard data processing was expanded to incorporate a new technique developed by
Geophysical Associates (GPA) for dealing with magnetic soils in Hawaii. This technique is GPA
Soil_Metal Discrimination Analysis, based on statistical Chi-squared discrimination analysis of the
difference of time decay response across the four EM61 Mk II channels from that of soil and rocks.
Preliminary results show that GPA Soil_Metal Discrimination Analysis reduced the number of
targets per acre by approximately 80%. Most of the GPA-derived targets yielded significant metal
items.
As in other geophysical approaches, there remains some residual risk that not all buried
munitions will be detected. Some metallic objects, at certain orientations, have decays similar to
magnetic soil and will yield corresponding Chi-squared results.
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Application and Extension of Texture Feature Coding Methods To Anti-Tank Landmine Detection
Authors Peter Torrione and Leslie CollinsRecent advances in ground penetrating radar (GPR) fabrication and related signal processing have
yielded robust performance on government sponsored blind tests of anti-tank landmine detection
capabilities on test lanes. Recent data collections with the NIITEK GPR system have focused on more
difficult “off-lane” soil conditions that typically contain higher levels of sub-surface GPR anomalies and
provide more difficult tests of anti-tank target detection capabilities. Our recent research in this field has
focused on the application of advanced signal processing techniques to target/clutter discrimination at
pre-screener-flagged locations of interest. In this work we discuss the applications and extensions of a
texture feature coding method (TFCM) for landmine detection in off-lane soils. First we consider
application of the TFCM technique to target detection in 2-D GPR data slices. We also consider
application of the TFCM to “tiled” images containing multiple instantiations of a target response.
Finally we consider a 3-D extension of the TFCM and apply our extension to target detection in 3-D
time-domain GPR data. Our results indicate performance increases for TFCM-based processing of prescreener
generated alarms, with the most robust performance increases resulting from application of our
3-D TFCM extension.
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Multisensor, Remotely-Operated force Protection Mine Detection
Authors Gregory M. Schultz, Karen Norris Zachery and Leslie M. CollinsDespite improvements in both military and civilian mine detection equipment, humanitarian
demining remains a slow, hazardous, and labor-intensive task. The Force Protection Demining System
(FPDS) seeks to provide an efficient and reliable, teleoperated, mine detection platform to eradicate
explosive ordnance and allow reoccupation of mined areas by local inhabitants. The FPDS is equipped
with electromagnetic induction (EMI) and ground penetrating synthetic aperture radar (GPSAR) sensor
arrays integrated onto a remote-controlled, rubber-tracked vehicle. The multi-sensor detection system
fuses source sensor data and/or extracted sensor information to provide an increased level of mine
detection while minimizing false alarms. Extensive preliminary testing at test facilities yields results
that define system integration issues and constrain detection performance for each sensor array. An
important focus of our testing is the characterization of signal-, image-, and physics-based features used
in discriminating targets from clutter. Acquisition of independent and dual-mode data collected over
simulants, landmines, and UXO facilitates the development of a preliminary library of system target
responses from which optimal features are determined. The developed detection software system also
exploits spatial registration and multi-sensor data fusion algorithms to provide real-time automatic target
recognition information to the user.
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A Multisensor System for The Detection and Characterization of Uxo
Authors Alex Becker, Erika Gasperikova, H. Frank Morrison and J. Torquil SmithA prototype active electromagnetic system has been developed for detecting and characterizing
UXO. The system employs two orthogonal vertical loop transmitters and a pair of horizontal loop
transmitters spaced apart vertically by 0.7 m. Eight vertical field detectors are deployed in the plane of
each of the horizontal loops and are arranged to measure offset vertical gradients of the fields. The
location and orientation of the three principal polarizabilities of a target can be recovered from a single
position of the transmitter-receiver system. Further characterization of the target is obtained from the
broadband response. The system employs a bipolar half sine pulse train current waveform and the
detectors are dB/dt induction coils designed to minimize the transient response of the primary field
pulse. The target transient is recovered in a 40 μsec to 1.0 msec window. The ground response imposes
an early time limit on the time window and system/ambient noise limits the late time response.
Nevertheless for practical transmitter moments and optimum receivers the size and the ratio of
conductivity to permeability can be accurately recovered. The prototype system has successfully
recovered the depths and polarizabilities of ellipsoidal test targets.
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The Advanced Ordnance Locator (AOL): A Dual-Mode Tem and Magnetics System for Detection and Classification of Uxo
Authors D.D. Snyder and D.C. GeorgeWith funding from NAVEODTECH (Indian Head, MD), Blackhawk GeoServices is
collaborating with G&G Sciences and Geometrics to assemble and test a dual-mode system for ordnance
location combining a multi-sensor, multi-component TEM system with a total field magnetic
gradiometer. The new sensor array is integrated with an efficient computer-controlled transmitter and a
high-speed multi-channel (28-channel, 800 kilosample/sec) data acquisition system. The resulting
system has unprecedented capability and flexibility for the study, in particular, of the performance of
TEM systems and the noise that affects them. In this paper, we describe our dual-mode system and
present results from both local tests and tests at the Navy’s Baseline Ordnance Classification Test Site
at Blossom Point, MD. We compare data from our TEM system with the Naval Research Laboratory’s
man-portable MTADS and other systems as a baseline. Subsequent to our Blossom Point
demonstration, we experimented with alternative transmitter waveforms in an effort to improve the
signal to noise ratio (SNR) for detection. Our results demonstrate that the AOL system has a
significantly better SNR for detection during dynamic surveys than the other baseline systems with
which we compared. We attribute this improved performance to careful design of system components
together with an appropriate choice of the current waveform parameters (e.g., base period, pulse
polarity, etc) for dynamic data acquisition.
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Multi-Sensor Uxo Detection System
Authors Noel T. Rogers, Stewart K. Sandberg and H.H. BennettA multi-sensor unexploded ordnance (UXO) detection system has been developed in which the
various sensors have been positioned in very close proximity to one another. This system is capable of
simultaneously deploying magnetic and electromagnetic sensors, along with real-time decimeter GPS
positioning, and real-time data monitoring by a single operator.
In one mode of operation, a Geonics EM-63 electromagnetic system is deployed along with a
Gem Systems GSMP-40 optically-pumped potassium vapor magnetometer. In this mode, the distance
between these two sensors on a rigid platform is approximately 1.5 meters. Positioning coordinates,
obtained in real-time, are accurate at the decimeter scale, using a NavCom SF-2050M GPS receiver,
which utilizes the StarFire satellite-based augmentation system (SBAS). Data from the electromagnetic,
magnetic, and positioning systems are recorded in a laptop computer in real-time, and available on a
visual display as profile data plots, and tracking using a map-type display.
Electromagnetic and magnetic sensor placement in close proximity is achieved using a secondary
electromagnetic transmitting coil operating in tandem with the Geonics EM-63. The magnetic sensor is
placed at a null point of the total primary magnetic field generated from the EM-63 transmitter coil and
the secondary coil combination. Magnetic field mapping in the vicinity of these coils was accomplished
using a magneto-resistive chip, which provided several candidate null positions. Extensive testing of the
effect of the secondary transmitter coil on anomalies over known UXO targets provided refinement of
the sensor placement. The result of this work is that the sensor platform configuration produces a
distortion of total magnetic field intensity anomaly measurements less than 10%. Anomaly shape is
only affected to a minimal degree, allowing confidence in resulting computer modeling of these data for
target geometry, and hence discrimination.
Field testing of the platform and sensors took place at two locations near Albuquerque, New
Mexico, and at the U.S. Army Corps of Engineers UXO test bed in Vicksburg, Mississippi. The
instrument platform proved to be robust in these field-scale tests, and the value of multi-sensor datasets
was apparent upon analyses of these data.
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Digital Geophysical Mapping In Wooded Conditions Using An Integrated Em61 Mkii and Robotic Total Station Navigation Vehicular-Towed Deployment System
Authors Martin J. Miele, Ji Ma, Sandra A. Takata, John P. Dolynchuk, Mark Sellers and Larry FowlerShaw Environmental Inc., (Shaw), is conducting digital geophysical mapping (DGM) at the
Former Conway Bombing and Gunnery Range (Conway), Myrtle Beach, SC, using an array of 2 EM61-
MKIIs integrated with a Leica Robotic Total Station (RTS) navigation system towed behind a John
Deere Gator. The site is over 900 acres of Carolina Forest vegetation ranging from rows of trees to
densely wooded areas with interspersed wetlands. The performance objective is to locate all detectable
unexploded ordnance (UXO) items (including 50 mm) to a depth of 4 feet below the surface. This
project is the first high-volume munitions of concern (MEC) survey conducted under these challenging
site conditions using high-tech methodologies.
Given the size of this effort, high-productivity acquisition of high-fidelity data is key to this
project. The survey results show that this deployment is able to detect and characterize 50 mm (and
larger) UXO and UXO-like items with accurate target location in the wooded areas where global
positioning systems (GPS) are in-operable.
This case study discusses the challenges of high-productivity deployment systems in wooded
conditions. Additionally, it demonstrates that aggressive data collection using stable deployment
systems results in both superior quality and cost effective DGM projects.
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A Simple Magnetic Charge Model for Classification of Multiple Buried Metallic Objects In Cases With Overlapping Signals
Authors Fridon Shubitidze, Kevin O’Neill, Irma Shamatava, Keli Sun and Keith PaulsenThis paper presents an application of a simple surface magnetic charge model to discriminate
between objects of interest such as unexploded ordnance (UXO) and innocuous items, in cases when
signals from buried objects are a mixture of responses from two or more items. In the low frequency
(ten’s of Hertz up to several hundred’s of kHz) electromagnetic induction (EMI) sensing considered
here, both conduction and displacement currents may be neglected within the medium surrounding a
metallic object. Therefore, the scattered magnetic field outside the object is represented in terms of
scalar potential fields, from which one can obtain all scattered magnetic fields. While these are time
dependent by virtue of forcing functions or boundary conditions, they correspond in structure to static
fields. Such fields are appropriately, and readily, produced mathematically by equivalent elementary
magnetic charges placed on a convenient fictitious closed surface [1, 2, 3]. This forward model is
physically complete in the sense that all heterogeneity, near and far field, and internal interaction effects
within the object are included. It is very fast; in particular it can be implemented in inversion
calculations on a PC. According to the Gauss’s law,∇⋅B=ρeq , the net flux of magnetic field through any
closed surface equals the total (equivalent) magnetic charge ρeq inside the surface. The frequency
spectrum of this total induced equivalent charge is used here as a discriminant. Based on measured data
from two scatterers together, an iterative two step procedure is used in conjunction with the differential
evolution (DE) algorithm [4, 5]. One step determines of each object’s location and orientation and the
other determines the amplitudes of the responding fictitious magnetic charges. Once the objects are
isolated, the total magnetic charge for each is calculated as a function of frequency and compared to
cataloged/library data. Finally, blind classification analyses are performed for a single object as well as
for multiple subsurface scatterers, when two objects appear simultaneously within the field of view of
the sensor.
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An Empirically Based Ellipsoidal Model for Multiple Frequency Emi Signatures From Uxo
Authors Bruce Barrow, Nagi Khadr, Jonathan Miller and Herbert H. NelsonEfforts to model electromagnetic induction (EMI) signatures from unexploded ordnance (UXO)
have focused on the magnetic polarizability constants that characterize the dipole response of the UXO
along its axes. These polarization factors are a function of time or frequency depending on the EMI data.
While these curves can be analytically modeled in terms of size and composition for simple shapes like
spheres, an analytic representation for the response from general objects has proven elusive.
Functional forms have been found that fit polarization response functions for a wide variety of
objects. One such form is presented here in the frequency domain. This matches the analytic cases for
both a sphere and an infinite cylinder in the high permeability limit. The parameters for this empirical
function can be related to the demagnetization and time constants, as well as the volume, of the object.
For the case of an ellipsoid, the demagnetization constants and volume are analytically well-defined,
while the time constants are not. Here, we assume the time constants to scale as the ellipsoid dimensions
squared. While this assumption is a weak one, data inversions will be presented that return consistent
size dimensions for a given UXO item.
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Decay Time Constant Analysis for Uxo Target Prioritization
Authors Bart G. Hoekstra, Amy Walker and Andrew SchwartzThe Geonics EM-61MK2 is currently used for many surveys for UXO detection. The system has
the capability to record the voltage over four different times after current turn off in the
transmitter/receiver coil. This allows the computation of the time constant of decay between these four
channels and the possibility that this additional information could be used to prioritize geophysical
anomalies for intrusive investigation. Previous work has demonstrated that the four time gates are
located too soon after the current turn-off to perform an inversion capable of characterizing the buried
objects in terms of its size, shape, depth and composition. This paper will analyze the potential for the
apparent time constant to prioritize the geophysical anomalies and reduce the number of anomalies that
are intrusively investigated, by lowering the priorities of anomalies that have clearly different
characteristics from the objects of interest.
This paper will focus on geophysical surveys conducted for the Chemical Warfare Materiel
(CWM) Scoping and Security Study for the purposes of detecting steel Chemical Agent Identification
Sets (CAIS) shipping containers. There objects are larger and heavier than typical munitions and
explosives of concern (MEC) items and thus offer a better opportunity to prioritize target selection.
Additional data from test are also discussed in this paper.
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Support Vector Machines for EM Data Interpretation
Authors Heidi Anderson Kuzma and James W. RectorSupport Vector Machines (SVMs) are a class of trained algorithms which were introduced in the
mid-1990s which have rapidly reproduced state-of-the-art computer learning results. They are much
easier to understand than neural networks because they mimic the natural way that geophysicists think.
Using examples of objects and associated synthetic electromagnetic signatures, an SVM can be trained
to find the conductivity of similar objects from new EM signatures. Normally such an operation would
fall under the domain of geophysical inversion. SVMs do it in a way that is much more akin to
interpolation, producing values for the thickness of a spherical shell based on the similarity of its
signature to the training signatures. An SVM can iinterpret a signature with an accuracy of 10% or
better. The technique is general enough to apply to a wide variety of geophysical inverse problems.
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Uxo Discrimination Using Blind Source Separation
Authors Yingyi Tan, Stacy L. Tantum and Leslie M. CollinsStatistical signal processing techniques have shown progress in discriminating UXO from clutter
when the objects occur in isolation. Under this condition, only a single object contributes to the sensor
measurement. For multiple closely-spaced subsurface objects, however, the unprocessed sensor measurement
is a mixture of the responses from several objects. Consequently, the unprocessed measurements
cannot be used directly to discriminate UXO from clutter. In this paper, we implement blind source separation
(BSS) techniques, specifically independent component analysis (ICA), to recover the unobserved
object signatures from the mixed measurement data obtained by electromagnetic induction (EMI) sensors,
and then use the recovered signatures for UXO/clutter discrimination. Discrimination performance
depends on multiple factors, including the number of clutter objects in proximity to the UXO and the
separation distance between the UXO and clutter. Simulation results are presented illustrating the impact
of these factors on discrimination performance.
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Synthetic Crosshole Tomography Studies of Bridge Foundation Sites in Karst Terrane
Authors Dennis R. Hiltunen and Gye-Chun ChoSeismic crosshole tomography systems for characterization of the subsurface at bridge sites in mantled karst terrane were evaluated in this study. There is great need for a reliable testing method to map competent rock and estimate pile tip elevations during design of a structure. Prior to conduct and detailed analysis of crosshole tests at actual bridge sites in karst, important “synthetic model” investigations were first undertaken. For purposes herein, “synthetic models” refer to earth models whose velocity profile is assumed or known a priori. The synthetic model studies conducted indicate that the crosshole system is capable of accurately identifying and delineating horizontal and slanted layer interfaces, and that the method can produce valuable information for subsurface profiles that include dropping and pinnacled interfaces, and anomalies such as boulders or cavities. Based upon these findings, it can be concluded that seismic crosshole appears to be technically feasible for developing basic layer structure and top of rock profile in karst terrane.
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Meeting Tighter Navigation and Data Qc Requirements for Mec Investigations
Authors Kent Boler, Martin Miele, Ji Ma and Richard GrabowskiCurrently, good DGM implementation for MEC can get 1 to 2-ft radii navigational accuracies
during production and better during demonstration surveys. The differences between a 1-ft and a 2-ft
implementation are the selection of appropriate navigation systems, up-front thought in survey design,
and rigorous QC including inspections, audits, and failure analysis.
At two sites at the former Seneca Army Depot, where the performance objective was to detect
small (down to 20mm) target items, Shaw Environmental (Shaw) used an EM-61MK2 towed array with
robotic total station navigation that reliably got close to 1-ft total positional error site-wide. Keys to the
implementation included optimization of the form factor (deployment system and geometry) to
minimize bounce, pitch, roll, and yaw, software and logger selections to minimize latency issues, and
establishment of many QC control points.
The volume of DGM production data, QC data, QC assessments, and documentation can be
overwhelming. Although fixed calibration site data were good indicators of field data quality, the most
time efficient indicators of positional data quality were known location QC points.
The operational demands of navigation, logging, and monitoring multiple simultaneous data
streams on the equipment operators are a challenge. Audits and failure analysis are performed to ensure
compliance with operating procedures and, equally important, to optimize those procedures to minimize
the potential for incidental or catastrophic errors.
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The Effects of Speed and Platform Stability On Em61 Mk2 Anomaly Responses
Authors Andrew B. Schwartz and Amy N. WalkerThe U.S. Army Engineering & Support Center, Huntsville collected and evaluated EM61
MK2 electromagnetic data over a controlled test site using five different platforms, each
deployed multiple times at different tow speeds. We evaluated the effects of sensor speed on the
noise levels and peak anomaly amplitudes of measured EM data and we evaluated the effects of
different tow platforms on both background and anomaly responses. We analyzed our data to
derive background noise statistics for each dataset, anomaly peak responses corrected for
background noise, the spatial extent of anomalies, and anomaly signal to noise ratios.
We found that background noise increases as tow-speeds increase. With few exceptions,
anomaly peak responses and SNRs decreased as the tow speed was increased. On average, the
data show that the spatial extent of anomalies increased as tow speed increased. Our results
suggest that improvements to platform stability result in lower background noise levels and
increased SNRs. We also found that the background noise and anomaly responses increased
when the sensor height above the ground was reduced.
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Evolving Geophysical Standards 2004
More LessWhat geophysical methods may be the most applicable to a project, and how are these individual
methods applied? Ten years ago, the answer was dependent on individuals and their experience. Today,
geophysical standard guides are available to assist individuals unfamiliar with geophysical methods and
provide answers to this question. This paper identifies current ASTM Standard Guides and Standard
Test Methods associated with surface and borehole geophysical methods. The paper discusses available
standards, the rationale for developing the standards and ASTM committee makeup. Finally, the
Standard Guide to Selecting Surface Geophysical Methods (D 6429) is used to identify the geophysical
techniques that are, by consensus, the most appropriate for your projects.
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A Ground-Bas ED Squid Magnetometer
Authors W E. Doll, T.J. Gamey, L.P. Beard, J.R. Sheehan and Wayne MandellOver the past five years, our group has focused on development of airborne magnetic and
electromagnetic systems for mapping and detection of unexploded ordnance (UXO). Among
these projects is one involving testing and development of a high temperature airborne
Superconducting Quantum Interference Device (SQUID) tensor magnetometer. In association
with the airborne SQUID development, we are conducting tests of the SQUID as a ground-based
instrument for UXO mapping and detection.
The ground-based SQUID is mounted on a man-portable litter with ancillary
instrumentation mounted on a cart and connected by an umbilical. High-temperature SQUIDs
use liquid nitrogen as the cryogen, which is more convenient than the helium-cooled lowtemperature
devices. Preliminary data from the ground-SQUID prototype show good agreement
with cesium vapor total magnetic field data along a test line. Noise frequencies and levels
appear manageable and we anticipate that specialized processing should make it possible to
extract more details about UXO items from their tensor magnetic properties.
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Alltem, A Triangle Wave On-Time Time-Domain System for Uxo Applications
Data from tests of two electromagnetic prototype systems, the High Frequency Sounder (HFS),
and the Very Early Time Electromagnetic (VETEM) system at the Unexploded Ordnance (UXO)
Calibration Test Lanes at the Yuma Proving Ground, Arizona, showed good sensitivity to UXO objects,
but also considerable response to the electrical conductivity of the ground at frequencies above 20 kHz
or at the very early times provided by VETEM. To overcome these problems, we use a triangle wave
excitation similar to the one used by the UTEM system developed at the University of Toronto. One of
the favorable characteristics of such a system is that ferrous and non-ferrous metals behave quite
differently so that an operator can tell at a glance whether a response involves ferrous metals or not.
With such a system we can measure the transients to much later times than were possible with VETEM.
In addition, the ability to generate and measure three field components aids target discrimination and
identification. Results from laboratory test prototypes show good sensitivity to surrogate and actual
inert UXO items.
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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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Noise In Time Domain EM Systems
Authors D.C. George and D.D. SnyderThis paper considers the signal-to-noise performance of a time-domain electromagnetic (TEM)
system that we developed with funding from NAVEODTECH (Indian Head, MD). TEM systems
require wide bandwidth and large dynamic range. Wide bandwidth is not conducive to low noise levels.
However, large dynamic range requires low noise levels. Stacking of successive repetitions of the signal
is a commonly, if not ubiquitously, applied method for effectively narrowing the bandwidth and for
lowering the effective noise levels. The method of signal repetition and the corresponding stacking
algorithm can have a significant influence on the performance of these systems.
We define noise sources for this system as environmental noise, system generated magnetic field
radiation noise, system electronic noise, motion noise, transmitter noise, and mechanical and physical
noise.
The TEM system we have developed includes a novel transmitter and a versatile computer
interface that provides flexibility in controlling the period, repetition rate, and successive polarities of
transmitted signals. This unprecedented control of the transmitter output has provided us with the
opportunity to test transmitter waveforms and stacking algorithms.
In this paper, we compare relative contributions and importance of each noise source and we
further compare the effects of using different transmitted waveforms and corresponding stacking
algorithms. Careful consideration of noise has helped us build a prototype system with superior
sensitivity.
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Coaxial Electromagnetic Sensor for Uxo Detection
Authors Haoping Huang, Bill SanFilipo, Alex Oren and I.J. WonA new broadband electromagnetic (EM) sensor, called GEM-5, for shallow earth investigation
including detection and discrimination of UXO has been developed. The sensor consists of a central
transmitter coil and two coaxial receiver coils located at the same vertical distance above and below the
transmitter coil. The difference in output between the receiver coils would be zero in free space. This
particular configuration provides a high degree of primary field cancellation, reduction of power-line or
other environmental noise, and immunity from noise induced by sensor motion in the earth’s magnetic
field.
We have first studied the detection capability of the sensor. The EM responses are numerically
simulated using a sphere model for UXO-like objects, and a half-space for geology. Algorithms to invert
the EM data into apparent susceptibility and conductivity show that the sensor can detect small metal
targets. Transforming the EM data into the apparent conductivity can enhance the anomalies from metal
targets and reduce geological noise. Our field test indicates that the gradiometer measurements yield a
lower static and motion noise over the critical low frequencies than that of existing sensors, and detect
all targets at a seeded UXO test site.
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Borehole Geophysics and Hydrophysical Logging of Fractured Bedrock at The Alark Hard Chrome Facility, Riverside, California
Authors Lewis E. Hunter, William H. Pedler, David W. Henry, Nancy Barnes, John Erwin and David StensbyBorehole geophysical investigations were conducted at the Alark Hard Chrome Superfund Site during the spring of 2003 and winter 2004. The Alark site housed a former electroplating facility in Riverside, California. Operations, including the dumping of electroplating solutions into a 40-foot deep dry well and storm drains. These activities released hexavalent chromium solution to the soil and underlying fractured bedrock. Previous investigations have found up to 17 mg/L in a well located about 80 ft southwest of the facility. The most contaminated monitoring wells are screened in granitic bedrock; however, the migration pathway is yet to be determined. Current investigations are being conducted to validate previous investigation results, examine extent of lateral and vertical contamination, and to evaluate the hydraulic characteristics of the fractured bedrock for the purpose of remediation. Data to be presented will highlight results of the borehole geophysical and hydraulic investigations of five existing and three new boreholes. Five existing PVC-cased monitoring wells (MW-2, MW-3, MW-7, MW-8, and MW-9) were
investigated in the spring of 2003. Geologic parameters were determined using a natural gamma and electromagnetic induction tool. This tool documented overburden thickness of 11 to 22 feet. Weathered bedrock extends from 17 to 30 feet below ground surface and ranges in thickness from 2 to 20 feet. The screened intervals were subsequently investigated to define hydraulic properties using an advanced hydrophysical tool (NxHpLTM) and a Scanning Colloidal Borescope (SCBS). Zones of preferential flow were identified in MW-2, MW-3 and MW-7.
Three new bedrock wells were drilled in December 2003. Each of these wells extends to about 60 feet below the bedrock-alluvium interface. These wells were geophysically investigated in January 2004 with PVC casing extending to just below the top of bedrock. Geophysical investigations included caliper, acoustic and optical televiewer, natural gamma, electromagnetic induction, SP, single point resistivity and long- and short-normal resistivity. Hydrologic evaluation of these wells was conducted using the same hydrophysical tool. Hydraulically conductive intervals identified with hydrophysical logging with were then isolated using an advanced wireline straddle packer (WSP). The WSP provided both interval specific formation fluid sampling and hydrologic information. To evaluate the large-scale hydraulic connections between the tested wells, a pump test was conducted at the conclusion of the WSP testing and sampling.
The preliminary results of this study suggest that previously undetected preferential contaminant pathways exist in both the saturated and unsaturated portions of the fractured bedrock. This spatial evaluation of contaminant distribution, permeability and contaminant mass flux, will provide the basis for remedial activities.
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3D Resistivity and Ip Tomography As Efficient Tools To Monitor Remedial Actions Over A Gas Plume
Authors John Deceuster, Mélanie Chaballe and Olivier KaufmannLast year, we assessed the efficiency of 3D resistivity tomography to delineate a hydrocarbon contaminated area situated near a gas station at Brugelette, Belgium. The site subsurface consists of dolomitic bedrock which is overlaid with 5 to 9 meters of clayey sands. The fresh hydrocarbon plume was detected through an increase in resistivities plotted especially on the modelled groundwater table. Geophysical investigations results were consistent with boreholes data and samples analyses. To cleanse groundwater at the site, 9 pumping wells were drilled in contaminated areas. In order to evaluate the efficiency of remedial actions, three 3D cross-diagonal resistivity and induced polarization surveys were conducted within seven months. Data were inverted using Res3DInv, resampled within Matlab and plotted on the modelled rockhead topography and water table. 3D resistivity tomographies showed higher resistivities in hydrocarbon-contaminated areas. Lithologic variations on the site do not stand out on IP tomographies. Moreover, hydrocarbon-impacted areas showed strong IP response compared to clean soils. Finally, time-lapse resistivity and IP tomographies revealed very efficient tools to follow hydrocarbon plume movements and to assess the efficiency of remedial actions.
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Study of Petroleum Contaminated Sites In Mexico With Resistivity and Em Methods
Hydrocarbons are among the main factors of geological medium contamination. We differentiate long-term contaminations lasting years or decades of years and short-term contaminations or single accidents. The first produces give more evident geophysical anomalies, whereas anomaly strength of the second depends on the time since the accident occurred. After 6-12 months following the accident this type of contamination gives measurably low resistivity anomalies. Our experience with contaminated sites characterization in Mexico shows that low resistivity
anomalies caused by hydrocarbon contamination is possible to localize with the help of vertical electrical sounding (VES) or with electromagnetic profiling (EMP). Such contamination gives low resistivity anomaly as a result of petroleum biodegradation at shallow depth in the earth. It is more difficult to characterize the second type of contaminated sites because the anomalies are not as intensive. Short-term contamination is more abundant in the oil industry.
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Resistivity Investigation for Mapping Polluted Areas In and Around Hyderabad, India
Authors B. Venkateswara Rao, Ch. Prem Chand and Md. AnwarGeoelectrical, Physiographic and Hydrogeochemical investigations were carried out along and across the Musi River covering an area of 113.75 Km2 stretching from Amberpet to Nallachervu to know the ground water pollution potential due to discharge of sewage in and around Musi. 35 Vertical Electrical Soundings were conducted on 6 Profiles across the Musi. Topographic levels and water levels were also collected near the VES points.30 Ground and Surface water samples were collected to know the concentrations of various chemical parameters. Using the data, contour maps of the chemical parameters and physical parameters has been prepared. Moreover geoelectrical cross sections along and across the Musi are studied. When we face down stream of Musi in general it is found that the North side of the Musi is at lower elevation so also the aquifers when compared to South side. In view of the continuity of the aquifers from high elevated areas to low elevated areas the ground water is moving towards low lying areas. The high concentrations of TDS and other major chemical parameters are found at low-lying areas that are reflected in the resistivity contour maps.
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Soil Composition Determination Based on Soil Optical Properties In The Thermal Infrared
Authors J. Michael Cathcart and Robert D. BockCurrent research into the use of electro-optical sensing modalities for the detection of landmines and other objects in complex backgrounds has led to the need to understand the optical properties of background materials (e.g., soils) in more detail. In particular, the use of spectral and polarimetric signatures in the optical and infrared domains has been the subject of much study; an understanding of soil, foliage, and other background optical properties and their variations is critical to evaluating the utility of these signatures. Our research examined both soil polarimetric signatures and emissive characteristics in the context of a real world environment; specifically, we examined the infrared spectral and polarimetric properties of soil components within the complex natural radiative environment. A modified Hapke radiative transfer model was employed to compute these properties. This paper will present a brief overview of this computational model and results from the optical property calculations for several mineral types and soil compositions. In a parallel study, hyperspectral data of soils were analyzed and compared to evaluate the influence of local environmental conditions on soil spectral features. The results of this study will also be presented and compared to the modeling computations.
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Frequency Domain Analyses of TDR Waveforms for Soil Moisture Measurements
Authors Chih-Ping Lin, Chung-Chih Chung and Shih-Hung TangWhen time domain reflectometry (TDR) is applied to the field characterization of soil moisture, the waveforms have typically been analyzed using traveltime along the waveguide. The apparent dielectric constant traditionally determined by the traveltime analysis using a tangent-line method does not have a clear physical meaning and is influenced by several system and material parameters. The frequency domain analysis, however, can determine the actual frequency-dependent dielectric permittivity and can be performed using a very short probe. This study presents a numerical modeling approach for common unmatched TDR probes to analyze the TDR signals in the frequency domain. This approach is also adopted to examine how dielectric relaxation, and electrical conductivity affect traveltime analysis. Simulation results indicated that, although the effects of TDR bandwidth and probe length could be quantified and calibrated, the calibration equation for soil moisture measurements was still affected by dielectric relaxation and electrical conductivity, due to differences in soil texture and density. The effects of density can be removed by adding a density term to the calibration equation. Correlating with water content the dielectric permittivity at frequencies between 500 MHz and 1 GHz, rather than the apparent dielectric constant, can minimize the influence of soil texture.
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A New Approach for Real Time Pipe Locating
More LessA new method of detecting and following pipes has been developed that should facilitate efforts to track and locate natural gas leaks. This ground penetrating radar (GPR) tool is deployed like a metal detector; the user swings a 400 MHz antenna back and forth over the ground. The data are then reduced and processed in real-time to automatically display the location and depth of targets found along the arc. Swinging the device around the body guarantees that any pipe crossing the arc will produce a “hyperbolic” shape in the data. This shape is then recognized and displayed in real time according to recognition confidence, depth and X,Y GPS position. The advantages of mapping pipe (metal, PVC, clay) are clear. But perhaps the more intriguing application has to do with locating gas leaks. As one follows a gas pipe along its course, GPR has demonstrated the ability to detect gas leaks by sensing changes in the surrounding soils. This means GPR can detect the leak site rather than the surface exit point of the gas. Our goal is to map these subtle changes in soil properties in real-time, offering a new method for constraining, and hopefully pinpointing, natural gas leaks.
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Statistical Comparison of Models for Soil Dielectric Spectrum
Authors X. Yu, V.P. Drnevich and R.L. NowackDielectric spectra have been widely explored to obtain information about material structural characteristics and physical constituents. As a mixture of three phases (cohesionless soil) or four phases (cohesive soil with significant amount of bound water), soil dielectric behavior is complicated by the phase interactions. Understanding the soil dielectric behavior is the prerequisite for application of technologies such as active remote sensing, ground penetration radar, Time Domain Spectroscopy, and other electromagnetic wave technologies for soils. The soil dielectric spectrum can be measured in the frequency domain, using a Network Analyzer or in the time domain by model-based inversion of recorded electromagnetic time signal. The model-based inversion is advantageous in that it is fast and the spectrum is less influenced by the presence of signal noise at individual frequencies. This paper evaluates the performance of several commonly used models for soil dielectric spectrum, including Debye’s model, Cole-Cole’s model, a power-law type volumetric mixing model, and a simplified model based on the concept of apparent dielectric constant. The model parameters were obtained from inversion analysis of TDR (Time Domain Reflectometry) measurements on soils. The performances of individual models are compared using statistical analysis of residuals. The analyses indicate that there is no significant difference between the capability of Debye’s model and Cole-Cole’s model for describing soil dielectric behavior, which possibly indicates that there are no pronounced distributive relaxation mechanisms inside the measured soil mixtures. While the volumetric mixing model better describes the trend of actual soil dielectric spectrum, it also causes increased non-uniqueness in the inversion process. The simplified model, although it could not fully characterize the soil dielectric spectrum, provides robust indication of soil dielectric behaviors within the TDR measurement range. The relative merits and shortcomings of each individual model are discussed in this paper.
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Hydraulic Conductivity Estimation From Spectral Induced Polarisation Data – A Case History
Authors Andreas Hördt, Roland Blaschek, Andreas Kemna, Julia Suckut and Norbert ZisserWe carried out a spectral induced polarisation (SIP) survey at the Krauthausen hydrogeological test site in Germany. Multifrequency data were recorded along two profiles, and single frequency data along ten densely-spaced profiles with a dipole-dipole configuration and 2 m electrode spacing. The data were inverted with a 2-D inversion code to obtain images of resistivity and phase distribution for each frequency. We used a focusing regularisation scheme that does not suppress sharp boundaries and yields realistic images of the subsurface. From the complex conductivity distribution we calculated the hydraulic conductivity using different empirical equations. The results obtained with the constant phase angle model are encouraging in the sense that the hydraulic conductivities have the same order of magnitude as those determined from grain size analyses. The gross structural features compare well with the known lithology,
which consists of an aquifer over an impermeable clay layer at approx. 10 m depth.
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An Interpretation of Various Well Logs Acquired In Unconsolidated Soil for Hydraulic Property Estimation
Authors Chisato Konishi and Genyuu KobayashiIn order to obtain some parameters for hydraulic property estimation, we carried out various well loggings, including velocity (Vp, Vs), electric, induction, IP, density, natural gamma, water conductivity, and temperature. This time, we considered a simple sand-clay model and tried to estimate its hydraulic conductivity from well logs. In this process, we recognized that the grain size or pore size crucially affects hydraulic conductivity if we calculate it from porosity by such kind of Kozeny-Carman equation. Although the natural gamma log is commonly used for discriminating sand and silt formations, we could not use it, because not all part of high gamma zone of the well corresponded to clay rich zone. Therefore, we examined a possibility of discriminating sand and clay by fuzzy clustering techniques (Imamura and Nakayama, 1997). The computed soil classification by fuzzy clustering using following three logs: conductivity, P-velocity, and density, showed good agreement with the soils determined by core observation. Among these three logs, we tried to apply conductivity log as an indicator of content of small particles. Using the conductivity log, we calculated pseudo clay content log and average grain size of sand-clay mixture. The calculated average grain size and porosity were substituted to Kozeny-Carman equation, and then hydraulic conductivity could be estimated. The estimated hydraulic conductivity was consistent with that measured by slug test and EKL deeper than 50m depth.
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2-D Resistivity Imaging and Determination of Hydraulic Parameters of The Vadose Zone In A Cultivated Land In Southwestern Nigeria
Authors Abel I. Olayinka and Michael A. OladunjoyeA combined 2-D resistivity imaging survey and determination of hydraulic parameters of the vadose zone in a seasonally-flooded part of southwestern Nigeria that has been cultivated for the past 18 years is presented. The survey area is about 22 000m2 (2.2 ha) with the bedrock geology comprising Pre-Cambrian granite gneiss. Imaging data were measured along 11 lines, with a minimum inter-electrode spacing of 1 m; control data were obtained along two lines nearby. Soil samples were collected from 30 shallow pits at depths of 0.5, 1.0 and 1.5 m, respectively. The resistivity of the near-surface sandy-clay lithology is low at less than 50 Ωm while that at the control site is much higher. The natural moisture content ranges between 3.3 and 27.4% with a mean of 12.2%; porosity ranges between 9.9 and 36.8%, with a mean of 14.4%. The coefficient of permeability ranges between 9.61x10-7 and 1.01x10-5 cm sec-1. The study has shown that
changes in electrical properties of the soil could be due to variations in moisture content and that there is a close relationship between the sandy clay lithology and the recharge. Moreover, the soil could constitute an environmental problem at the peak of the rainy season.
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Evaluation of Mine Seals Using Ground Penetrating Radar
Authors William D. Monaghan, Michael A. Trevits and Michael J. SapkoMine seals are used extensively in underground coal mines to segregate mined-out areas and to isolate fire zones or underground areas that are susceptible to spontaneous combustion. Over the years, 30,000 mine seals have been erected in underground coal mines in the United States. Mine seals, along with rock dusting and ventilation, represent the fundamental means of preventing underground coal mine explosions. In this study the National Institute for Occupational Safety and Health (NIOSH) used ground penetrating radar (GPR) technology to determine if each of three test mine seals were uniformly constructed. Three mine seals representing different construction methods and materials (cast-in-place foamed cement; solid block, polyurethane foam and gravel; and wire mesh, rebar and gunite) were erected at the NIOSH Lake Lynn Laboratory’s underground mine as part of an on-going research program. The seals were imaged using GPR with 400-, 500-, 900-, and 1,000-MHz antennas. We found that variations in uniformity existed in the material for the cast-in-place foamed cement seal. Variations in uniformity also existed in the seal made from solid block, polyurethane foam and gravel. The postprocessed radar records correlated closely with recovered core samples (areas of gravel and polyurethane foam, and areas containing only polyurethane foam). Finally, the radar records of the seal made of wire mesh, rebar and gunite showed the various components. The results of this work suggest that GPR could be used as a tool to evaluate mine seal characteristics and construction uniformity.
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Electrical Imaging of Deep Clay-Mine Voids
More LessIn the late 1890s through the 1920s, deep mining of a local clay bed beneath 35 to 50 feet of Pennsylvanian-aged rock resulted in mine voids in the St. Marys, Pennsylvania area. Recently, long subsidence troughs have been noted locally in this area. The history of the deep mining has been all but lost to the local residents; however, recent mine maps show that deep mining has occurred near these residents. Shallow-focused continuous vertical electric sounding profiles imaged subsurface haulways and rooms. Inverted measured apparent resistivity values indicate areas of subsurface voids presumable saturated. The deep mine map prepared in the 1920s shows an excellent correlation between the electrical imaging prediction of voids and the mapped voids.
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Electrical Properties of Leach Ore, Cripple Creek Mining District, Colorado
Authors Gary W. Horton and Will FrangosThe Cripple Creek and Victor Gold Mining Company in Victor, Colorado is a leading gold producer in North America. The mine is an open pit/heap leach recovery facility where the ore is placed in an unconsolidated pile and treated with a dilute cyanide solution. The pregnant solution is then pumped to an adsorption facility for gold extraction. Annual recoveries of gold reach 365,000 oz/yr at a mine cutoff of 0.018 opt. The project investigated 1) saturation characteristics within the heap itself through the use of a series of Schlumberger soundings at locations with different solution saturation (e.g., fresh ore with no solution as opposed to fully leached rock), and 2) laboratory induced polarization spectra of ore samples to determine electrical properties that may characterize saturation conditions. Spectral induced polarization electrical frequencies used were from 0.01 to 500 Hz. If there were a characteristic difference in the electrical properties of saturated vs. unsaturated ores then these differences could be detected by surface measurements, revealing subsurface regions of under-saturation and insufficient leaching. Findings indicate the competent, low-sulfide rock is highly resistive and has very low IP response. Conclusions that could be drawn from the Schlumberger arrays on the heap showed that subtle variations in bulk resistivity could be differentiated using a vertical electrical sounding (VES) program in addition to conventional log/log plots. Results of resistivity and induced polarization measurements on individual samples exhibit wide variation in resistivity, but when normalized to 1Hz all samples regardless of mineralogical composition showed little variation. With respect to IP effect little variation existed but more study needs to be performed to determine the dominant mechanism (ionic or electrode polarization). Column tests showed distinct differences in resistivities based on whether the ore was non-leached (fresh) or leached which is consistent with heap characteristics described by the Schlumberger soundings. IP effect was consistent with field result also. An important conclusion that can be drawn from the column tests is that it is possible to replicate field conditions in the laboratory.
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Internal Structure and Preferential Flow of A Waste Rock Pile From Geophysical Surveys
Authors Michel Chouteau, Jérôme Poisson, Michel Aubertin and Daniel CamposThe authors have carried out a geophysical investigation on a waste rock pile at the Laronde gold mine (Agnico-Eagle Ltd, Quebec). GPR, low-induction number EM and 2D resistivity tomography measurements have been conducted during consecutive field surveys in 2002 and 2003 on top of the pile to map its internal structure. Repeated infiltration tests and geophysical monitoring from ground surface have also been performed to map preferential flow paths. Using the field data, the authors propose here a three-layer model for the first 5-6 m of the pile, mainly based on electrical conductivities. Electrical conductivity increases with the degree of oxidation of the sulphidic minerals contained in the waste rock. The contact between unoxidized-oxidized waste rocks is typically marked by a well-defined resistive-conductive boundary, as shown by digging a 2.5 m-deep trench. Integration of geophysical and hydrogeological information indicates that there is a lateral conductivity variation, which may be due to the deposition method, mechanical alteration caused by machinery, and by increased oxidation of sulphides near the external boundary of the pile. GPR monitoring during infiltration tests seems to indicate preferential flow paths towards a zone of low sulphide content materials and low oxidation by products. The resistivity and GPR methods appear to be efficient geophysical methods to characterize the internal structure of the Laronde waste rock pile.
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Spectral Induced Polarization (SIP) Measurements for Stability Assessment of Tailings Dams
Authors Ernst Niederleithinger, Sabine Kruschwitz and Matthias BehrensTailings dams pose major risks to human life and the environment. The often poor construction and low maintenance level has caused serious accidents in both developed and developing countries. The EC funded project TAILSAFE is aimed to find ways for better investigation, stabilization and management of these facilities. Geophysical methods play an increasing role in the investigation of tailing dams. But the interpretation of the results and the translation into something useful for the engineers is often difficult. Thus several geophysical techniques have been tested at tailing dams in Germany, Hungary and Romania. The focus was on detecting structures in the dams and determining the water level. The measurements are accompanied by sampling and lab investigations. The focus of the paper is on the use of the SIP technique.
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Using Helicopter Electromagnetic Surveys To Identify Potential Hazards at Coal Waste Impoundments
More LessIn July 2003, the National Technology Transfer Center contracted with Fugro Airborne Surveys
to perform helicopter electromagnetic surveys of 14 coal waste impoundments in southern West
Virginia. The purpose of these surveys was to detect conditions that may lead to impoundment failure
either by structural failure of the embankment or by the flooding of adjacent or underlying mine works.
Specifically, the surveys attempted to: 1) identify saturated zones within the coal waste, 2) delineate the
paths of filtrate flow through the embankment and into adjacent strata or receiving streams, and 3)
identify flooded mine workings underlying or adjacent to the waste impoundment. In-phase and
quadrature data from the helicopter surveys were inverted using EM1DFM software to generate
conductivity/depth images. Conductivity/depth images were then spatially linked to georeferenced air
photos or topographic maps for interpretation. Preliminary analysis of the data indicates that helicopter
electromagnetic surveys can provide a picture of the hydrologic conditions that exist within the
impoundment. However, results from the helicopter electromagnetic surveys must be corroborated with
data from other sources to accurately assess the potential for impoundment failure or the potential for
filtrate from the impoundment to contaminate local streams and aquifers.
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Efficient Large-Scale Utility Mapping With Radar and Induction Arrays
Authors Michael Oristaglio, Ralf Birken, Thorkild Hansen, Ross Deming, Scott MacIntosh and Qifu ZhuA project sponsored by the United States Department of Transportation (U.S. DOT) has developed a new mobile geophysical system combining an array of broadband electromagnetic
induction (EMI) sensors with an array of ultra-wideband ground-penetrating radar (GPR) antennas. This “dual-array system” was designed for mapping underground utility networks efficiently over large areas, but can also be useful in environmental surveying for applications such as leak detection and hazardous waste monitoring. The project was part of the Pipeline Safety Research and Development Program1 of the U.S. DOT Research and Special Programs Administration. Several utility companies, including Consolidated Edison Company of New York and Regional Water Authority of South Central Connecticut, participated in the project. The EMI array consists of 16 vector magnetometers (induction coils) with a flat frequency response from about 1 to 100 kHz. Signals from each sensor are recorded and digitized as time series, with a sampling rate of 1 MHz. The EMI sensors are arranged in two linear arrays, each consisting of 8 sensors with a spacing of 30 cm; the arrays are offset vertically by about 50 cm. The system works with two types of transmitters: “clamp-on” transmitters which can inject current at discrete frequencies onto individual pipes (by galvanic or toroidal clamps) and a 3-axis induction coil which rides “on-board” with the transmitters and operates over the same frequency range as the sensors. The GPR array, which is based on the commercial CART Imaging System (Birken et al., 2002), consists of 17 antenna elements in an arrangement that creates 16 independent radar channels (transmitter-receiver pairs). The GPR array system has two antenna sets: one set has a central frequency of about 200 MHz and a channel spacing of about 14 cm; the other, a central frequency of 400 MHz and a spacing of 8 cm. The positioning system is designed to allow surveying in arbitrary patterns. Each array is mounted on a trailer whose position is monitored by a laser surveying instrument as the array moves over the survey area. Special algorithms merge data from different passes of each array to create a regular data grid. The radar data are imaged into a 3D volume using standard synthetic-aperture seismic imaging techniques adapted for GPR. The EMI data are inverted using a parametric model that assumes currents in the subsurface flow mainly along a network of (possibly interconnected) pipes. Two large surveys have been conducted with the dual-array system. One survey in the spring of 2004 successfully mapped a complex network of subsurface water, electrical, gas and telecommunication lines in Connecticut for a local water utility company. This survey covered over 2000 sq m with radar traces on a 10 cm grid and EMI data on a 30 cm grid. A second survey done in the summer of 2004 mapped electrical lines emerging from an electrical substation in New York. 1The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the RSPA or the U.S. Government.
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Voodoo Methods: Dealing With The Dark Side of Geophysics
By Greg HodgesThe exploration industry has been plagued since the dawn of technology with near-magical oil,
gold and waterfinders. They do untold damage to the reputation and business of honest geophysical
applications and research. A geophysicist with sound scientific knowledge can usually recognize when
geophysics is "from the dark side", but it can be difficult to convince non-scientists. Exposing the
voodoo methods can be a complex and expensive nightmare of politics, marketing, and litigation.
Some common characteristics of voodoo geophysical methods are: dubious theoretical bases,
fantastic levels of instrument sensitivity, phenomenally accurate interpretations, extraordinary levels of
secrecy, and combative or evasive response to challenges. The evaluator should also determine whether
the questionable method is the product of over-zealous marketing, misguided science, or fraud.
Funding agencies and corporations must insist on assessment and approval by technical experts
before investing in a new system. The technical investigators must be open-minded, but rigorous. The
tests must be definitive, and the testers must have the right to publish results. Fraudulent methods shy
away from technical testing and publication, and refusal of the purveyor of a new system to comply with
evaluation and publication of results must be viewed with the greatest suspicion.
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Human-Sensed Fields: Does Dowsing Response Correlate With-Self Potential Or Conductivity Anomalies?
Authors Charles T. Young and James TrowDowsing is the use of metal rods, forked sticks, pendulums, etc to locate minerals, water, pipes
etc. Dowsing has been practiced for centuries and in various world cultures, and there are abundant
reports of dowsing success in the non-scientific, popular literature. The natural question arises whether
the dowsing response correlates with any geophysical measurement such as self potential or
electromagnetic response. The dowsing reported here uses U-shaped bent metal rods carried along a
survey line. Both authors have found that the rods move in their hands. We report dowsing responses for
the two authors and compare these responses with self-potential and electromagnetic response across a
folded graphitic slate body and across metallic copper deposits, both in Michigan’s Upper Peninsula.
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Smoke Without Mirrors: A New Tool for The Geophysical Characterization of Shallow Karst Cavities
Authors Jonathan E. Nyquist, Mary J.S. Roth, Sandra Henning, Ron Manney and John PeakeWe are involved in a multi-year study to evaluate electrical resistivity as a tool to characterize
thinly-mantled karst. Guided by the results of a multielectrode resistivity survey, we drilled into a cave
7m below the surface. While mapping the cave's dimensions, triangulating using laser pointers and a
downhole camera, we observed a snake crossing the floor of the cave. Hypothesizing that one or more
animal burrows must intersect the karst network, we purchased a Hurco Technologies, Inc., Power
Smoker II, designed for smoke testing plumbing systems for leak detection. It is a 2.5 hp Honda Motor
modified to heat and blow Hurco's LiquiSmoke into plumbing systems. We injected non-toxic smoke at
a rate of approximately 700 cfs, but did not discover an animal burrow. After about half an hour,
however, we noticed puffs of smoke coming up through the grass about 30 ft from the borehole along a
straight line about 10 ft long. Comparison with the resistivity data shows that the smoke emerged downstrike
from the borehole along the same limestone ridge as the cave. We conclude that "leak-testing"
with non-toxic smoke may prove to be an effective complement to geophysical investigations of karst.
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Application of 2D-Ert In Feasibility Study of Hydropower Projects In Nepal
More LessIn Nepal seismic refraction is the traditional method of subsurface exploration for geotechnical
applications. Until the mid-90s seismic refraction was routinely used to investigate proposed sites for
hydropower projects of Nepal. These areas lie in the mountain and are inaccessible for vehicular
transportation. Very high heterogeneity of surface material creates poor energy coupling for both source
and receiver. Because of the high-energy content and easy transportability, explosives were the preferred
source for the generation of seismic waves.
Due to the rebel activities in the recent years, however, the government has tightened regulations
on transporting and using explosives. This situation has forced the hydropower projects to search for
alternative geophysical methods in place of seismic refraction. Electrical resistivity tomography (ERT)
was selected as a possible replacement method. In the initial stage of the application of ERT, various
electrode arrays and data acquisition approaches were evaluated for data quality and subsurface
resolution. Experience in different projects reveals that the pole-dipole electrode arrangement with data
levels at different electrode spacing is suitable for better resolution of the subsurface material.
Furthermore, manual acquisition, although less productive than automatic acquisition, seems to secure
higher quality data. With these systems of data acquisition ERT was able to fulfill most of the objectives
of the subsurface investigation at hydropower projects.
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Geophysical Exploration at The Giza Plateau, Egypt A Ten-Year Odyssey
More LessThe Giza Plateau of Egypt has many above-ground marvels (Pyramids and the Great Sphinx) and
many hidden or suspected tunnels/rooms that may lurk in the limestone beneath the sands. Since 1991,
we have been granted several opportunities to look for buried structures on the Plateau as well as on, in,
and under some of the Plateau’s more famous structures. Five separate campaigns have included
geophysical surveying utilizing ground penetrating radar (2D and 3D), electromagnetic conductivity,
magnetic field, and seismic reflection, refraction, and refraction tomography techniques. The results of
these surveys can be qualified as ranging from glowing successes, to tantalizing opportunities, to
fascinating failures. 2D and 3D GPR surveys within the Sphinx Enclosure and in the sands west of the
Sphinx have detailed anomalous structures within the shallow limestone that defy geologic interpretation
(man-made?) and are waiting to be excavated. Whether or not they will ever see exposure may be the
source of the Sphinx’s inscrutable smile.
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Measuring Sub-Seabottom Seismic Velocities - Some Unusual Experiments
During the last 30 years, we have performed a number of unusual experiments to measure P- and Swave
velocity structure of unconsolidated sediments below the seafloor at sites to water depths of 700
m. These experiments were designed to aid in specific geotechnical problems related to soil stability
along pipeline routes or beneath bottom-founded structures as well as to provide regional information
for military purposes. Some of these experiments were done in open-water conditions, but many were
performed beneath ice-cover.
Early testing was directed towards mapping sub-seafloor ice-bonded permafrost in the Beaufort Sea
and elsewhere in the Arctic Archipelago, mainly using dynamite sources. Continuous ice-cover at high
latitudes presented specific challenges for refraction methods and various devices and hydrophone
arrays were designed for deployment beneath the sea-ice through open leads or drilled holes. Later
testing evolved from seabottom-laid hydrophones to towed, near-bottom arrays and non-dynamite
sources. As well, mapping of shear wave velocity structure was tested using seabottom coupled
interface waves.
All of these experiments (successful or otherwise) were learning experiences in one form or other,
and it is hoped that documentation of these experiments herein will be of some future research value.
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How Far Will You Go To Collect The Data?
Authors K. Michael Garman and Scott F. PurcellSubsurface Evaluations, Inc. has been using three-dimensional multi-electrode resistivity (3DMER)
surveys in the Tampa, Florida, area to image karst features beneath undeveloped sites; around and
beneath buildings; beneath bridges, canals, and tertiary treatment ponds; and around bridge and highway
support structures. The surveys have been very successful for identifying karst features such as buried
depressions, breached confining layers, cavities and caves. Although the survey data have been very
useful for site evaluation and positioning test borings, many interesting and unpredicted problems
develop during the long days it takes to setup and collect the surveys when over a mile of cable has been
deployed in urban areas and the unpredictable Florida weather turns bad. Alligators, Soccer Moms in
minivans, lightning, flash floods, traffic, and small boats create unusual situations that often require
novel solutions. Somehow, the data usually gets collected even though the field crew often believes
management has acted irresponsibly. We provide some interesting examples of 3D-MER surveys with
the stories of how the data were collected.
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Integrated Geophysical Methods for Lng Site Characterization In A Jungle Environment
Authors Finn B. Michelsen and Martin MieleThe contribution of integrated geophysical survey methods to site characterization in difficult
and challenging environments is extremely valuable to design engineers and planners of new Liquid
Natural Gas (LNG) facilities, especially if the site is located in a remote and geologically complex area.
In particular, subtropical jungle environments present unique equipment and instrument preparation
requirements, field operation problems, and have certain human risk factors that must be taken into
account. Qualifying and characterizing jungle sites for installation of an LNG facility is costly, time
consuming, and requires the application of a variety of geoscientific investigation methods, including
geophysics. In conjunction with geotechnical and other site investigation methods, data from
continuous resistivity profiling, seismic refraction tomography, P-S wave downhole seismic, and
Wenner sounding surveys are used as an aid the design and planning of an LNG expansion facility
located on Bioko Island, Equatorial Guinea.
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The Pentagon Building Performance In The 911 Crash
Following the 911 crash of an airliner at the Pentagon, the American Society of Civil Engineers
established a team to study the damaged structure and make recommendations for the future. The team
reviewed available information on the structure, the crash loading, and focused assessments by others.
The team then analyzed the essential features of column response to impact, the residual frame capacity,
and the structural response to the fire. Plausible mechanisms for the response of the structure to the crash
were determined and recommendations were offered for design and construction. This paper
summarizes the study and discusses the role of geophysics in such forensic inquiries.
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Testing A Safe Acquisition Procedure for An Effective Application of GPR To Security Operations
Authors Maurizio Lualdi and Luigi ZanziTerrorism is posing new technological problems to security forces. The need of checking for
hazardous targets such as explosives, bombs, weapons, etc., hidden inside unexpected packages or
embedded within blocks of other materials is increasing. GPR has been already explored for Anti-
Personnel Mines (APM) detection and most prototypes are based on single or arrays of antennas
surveying the suspected area from a distance of a few centimeters. We are exploring a flexible solution
that is expected to be: a) applicable to different scenarios such as soil inspections (as for landmines) but
also investigations on packages, blocks of materials, walls, etc., b) adaptable to a set of antennas to
ensure proper penetration and resolution, c) safe for operator and equipment, d) accurate in antenna
positioning to produce 3D reconstructions, e) based on a survey procedure that allows the contact with
the medium to eliminate the energy loss produced by the non-contact approach. A single antenna plus an
armored PSG (Pad System for Georadar) seem to fulfill the requirements. Radar performances were
tested to explore the effects of the armor material. Tests were executed on the landmine field managed
by the Joint Research Center (JRC) in Ispra (Italy). The results demonstrate the feasibility and the
advantages of this solution.
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Tunnel Detection Along The Southwest U.S. Border
The U.S. Army Engineer Research and Development Center (ERDC) has worked with U.S. Law
Enforcement Agencies (LEAs) since 1995 to address the problem of clandestine tunnels beneath the
U.S./Mexico border. ERDC has performed tunnel-related research, equipment development, or tunneldetection
missions at the request of the LEAs, coordinated by Joint Task Force 6 (JTF-6, Fort Bliss, TX,
now known as JTF-N for Northern Command, US Army). This support to LEAs has revealed the
importance of understanding the geologic context of a suspected tunnel site as a basis for selecting the
appropriate geophysical tools and interpreting anomalies indicated by geophysical data. Tunnel detection
missions always involve multiple tools and techniques. A combination of geophysical instruments is used to
record data based on different physical principals. When interpreted in a regional geologic context, the
combined geophysical methods improve the likelihood of success for tunnel detection.
A variable-frequency electromagnetic survey tool was developed in the 1990s as part of tunneldetection
research, and proven at a tunnel test bed near Otay Mesa, CA. Also at the Otay Mesa site, an
ERDC-led team installed and tested a prototype passive-seismic fence, a system that can detect machine and
impact noise during the tunnel excavation process. This seismic fence concept has strong potential for
deterring tunneling in geographic areas where tunnels have been found most frequently and where cultural
clutter limits the usefulness of surface geophysical techniques and tunnel detection.
Current ERDC tunnel detection efforts (March 2005) are coordinated with the National Geo-
Intelligence Agency (NGA, formerly NIMA) to combine electromagnetic and radar methods with emerging
technology in microgravimetry.
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Simultaneous Geophysical Surveys Using a Towed Multi-Sensor Platform and Its Application to Characterisation of Brownfield Sites
Authors Chris Leech, Ian Hill and Tim GrosseyThe multi-sensor platform allows dense data sampling with an array of geophysical sensors, chosen for the particular purpose e.g. magnetic, EM, Radiometric, VLF or metal detection. Geophysical data and navigation parameters are telemetered in real-time to a base station where they are viewed and logged on a laptop PC. Data can be collected from up to 6 geophysical sensors plus navigation at data rates in excess of 10 Hz. The system can be pulled at up to 7-8 km per hour in normal terrain, using a lawn tractor, or ATV. The combination of multi-parameter measurements at each point of the survey maximises the ability to characterise the material underlying the site. The real-time transmission of the data allows real-time data QA, survey design modification, and facilitates rapid in-field processing of the data. The method is applied to the geological, and archaeological sites in addition to part of a large integrated site investigation over a land fill site.
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Final Disposal of Spent Nuclear Fuel Triggers Security-Related Geophysics
By Olli T. OkkoA geological site investigation including a comprehensive geophysical program confirmed the suitability
of the studied crystalline bedrock sites for the safe disposal of highly radioactive spent nuclear fuel.
According to a legal decision endorsed by the Parliament in 2001, the final repository of Finnish spent
nuclear fuel shall be located at the Olkiluoto site in Eurajoki, western Finland. The next phase of the site
investigations includes the construction of underground premises for rock characterization purposes. The
excavation of these galleries began in 2004. Since these premises are scheduled to form a part of the
final repository, these investigations are subject to regulatory control with safety, security, safeguards
and societal aspects. The national geophysical site characterization and monitoring program planned for
the long-term safety assessment shall also be applied to other repository performance related matters.
The construction documentation and the adjoining geophysical monitoring program are applied to
safeguard the repository for homeland security purposes. The regulatory use of geophysical methods
creates new challenges to the geophysical community.
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Detection of Subsurface Utilities Using GPR Techniques
Authors Sherif M. Hanafy and Mohamed A GamalThe nondestructive evaluation of buried utilities is of interest for engineering and environmental
applications. The use of Ground Penetrating Radar (GPR) for buried utility detections is one of the most
common geophysical techniques due to its capacity for accurate results in either horizontal or vertical
directions. Electric cables and metallic pipes are easily recognized on GPR profiles due to their high
dielectric values relative to surrounding soils. In this study, 12 different sites are investigated using GPR
techniques to detect buried utilities, such as electrical cables, water, and derange pipes located within the
first 3 meters of the ground’s surface. At each investigation site, two perpendicular GPR profiles, using
a 200 MHz antenna, are collected in order to detect any buried utilities. The area under study is
excavated after GPR data acquisition for manual verification. It is found that electric cables are easy to
recognize on radar profiles by their sharp hyperbola edges and multiples extending to the bottom of the
data set. Water pipes have especial characteristics on GPR lines, indicated by a thick, small hyperbola
without any multiples. The presence of cement blocks masks any object beneath them, but they are
prominent on radar lines as high amplitude reflections.
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Underground Utility Location Challenges Using GPR: Life On The Streets
Authors Randy Sorenson and David TillsonThis paper presents a historical account of our evolutionary experiences over the last 8 years
using conventional GPR to locate underground utilities, typically prior to the installation of new water
lines, sewer lines, gas lines, telecommunication cables, etc. Brief summaries of several case histories
are presented to illustrate how GPR can be utilized, with and without, other geophysical techniques.
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Location of Mammoth Remains In Perma-Frost of Northern Siberia Using GPR and Multifrequency Em
Authors Ken-Ichiro Makino and Hidetoshi MiuraIt is generally considered that that Wooly Mammoths have been extinct for around 10,000
years BP, however it is unknown whether the reasons for their extinction were due to an abrupt
change in the Earth’s climate, disease, or to indiscriminate hunting. This paper describes a
geophysical investigation conducted in 2004 to hunt for remains of Mammoths to assist in
studying the causes of their extinction. Ground Penetrating Radar (GPR) and
Multifrequency Electromagnetics (MF-EM) were used to map the subsurface of the
permafrost at sites along the Artic coastline of Northern Siberia. The sites were preselected by
helicopter reconnaissance, followed by ground geophysical surveys. The results obtained from
one control site plus two field sites are presented here. It is concluded that GPR is a valid method for mapping skeletal remains from Mammoths,
however there is little signature obtained with MF-EM due to the poor contrast between the
remains and the properties of the surrounding permafrost.
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Towed Geo-Electrode Arrays for Analysis of Surface Water Groundwater Interaction
Authors David A. Allen and Noel P. MerrickHigh productivity analysis of surface water groundwater interaction is possible using GPS
positioned vertical electrical conductivity imaging along with depth recording. Short submerged geoelectric
arrays provide a great deal of detail right at the base of surface water bodies; however they
cannot be used in treacherous waterways with debris scattered through them. Long floating arrays can
be towed through such waterways and have the additional benefit of greater depth of exploration.
Floating arrays also can have reasonable resolution at the base of surface water bodies provided that they
are designed optimally and that data from them is inverted effectively. Array design must optimize a
balance of signal strength maximization, exploration depth resolution, weight and drag minimization,
crosstalk and current leakage minimization and simple, minimal response to three dimensional
heterogeneity. Pragmatic interpretation of the huge volumes of data that are typically acquired is best
achieved using three dimensional ‘ribbon’ images. Prominent features of interest are, in most cases
related to groundwater salinity. Low conductivity anomalies often indicate fresh surface water seepage
into more saline groundwater. High conductivity anomalies that intersect the bases of surface water
bodies usually indicate saline inflow to the surface water bodies.
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Application of Non-Desructive Geophysical Techniques for Risk Evaluation (Sannour Protected Area, Beni Suef, Egypt)
Authors Sherif M. Hanafy and Essam A. MorsA large cavity was recently discovered in the Wadi Sannour area, near Beni Suef City, Upper
Egypt, in the same general area in which alabaster is known to have been quarried since approximately
200 years ago. This cave has a span of up to 300m, is 10-30m in width, reaches a height of 15m, and
generally forms a crescent cross section. The opening is situated on a raised outcrop approximately
39.5m below the original ground surface. In order to evaluate the site for potential risks, to protect the
cave features from irreversible damage, and to make the site more secure for visitors, a Ground
Penetrating Radar (GPR) study was conducted inside, over, and in areas adjacent to the cavity. Here,
twenty GPR reflection lines were acquired uniformly over a 35 x 11m area in front of the cave’s
entrance, and during the course of this survey a new subsurface cavity was discovered. The depth and
dimensions of this new cavity were determined from the acquired radar data.
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Estimating Source Depth Extent From Inversion of Magnetic Field Data – A Synthetic Modelling Study of The Cretaceous Volcanic Intrusions Into The Edwards Aquifer, Texas
More LessDepth to the top or base of a small magnetic source body cannot be reliably determined from
single profile measurements of any one magnetic field parameter. However simultaneous measurement
of transverse horizontal gradients or the magnetic gradient tensor allows estimation of any offset of the
source from the profile and this substantially improves the ability to interpret sparsely sampled
anomalies. Surveys at low elevations improve resolution of detail and detection of weak magnetizations
but the measured anomalies are dominated by the topmost section of the source bodies thereby reducing
sensitivity to their base depths. Aeromagnetic surveys flown to help resolve structure of the Edwards
Aquifer have discovered many Cretaceous intrusions but many of these anomalies are detected only on
single profiles and depths to their top and base are poorly constrained. Models derived from inversion of
the anomalies can however be used in the design of any follow-up survey and to investigate the possible
benefits of transverse gradient or gradient tensor measurements. Inversion of a multi-line anomaly has
resolved the source to be a steeply-dipping sheet illustrating implications of the magnetic modeling to
local groundwater flow models.
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On Time-Domain Transient Electromagnetic Soundings
Authors Jia Ruizhong and R.W. GroomWe have developed two techniques for simulating EM responses of a layered earth model; a
general and an approximate method. The general method allows the computation of the magnetic field
produced by systems with various current waveforms and survey configurations, including in-loop and
out-of-loop for both moving and fixed transmitter with arbitrary location and orientation of receivers.
The approximate method only allows the calculation of the vertical transient responses of the secondary
currents during the off-time with receiver inside of the transmitter loop. I ncorporating these two forward
modelling techniques and both Marquardt and an Occam s inversion algorithm approaches, we have
developed four methods to perform inverse modeling of transient electromagnetic soundings. A time
domain conductivity-depth image (CDI) technique is also implemented. To prepare the data for this
technique, an algorithm converting impulse response into step response has been developed. Armed with
these inversion techniques, we can process ground and airborne data collected with systems using
various current waveforms and survey configurations. The applications of these inversion techniques to
synthetic layered-earth models demonstrate the effectiveness of these techniques. Interesting field data
uses are also shown.
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Laterally Constrained 1D-Inversion of 3D TEM Data
Authors E. Auken, A.V. Christiansen, L. Jacobsen and K.I. SørensenIn this presentation we show that the interpretation of spatial densely sampled TEM data over 2D
earth structures is greatly enhanced by inverting the data using the 1D-LCI algorithm (Laterally
Constrained Inversion). In the 1D-LCI algorithm neighboring models are constrained laterally on layer
resistivities and interfaces.
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Incorporation of Variable Background Velocity In Subsurface Imaging With GPR
Authors Beiyuan Liang and Lanbo LiuFor fast imaging subsurface targets with ground penetrating radar (GPR) it is common to assume
a homogeneous background of electromagnetic (EM) wave velocity. Nevertheless, if some information
about EM velocity is available, it is desirable to use more realistic velocity distribution in subsurface
target imaging. This paper introduces a ray tracing function into GPR imaging process with using
reflection energy stacking. We demonstrate the effectiveness of this approach by comparing the imaging
results with different velocity layers for a typical subsurface imaging example. Use of the ray tracing
technique in the imaging process built in GPR software, GIT-Radar, is better to image and determine the
depths and shapes of the targets. Also, the application of the ray tracing in a 3D imaging process does
not substantially retard the speed of calculation if a layered velocity model is used.
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Comparison of Discrete and Voxel 3-D Gravity Inversions Over Karst-Related Gravity Anomalies On The Coastal Plain of North-West Malaysia
More LessA negative gravity anomaly of amplitude 12 μm/sec2 and diameter 400 metres on the coastal
plain of north-west Peninsula Malaysia has been modelled as due to cavities in buried limestone
bedrock. A model of equi-dimensional cavities using spheres does not reproduce the sharpness of the
measured anomaly but a model using ellipsoids is successful. The value of the expected density contrast
is the only constraint required to develop these parametric models. Maximum-smoothness voxel
inversions are poorly suited to the binary density distribution of cavity models, but when guided by
earlier parametric inversion results the voxel inversions enable the testing and development of more
complex source shapes. For the deeper equi-dimensional cavity model the computed field is only weakly
sensitive to source shape but for the shallower ellipsoid model the voxel inversion may provide a valid
improvement in representing the shape of the source body.
The anomalies can also be modelled as due to changes in overburden thickness. Parametric and
voxel models of basement topography are similar to each other and differ significantly from simple
Bouguer slab predictions establishing the importance of 3-D modelling even for such a simple
geological model. There appears to be no inherent superiority of either parametric or voxel inversions
provided each has the versatility to select models according to guiding geological concepts.
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Simultaneous Geophysical Surveys Using A Towed Multisensor Platform and Its Application To Characterisation of Brownfield Sites
Authors Chris Leech, Ian A. Hill and Tim M. GrosseyA rapid measurement technique for assessment of landfill areas and other environmentally sensitive areas. A multi sensor platform with real time data logging and navigation has been built to
allow rapid reconnaissance surveys over large areas thereby reducing survey time and costs dramatically. Sensors from many different disciplines can be mounted on the platform and data acquired simultaneously. Case histories from geological, environmental and archaeological sites are presented showing the attributes of the technique in all three applications.
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Time-Lapse Azimuthal Resistivity Survey In A Highly Industrialized Area: Problems, Diagnosis, and Solutions
Authors Les P. Beard, Jacob R. Sheehan and Scott C. BrooksA time-lapse azimuthal resistivity survey was proposed to support a fluid flow test pertinent to
an upcoming bio-remediation experiment. The site is in a highly industrialized area, and preliminary
resistivity measurements showed steady, but substantial, drift in apparent resistivities measured over
tens of minutes. The drift was of such magnitude that collection of time-lapse data would be precluded
unless a solution could be found. We hypothesized that the problem was either instrumental, was
induced by the electrodes, or was site specific. Internal checks of the Sting R1 indicated a properly
functioning instrument. We tested three different electrode types and found no particular differences in
the rate of drift with any of the three types chosen. A test of the resistivity system in a plastic tub filled
with a sodium chloride solution produced steady measurements over a span of about an hour. We
concluded that stray currents at the site itself must be producing the drift. We found that by averaging
two measurements at a given azimuth, one with electrodes positioned AMNB, the other BNMA, we
could obtain steady resistivity results over acceptably long durations, and so we were able to acquire
useable data during the flow test.
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A Wide-Band Electromagnetic Impedance Profiling System for Non-Invasive Subsurface Characterization
Authors Hung-Wen Tseng, Ki Ha Lee and Alex BeckerA non-invasive, wide-band electromagnetic (EM) impedance difference system for shallow subsurface electrical structure characterization in environmental and engineering problems has been
developed at the Lawrence Berkeley National Laboratory (LBNL). Electrical parameters of interest are electrical conductivity and dielectric permittivity that are deduced from the impedance difference data. The prototype system includes a magnetic loop transmitter, which operates between 0.1 MHz and 100 MHz, an electrical dipole antenna for observing the electric field, and a loop antenna for measuring the magnetic field. All antennas are mounted on a cart made of non-metallic material for easy movement of the whole array for profiling. Surface EM impedance difference is obtained by taking the difference of the ratios of the electric fields to the magnetic fields at selected frequencies at two different levels. Numerical simulations verify this approach. A set of the impedance difference data acquired at the University of California’s Richmond Field Station compares reasonably well with simulation results based on a model obtained with the resistivity method and in situ TDR (time domain reflectometry) measurements.
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Application of Multi-Channel Electrical Method In Investigation of Hydraulic Channels In Qiu Jia Wan Main Dike of The Yangtze River, China
Authors Yixian Xu and Jianghai XiaQiu Jia Wan, the main dike of the Yangtze River in Jia Yu County, Hubei Province,
China, is a section scheduled for reconstruction due to underground water leakage. Prior to the
reconstruction, the paths of water movement (sand layers or local bodies) and their covering
material must clearly be investigated. The traditional methods, e.g. borehole drilling, are
expensive and may re-damage the dike, so the noninvasive multi-channel electrical method was
chosen to perform the investigation. One surveying profile parallel to and other three profiles
perpendicular to the main dike were carried out in the Qiu Jia Wan section. Acquired data were
inverted by a two-dimensional inversion method. Our interpretation was able to relate anomalies
in resistivity cross sections to water leakage through a sand and gravel layer from the Yangtze
River to the back of the dike. The results were consistent with hydro-geological analysis and
therefore supplied basic data for the reconstruction task.
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A High Resolution 3-D Imaging Radar Survey of A Concrete Ring-Wall Collar for Mapping The Extent of Concrete Deterioration and Delamination
Authors Brian Jones, Mario Carnevale, Daniel J. Delea and Jutta HagerA high-resolution 3-D GPR survey was conducted on a large concrete structure in the central
coastal area of California. The survey imaged the top 6-meter-high collar of a 19.81-meter-diameter
ring-wall. The purpose was to determine the presence and extent of concrete deterioration in the form of
air or water-filled cavities, as well as identify areas of concrete delamination or advanced stages of
concrete deterioration. The imaged section of the ring-wall was 0.91 meters thick. A 900-MHz GPR
system was used to image the full circumference of the ring-wall from the outside. A 1.5-GHz GPR
system was used concurrently to image the full circumference of the top of the ring-wall. This survey
plan was developed to provide a suitable combination of resolution and GPR signal penetration to image
the upper ring-wall structure.
The high-resolution top-of-wall and outer-wall surveys were conducted to identify delamination
zones and discontinuities within the concrete. A total of 6,047 data files were collected. This equates to
a data set size of 1.725 GB. The total linear distance surveyed was 3,907 meters. 3-D analysis of the
GPR data was used to create a continuous 3-D composite model for both the 900-MHz and 1500-MHz
data sets. “Wrap-around” time-slice views were provided to the client in video format to illustrate the
ring wall in quarters and octants. The investigation indicated that approximately 25% of the surveyed
wall contained some degree of rebar rust jacking and concrete defects in the form of voids and/or
delamination. This study demonstrates that a comparison of the 3-D results of two concurrent surveys at
different frequencies can provide comprehensive, high-resolution images of defect distributions in large
concrete structures.
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Use of Ground–Penetrating Radar (GPR) As An Evaluation Tool In Nineteenth-Century Building Restoration
Authors James S. Mellett and Shelley SmithA GPR survey using a 1.5 GHz transducer was carried out on the inside walls of a nineteenth
century church tower in Westchester County, NY to gather information on moisture infiltration and to
guide remediation. The tower walls have a thickness of 40 cm (16 in) and are faced with granite ashlar
blocks about 15 cm (6 in) thick. The inner face is a rubble wall, with stones having diameters from 7 to
20 cm (3 to 8 in). In between is a mortar mix that filled in the spaces between the rubble stones. Over
the past decade, the inner walls in the church have been showing evidence of leaks and water damage.
Recent remediation included masonry repointing and injection grouting of the parapet walls. The leaks
subsided significantly, but evidence of moisture infiltration continued. GPR scans detected apparent
voids in walls below the parapet, with large void spaces around the central windows. In the area below
the windows, just above the church crossing, the voids appeared to increase in frequency and size. We
believe that voids were initiated around the windows and the entrained water developed a hydraulic head
that pushed it into lower levels of the church. Additional injection grouting will be performed in all wall
areas showing voids.
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Time Domain Reflectometry Uses In Geotechnical Engineering
More LessTime Domain Reflectometry (TDR) makes use of a device that propagates transverse
electromagnetic waves through soils by means of special soil probes as shown in Figure 1 for field
testing and in Figure 2 for testing a specimen. For field testing four spikes are driven into the soil while
for testing a specimen in a mold, only a center rod is driven into a metal mold on a non-conducting base.
The same probe head is used in both situations.
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Pavement Evaluation and Design With Seismic Methods
More LessThe primary parameters that affect the performance of a road or an airfield pavement are the
thickness, the Poisson's ratio, and more importantly, the modulus of each layer in the pavement
structure. Stress wave propagation or seismic techniques have been used to measure these parameters
through nondestructive testing (NDT). Other factors affecting the performance of a pavement are the
interfacial bond quality of asphalt concrete (AC) or Portland cement concrete (PCC) overlays and the
existence of flaw or delamination in a PCC slab. They can also be detected with seismic methods. To
make field tests and data analyses with seismic methods faster, more practical and cost-effective,
automated testing devices have been developed. The principles of seismic methods as applied to
pavement analysis, design and diagnosis are discussed through several case studies.
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Reflection Seismology: Application To Identifying Fault Structure
Authors Richard T. Williams, Vladimir A Babeshko and Ernest KutsenkoReflection seismology has been developed to an advanced state by the petroleum industry,
capable of determining not only the geometric shapes of subsurface layers and faults, but also yielding
substantial information about the elastic properties of the rocks and the nature of any fluids present. The
technology is expensive, but highly successful and justified in comparison to the cost of drilling wells
several thousand feet deep to discover and produce petroleum. Geologists drill few oil wells and no
deep ones without the support of seismic reflection data. Geotechnical investigations share some of the
same goals, to image subsurface layers and faults, and to determine the elastic properties of the strata
without the expense of excavation or drilling, but the economics are quite different. This paper reviews
the state-of-the-art as it is practiced in the petroleum industry, and examines which elements of the
technology can be adapted in a cost-effective way to geotechnical site investigations. Not all of the
petroleum industry technology is appropriate for engineering applications, but the basic field equipment
and software can be easily adapted. Recommendations are presented as to the kinds of problems where
reflection seismology may be the more effective than other geophysical methods including refraction
seismology and ground penetrating radar.
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Shear-Wave Velocity and Liquefaction Potential for Soils Under Dynamic Loading
More LessThis paper presents an overview of the procedure for evaluating liquefaction potential of soils
using shear-wave velocity measurements described in recent journal papers by the author and others.
The procedure follows the general format of the Seed-Idriss simplified procedure, and was developed
using suggestions from participants of two workshops and several independent reviewers. Comparative
studies show good agreement between predictions by the shear-wave-based procedure and the
penetration-based procedures.
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Shear-Wave Velocity (VS) Profiling By Surface Wave (MASW) Method
More LessThe multichannel analysis of surface waves (MASW) method (Park et al., 1999) is a non-destructive
(NDT) seismic method to evaluate shear-wave velocity (Vs) (or stiffness) of the ground. It analyzes
dispersion properties of seismic surface waves propagating horizontally along the surface of
measurement. It gives the Vs information in either 1-D (depth) or 2-D (depth and surface location)
format in a cost-effective and time-efficient manner. The main advantage of the MASW method is its
ability to take a full account of the complicated nature of seismic waves that always contain harmful
noise waves such as higher modes of surface waves, body waves, scattered waves, traffic waves, etc
(Figure 1). Since its first introduction to the related communities in mid 90s, there have been various
types of applications with the MASW method at different parts in the world. Nowadays, the keyword
MASW can be effectively used to search for related information on the Internet. Complete list of the
MASW publications made at the Kansas Geological Survey (KGS) during the last one decade can be
found at www.kgs.ku.edu and includes various types of case studies as well as theoretical ones.
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Surface Wave Analysis Using Wavelet Transforms
Authors Nenad Gucunski and Parisa ShokouhiComprehensive and efficient shallow evaluation of layered systems, like soils and pavements,
can be done using surface waves. The most commonly used surface wave techniques include the
Spectral Analysis of Surface Waves (SASW), Multichannel Analysis of Surface Waves (MASW), and
its high frequency counter part for paving layer evaluation, the Ultrasonic Surface Wave (USW). The
techniques have been primarily used in evaluation of elastic moduli and layer thickness of layered
systems, and to a certain extent in evaluation of profile variations and detection of anomalies, such as
cavities, soil lenses and buried objects. Since the SASW is using only a pair of receivers and data
evaluation is based on a standard spectral analysis, it has limitations in the accuracy and completeness of
the obtained information. The MASW, with its multi-channel data collection and analysis based on
double transformation of the signal from the spatial-time domain into the frequency-wave number or
frequency -phase velocity domain, can provide a more comprehensive description of the subsurface
conditions. Further enhancements can be achieved through surface wave analysis using wavelet
transforms. Application of three types of wavelet transforms of surface waves is presented.
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Applications of Controlled Source and Natural Source Audio-Frequency Magnetotellurics To Groundwater Exploration
Authors Norman R. Carlson, Phillip M. Paski and Scott A. UrquhartAudio-frequency magnetotellurics (AMT), using either a controlled source (CSAMT) or natural
source (NSAMT), has become an efficient, cost-effective tool for groundwater exploration.
Advancements in field equipment have improved data quality and increased data acquisition speed, and
the availability of 2D inversion modeling has significantly improved data interpretation. Since depth of
investigation is not related to the receiver electric field dipole size, AMT can be used as either a highresolution
tool (using short dipoles) or as a reconnaissance tool (using large dipoles).
Several recent field examples are presented, including fractured bedrock targets, in which lateral
resolution is important, as well as reconnaissance-style basin mapping, in which speed and economic
efficiency is critical. In one project, located in Tule Desert, Nevada, CSAMT was used successfully to
map an undeveloped basin, and was instrumental in subsequent court hearings to support water right
applications to develop groundwater resources from this basin.
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