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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 - 50 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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