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14th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 04 Mar 2001 - 07 Mar 2001
- Location: Denver, Colorado, USA
- Published: 04 March 2001
101 - 120 of 129 results
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Innovative Technologies For In Situ Detection, Speciation, And Quantification Of Radionuclide And Heavy Metal Contamination
Authors John H. Ballard, J. Cliff Morgan and M. John CullinaneThe U.S. Army Engineer Research and Development Center (ERDC)
developed sensor technologies in response to a critical need of the U.S.
Government to rapidly characterize soil and groundwater conditions on
Department of Energy and Department of Defense installations. While
conducting the Site Characterization and Analysis Penetrometer System (SCAPS)
Research and Development Program, the ERDC developed a suite of direct push
sensor technologies with the capability to interrogate subsurface soil and/or
groundwater in situ. The sensor systems provide cost-effective speciation and
quantification of contaminants in near real-time. Direct push multisensor
configurations have also been developed with the capability to provide the
simultaneous interrogation of subsurface media for multiple contaminants, and for
soil geophysical properties and layering classifications in situ. These
configurations have helped optimize the placement of conventional monitoring
wells. This paper will address new innovative radionuclide and heavy metal
detection technologies and multisensor configurations applicable to well logging
and direct push subsurface and surface site characterization for the in situ
detection and quantification of depleted uranium, gamma-emitting radionuclides,
and heavy metal contaminates in near real-time. The utilization of in situ multisensor
site characterization technologies and the deployment of multisensor longterm
monitoring systems will provide near real-time onsite interrogation results
and is expected to typically save from 25-50 percent per site when compared to
conventional drill, sample, and offsite laboratory analysis techniques.
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Developments In A Normal Mode Helical Electrical Antennae Crossole Instrumentation And Integrated Interpretation System
Authors Ross Groom and Cliff CandyElectromagnetic crosshole methods are promising for imaging conductivity, permitivitty and
permeability variations in the earth. Radio and radar frequencies with electric field antennae offer advantages
for resolution and sensitivity in a wide range of environments. However, scattering difficulties created by
small, near-field inhomogenities and attenuation issues have limited the use of such systems. Our modelling
studies have indicated the usefulness of lower frequency antennae in a wide range of applications but
antennae lengths and efficiencies under conventional design have prohibited application at this lower range.
The use of normal mode helical antennae (NMHA) provides the opportunity to exploit a range of
frequencies lower than those employed with conventional antennae design while retaining the resolution and
sensitivity advantages of these techniques. We have found these lower frequencies are accessible using a
NMHA system with the antennae providing acceptable efficiencies from a compact downhole device both in
length and thickness. The broadband effectiveness of the NMHA allows for effective multi-frequency
investigation with a single antennae.
In conjunction with instrument development, we have developed simulation capabilities both for the
antennae radiation pattern and electromagnetic wave scattering in a three-dimensional environment allowing
for contrasts in resistivity, electrical permittivity and magnetic permeability. A variety of useful data
representation techniques have been developed to display data effectively in one, two and three dimensions.
Useful tomographic techniques have been developed for the system allowing for near-field scattering and
curved boreholes.
Survey techniques and data will be presented for several different types of applications. Data will be
presented from a calibration site with detailed logs, a landfill site with leachate contamination, an industrial
site with industrial cleaning solution contamination, and a survey to study groundwater geometry in a glacial
till environment.
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Use Of Shear Wave Velocities To Estimate Thick Soil Amplification Effects In The Fraser River Delta, British Columbia
Authors J.A. Hunter and H.A. ChristianShear wave velocity-depth profiles are critical inputs to the estimation of ground motion for thick soil
sites, where ground motion amplification, resonance and liquefaction effects related to earthquake shaking
can pose serious engineering hazards. In the Fraser River delta just south of Vancouver, British Columbia,
over 500 shear wave velocity-depth measurements have been made in a 24 by 26 km area, using both surface
and borehole techniques. These measurements were made to improve our understanding of the subsurface
in this area of rapid urbanization in a zone of high seismic risk. The area is underlain by thick,
unconsolidated Holocene and Pleistocene sediments; soft, deltaic sediments (>300 m thick in places) overlie
glaciofluvial sands and gravels as well as over-consolidated glacial tills, and the depth to Tertiary bedrock
varies between 200 and 1000 m below surface. The shear wave velocity database is now extensive enough
to allow regional estimations and maps of ground motion amplification and site resonant periods to be
derived using simple linear elastic approximation methods. These maps show considerable spatial variations
correlated with the variation in the depth to the Holocene-Pleistocene boundary, which commonly exhibits
a high seismic impedance contrast. Records of local earthquakes from strong-motion seismic stations within
the delta show variations in ground motion response that correlate well with the derived maps. This
information dramatically highlights the importance of considering thick soil response in future land-use
planning and engineering design in the Fraser River delta, and demonstrates how geophysical surveys can
be used to provide the required shear wave velocity-depth information in this and other areas of earthquake risk.
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Soil Investigation In An Area Of A Disposal Site For Blast-Furnance Slag Using Rayleigh Surface Waves
Authors Dirk Orlowsky, Christoph Witte and Bodo LehmannA disposal site for blast-furnace slag shall be recultivated and used as building ground for large
industrial halls. It is well known, that during the dumping period the liquid slag may had build
cavities or include loose material which can decrease the stability of the underground. The common
technique to detect and to close these inhomogeneities is the consolidation of the whole area with
accelerated weight drops. However, this technique is not environmently friendly because of the noise
impact and therefore this area was investigated analysing the propagation and dispersion
characteristics of Rayleigh surface waves. The P- and S-wave velocities in the compact slag are
about 2- to 3-times higher than those of the underlaying soil. Thus an inverse dispersion character of
the phase velocities of the Rayleigh surface waves is expected. The whole area was investigated
using a pseudo 3D-seismic technique with a hammer blow as a seismic source. Low amplitudes of
the P- and S-waves, but strong amplitudes of the surface waves could be recognised. The velocity of
the surface waves for three different frequencies were determined at each geophone location and
displayed on a map. Three maps of the velocity distributions showing the results for different depths
ranges were generated. Additionally, locations were marked at those geophone positions where the
surface waves were reflected or show not a normal propagation characteristic. Although the area of
blast-furnace slag was expected to be very complicated for the investigation with surface waves the
localisation of inhomogeneities with velocity and reflection maps worked perfectly well and
achieved the demands of the customer. The consolidation of the slag area with accelerated weight
drops has been reduced to those locations only where cavities and loose material have been detected,
such that the environmental noise has been reduced to a minimum.
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Evaluation Of Local Effects In The Study On Seismic Risk Reduction: Discussion About The Resolution Capability Of The Updated Techniques
It is well known that many factors contribute to the definition of Seismic Risk and that the study of
these factors calls for a multidisciplinary approach.
Moreover, the complexity of seismic phenomena and their consequences on the human and natural
environment contain a margin of uncertainty, as for example in surface effects estimates. Studies on
seismic risk mitigation, particularly those on the evaluation of local effects, are based on a wellestablished
methodology.
Nevertheless, open questions are: can the updated techniques still in use provide reliable data on local
effects? What is a rough estimate of the order of magnitude of local effects?
Seismic events which recently occurred in Italy were characterized by strong differences in the damage
level for similar types of buildings, so they offer a great opportunity for the study of such surface
phenomena.
In the present study, on the basis of this experience, we provide a critical overview and discuss
established methodologies through case history examples and modeling.
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High-Resolution S-Wave Reflection Survey Using Land Streamer For The Safe Driving Of A Shield Tunnel
Authors Inazaki Tomio, Kusaka Naoki, Ashida Yuzuru, Takigawa Shinji and Yoshimi ShigenoriA high-resolution S-wave reflection survey using the Land Streamer was conducted
over a trunk highway in Kyoto Prefecture, central Japan. The purpose of the survey was to
delineate the detailed geologic structure of the surficial part of a faulted zone of an active
fault named “Katagi-hara Fault” for the safe driving of a shield tunnel, which was planned to
undermine the highway about 15 m in depth. At the excavation stage of the tunnel, potential
influence of the active fault for the tunneling was reminded, which had been overlooked at
the planning stage of the tunnel. Additional boring survey was carried out, however, a
detailed structure in the faulted zone had been still uncertain. We therefore applied the highresolution
S-wave reflection survey using the Land Streamer, which can provide high quality
data even on the paved surface resistant to heavy traffic noises. The survey result profiled
deformed structure and small scaled faulting in the surficial layer within the section to be
excavated. Interpretation of the reflection survey result was utilized for the informed control
of tunneling. As a result, the shield tunnel was driven safely without encountering any
geotechnical trouble.
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Avo Analysis Of Shallow Seismic Data: Feasibility And Analysis Of 2D And 3D Ultra Shallow Reflections
Authors Ran Bachrach and Tapan MukerjiAmplitude versus offset (AVO) analysis can provide information about the change in the
shear wave properties between two layers given only P wave reflection data. AVO
techniques have been used for many years in the petroleum industry for characterizing
deep reservoirs. Here we present an AVO analysis of shallow seismic sections (P-to-P
reflections) preformed over a water table in beach sand at depth of 2.4m. Threecomponent
seismic survey provides an independent measurement of the shear and
compressional wave velocity in the sand. We compare the results of the AVO analysis of
ultra-shallow seismic data to the theoretical prediction given by the complete Zeoppritz
equations. We show that reliable AVO analysis can be preformed when there is a
calibrated reflector with known response. Some of the uncertainty about the results is
related to our reference reflector. Our results suggest that non-linear response of the
geophones in shorts offsets contributes to this uncertainty. We also show that shallow
seismic data can provide useful AVO estimates, and specifically information about the
AVO gradient. We show that AVO analysis of 3D seismic provides better estimates of
the near offset section due to high fold. Both 2D and 3D show that at the interface
between dry and water saturated sand there is an increase in reflection amplitude with
offset that picks at the critical angle.
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Near-Surface Seismic Studies To Estimate Potential Earthquake Ground Motion Amplification At A Thick Soil Site In The Ottawa River Valley, Canada
Authors Beatriz Benjumea, Jim A. Hunter, Susan E. Pullan, Robert A. Burns and Ron L. GoodIn an area approximately 100 km east of Ottawa, post-glacial marine sediments of the Ottawa
River valley (Ontario, Canada) exhibit surface disturbance that has been postulated to be a result of
paleo-earthquake shaking circa 7060 years BP. A high-resolution shallow seismic survey has been
conducted in this area to obtain information about the subsurface structure that may have locally
influenced the ground motion response. Seismic reflection quality is excellent with dominant reflection
frequencies in the 200-300 Hz range. Forty seismic reflection test sites outline a deep, bowl-shaped,
bedrock basin beneath the zone of disturbed surface sediments. The circular basin is approximate 8 km
in diameter with sediment thicknesses reaching at least 175 m. Follow-up P-wave reflection profiling
along a 4 km long survey line shows a detailed image of the stratigraphy within unconsolidated
overburden as well as the bedrock topography.
Shear wave velocity structure within unconsolidated overburden as well as the impedance
contrast at the bedrock boundary are being used to estimate ground motion amplification, resonance, and
liquefaction potential for significant earthquakes. SH refraction/reflection surveys have been carried out
in the project area to map shear wave velocity structure and to calculate fundamental site resonance
frequencies directly from the two way travel time of the SH bedrock reflector. Preliminary results
indicate substantial variations in amplification effects and fundamental site periods across the survey
area. Low near-surface shear wave velocities and the presence of water-saturated sand indicate a
significant potential for cyclic (seismic) liquefaction to occur.
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Detection And Location Of Obstructions Ahead Of A Tunnel Boring Machine Using The Tunneling Vibrations As A Seismic Source: The Firist Successful Example
More LessTunneling with underground boring machines is quickly becoming the preferred method for
efficient, safe, and cost-effective construction of underground utilities and access tunnels. However, one
of the significant problems for mining with tunnel boring machines (TBM’s) is the variation (sometimes
sudden) in the ground conditions into which the TBM is drilling. In this study, for the first time, an
obstruction ahead of a TBM was located using the vibrations from the actively mining TBM as a seismic
source.
At the original study site in Columbus, Ohio, the vibrations from an active TBM were passively
recorded using a 36-channel Strataview seismic system. Single 30 Hz geophones were deployed at 10 m
intervals, starting at a position 10 m from the epicenter of the active TBM and at 10 m spacing along the
alignment of the future tunnel for a distance of 360 m. Vibrations from the TBM were easily recognized
across this entire spread. Various sample rates and recording times were tested, with the longer records
producing better results. Because of hardware limitations on record length (i.e., number of samples per
trace), data processing included appending individual records together into super-records. This provided
simulated extra long recording times for improving the signal/noise ratio through the correlation process.
Appended super-records up to 96 seconds were used in the correlation process. Various near-source
(near-TBM) data traces were tested for use as the optimal “pilot sweep” for correlation with the
remainder of the data traces. The utility of this acquisition and processing approach is the lack of a need
to interfere with the TBM operation in the subsurface or the necessity to synchronize time with a
geophone underground at the TBM.
The processed records revealed a reflection of TBM-generated energy back along the spread
from a steeply dipping boundary at a location ~240 m in front of the TBM. At that location, the TBM
later encountered a 3 m wide clay-filled vertical cavity in the limestone. These data and interpretation
have also been modeled using a ray-trace modeling program.
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Fast 3D Ultra Shallow Seismic Reflection Imaging Using Portable Geophone Mount
Authors Ran Bachrach and Tapan MukerjiIn this paper we present the results of a suite of seismic experiments designed to develop and
evaluate the use of 3D Ultra Shallow Seismic Reflection System for high resolution near surface
imaging. A feasibility study and the first implementation of 2D portable geophone mounted array for
fast and cost effective ultra shallow 3D seismic data acquisition are discussed. The new portable
geophone mount is made out of anelastic base with a frame and 72 geophones spaced in an array.
Geophone interval of 0.25m both in the inline and crossline direction enables acquisition of very highresolution
3D data cubes with bin size of 12.5x12.5cm and wavelets of 350Hz. The time of re-planting
the 72 channels portable geophone mount is about 5min in the field. We also developed a data
acquisition quality control and real time imaging that can be preformed on a portable computer in the
field and produce 3D images in real time.
Thus, we show that high-resolution near-surface 3D seismic images can be obtained rapidly, and
in a cost-effective manner. This has the potential of providing effective solutions for many geotechnical
and geoenvironmental applications.
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Application Of The Surface Wave Method To Delineate Two-Dimensional Shallow Subsurface Structure
Authors Haruhiko Suzuki, Koichi Hayashi and Dai NobuokaS-wave velocity measurements using surface waves usually estimate one-dimensional structure.
If the method can be expanded to two-dimensions, one can estimate two-dimensional S-wave
velocity structure effectively. In this study, we examined property of surface waves under the
two-dimensional structure using finite difference method. The result was that surface wave method
could estimate velocity structure like a fault model. And we conducted field experiments of surface
wave method to estimate two-dimensional structure. The result of surface wave method agreed well
with the results of reflection and refraction survey. So we could confirm the applicability of surface
wave method for near surface two-dimensional S-wave velocity survey.
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Modal Separation Before Dispersion Curve Extraction By Masw Method
Authors Julian Ivanov, Choon B. Park, Richard D. Miller and Jianghai XiaAccurate extraction of dispersion curves is the most critical part with any surface-wave method.
Although a multichannel method like Multichannel Analysis of Surface Waves (MASW) method proves
most effective for this purpose, there are several acquisition parameters that need to be set properly for
the optimal extraction. One of them requires a long range of receiver spread to separate phase velocity of
one mode from those of other modes. Sometime, however, this requirement can not be met due to an unacceptable
lateral inhomogeneity in the near-surface materials being surveyed and therefore multichannel
records are often collected with a relatively short spread length. In this case the interference
between the different modes of surface waves can be so severe that neither fundamental nor highermode
dispersion curve can be extracted with a reliable confidence. In addition to this multimodal effect
by surface waves, other types of seismic event such as channel (guided) waves can cause similar harmful
effect during the analysis. When this occurs, a simple multichannel processing technique that mutes
the interfering wavefields in the offset-time (x-t) domain can significantly enhance the resolution of
multimodal dispersion curves. This is demonstrated by using both synthetic and real shot gathers.
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Offset And Resolution Of Dispersion Curve In Multichannel Analysis Of Surface Waves (Masw)
Authors Choon Byong Park, Richard D. Miller and Jianghai XiaInfluence of offset-related parameters on the resolution of dispersion curve in multichannel
analysis of surface waves (MASW) surveys is described from the theoretical perspective of the
dispersion curve imaging method used during a normal implementation of MASW. The examined
parameters include total number of channels (or traces), closest-to-source offset, receiver spacing, and
total length of the receiver spread. The influences of different phase velocities and frequencies are also
briefly described. It is shown that a larger total receiver spread length is always preferred to produce a
higher resolution. This means that with a given number of channels available a greater receiver spacing
is preferred as long as it does not cause a spatial aliasing problem. This shows that with MASW method
the general notion that more channels are always better can be misleading.
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Lax Pair And The Riemann-Hilbert Method For Solving Diffraction And Scattering Problems In Geophysics
More LessNew Riemann-HIlbert method was suggested recently by A. Fokas for studying boundary problems
for linear and integrable nonlinear PDEs. In this paper we extend this approach for solution
of the vector elastodynamic equation in inhomogeneous geomaterials. Scattering of Rayleigh wave
in an elastic quarter space is considered. The suitable Lax pair formulation of the elastodynamic
equation is obtained. The integral representations for the solutions of the Lax pair equations are
found. This reduces the problem to the analysis of certain Riemann-HIlbert problems. The results
of this analysis, which uses the apparatus of the Jacobian elliptic functions will be reported
at the meeting.
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Investigation Of Infrastructures With Microseismic Methods
More LessWith the objective of contributing to a more efficient management of the funds required to
maintain concrete or asphalt surface works, Geophysics GPR International Inc. has developed the
conventional seismic method into a new non-destructive and continuous evaluation tool for bridges,
roads and structures in general. By producing high frequency S and P seismic waves and recording the
waves an ultra rapid acquisition system, it is possible to measure the mechanical parameters of the
material such as the Young, Shear and Bulk moduli, or the Poisson ratio. These parameters in turn
characterise the quality of the investigated material. The development of a specialised processing
software, in parallel to the development of the equipment, allows to complete these measurements and
calculations more rapidly and economically.
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Automatic Detection Of Uxo From Airborne Magnetic Data Using A Neural Network
Authors Ahmed Salem, T. Jeffrey Gamey, Dhananjay Ravat and Keisuke UshijimaRecent developments in airborne magnetic detection systems have made it possible to
detect much small magnetic objects such as UXO. However, the data sets are extremely large
and the measurements must be processed and interpreted. Therefore, there is an increasing
need for a fully automatic interpretation technique that can be used to make decisions
regarding the nature of the source in real time. The massively parallel processing advantage
of artificial neural networks makes them suitable for hardware implementation; therefore,
using these networks in conjunction with a magnetic system has the potential to greatly speed
the detection of ferro-metallic objects. In this paper, we have developed a new method for
detection and characterization of UXO using a Hopfield neural network as applied to airborne
magnetic data. The Hopfield network is used to optimize the magnetic moment of the
equivalent dipole source at regular locations. For each location, the Hopfield network reaches
a stable energy state, and the location of the object corresponds to the minimum Hopfield
energy. Output results include position in two dimensions (horizontal location along the line
and depth), magnetic moment, and effective magnetic inclination. Theoretical and actual field
examples show that the Hopfield network in an accurate and objective tool for UXO
detection.
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Low Level Helicopter Magnetic Survey For Uxo Detection
Authors Raye Lahti, Doug Lam, Allen Duffy, M. Blohm, Ed Wightman and LtC. J. KnottA helicopter-mounted magnetometer survey was used to locate unexploded ordnance (UXO) at
the Massachusetts Military Reservation (MMR). The system, called the HM3, was designed and built by
High Sense Geophysics of Toronto, Canada, now Fugro Airborne Surveys. The system has threecesium
vapor magnetometers housed in three booms attached to the base of the helicopter. Two of the
booms extend 6 meters from either side of the helicopter, and the other boom extends 6 meters from the
front of the helicopter. Positioning is accomplished using real time Differential Global Positioning
System (DGPS). In addition to the cesium vapor magnetometers, a fluxgate magnetometer installed on
the aircraft is used to facilitate magnetic compensation due to aircraft movement.
The objective of the survey was to locate significant areas containing UXO as well as relatively
clear areas. The survey covered about 2200 acres at the MMR, where many years of artillery practice
has resulted in munitions scattered across the area, some of which may be unexploded. The area is
covered with scrub oak bushes, sometimes quite dense, with heights of about 2 to 3 meters. Also present
are areas of pine trees about 10 meters in height. The thick vegetation, along with safety considerations,
makes an airborne survey the most realistic option for locating areas of UXO debris. The helicopter
flew 3 to 4 meters above the vegetation, giving a flight height of 7 to 15 meters depending on the
vegetation height.
Many magnetic anomalies were observed, including large anomalies from tanks and other metal
objects deliberately positioned as targets. Also observed were linear anomalous patterns with
amplitudes of one or two nano Teslas (nT) extending in a northerly direction. These anomalies are
thought to result from magnetite minerals in the soil, with the distribution of magnetite being controlled
by glaciation and subsequent weathering. In addition, data were recorded at various heights over typical
targets buried in the ground. This was done to allow the airborne system to be calibrated. This
calibration data shows that magnetic anomaly amplitudes of 2 to 4 nT could be resolved at a flight
height of 7 meters, decreasing to background noise above about 15 meters.
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Rapid Computation Of Time-Domain Response Of Metallic Scatterers For Real-Time Discrimination
More LessApplications of a new method for rapid computation of multi-exponential timedomain
responses of compact metallic scatterers with arbitrary transmitter and receiver
loop geometries is presented. The algorithm is based on computing a large (several
dozen member) database of freely decaying eigenmode shapes and associated decay
rates for a large range of scatterer geometries. The latter calculation is performed
using a new, extremely efficient, high-order mean field approach recently developed
by the authors. This database, which encodes only the intrinsic properties of the
scatterer itself, may then be accessed and combined with the transmitter and receiver
loop data for rapid computation of the predicted time-domain response. The method
now provides the basis for an efficient solution to the inverse problem, which potentially
allows real-time scatterer discrimination capability. We illustrate the method using a
database consisting of spheroidal scatterers with a large range of aspect ratios, allowing
modeling of long cylindrical UXO shapes at one extreme and of flat circular plates at
the other.
Unexploded ordnance (UXO) remediation sites are characterized by a tremendous range
of buried and surface metallic targets, including scrap metal and exploded bomb parts of
various sizes, only a small percentage of which correspond to the UXO of interest. The
development of data collection and analysis technologies that are able to rapidly discriminate
between UXO and "clutter" under the full range of different soil and topographical conditions
constitutes a scientific and engineering grand challenge.
On tile data collection side, the aim is to collect more detailed, less noisy data, correlated
as closely as possible to key discriminating features of the UXO, such as its shape, size, and
material constituents. Unfortunately, to date no detection technology has been developed
that is able to transparently exhibit these key features, at the level, say, that conventional
radar exhibits flying aircraft, under anything approaching realistic conditions.
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Robotic And Remotely Operated Geophysical Platforms For Terrestrial And Marine Uxo Detection
Authors Noel T. Rogers, Stewart K. Sandberg and George PrinceTwo remotely operated geophysical platforms were deployed for terrestrial and
marine unexploded ordnance (UXO) detection. The terrestrial system is a fully robotic
system and the marine system is a remotely towed platform. The remotely operated
geophysical UXO explorer (ROGUE) is a semi-autonomous robotic platform equipped
with a Geonics Ltd. EM-61™ time-domain electromagnetic metal detector, a pair of
GEM Systems, Inc. GSMP-30™ potassium magnetometer sensors, pan and tilt video
camera and, tracked with real-time kinematic GPS (±2 centimeter accuracy). Both
magnetometer and EM data, video, and GPS are simultaneously collected and transmitted
via radio modems to a remote data acquisition center. Once the extents of the survey are
input into the ROGUE command and control system the robot autonomously collects the
geophysical data and transmits the data back to the command center in real-time. The
marine geophysical system (ROHICA) consists of a non-magnetic vessel equipped with a
magnetometer and differential GPS (± 0.5 meter accuracy) and a moored offshore barge
for towing the geophysical vessel. The offshore barge has a set of four winches, two
winches control the on-line positioning of the barge and two winches control the
transport of the geophysical vessel down lines. Navigation for all marine vessels is
controlled from the remote data acquisition system. The geophysical and positional data
from the marine system are transmitted in real-time via radio modems to the remote data
acquisition system.
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A Novel Characterization Technique Using The Em61-Hh
Authors T. Jeffrey Gamey and Robert J. SelfridgeObservation of results from pull-away experiments to determine the lateral
footprint of electromagnetic instruments such as the EM61 and EM61-HH lead to an
interesting potential characterization technique for UXO. This paper details an empirical
study of target responses in the vicinity of an EM61-HH instrument. A wide variety of
geometric shapes were tested and the results imaged in two different ways in an attempt to
determine the general target shape. The overall result is a simple “rule of thumb”
discrimination technique that can be applied in the field with instruments and data that is
already being collected during target reacquisition.
The orientation of the field lines from the electromagnetic transmitter coil range
from vertical at the coil center to horizontal at the coil edges. The expectation was that the
ratio between early and late responses is driven largely by the cross-sectional area
presented to the field lines. When the field lines couple favorably with a target crosssection,
eddy currents are stronger and last longer into the late time gate. This appears as a
change in the ratio between the two gates. In this way, it was hoped that information about
the shape of the target could be derived from the ratio of responses at various points in the
vicinity of the target. While this cross-section/field line coupling theory was not supported
as the cause of the variation in ratios, the ratios did indeed vary in a way which can be
related to basic target geometry.
In general, 0-1D targets showed very low variance from a linear trend in the late vs.
early distribution line. The 2D objects showed much higher variance, and some unique
two-part distributions, with generally shallow slopes. The 3D objects showed similarly
high variance, but with steeper slopes and occasional three part distribution curves.
In addition, the shape of the response in the vicinity of the target was more
complex than is usually appreciated, and this has some serious implications for UXO
survey design and discrimination techniques.
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