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6th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 18 Apr 1993 - 22 Apr 1993
- Location: San Diego, California, USA
- Published: 18 April 1993
41 - 60 of 65 results
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Geophysical Characterization Of Mineral Waste Sites
Authors J.J. Snodgrass and C.M. LepperSurface geophysical methods including electromagnetic conductivity,
spontaneous potential, seismic reflection and refraction, ground
penetrating radar (GPR), and magnetometry were evaluated at mineral
waste sites to characterize subsurface geology and hydrology.
Three metal mine waste sites were investigated: (1) an open pit
operation with extensive spoils and sub-ore grade materials
distributed over an area approximately one square mile; (2) a
typical mill waste dump site, a few acres in size; and (3) a small
mill waste tailings pond. At the open pit mine, four different
locations were investigated to determine pre-mining topography,
characterize the subsurface groundwater flowpaths, and design the
most efficient drilling program for monitor wells. Interpretation
of the geophysical surveys at the waste dump site provided a model
of the ground water conditions which was crucial to the development
of a remediation plan. Results of the geophysical surveys at the
tailings pond provided information on the thickness of tailings and
water channelling for subsequent modeling of the hydrology, in
addition to determining feasibility of new methods for
characterization of mine wastes. The work to date has emphasized
the need for a multi-disciplinary approach to the characterization
of mineral waste sites with geological, hydrological, and
historical input to determination of the best technical evaluation
of site conditions. An integrated approach using multiple
techniques provided a high degree of confidence in interpretation
of conditions that were relevant to design of drilling plans,
development of groundwater models, and remediation efforts.
Efforts at two mill tailings piles, and at the abandoned open pit
mine indicate that each investigation must necessarily be
considered an applied research study and that the best methods will
evolve from ongoing surveys.
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Assessment Of Mississippi River Revetments Using The Self-Potential Method
Authors Keith J. Sjostrom, Dwain K. Butler and Robert F. CorwinTwo of the U.S. Army Corps of Engineers (USACE) primary mission
areas are the maintenance of the nation's navigable waterways and
development of flood control measures. These tasks are most evident
along the Mississippi River where revetments are used to maintain the
river channel and prevent riverbank failure. Revetments are structures
placed parallel to the river current for the purpose of stabilizing the
concave banks in river bends and to form a smooth bank line. Along the
lower Mississippi River, articulated concrete mattresses (ACM's) are
used exclusively for river bank protection (see Figure 1). An ACM
consists of an assemblage of concrete blocks reinforced and held
together by a metal fabric to form a flexible, protective blanket over
the river bottom (Petersen, 1986). A section of ACM is typically 150 ft
wide and extends from the river bank to the center of the navigable
channel. Adjacent ACM sections are installed by overlapping the
downstream mattress by five feet. The ACM are secured to the river bank
with launching cables anchored into the bank material.
Over time, ACM's are subjected to numerous cycles of high and low
water levels, changing river velocities and currents, and structural
fatigue. These factors may cause the ACM to become damaged, displaced
by the undermining of streambed material, or buried by sediment. The
condition of ACM's must be routinely monitored to determine whether or
not remedial or replacement measures need to be undertaken.
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Neural Networks For Highway Maintenance Investigations Using Ground Penetrating Radar*
Authors Dorothy V. Minior and Stanley SmithThe early detection of potential subsurface erosion in highway systems allows for the application of appropriate
maintenance rather than more expensive rehabilitation or reconstruction. Currently, there are few practical test
procedures to assist a highway maintenance engineer in detecting these problems or in evaluating how effective
remedial measures were in repairing the problem. Existing procedures rely on complex algorithms that are
computationally expensive and are not amenable to real time applications. Our research evaluates the synergy
between Neural Networks (NNs) and Ground Penetrating Radar (GPR) as tools for early detection of subsurface
highway problems. We trained NNs to identify the following pavement characteristics in GPR SC~IIS:
(i) pavement thickness (ii) the degree of moisture in the surface layer, (iii) the degree of moisture in the base
layer, (iv) voids or loss of support beneath slabs, and (v) overlay delamination. Undesirable conditions can then
be treated by maintenance, and major structural problems requiring extensive repair can be avoided. Our research
shows that the NN/GPR approach is a viable solution. The NNs extrapolate over noisy or incomplete data, and
they provide faster responses than existing methods.
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Gas-Rich Sediment And Coastal Wetland Loss In Louisiana
Authors M.D. Thompson, L.D. McGinnis, P.L. Wilkey and S.F. MillerHigh rates of wetland loss in southern Louisiana provide the impetus for
examining the role that trapped, biogenic gases play in regulating subsidence of
coastal areas. A significant cause for wetland loss in this region is relative sea-level
rise produced by sediment-volume reduction. Dewatering, grain reorientation and
packing, and oxidation of organic-rich sediments are thought to be the main
processes for volume loss. It is argued that natural and anthropogenic causes for
sediment degasitication play a critical role in sediment-volume reduction.
Compressional wave velocities were measured at 34 sites in both the
abandoned (Holocene) and modem parts of the Mississippi Delta. A low-frequency
source (~200 Hz) was used to maximize sound-wave dispersion caused by
interstitial gas bubbles. Compressional wave velocities measured at low
frequencies relative to the gas-bubble resonant-frequency undergo maximum
change from the velocity for a gas-free sediment.
Seismic refraction studies and velocity measurements using standard
reverse-spread profiling indicate that the upper 40-70 m of the sedimentary section
has sound velocities significantly less than the speed of sound through water
(1,500 m/s), despite the fact that the water table is within 3 m of the ground
surface. The low-velocity zone has average velocities ranging from 800 to 1,150
m/s in the Holocene delta and from 900 to 1,300 m/s in the modem delta. Uphole
traveltimes obtained from approximately 90 km of continuous-coverage, seismic
reflection profiles yield velocity estimates that range from 900 to 1,300 m/s. First
and secondary arrivals determined from the reflection profiles are currently being
used to model the low-velocity zone.
Theoretical velocity modeling, using material properties consistent with
observed sediment types, suggests that approximately 6-20s of the available pore
space must be occupied by gas to account for the observed velocities. This gas is
believed to be trapped in unconnected pore spaces to exist in the form of methane
gas bubbles within the interstitial pore waters.
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Mapping Fracture Connectivity Between Boreholes Using High Resolution Temperature Logs
More LessThe use of temperature logs in detecting fracture zones within a borehole is
sufficiently documented in the literature. This is an indirect technique and has been
used primarily for single-hole fracture characterization. Drilling fluid circulation and/or
groundwater movements induce temperature anomalies that are interpreted to indicate
permeable fractured zones. The use of temperature logs to map fracture
interconnectivity between boreholes is not well documented. This paper presents an
application of high resolution temperature logs in detecting fractured zones, mapping
fracture connectivity between holes and in monitoring groundwater movements. A
well system consisting of five vertical boreholes was drilled through sandstone to a
depth of approximately 55 m, one hole at each corner of a 7.62 m (25 ft) square and
one in the centre. These boreholes were drilled for experiments in connection with
mine technology development conducted by Centaur Mining Exploration Limited. High
resolution temperature logs and several other geophysical logs were acquired in these
boreholes with the Geological Survey of Canada R&D logging system. Borehole fluid
temperature logs recorded immediately after drilling and well development located
several water-producing fracture zones by their characteristic temperature signatures.
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Integrating Geophysical Well Logs, Surface Geophysics, And Hydraulic Test And Geologic Data In Ground-Water Studies - Theory And Case Histories
By F.L. PailletIntegration of surface geophysical surveys, borehole geophysics,
hydraulic tests, and geologic data has the potential to: 1) relate detailed
aquifer properties at specific well locations to large-scale structure mapped
across an investigation site; and 2) calibrate geophysical measurements in
terms of hydraulic variables of direct interest in most contaminant dispersion
studies. This paper reviews the physical and mathematical constraints on the
formal inversion of such data sets as illustrated by a number of case studies.
Our analysis demonstrates that the coupled inversion of these data depends on
1) formulation and verification of a specific site model, 2) statistical
sampling of the distribution of formation properties which depends mostly on
the number of boreholes, 3) specific location of boreholes with respect to
contacts and boundaries given in the model and interpretation. The importance
of these considerations is demonstrated by the synthesis of various field
studies including photo lineament analyses, magnetic surveys, surface seismic
reflection profiles and electrical resistivity soundings with well logs, and
with the results of various hydraulic tests. The case histories illustrate
the importance of logs in formulating and testing specific site models. These
results repeatedly demonstrate the need for obtaining logs from enough
boreholes to relate the boundaries and contacts inferred from surface surveys
to the much greater vertical spatial resolution given by the logs.
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Shallow Seismic Studies Of A Large Playa Basin Near Amarillo, Texas
Authors J.G. Paine, A.R. Standen, G.T. Jones, L.B. Kelley and S.M. RooksSevenmile Basin is a playa basin in the Texas Panhandle that is being studied as
part of a geological and hydrological characterization of the region surrounding
the U. S. Department of Energy’s Pantex Plant near Amarillo. Shallow seismic
data were collected in an attempt to understand the origin, development, and
hydrogeological framework of playas, which are recharge points for the
economically important Ogallala aquifer. This exceptionally large playa basin,
which is 5.5 km across in an east-west direction and 3.6 km across in a northsouth
direction, is less than 0.5 km from the southern boundary of the Pantex
Plant.
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High Resolution, Shallow 3-D Seismic Survey For Environmetnal And Geotechnical Sites
Authors Dr. James Rector and Paul MilliganThe University of California at Berkeley has a multiple test well site in Richmond, that is a
prime candidate for high resolution 3-D surface reflection seismic survey techniques. More than ten
wells, covering an area of 100m by 100m, have been drilled into looseIy consolidated sand and gravel
units seperated by clay layers, with a basement unit of a hard shale at an approximate depth of 37m.
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Hydrogeologic Characterization Using A Shallow Seismic Reflection Survey At Fort Ord, California
Authors Carlene Merey, Richard D. Miller, Edward J. Ticken and J. Scott LewisShallow seismic reflection profiling was successful in delineating a shallow impermeable layer acting to perch fluids at depths ranging from 125 to 175 ft. The near-surface material was dry dune sands with varying grades of sands and gravels within the upper 20 ft. The seismic data possessed dominant frequencies in excess of 150 Hz resulting in bed resolution on the order of 3
ft at depths less than 150 ft. Several key hydrogeologic features were identified and their significance more completely ascertained using the reflection data.
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Mapping Hydrogeologic Properties Using Ground Penetrating Radar
Authors Michael D. Knoll and Rosemary KnightGround penetrating radar (GPR) images provide information about the location, scale
and geometry of subsurface heterogeneities. With well control or geologic intuition, this
information can be used by hydrogeologists to fix the location of boundaries in fluid flow
and contaminant transport models. While the location of boundaries is important, the
hydrogeologist also requires estimates of the physical properties within the boundaries, for
instance the hydraulic conductivity and porosity of different stratigraphic units. In essence,
what the hydrogeologist really wants are the input parameters to his or her model. If the
model is deterministic, these inputs are a discretized map of hydrogeologic properties. If
the model is stochastic, these inputs are geostatistical parameters such as the correlation
length, distribution, mean, variance and trend of hydrogeologic properties. With our goal
being to help the hydrogeologist, we are investigating the capabilities of GPR to map
hydrogeologic properties and characterize the heterogeneity of shallow aquifers.
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Use Of Focused Electromagnetic-Induction Borehole Geophysics To Delineate The Saltwater-Freshwater Interface In Great Neck, Long Island, New York
More LessGreat Neck is a peninsula about 1.5 miles wide and 3.0 miles long on the northern shore of
Long Island. It is surrounded by saltwater embayments and is underlain by 250 to 600 feet of
unconsolidated deposits that form a sequence of aquifers and confining units. Ground water at
several public-supply wells screened within the primary aquifers (Lloyd and Port Washington]?])
has become contaminated by saltwater, and two wells have been abandoned. Ten wells were
drilled in 1992 for collection of geologic, geochemical, and geophysical (focused-induction and
gamma) data to delineate the degree and vertical extent of saltwater intrusion through focusedinduction
electromagnetic (EM) logging.
Two areas of saltwater intrusion were identified in the northern part of the peninsula: One
is 125 to 50 feet thick in the Port Washington(?) aquifer with a maximum chloride concentration
of 15,300 mgJL (milligrams per liter); the other is 20 feet thick in the Lloyd aquifer with a
maximum chloride concentration of 1,900 mg/L.
Chloride-concentration data from filter-press samples and ground-water (screen-zone)
samples correlated closely with focused-induction EM logs’ responses, and the geologic samples
correlate well with natural gamma logs’ responses. Thus, focused electromagnetic induction EM
logs provide an accurate indication of the vertical extent and degree of saltwater intrusion and can
be used to delineate the saltwater-freshwater interface.
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Tracking Fluid Flow In The Unsaturated Zone Using Cross-Borehole Resistivity And Ip
Authors Susan Schima, Douglas J. LaBrecque and Michela MilettoContaminant plumes occurring in the unsaturated (vadose) zone often reach
ground-water aquifers, where they pose as a threat to human health. Current
hydrological methods of detection cannot adequately track the complex nature of
plume movement. A more useful and accurate method of tracking fluid flow is the
technique of resistivity and induced polarization (IP) tomography. Resistivity
tomography has been used in previous studies to reconstruct images of the vadose
zone. However, the use of IP data in a tomographical array to image the subsurface
is still under investigation. The study presented here attempts to explore the data
that can be obtained through IP measurements, which is complimentary to
information from resistivity image reconstruction.
Cross-borehole resistivity and II? experiments were carried out at the Avra
Valley Geophysical Test Site, west of Tucson, AZ, to follow water flow in the
unsaturated zone within a 30 m by 30 m sediment filled basin, The study involved
cross borehole resistivity and IP measurements using four, colinear wells, two
meters apart, to monitor the flow of 7600 gallons of tap water injected along a line
running the length of the basin.
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Resistivity Changes Over Time Related To Modeled Fluid Fl,Ow In Room-Q Of The Waste Isolation Pilot Plant
Authors M.G. Truskowski and H.T. AndersenThe Waste Isolation Pilot Plant (WIPP Site) is located in southeastern New Mexico about
35 miles east of the town of Carlsbad. The purpose of the WIPP Site is to evaluate the
storage of transuranic waste in mine excavations within the layered Salado salt formation.
Consequently it is important to understand the potential for fluid inflow into the
underground facility. Room-Q is a horizontal cylindrical room that was excavated and sealed
to measure brine influx. The Colorado School of Mines installed a stationary, permanent
DC resistivity grid in Room-Q in late 1989 and following electrode stabilization, the grid has
been read periodically since then.
A fluid flow model of Room-Q was constructed using the MODFLOW modeling
program. The profile-oriented model incorporated geologic information from underground
mapping and hydraulic parameters available from limited borehole permeability testing in
Room-Q. Electrical geophysical data gathered in the room from January to March, 1990,
were correlated with modeled fluid flow for the same length of time and a similar overall
trend was observed. The introduction of minor geological heterogeneities produced an
improved fit. A correlation was found to exist between the change in resistance over time
and the change in fluid content within the unsaturated fracture porosity zone surrounding
the room.
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Seismic And Radar Data Compression Using A Wavepacket Transform
Authors Yi Luo and Gerard T. SchusterA wavepacket transform is used to compress earthquake records, crosswell radar
data and crosswell seismic data. Results show that this transform can compress the
data by factors of 4 to 8. The speed of the transform is comparable to that of an PPT,
so this method may significantly reduce the data storage requirements for 3-D engineering
geophysical surveys (shallow 3-D seismic or shallow GPR surveys).
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Processing Ground Penetrating Radar Data To Improve Resolution Of Near-Surface Targets
Authors Kevin Gerlitz, Michael D. Knoll, Guy M. Cross, Robert D. Luzitano and Rosemary KnightAlthough unprocessed ground penetrating radar (GPR) data are capable of producing detailed images of the shallow subsurface, these images may be difficult to interpret reliably. Here, we
investigate characteristics of GPR signals and various techniques to enhance GPR images. The processing steps are tested on GPR datasets acquired at a well-controlled site where two concrete blocks, simulating archaeological structures, are buried less than two meters from the surface. Short range GPR signals often possess a low-frequency component (commonly referred to as a ttwowtt) that causes amplitude distortion along an individual trace. Characteristics of this noise are considered in light of its physical cause, i.e., saturation of the receiver electronics. Three filter techniques were tested to remove the wow: (i) residual mean filtering, (ii) bandpass filtering, and (iii) residual median filtering. We found that the residual median filter performed best. Additional signal processing steps included static corrections, normal moveout (NMO) corrections, migration and eigen filtering. Static corrections compensate for drift of the time-zero sample that occurs during acquisition. NM0 corrections remove spatial distortion due to source-receiver offset. Migration focuses diffracted energy and corrects dipping reflectors thereby improving spatial resolution. Eigen filtering removes the coherent direct arrivals to improve the detection of near-surface features. Application of these processing techniques significantly improves the quality of the GPR images from the test site, both in terms of the positioning of events and resolving capability. By testing various enhancement techniques on data collected at wellcontrolled sites, a better understanding is gained of their relative benefits and the ultimate capabilities of GPR as a shallow exploration tool.
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Accounting For Near Field Conditions When Interpreting 3-D Gpr Data
Authors Roger Roberts, Jeffrey J. Daniels and Leon PetersMany GPR data sets are collected to characterize the earth's subsurface at depths less than 5.0 m. Depending on the soil's electrical properties and the frequency of the GPR antenna, near field conditions may extend to this depth and deeper. GPR measurements of a half-wavelength calibration sphere buried at different depths in the Ohio State University test pit have permitted near field calibration of the OSU bistatic antennas. Data collected along multiple profile lines at increasing offset distances from the buried sphere have been processed to extract the amplitude and time arrival of the 125 MHz component in the sphere reflections. Contour maps of the amplitude indicate strong antenna orientation sensitivity in the extreme near field, At
increasing depths the reflection strength becomes less sensitive to antenna orientation. Time arrivals from profile lines with antennas oriented transverse to the profile line direction are amenable to accurate velocity-depth inversion, even in the extreme near field. Accurate velocity-depth inversion was not successful on profile line data with transmit and receive antennas
oriented parallel to the profile line direction.
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2D Inversion Of DC Resistivity And Induced Polarization Data
Authors Douglas W. Oldenburg and Yaoguo LiDC resistivity and Induced Polarization (IP) surveys are commonly used in
engineering, ground water and environmental problems. The principle difficulty
arises with the interpretation of such data since neither the apparent resistivity
pseudosections nor the apparent chargeability pseudosections are adequate representations
of the subsurface structure. Such information can only be obtained
through rigorous inversion of the data. The solution for the distribution of electrical
conductivity that arises in a DC rtxistivity survey is a nonlinear inverse
problem. To solve this problem we discretize the earth into cells of constant conductivity
and then find a minimum structure model which adequately reproduces
the observations. This inversion is efficiently carried out using a generalized subspace
techniques so that the dimension of the matrix to be inverted is kept small.
The conductivity model obtained from inverting the DC resistivity data is used
as a background model for the inversion of IP data. The sensitivities from the
background conductivity provide a linear mapping between the apparent and intrinsic
chargeabilities. The intrinsic chargeability is therefore obtained by solving
a linear inverse problem. Again we use the subspace methodology but we also
impose the constraint that the intrinsic chargeability is positive. The inversion
algorithms are applied to synthetic and field data.
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Electrical And Electromagnetic Honltorlng Of A Salt Water Injection Experiment
Authors David L. Alumbaugh and H.F. MorrisonSince early 1988 several experiments involving the injection
of salt water into an aquifer at depth have been conducted at the
University of California’s Richmond test site in Richmond,
California. The purpose of these tests has been to evaluate the use
of electrical and electromagnetic geophysical methods to detect and
monitor the location of contaminant plumes. The geology of the test
site consists of 35 to 40 m of electrically conductive clays, silts,
sands and gravels overlying a more resistive basement. Injection of
the slat water plume occurs in a gravel aquifer just above the
overburden-basement interface at approximately 30 m depth.
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Electromagnetic Data Evaluation Using A Neural Network: Initial Investigation-Underground Storage Tanks
Authors Darrin Cisar, John Dickerson and Timothy NovotnyEnvironmental and engineering studies utilize multiple methods of investigation with an
integrated geophysical approach. This complicates data interpretation because while integrating
data collected by a single method is no simple task, that of integrating multiple data types is
even less so. Traditionally, simple analytical and numerical models have been used to interpret
geophysical anomalies. However, interpretive models for the types of objects frequently
encountered at environmental restoration sites have not been available. Simulating
environmental targets as mineralized ore bodies has been our only available capability. This is
neither cost-, nor labor-effective, and is wholly inappropriate for cultural artifacts.
Artificial intelligence neural network concepts can be applied to these processes of discriminating
anomalies of interest from the host matrix, Neural networks are taught the defining response
parameters for a particular archetype. The trained network is then used to interpret geophysical
data anomalies relative to the response characteristics of the archetype.
For our investigation, frequency-domain electromagnetic (EM) data from known test
configurations were used to train a neural network to discriminate underground storage tanks.
The trained neural network was then used to “interpret” EM data collected at Hickam Air Force
Base, Hawaii. The results were compared with known “ground truth” excavation information
to determine the accuracy of the performance of the neural network.
The program demonstrated that artificial intelligence concepts can facilitate interpretation of
geophysical anomalies. In addition, the neural network offers the potential of truly integrating
collected data sets. The current state-of-the-art of integrated interpretation is to overlay separate
interpretations of the multiple methods employed and derive a “best fit” conclusion from this
collage. The neural network provides a means of integrating the separate data sets in either a
multiple (parallel), or in a step-wise (serial) integral response.
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Object Delineation Using Euler’S Momogeneity Equation Location And Depth Determination Of Buried Ferro-Metallic Bodies
Authors Armik Yaghoobian, Glenn A. Boustead and Tim M. DobushThe most universal application of magnetic field surveys in environmental studies is the
determination of the plan location and depth of buried ferro-metallic objects, such as drums or
pipes. Conventional methodology involves presentation of the measured magnetic field as a
contour map, manual selection of suspicious magnetic anomalies, and manual or computerassisted
interpretation of the plan location and depth estimation of the magnetic sources. The
results are dependent on the selection abilities of the interpreter and complicated by the fact
that virtually all man-made magnetic objects have significant permanent magnetisation that
distorts the observed field.
Euler’s homogeneity relationship offers a quasi-automated way to derive plan location and
depth estimates of buried ferro-metallic objects from a gridded magnetic data set. Euler’s
homogeneity equation relates the magnetic field and its gradient components to the location of
the source, with the degree of homogeneity expressed as a structural index. The structural
index is a measure of the fall-off rate of the field with distance from the source and provides a
way to discriminate between different source shapes. The method is also insensitive to field
distortion caused by permanent magnetisation of source objects.
This method has been applied to data collected over several test sites, including the Columbia
Test Site in Waterloo, Ontario, the US Army Engineer Waterways Experiment Station site in
Vicksburg, Mississippi, and EG&G Geometries’ environmental test site at Stanford University,
California. The objects buried at these sites are typical of those encountered at sites
undergoing environmental assessment (drums, pipes, sheets). The analyses of these data sets
have provided characteristic Euler deconvolution signatures and structural indices associated
with typical ferro-metallic objects. The solutions obtained indicate the ability to quickly and
accurately map the location and depth of buried ferro-metallic objects from gridded magnetic
survey data.
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