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11th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 22 Mar 1998 - 26 Mar 1998
- Location: Chicago, Illinois, USA
- Published: 22 March 1998
101 - 111 of 111 results
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Controlled-Source Electromagnetic Mapping Of A Faulted Sandstone Aquifer In Central Texas
Authors E.M. Gorman, M.E. Everett and B. JohnsonWe have used a standard controlled-source electromagnetic (CSEM) mapping technique
to identify lateral variations in electrical conductivity associated with faults and lithological
contacts in a shallow sandstone aquifer in Mason County, central Texas. Faulting
often plays a significant role in affecting groundwater flow. Faults can act as barriers or
conduits to fluid flow. The structural geology and role of the faults in affecting groundwater
flow in this aquifer, the Hickory Sandstone, have been previously studied. Our results
indicate that the CSEM technique provides useful complementary subsurface structural
information that can be used to refine hydrological models and their interpretation for
local water resources management and conservation.
Electrical conductivity profiles are inexpensive and convenient to obtain, compared to subsurface
information acquired by conventional invasive methods such as drilling. However,
electrical conductivity profiles are indirect hydrogeological indicators that require prior
geological knowledge for accurate interpretation. In the Hickory sandstone aquifer the
distribution of clays affects both hydraulic and electrical conductivity. Previous geological
studies have determined that different stratigraphic units within the Hickory aquifer
contain characteristic amounts of clay. We have interpreted lateral variations of apparent
electrical conductivity profiles in terms of contacts or faults juxtaposing different stratigraphic
units. CSEM measurements have much better shallow fault-resolving power than
the gravity data collected earlier at this site. The gravity data appear to respond primarily
to undulations of the crystalline basement at this site.
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Geophysical Identification Of Karst Fissures Near A Landfill In Southwestern Illinois
Authors James C. Schneider and Philip J. CarpenterMississippian limestone underlying most of Monroe and St. Clair counties in
southwestern Illinois forms a mature karst terrain. This is evident in hundreds of
sinkholes, a well-developed epikarst surface, and by the presence of caves and springs in
the area. Groundwater probably moves through conduits and fissures in the bedrock, in
an extremely complex groundwater flow system.
The City of Columbia Municipal Landfill is located in, or on the edge of, this karst
area. The landfill, primarily containing construction waste, is being closed, and monitoring
wells need to intercept any landfill leachate. Initially the area was investigated by mapping
fractures from rock exposures, sinkhole orientations, and lineaments. Major fracture
orientations are N6O”E and N40”W. Electrical geophysical methods were then employed
to locate subsurface karst features. Electromagnetic (EM) profiling with a 10 meter
vertical dipole coil spacing showed conductivity increases of about 5-15 mS/m over
possible karst fissures, and these anomalies are oriented at N6O”E or N40”W. Using the
horizontal dipole at a ten meter spacing, which “sees” only about half as deep, these
anomalies do not show up. This suggests that vertical dipole anomalies are from the
epikarst surface, and not from shallow soil structure or waste outside the landfill. An
electrical resistivity 2-D cross-section shows an undulating bedrock surface between the
low resistivity soil (40-50 Qm) and the high resistivity limestone (2000 Qm) with
increased depth to bedrock in the conductive zones mapped with EM methods.
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An Airborne Electromagnetic (Em) Survey Used To Map The Upper San Pedro River Aquifer, Near Fort Huachuca In Southeastern Arizona
Authors Jeff Wynn and Mark GettingAn airborne EM survey using a 3-component, 20-channel GEOTEM system with an
additional airborne magnetic channel was flown over Fort Huachuca and the area north and east
as far as the San Pedro River, southeastern Arizona. Inversions of the EM data, so-called
Conductivity-Depth Transforms (CDTs), provide remarkable information on the upper San Pedro
River aquifer down to 400 meter depths in some cases; results agree with limited well-control.
Depths derived from the magnetic data set, when inverted using an Euler transform, agree well
with a map of the basement derived earlier from gravity data. Together, the complete data set
allows us to map the Upper San Pedro River aquifer in a region where there is dispute over
allocation of scarce water resources and the San Pedro Riparian Conservation Area.
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Vertical Seismic Profile And Crossborehole Ground Penetrating Radar Measurements In Basalt Intraflow Structures
More LessVertical seismic profiles and cross-borehole ground penetrating radar experiments were carried
out in a basalt flow having a nearby outcrop exhibiting common intraflow structures (upper and
lower colonnades, and entablature). Results of velocity and attenuation analyses of the seismic
and radar data sets show clear differences between the different intraflow structures. Seismic
velocity (P and S) was highest and attenuation (P) lowest in the entablature and lower colonnade
compared to the upper colonnade. Radar velocity was highest and attenuation lowest in the upper
colonnade. Radar velocity was lowest and attenuation highest in the entablature. Radar velocity
and attenuation were intermediate in the lower colonnade.
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Tomographic Pseudo-Inversion Of Pole-Pole And Pole-Dipole Resistivity Profiles
Authors P. Cosentino, D. Luzio, R. Martorana, L. D’Onofrio, M. Marchisio and G. RanieriHere is presented an approach to carry out a fast construction of pseudosections using resistivity
data. The purpose of the procedure is to obtain maps and sections which match as closely
as possible the geometry of the structures which are investigated. Even if such a procedure is not
a real inversion but only an arrangement of the experimental data, it can nevertheless be used as
a useful tool in their interpretation or, at least, as a first step before the inversion, that is in the
choice of a preliminary inversion model.
In principle the method works like a back-projection in a pixel grid, which is performed by
arranging the experimental data in a sort of a sets of convolutions using 2D or 3D filters. The
coefficients can be calculated depending on the geometry (type, size, relative position and direction)
of the electrode array which is used. Consequently the filters for pole-pole and pole-dipole
arrays are presented. Furthermore some aspects of the method are discussed together with some
applications, using both synthetic as well as experimental data. The former ones were calculated
for a simple model (a buried resistive sphere) while the latter ones concerned some resistivity
profiles acquired in Sardinia and in Tuscany, in the course of some archaeological investigations.
Key words: pseudosection, resistivity profile, pseudo-depth section, pseudo-inversion, electric
tomography, pole-pole andpole-dipole arrays.
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Distributed Em Parlmeters Of An Ungrokmded Horizontal Loop .4Nd Their Relation To The Near-Si-Rface Geoei,Ectrical Features
Authors N.O. Kozhevnikov and S.P. NikiforovA horizontal transmitter loop is the principal part of many prospective TEM and FEM systems. In shallow
TEM geoelectric studies a loop with low electrical inertia has to be used to avoid a convolution problem. Up
to now an equivalent circuit with lumped inductance, capacitance, and resistance is widely used to simulate
loop’s parameters (e.g., resonance frequency) and intrinsic response. However, in-field measurements with
loops of different size have shown a lumped circuit to be unadequate in simulating early time and/or high
frequency loop responses. To account for these experimental results a system with distributed parameters has
to be used rather than an equivalent lumped circuit. The system is formed by the loop itself and the underlying
it shallow ground. Both oscillating and damped current and voltage transients as well as the frequency
response of such a system could be studied in detail on the basis of the transmission line theory and specially
designed computer simulating technique. It has been found from the in-field measurements that oscillating
transients are sensitive to local earth’s geoelectric structure and meteorological conditions.
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Approach To The Interpretation Of Potential Electrical Data In The Presence Of Uneven Surface
More LessThis paper describes how the integral equation system for solving a direct task of electrical
reconnaissance with direct current regimes in the presence of uneven surface is obtained. This
system is a combination of surface and volumetric integral equations. Solving of these equations
allows to model complicated situations. This paper also describes how to use the obtained results
for solving reverse tasks. The paper is illustrated with some of the obtained 2D results.
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Rapid 3D Refraction And Reflection Raytracing
Authors Jie Zhang and Eugene LavelyWe present a rapid 3D raytracing method optimized for the computation of reflection and refraction
wavefronts from a point source in a class of simplified models. The models consist of constant
velocity layers separated by arbitrary (and possibly complex) interfaces. We demonstrate that the
method simulates wave phenomena such as diffraction and head wave propagation. The approach
is extremely fast since it avoids traveltime expansion in the volume between interfaces, and solves
a simple 2D problem on each interface. Other methods require local propagators (even in constant
velocity regions) whereas our approach enables large jumps of wavefronts from interface to interface.
From interface to interface for reflection/transmission or on the same interface for refraction calculation,
graph theory and Snell’s law are applied to optimize raytracing. The calculation of raypaths
for an actual 3D survey in the Wind River Basin with 1690 sources (700,000 source-receiver pairs)
and a velocity model with two complex interfaces required less than 12 minutes of CPU time on
a 150 MHz Silicon Graphic 02 workstation. The computation time scales approximately linearly
with the number of interfaces.
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Hum Filter: Power-Line Noise Eliminator For Shallow Seismic Data
Authors Jianghai Xia and Richard D. MillerA unique filtering approach designed to eliminating power-line noise on shallow seismic
data without affecting the frequency content of signal provides a powerful harmonic noise
suppression tool for data acquired with modern large dynamic range recording systems.
Amplitudes and phases of sinusoids (functions of power-line noise) present before the first
arrivals can be estimated using the Levenberg-Marquardt (L-M) method. Initial amplitudes of
sinusoids are determined in the frequency domain using fast Fourier transform (FFT) methods
while initial phases are obtained by time domain correlation. Well-defined initial values
guarantee convergence of the L-M method. Modeling results suggest the relative error of initial
estimates are less than 50 percent. Calculation efficiency is achieved by simplifying the L-M
solution using the singular value decomposition (SVD) technique. The approach can handle
cases where power-line noise with frequencies of 60 Hz and/or its multiples exist simultaneously.
Once determined, the amplitudes and phases of sinusoids can be directly subtracted from the raw
data. Recorded frequencies of high-resolution shallow seismic surveys generally range from 30
to 300 Hz. Power-line noise (60 Hz) and its multiples (120 Hz, 180 Hz, and 240 Hz, etc.) are
within the optimum frequency range. This filtering technique only removes harmonic noise and
does not alter the spectra of signal. Real data examples demonstrate the efficiency and accuracy
of this method when implemented on a normal shallow seismic data processing flow.
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Radar~Equencestratigraphy: Anintegratedanalysisofascrollbarcomplex
Authors David P. Lesmes, David C. Roy and Scott M. DeckerWe have conducted an integrated geophysical survey of a scroll bar complex on the
Piscataquis River, near South Sebec, Maine. A sequence stratigraphic interpretation of the radar
data shows that the prograding scroll bar complex is composed of clinoforms, which are fining
upward successions of point bar sands and silts that top lap into the overbank deposits and
downlap onto channel lag deposits that overlie a basal unconformity. The basal unconformity
consists of a Pleistocene glacio-marine clay which pinches out onto a glacially eroded bedrock
surface. Radiocarbon dating of organic material obtained from cores should provide a
chronostratigraphic framework which can be used to reconstruct the three-dimensional evolution
of the river channel.
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Applications Of The Ground-Penetrating Radar Technique In The Detection And Delineation Of Homicide Victims And Crime Scene Paraphernalia
Authors Michael S. Roark, Jeremy Strohmeyer, Neil Anderson, Michael Shoemaker and Shauna OppertDuring the spring, summer, and fall of 1997, ground penetrating radar profiles (radargrams) were
acquired across a controlled/simulated homicide site near Rolla, Missouri. In this study, two deer
carcasses were buried in separate trenches at depths of 0.53 meters and 0.58 meters, respectively.
The trenches were excavated in compact, clay-rich alluvium. The carcasses were covered with less
compacted alluvium. Two dimensional and three dimensional radargrams were acquired/constructed
to optimize the interpretation of the data.
We have evaluated the radar data acquired to date and have made several interesting
interpretations/observations regarding the GPR signatures of the simulated homicide site and how
they varied over time. More specifically, the radar signature of the trench is mostly a function of the
fundamental differences (dielectric constant, velocity, and homogeneity) between the trench fill and
in-situ soils. On the radar profiles, the undisturbed soil is characterized by laterally continuous
reflection events. The trench/carcasses, in contrast, are characterized by a lack of laterally coherent
reflection events. Over time, the signatures of the trenches have changed subtly. These changes are
mostly attributed to compaction of the infill sediments and the decomposition of the deer carcasses.
However, even after a period of seven months, the trenches remain relatively easy to differentiate
from surrounding compact alluvium. In addition to the carcasses, a piece of metal the size of a
handgun was buried. The corresponding anomaly on the radargram is a high amplitude, hyperbolic
diffraction.
The ground penetrating radar survey was successful. Over the seven month period, there was a
notable change in the signature of the burial trench due to the decomposition of the bodies and the
compaction of the trench fill. The surveys have established the ground penetrating radar technique
as a useful tool in crime scene investigation for locating buried victims or evidence.
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