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7th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 27 Mar 1994 - 31 Mar 1994
- Location: Boston, Massachusetts, USA
- Published: 27 March 1994
61 - 76 of 76 results
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High-Resolution Marine Seismic Reflection Data Acquisition Using An Engineering Seismograph
Authors Jeffrey S. Waldner, Robert E. Sheridan, David W. Hall and Gail M. AshleyA digital single-channel seismic reflection system was developed to acquire continuous
profiling data by interfacing a conventional analog system with a land-based digital signalenhancement
engineering seismograph. Digital profiles are enhanced by trace static correction,
deconvolution, automatic gain scaling, weighted horizontal stacking and digital filtering.
Problems common to analog data, such as wave-motion effects of surface sources, waterbottom
reverberation, multiple reflections, and bubble-pulse width can be addressed by the
processing. The fast firing rate for analog records is maintained while a trigger-divider sends a
slower trigger rate to the digital seismograph. An auto-save feature with an automatic rollalong
switch groups adjacent traces to files, eliminating system limitations of the mass-storage device.
The digital data were collected to map sand ridges of the inner continental shelf off New
Jersey for beach sand replenishment surveys. The seismic tracklines are correlated to
vibracores and known geologic stratigraphy. The digital data exhibit reflections from layers
less than 1 meter thick. Synthetic seismograms constructed from the vibracore information
show these internal reflectors are caused by shelly gravel/medium sand interfaces. Global
Positioning System (GPS) was used to plot real-time position as well as obtain a navigational
accuracy within 10 meters. The combination of the analog system with the land-based seismograph
offers a less expensive technique for digital acquisition and processing. This provides
improved results over older analog data.
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Ground Penetrating Radar Survey Of A Waste Disposal Area At The Site Of A Former Lumber Mill
Authors J.R. Pelton, L.M. Liberty, M.W. Lyle, M.E. Dougherty, D.D. Crowther, R.D. Bolger, P.J. Gallaway and M.P. PetteysApproximately 1.5 km of 200-MHz fixed-offset ground penetrating radar (GPR) profiles acquired
at the site of a former lumber mill are dominated by two different reflection configurations: (1) a
chaotic pattern consisting of laterally discontinuous reflections and/or overlapping diffraction
hyperbolas; and (2) a more uniform pattern that is characterized by mostly continuous reflections
with smooth lateral variations suggestive of natural sedimentary features. Comparison of the
GPR profiles with geologic logs of nearby trenches indicates that the chaotic pattern is associated
with disturbed areas where near-surface burial of up to 3 m of mill waste (i.e., fill material composed
of woody or organic matter intermixed with sediments and minor metal, rubber, and plastic)
has resulted in strong lateral variations in subsurface dielectric or conductivity properties; the
chaotic pattern is distinctive and is not likely to be caused by natural sedimentary features. In
contrast, trench logs indicate that the more uniform pattern is associated with undisturbed areas
where native sedimentary materials are still in place, or with disturbed areas characterized by laterally
uniform fill material and no diffraction-producing trash zones. This ambiguity can be
resolved by directly mapping the base of the fill material (i.e., the contact between the fill material
and the underlying native sediments), but an easily recognizable radar reflection from that interface
is not consistently observed at the site. Although interpretation of the radar profiles is therefore
subject to some limitations, an overall evaluation concludes that the data are very useful for
reconnaissance, for supporting and extending the results of the site trenching program, and for
outlining the waste disposal area.
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A Preliminary Geophysical Survey Of An Inactive Landfill Scheduled For Reclamatio
Authors E. Jay Knopf and John E. McCallenLandfill reclamation can provide a cost effective alternative to traditional landfill closures. Reclamation may emerge as a viable option to help extend the life of a landfill, to recover soil for use as cover and other purposes, to reclaim certain recyclable material such as white goods, tires, metals, and wood wastes and for energy recovery. In 1990 the New York State Energy Research and Development Authority (NYSERDA) initiated, and in 1993 has provided majority funding for a number of research and demonstration reclamation projects in New York State.
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Gamma Logging To Define The Stratigraphy In A Buried-Valley Aquifer System
Authors David F. Dominic, Benjamin H. Richard, Paul J. Wolfe and Songlin ChengA number of methods were employed to determine the stratigraphy, bedrock topography, and
regional ground water flow system within a bedrock valley filled with Pleistocene sediments. A
proposed highway extension within the valley will pass within one mile of Cedar Bog, a relict
boreal fen; we are evaluating the potential for construction to disrupt the ground water supply to
the fen. Seismic refraction and gravity surveys were used to delineate water table and bedrock
depths along the eight miles of proposed highway routes. Drilling along the survey lines
verified the geophysical interpretation that several tributary valleys join the main bedrock valley.
Several previous studies have suggested that such valleys connect recharge areas (to the north
and east) to observed discharge areas in and near the fen. Critical to evaluating this hypothesis
is the stratigraphy within the main and tributary valleys.
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Identification Of Hydraulically Conductive Fractures From The Analysis Of Localized Stress Perturbations And Thermal Anomalies
Authors Colleen A. Barton, Mark D. Zoback and Daniel MoosA number of key questions need to be addressed before the relationship between stress and
fluid flow in the Earths crust can be understood. First, how is fluid flow related to the current
stress field in highly fractured crystalline rock? Second, is enhanced permeability and permeability
anisotropy principally the result of flow along joints (Mode I failure) or shear faults (Mode II
failure) or both? Is it possible that answer is neither - that in highly fractured crystalline rock,
flow is dominated by the orientation of faults and fractures introduced into the rock mass during its
long geologic history and the orientation of these structures bears no strong relation to the current
stress field?
It is well known that relatively few fractures in fractured rock serve to conduct fluids
through the rock. The use of fracture geometry to predict hydrologic flow in the crust is therefore
severely limited by the lack of knowledge of which fractures measured in a given survey actually
provide conduits for fluid flow. This study examines the relationship between in situ stress and
fluid flow using data from detailed analyses of wellbore breakouts and fracture geometry in
conjunction with precision temperature logs.
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Seismic Refraction And Electromagnetic Methods For Mapping Confining Layer In Central New Jersey
More LessThe Woodbury clay is one of the major hydrogeologic confining
units in the New Jersey coastal plain. It separates two main
aquifer systems; the Englishtown and the Potomac-Raritan-Magothy
aquifers. Therefore, at a hazardous waste site, it is important
to map the surface of this confining layer to help define the
local hydrostratigraphy, ground water flow and possible contaminant
transport.
In a contaminated site in central New Jersey two wells were
drilled to determine the vertical extension of a contaminant and
the direction of groundwater flow. The drilling results show a
difference in the depth of the Woodbury clay between the two
wells and some thin clay beds. At this stage geophysical support
was requested to map the Woodbury clay between the two wells and
to determine the depth to the water table.
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When Is 1D 2D? Interpretation Of Geoelectrical Sections
Authors Peter M. Duch and Kurt I. SørensenSurface geoelectrical measurements are routinely used in environmental and mapping
geophysics. The methods are used for the investigation of waste deposits, detailed mapping
of aquifers in ground water source areas and mapping of raw materials. In Denmark, that
means gravel, sand and clay.
The geoelectrical data may be presented as apparent resistivity pseudosections for
interpretation by the geophysicist. A pseudosection shows an unfocused and distorted image
of the subsurface which can be a starting point for a qualitative interpretation.
Quantitative interpretation of geoelectrical data is done with the purpose of finding the
parameters of an assumed model. Model assumptions include layered (ID), horizontal
cylinders of arbitrary cross-section (20) or 3D bodies. Traditionally, geoelectrical soundings
are interpreted using 1D models and tools for such inversions are very well developed with
regard to speed and accuracy. Thus an initial interpretation can be accomplished quickly and
used for the planning of further field work.
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Pulled Array Continuous Electrical Profiling
More LessThe electrical profiling method is a powerful technique for mapping of the near
surface geology, and the method has been used for decades. With the increasing need for
detailed and fast mapping of the protective clay caps of the aquifers the method has
achieved renewed attention. In this context a new method for electrical profiling: Pulled
Array Continuous Electrical Profiling, PA-CEP, has been developed. Electrodes are
mounted on a tail pulled by a small vehicle, and inside the electrodes and on the vehicle
data processing electronics are mounted. Data from several arrays are measured
continuously and simultaneously while the tail is actively being towed. The PA-CEP
method has enabled fast and reliable mapping of large areas (lo-25 km2). With areas this
large, surprising insight into the morphogenesis of the survey areas has been obtained.
The density of the data reveals hitherto unseen geological details. The PA-CEP method
has been used intensively in hydrogeological investigations, in prospecting for raw
materials, and in the construction of highways and pipelines.
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The Ellog Auger Drilling Method
More LessWith the Ellog Auger Drilling Method a detailed electrical log and gammalog are
measured with tools integrated in the drilling stem while actively downward drilling, and
undisturbed level specific water samples are taken through inlets placed close to the
cutting head and pumped to the surface. The drilling stem is a hollow auger with
continuous flight, and no drilling mud is applied. Hence the measured logs are not
disturbed by mud or invasion zone, and the collected water samples have a chemical
composition that reflects the undisturbed conditions within a narrow depth interval. By
the new technique a detailed mapping of the lithology and a vertical chemical profile are
achieved with a resolution surpassing that of traditional techniques. The method provides
data which are essential for locating pollution plumes, chemical interfaces and mapping of
the variation in the water quality of aquifers. The combination of the chemical analysis of
the water samples and the hydrogeological conditions inferred from simultaneously
recorded geophysical logs opens up the possibility of understanding the relationship
between chemical and hydrogeological processes.
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Elastic Wave Equation Travel Time And Waveform Inversion Of Crosshole Seismic Data
Authors C. Zhou, G.T. Schuster and Sia HasanzadehAn acoustic wave equation traveltime and waveform (acoustic WTW)
inversion method was presented by Zhou et al. (1993) to invert for
the P-wave velocity distribution from crosshole seismic. data. The
acoustic WTW tomograms showed about 6 times better spatial resolution
relative to the associated travel time tomogram. In this
paper we present the elastic wave equation travel time and waveform
(elastic WTW) inversion method which inverts for the interwell
P-wave and S-wave velocity distributions. Comparison of the elastic
WTW tomograms with the acoustic WTW tomograms shows
that both methods can invert for the high resolution P-wave velocity
structure when the S-wave energy is very weak in the recorded
seismograms. Although the field data we used are dominated by unconverted
P-wave arrivals, the elastic waveform inversion can still
invert for the S-wave velocity structure. Results show that elastic
WTW inversion is superior to acoustic WTW inversion when there
are large amplitude converted S-waves in the recorded seismograms.
The disadvantage of elastic WTW inversion is that it requires about
3 times more CPU time compared to acoustic WTW inversion.
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Use Of I!Iigh Resolution Geomagnetic& 2D-Dc-Geoelectrics And Induced Polarisation In Environmental Investigations
More LessFor more then 10 years geophysical methods are used for environmental investigations in
Germany. Landfills and abandoned industrial sites are focal points.
Especially in urban areas High Resolution Geomagnetics has become an important tool. Up to
4 points/m2 are measured with Fluxgate Simultaneous Gradiometers. This leads to a horizontal
resolution and a signal/noise ratio not within reach with the conventional geomagnetic method.
Geoelectric methods are able to detect certain types of contaminations in the subsurface. But
the traditional kind of use (profiling, 1D soundings, pseudosections) often can’t describe the
structures exactly. Recently developed algorithms for 2D-inversion are showing us sharper
pictures of what’s going on under our feet.
In many cases measuring DC-resistivity is not enough. To distinguish between geological and
anthropogenuous features by registrating just this parameter sometimes is impossible.
Measuring Induced Polarisation can be a great help in delineating the boundaries of
contaminated soil.
Examples from landfills and an anbandoned gas works are presented.
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Enhancement Of Ground Penetrating Radar Data Through Signal Processing
Authors M.E. Dougherty, P. Michaels, J.R. Pelton and L.M. LibertyRecently collected ground penetrating radar (GPR) data from a former lumber mill waste site
near Boise, Idaho have proven to be a useful reconnaissance aid for delineating possible areas of
fill from surrounding fluvial sediments. Several lines of data were collected at coincident stations
using antennas with central frequencies of 50, 100, 200 MHz. Some ambiguities exist in the interpretation
of the original 200 MHz data due to lack of a distinct fill signature in the data processed
with standard seismic processing techniques. We present here an effort to enhance the original
GPR data through air wave removal, bandwidth enhancement, and cross-correlation analyses. In
general, these analyses appear to improve the usefulness of the data and in some environments
may become standard techniques for future GPR data collection and processing.
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Amplitude Control In Shallow Seismic Surveying
Authors Jan Brouwer, Peter Bakker and Klaus HelbigThe amplitude of seismic waves contains valuable information on sub-surface soil parameters. In order to extract this information from the seismic records we have to: 1) preserve seismic amplitudes during acquisition and subsequent (pre-) processing; 2) determine (preferably) absolute reflection strength-versus-offset distribution from the true amplitude data; 3) interpret reflection strength information in terms of sub-surface soil parameters. In this paper we deal with the implementation of amplitude-versus-offset analysis in shallow seismic exploration. We show that use of seismic amplitude information can be successful at the cost of extensive (quantitative) monitoring of, e.g., source-signature and geophoneresponse and compensation for any changes or differences observed. A method to determine absolute reflection strength as a function of offset followed by an estimation of soil parameters is presented. This method is based upon the interpretation of amplitude ratios in primary-multiple chains. Practical application of this method has been tested on several datasets, two of which are presented here.
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Electromagnetic Modeling Of Large Structures Using Integral Equations
Authors Zonghou Xiong and Alan C. TrippApplication of electromagnetic techniques to high resolution geotechnical problems will require
effective means of calculating the response of complex models having many cells of possibly
differing conductivity. To address this need, we have developed an electromagnetic modeling
algorithm for large conductivity structures based on the method of system iteration and spatial
symmetry reduction using integral equations.
Electromagnetic scattering of conductivity structures using the integral equation method has
been widely used in geophysical applications [l]-[8]. Recently, [9] introduced the method of
system iteration, where a scatterer is divided into substructures and the direct matrix inversion
is applied to each substructure only, while the mutual interactions among the substructures
introduce equivalent sources. This technique greatly reduces the matrix factorization time and
storage requirements. Using the lateral homogeneity of a layered earth, [lo] developed a spatial
symmetry reduction scheme which drastically reduces the computation time for forming the
scattering impedance matrix by identifying and reducing the redundancy of calculating the
scattering matrix elements. This scheme permits enhanced use of the method of system iteration.
The method of system iteration is applicable to arbitrary scatterers with arbitrary discretizations.
It divides a structure into many substructures and solves the resulting matrix equation
using a block iterative method. The block-submatrices usually need to be stored on disk in
order to save computer core memory. However, this requires a large disk for large structures. If
the body is discretized into equal size cells, it is possible to use the spatial symmetry relations
of the Green’s functions to re-generate the scattering matrix in each iteration, thus avoiding
the expensive disk storage. This will allow us to calculate the responese of models comprised
of tens of thousands of cells on workstation type of computers. Numerical tests show that the
algorithm effectively reduces the solution of the modeling problem to an order of O(N2), instead
of O(N³), as with direct solvers.
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Marine Sp Surveys For Dam Seepage Investigations; Evaluation Of Array Geometries Through Modeling And Field Trials
Authors John Jansen, Edward Billington, Frederic Snider and Patrick JurcekMarine SP surveys have been periodically used on reservoirs for several decades.
Most surveys have used gradient arrays towed at the surface or at some depth in the
water column. Water column gradient arrays are efficient, but at the cost of some
sensitivity. Some surveys have used a single fixed electrode and a roving electrode
dragged across the reservoir bottom. These surveys have typically been labor
intensive and logistically limited. The authors used finite difference models and field
trials to evaluate the sensitivity and field productivity of gradient arrays and roving
electrode arrays, in the water column and on the reservoir floor, for detecting narrow
seepage pathways in a large reservoir.
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Electrical Detection And Monitoring Of Leaks In Lined Waste Disposal Ponds
More LessWaste disposal ponds or landfills may be lined with
impermeable plastic liners to prevent escape of noxious or toxic
fluids into the environment. Verification of the integrity of the
liner is important, both to assure quality at time of installation
and to guard against damage during the life of the pond. Several
leak-detection techniques have been employed which exploit
similarities between fluid flow and electric current in such an
environment. In this work, I describe a method which overcomes the
short-comings suffered by previously described systems.
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