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9th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 28 Apr 1996 - 02 May 1996
- Location: Keystone, Colorado, USA
- Published: 28 April 1996
1 - 20 of 134 results
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Pulled Array Continuous Vertical Electrical Sounding (Pa-Cites) With An Additional Inductive Source.
Authors Niels B. Christensen and Kurt SørensenElectrical and electromagnetic profiling methods are used extensively in environmental
geophysical investigations for many different purposes. The PA-CVES method (Pulled-
Array Continuous Vertical Electrical Sounding), where a tail of electrodes is towed behind
a small vehicle while continuously measuring in many electrode configurations, has been
used extensively for mapping the vulnerabilty of aquifers. Measurements are done for
every 1 metre and lo-15 km of profile can be made in one day.
This paper presents a theoretical study of the resolution capabilities of PA-CVES
measurements, as they are now performed, and an experimental design study for
including an inductive source in the measuring equipment. The joint interpretation of the
galvanic data set of ordinary PA-CVES measuements and a small number of induc.tive
data from a horizontal magnetic dipole source will significantly enhance the resolution
capabilities of the data set.
The study is carried out by analyzing the uncertainty of the model parameters of a onedimensional
three layer model using the covariance matrix of the inversion problem.
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Development Of A Continuously Monitoring Resistivity Probe For Free-Phase Petroleum Hydrocarbons
More LessAn in-situ electrical resistivity probe was evaluated for use in monitoring the vertical distribution of petroleum
hydrocarbon in contaminated soil. By installing the probe to intersect the water table, it continuously monitors the
presence of free-phase hydrocarbons by detecting electrical resistivity changes corresponding to the degree of oil
saturation in the sediment pores. At a field site where diesel contamination occurs within a fine-grained sand, the
results from the probe were compared to chemical analysis of total petroleum hydrocarbons (TPH) of soil borings
and measured free product (light non-aqueous phase liquid-LNAPL) thickness and groundwater level in wells. The
resistivity probe was also used to study LNAPL behavior under fluctuating water table conditions in a controlled
laboratory environment. Results from the resistivity probe reflect the distribution of the contaminant within the
sediment on a continuous basis and more accurately than contaminant thickness measurements in wells.
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Seismoelectric Studies In An Outwash Plain
Authors Paul J. Wolfe, Jianming Yu and Naum GershenzonInitial studies of the seismoelectric effect in an outwash plain have been conducted near Yellow
Springs, Ohio. The purpose was to make seismoelectric measurements in a simple, well-understood
test site where the seismoelectric signals could be clearly recorded and theoretical predictions could
be calculated with as few assumptions as possible. Suppression of electrical noise was a major
concern because 60 Hz electromagnetic fields from the power grid system are unavoidable in most
parts of the United States.
The site was characterized by seismic refraction surveys, DC resistivity surveys, and two drill holes.
The site has a fairly uniform, 3 m thick unsaturated layer over a thick saturated sandy layer. The
water table was near the top of the outwash layer.
For the seismoelectric studies a sledgehammer source was used. Seismic signals and electrical signals
were recorded separately and jointly with a variety of electrode combinations. An engineering
seismograph was used to record both the seismic and electrical signals. The built-in 60 Hz and 180
Hz notch filters were effective in suppressing much of the power grid pickup.
Electrical signals were observed which were consistent in time and frequency with the expectation
of seismoelectric response due to the electrokinetic effect. The peak-to-peak electric field amplitude
was about 6 mV/m and the time delay corresponded with one-way seismic travel times to the water
table.
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The Electromagnetic Azimuthal Resistivity Method
Authors Stewart K. Sandberg and Donald L. JagelPumping ground water from a fractured rock aquifer commonly leads to preferential drawdown
controlled by bedrock fracture orientation. The azimuthal resistivity method has previously been used to detect
this fracture orientation in covered areas, which has led to a determination of the principal directions of
transmissivity by geophysical means. However, the azimuthal resistivity method often requires a large amount
of space, free from cultural interference. In additon, data collection can be both labor intensive and time
consuming.
In this paper, we describe the collection of azimuthal resistivity data using electromagnetic methods.
Commonly available terrain conductivity instruments can be used in the horizontal coplanar (vertical dipole)
configuration, and data collected in an azimuthal array. Field examples using this system are presented showing
empirical comparisons with azimuthal resistivity data, and both geologic and hydrogeologic data sets. Data
collection efficiency is increased six-fold by using electromagnetic methods in this application, compared to the
azimuthal resistivity method. Disadvantages of the resistivity method, such as high electrode contact resistance
and large spread lengths necessary for relatively shallow penetration, are avoided or lessened by the use of
electromagnetic methods.
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Determining Fracture Geometry From Azimuthal Resistivity Data
Authors John Jansen and Robert TaylorThis paper presents the results of a series of three-dimensional finite difference models conducted to
determine the response for Wenner array azimuthal resistivity surveys run over conductive fracture zones in resistive
bedrock beneath conductive overburden. Microanisotropic and macroanisotropic fracture systems with varying
overburden thicknesses were simulated. Microanisotropic models were simulated by applying a uniform anisotropy
factor to the bedrock layers. Macroanisotropic models were simulated by representing fracture zones as conductive
vertical dikes. The model results demonstrated that for both microanistropic and macroanistropic cases, the shape
and magnitude of the apparent resistivity ellipse varies significantly as a function of overburden and “a” spacing.
The magnitude of the variations is sufficient to cause significant misinterpretations of azimuthal resistivity field data.
Several important factors must be considered when interpreting fracture patterns from field data. In summary, these
are: 1) the choice of a macroanisotropic or microanisotropic conceptual model, 2) the thickness of overburden
relative to the “a” spacing, 3) the position of macroanisotropic fracture zones relative to the center of the array, 4)
the width of macroanisotropic fracture zone relative to the “a’ spacing and thickness of overburden, 5) the presence
of parallel or intersecting macroanisotropic fracture zones, and 6) the degree to which the field conditions are purely
microanisotropic or macroanisotropic, rather than a mixed model.
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Azimuthal Electrical Resistivity As A Tool For Determination Of The Orientation Of Preferred Hydraulic Transmissivity For A Dolomite Aquifer In Southeastern Wisconsin
Authors Douglas A. Carlson, Robert W. Taylor and Douglas S. CherkauerJoints in rock act as avenues for the flow of both electrical charge and water. Therefore methods which can efficiently map anisotropic distribution of electrical pathways should also provide
a good indication of hydraulic anisotropy. The azimuthal electrical resistivity survey method has been successfully used as a method for determining the trend(s) of generally local joint sets
in a variety of rock and soil types. This study, which is a regional study, demonstrates that the azimuthal resistivity method can be a reliable method for determining the trend of regional
joint sets within the Silurian-Devonian dolomite, a single rock type, in southeastern Wisconsin. In particular the study demonstrates on a regional scale the consistency of azimuthal
resistivity survey results with results from direct observation of joints and from the analysis of numerous multiple-well-aquifer tests conducted nearby. For this study, 26 azimuthal resistivity surveys were conducted at 17 sites scattered throughout the approximately 500 km2 area around Milwaukee, Wisconsin. Direct observation by previous workers have shown two major sets of joints within the dolomite at 24 sites throughout southeastern Wisconsin. Azimuthal resistivity surveys for this study have been able to reproduce the average observed joint trend to within 3 degrees. In addition, the transmissivity ellipses from 14 multiple-well-aquifer tests in nearby Mequon, Wisconsin are compared with the apparent resistivity ellipse results from azimuthal resistivity surveys. The average tend of maximum transmissivity obtained from this study's azimuthal resistivity surveys is within 6 degrees of the average trend of maximum transmissivity determined by multiple-well-aquifer tests. In addition, the azimuthal resistivity surveys may be can differentiated between which of the joints sets should dominate the flow of water.
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Very Low Frequency (Vlf) Geophysics: A Case Study On Locating Bedrock Wells In Water Bearing Fracture Zones For Use In Contaminant Migration Interception
Authors Christopher L. Covel, Darryn T. Kaymen, Ian M. Phillips and James C. HarrisonA Very Low Frequency (VLF) geophysical investigation was performed during June 1995 at an industrial facility in Waltham, MA. The VLF survey was used to locate ground water monitoring wells in water bearing bedrock fracture zones at the site to determine contaminate migration pathways and improve ongoing remediation of the ground water. The bedrock at the site consisted of Granodiorite and Gabbro-diorite and brittle fracture analysis indicated bedrock was permeable and highly fractured. Historical releases of chemicals have reportedly occurred at the site.
Previous investigations indicated contaminated ground water exists in the bedrock at the site. The objective of using VLF geophysics was to increase the likelihood that water bearing fractures in the bedrock were intercepted during drilling, and that the most conductive zones were intercepted. The VLF geophysical survey consisted of six days of field work by
two geophysicists, covering 5,290 meters of geophysical survey line, and nine days of data interpretation, to select the proper drill targets. Three separate VLF stations were used to collect
the data. The data was collected and interpreted using the ABEM WADIcTM' instrument and SECTOR software.
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The Very Early Time Electromagnetic (Vetem) System: First Field Test Results
Authors David L. Wright, Thomas P. Grover and Victor F. LabsonThe very early time electromagnetic (VETEM) system is a time-domain, surface-deployed system
that has potential applications to direct detection of non-aqueous phase liquids as well as to the
detection of buried objects. It is designed to operate in environments that are too conductive for
ground penetrating radar (GPR) to be effective, and too shallow for standard electromagnetic
systems. The VETEM system is a faster profiling complement to the frequency-domain high
frequency sounder (HFS).
First field tests of the VETEM were made at the Geophysics Performance Evaluation Range at
Rabbit Valley west of Grand Junction, Colorado in June and July, 1995. A number of well
defined targets of various types are buried there (Allen, 1995). The VETEM system was next
used as part of the Electromagnetics Integrated Demonstation (EMID) at the Cold Test Pit at the
Idaho National Engineering Laboratory (INEL) in November, 1995. The Cold Test Pit has a
number of subareas with buried objects of various types. For the EMID tests, VETEM was run
over three gridded areas: the Primary Grid (PG), the Large Object Pit (LOP), and the Calibration
Cell (CC). In this paper we display recorded lines from the LOP that show VETEM is
responding to electrical properties of the near surface and to buried objects.
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Vetem - A Very Early Time Electromagnetic System - Year 2
Authors Louise Pellerin, M. Cathy Pfeifer and Victor F. LabsonIn electrically conductive conditions common in environmental characterization studies the
minimum depth of investigation for traditional electromagnetic (EM) sounding techniques is
roughly 5 meters, while ground penetrating radar (GPR) systems often investigate no more than
the top meter or so when clay minerals are present in the soil. Hence, in many cases, there is a gap
in our detection capability between the lower limit of GPR and the upper limit of traditional
electromagnetics (EM). Bridging this gap is essential to the characterization of buried waste,
contaminant plumes, and other environmental and hydrogeological targets located in the shallow
subsurface. The Very Early Time Electromagnetic (VETEM) system is designed to ascertain the
conductivity and dielectric properties of the shallow subsurface in conductive terrain (Pellet-in et al.;
1994, 1995). Hence, the one-dimensional (1-D) and three-dimensional (3-D) numerical modeling
algorithms, developed in the first year of the project, contain the full solution to the EM problem
including both displacement and conduction currents.
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Resolution Of 3D Earth Structures - Detection And Measurement Configuration Using Different Sources.
Authors E. Auken and G.M. HoverstenIn this paper it is shown that a plane wave source, an infinite line source and a vertical magnetic dipole source
generate approximately equal peak responses from a thin three dimensional (3-D) conductor. The main difference between
source types come in the layered response. In general the plane wave source is greater than the infinite line source layered
response which in turn is larger than the dipole layered response. For the anomaly index, as measured by the ratio of
secondary to layered response, the magnetic field component is largest for the dipole source and smallest for the plane wave
source whereas for the electric field the magnetic dipole has the smallest anomaly index. This is true both when the dominant
mode of excitation is current gathered into the scatter from the surroundings (channeling currents) and current induced in
the scatter (vortex currents).
We consider large and small transmitter loops with in loop and out of loop receiver configuration in both the
frequency and the time domain, Most of the currently used prospecting system configurations in either mining or
environmental work are thus covered. In the time and the frequency domain we find that a system which measures the
horizontal magnetic field has a anomaly index one to two orders of magnitude larger than a system which measures the
vertical magnetic field. Furthermore, conclusions regarding the optimum source-receiver configuration are dependent on
the target orientation. In the time domain, for a vertical thin sheet, the anomaly index is largest when the loop size is of the
order of the depth to the conductor whereas in the frequency domain a much larger loop can be employed.
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The Quantitative Influence Of Sallnity On The Apparent Resistnity On A Physical Model Upon Saltnation
Authors Kamal Khair and Catherine SkokanThe excessive exploitation of groundwater aquifers leads to water table drawdown, and
subsequentfy to the contamination of these aquifers by the intrusion of sea water or other hazardous
sources. This wortdwide environmental problem is becoming increasingly critical in coastal agricultural
areas, where the fine grarned materials develop a thick fringe zone. By evapo-transpiration the moisture
of this zone pumps up the satt in the dry season, which cannot be efficiently washed away in the wet
season. Over the years the salt will accumulate at different rates for different terrarns and climates.
which will uttrmately cause the deterioration of the land Lebanon is not an exception and the moderate
precipitation rate together with a strong water flow in wmter cannot, in the long term, compensate for the
consequences of chaotrc and uncontrolled exploitation of coastal aquifers.
The current study investigates the possibility of an early detectron of salination. through
systematic observation of electrical resistivity in selected positions with fixed electrode arrays A direct
current electrtcal profiling system of Wenner configuration was tested in the laboratory using a physlcal
model. The model was constructed of wood and plastic filled with saturated sand and hawng a constant
water flow of 1.6 Vmn. The model size is 148 by 85 cm for lateral dimensions and 25 cm of sand
thickness. with a total porosity of 36%. Upon salination the salt was increasingly added to fhe system to
reach a concentration of 32 gn. Upon desalinahon satt water was replaced by fresh water to dilute the
water In the system to a concentration of 0.25 g/t. The results show that the relationship between
sallnrty and electrical resistivity is inversety proportional and characterized by linear loganthmlc function.
the velocity of water flow calculated by abrupt resistivity changes is tower than the hydraulic velocity; the
reslstivity values for low salinity upon desalination are much different (smaller) than those upon
salination of equivalent salt concentrations; the relative change of resistivity upon salination and
desatination involves almost equally all features of the tank that have distinctive resrstrvrty values.
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Mapping Of Impediments To Contamination Flow Using Multicomponent Reflection Seismology At The Savannah River Site
Authors Omar A. Dickenson, Gilein J. Steensma and Thomas M. BoydA major obstacle to the remediation of contaminated aquifers at the Savannah River Site in Aiken, South Carolina is
the presence of discontinuous sand and clay lenses that are difficult to map effectively using geologic and geophysical
well logs. In order to map these discontinuous sand and clay lenses we acquire two perpendicular nine-component
(9C) seismic lines, a 9C Vertical Seismic Profile, (VSP) and p-wave and s-wave sonic logs in a borehole south of the
Old Burial Ground at the Savannah River Site within which were available natural gamma ray and interpreted geology
logs. P-wave reflections are interpreted as originating from water table, the Tan Clay, the Green Clay, the top of
the Ellenton Clay, and a calcareous sediment layer within the Barnwell/McBean aquifer. Along the east-west trending
line, reflectors are generally continuous except for the occurrence of a discontinuity in the upper reflectors near the
east end of the line. This discontinuity could be interpreted as a sediment slump feature possibly related to the dissolution
of the calcareous sediment layer, or as the eastern terminus of a large scour feature. Along the north-south
trending line, reflectors are spatially less continuous and are interpreted as being cut by several channel/scour features.
The shear-wave stacks are interpreted as mapping the top of the Tan Clay and the calcareous sediment layer and show
evidence of azimuthal anisotropy. Using Alford rotations to maximize the energy in the fast and slow shear directions
gives a magnitude of anisotropy of 3% with a direction of anisotropy agreeing, near the center of the east-west trending
line, with the regional stress direction. Although the most consistent anisotropy is observed near the center of the
line, systematic variations along the line suggest that the observed anisotropy is not related solely to the regional
stress field. Other mechanisms that could be responsible include grain orientation, stacked stratigraphic features, and
depositional or erosional lineations. None of these mechanisms can be excluded until further analysis of the 9C VSP
and until laboratory measurements of core samples have been completed.
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Shallow Seismics When Access Across Contaminated Areas Is Prohibited
Authors James C. Hasbrouck and Rust GeotechBecause of health hazards and possible breach of contaminant caps, waste-site managers are
reluctant to grant permission to conduct geophysical surveys directly across repositories. Yet,
they need to know if the integrity of confined aquifers is being maintained beneath these sites
and if contaminant pathways may exist outward from these sites. One possible way to supply
these needed answers is the use of three-dimensional (3D) seismic reflection surveys. These
surveys can acquire data around the perimeter of a restricted-access area while lines that
traverse the area may be selected for processing and interpretation.
To evaluate the feasibility of this approach for an area where these problems exist, a threedimensional,
three-component (3D/3C) seismic survey was conducted at the U.S. Department
of Energy’s Savannah River Site (Figure 1) along the survey lines shown in Figure 2. Both
P- and S-wave seismic data were taken from a set of 13 source points along Line A at a
separation of 12 m into a spread of 24 three-component, 40-Hz geophones along Line C. The
geophones were set at a perpendicular offset of 36 to 105 m from the center of the source
point array (Figure 3). At each source point, a 9.1-kg hammer was repeatedly struck
vertically and then horizontally in four directions against a firmly embedded steel fixture.
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The Anomalous Behavior Of Sh-Waves Across The Water Table
Authors P. Michaels and W. BarrashMost theoretical studies of seismic wave propagation in a porous medium do not predict a significant
increase in SH-wave velocity with increasing water saturation. Although that type of behavior is commonly
predicted for P-waves (and confirmed by countless observations), the expectation for SH-waves is a slight
decrease in propagation velocity with increasing water saturations. While published measurements of SH-wave
velocity in laboratory studies have been supportive of such a slight decrease in velocity, the data have been
biased towards high pressures (typical of oil reservoirs at large depths of burial). On the other hand, the few
published low pressure laboratory measurements have revealed significantly different results.
The authors’ in-situ measurements of seismic wave velocities in a shallow, coarse grained, unconfined
alluvial aquifer document a significant SH-wave velocity increase in the transition from the vadose zone to the
water table. In one vertical seismic profile (VSP), the P-wave velocity increases by a factor of 4.2 and the SHwave
velocity increases by a factor of 2.6. What is not clear at this point is the true nature of the increase. Is
the velocity increase an expression of the presence of water in the pores, or does water alter the rigidity of the
soil matrix?
In addition to the broad-band velocity increase, we have also observed changes in the attenuation of SHwaves
across the water table. After correcting for geometric spreading, the amplitude decay observed in the
vadose zone has been found to be larger than that observed below the water table. However, the variation in
amplitude decay as a function of frequency and the measurements of body wave dispersion were found to be
larger below the water table than above. That is, the water saturated soil behavior is consistent with a Voigt
solid, but the dry material appears to follow a different model.
The authors will discuss these observations in the context of the current debate on Poisson’s ratio and the
Vp/Vs ratio.
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Multi-Component Seismic Acquisition In Engineering And Environmental Investigations
More LessShear waves and compressional waves are sensitive to different physical properties of earth materials. In addition
shear waves exhibit unique phenomena associated with anisotropic material which can be used to examine
variations in earth properties with direction. These facts have been exploited to advantage in the oil and gas
industry for over 10 years. In the engineering and environmental fields, similar advantages can be demonstrated.
The advanced utility of multi-component seismic will be illustrated using a number of examples from both the
shallow and deeper fields.
Fracture maDDin2. In bedrock controlled hydrology, fracture and shear zones often represent the dominant
pathways for groundwater flow and contaminant migration. On entering such anisotropic zones, shear waves split
into two distinct waves with different velocities and polarization directions. The split shear waves (s-wave
birefringence) can be used to determine not only the strike of fractured zone but can also be used as a tool to map
areas of increased fracturing.
ManDing stratiPraDhv in unconsolidated sediments. In unconsolidated sediment sequences, important objectives
often are mapping clay and sand strata. In particular, the continuity of clay layers as barriers or aquitards to
contaminant migration is a principal objective in characterization and remediation investigations. As noted by Clark
et al. (1994) and Dobecki (1995) shear wave reflection surveys can, for this purpose, under certain circumstances,
demonstrate significant advantages over compressional wave reflection surveys, such as; higher seismic impedance
contrasts at clay-sand interfaces, insensitivity to water table and better resolution for detecting “thin strata.
However, these benefits are not always realized.
In shallow investigations only a small part of the potential information available in shear wave investigations is
typically acquired and analyzed. The full amount of information available in multi-component data has been
illustrated by Crampin (1985) and by Hasbrouck (1987) for the shallow field. By recording a full nine component
(9C) data set or some subset thereof, properties associated with shear wave birefringence can be utilized. For
example, the difference between Sh, or cross line, and Sv, or inline, shear wave travel times at the far offsets on a
surface reflection survey can reveal the magnitude of the layer anisotropy in unconsolidated sediment sequences.
These additional properties and the information contained within them are reviewed in further detail.
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The Accuracy Of Shear Wave Velocity Measurements Taken Using The Seismic Cone Penetrometer
More LessThe purposeo f this study was to determinet he accuracy of shear wave velocity measurementsm ade using the seismic
cone penetrometer. The general framework for the performance of this test and the evaluation of the field data is from
the area of geophysical exploration, and is based on the premise that the first arrival time of a shear wave can be easily
deciphered from field seismic wave traces. However, when seismic wave traces are not clear, it is common to pick peak
arrival times for the calculation of interval shear wave velocities. Many of the important aspects of wave propagation
which can have an influence on the peak arrival times of the shear wave such as multiples, transmission effects, and
attenuation, are usually not corrected for, meaning that some amount of error may be introduced into the arrival times
used for calculating interval shear wave velocities.
Since development of the seismic cone, studies have been done to address this issue. However, they focused primarily
on using digital signal processing techniques to obtain more accurate arrival times from clear seismic cone wave trace
data whose first arriving shear wave times were distinct, rather than on accounting for the case where the first arrival
time could not be easily deciphered from the trace. For this research, we used acoustic modeling (SH wave equation) to
account for transmissione ffects, multiples, reflection, refraction, and ghosts. Using the acquisition geometry and the
velocities estimated from several seismic cone penetrometer test sites in the San Francisco bay area, we modeled the
theoretical seismic wave traces. We estimated the first arrival times of the theoretical seismic traces and then computed
interval shear wave velocities. We then compared these velocities to the field seismic cone velocities.
The results indicate that seismic cone velocities can be within *5% of theoretically derived velocities when the first
arrival time is used. When peak arrival times are used, seismic cone velocities, are approximately 20% higher than
theoretical velocities in soil with increasing density, and approximately 20% lower than theoretical velocities in soils
with decreasing density. At interfaces where there is a change in soil type, it is recommended that seismic cone
velocities not be used. Confidence in the resultso btained from seismicc one penetrometer test measurementsc an be
greatly increasedif knowledgeo f the specific soil types is available.
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High-Resolution Seismic Reflection Survey At Dover Afb: A Comparison Of Three Seismic Sources
More LessIn June of 1995, the Earth Sciences Division of the Air Force Phillips Lab, with survey equipment
from the University of Delaware and assisted by the Kansas Geological Survey and Elohi Geophysics,
conducted a geophysical site characterization of the SERDP-funded Groundwater Remediation Field
Lab (GRFL) located at Dover AFB, Delaware and administered by Applied Research Associates for
USAF Armstrong Lab. Seismic data were collected in order to 1) compare the results using three
different compressional sources and 2) cover the field site well enough to characterize the seismic
response of the shallow subsurface. This paper will focus primarily on the first of these two goals.
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A Vibratory Seismic System For High-Resolution Applications
Authors George W. Schneider and John VanderkooyA vibratory seismic system has been developed by the authors, utilizing an engineering seismograph a&
a low-cost vibratory source. The project was a collaboration between the Geophysics Group at the Waterloo Centre
for Groundwater Research (WCGR) and the Audio Research Group in the Department of Physics at the University
of Waterloo. The system was developed in the winter of 1995 and preliminary field work was completed in the
spring of 1995. The source consists of an electrodynamic vibrator driven by a portable car audio amplifier. Source
excitation signals are generated with a sound card for a personal computer (PC). Software was developed to
generate excitations and perform crosscorrelation on the data. The source was well instrumented during preliminary
field tests, with accelerometers on both the reaction mass and moving coil, and a strain gauge between the coupler
and the moving coil to monitor the force acting on the coupler. Three different coupling devices were tested: a
short auger, a long auger and a baseplate. The preliminary testing was done at the Bauer Warehouse area on the
University of Waterloo’s North Campus, where both surface and downhole data were acquired. Continuous core
from a drill hole at the site shows that the stratigraphy consists of a series of glacial tills and alluvium underlain
by limestone of Silurian age below 68 metres. The water table was about 1 metre below ground surface at the time
of the survey.
For the surface tests, 24-channel common shot records were recorded. Twenty-four 100 Hz geophones
were spaced 2 metres apart with a source offset of 2 metres from the end of the array. For the downhole tests, an
8-channel hydrophone array was used to acquire a vertical seismic profile (VSP) from ground surface to 70 metres
depth. Hydrophones were deployed every metre with a source offset of 20 metres from the borehole. A number
of acquisition parameters were used and compared: sweep range, sweep duration, sampling rate and stack count.
Cosine-tapered linear sweeps as wide band as 50-1000 Hz were used. As well, pseudo-random maximal length
sequences (MLS) were tested. The 14-bit sequence used was “clocked out” at rate of 5 kHz, then bandpass filtered
from 100-1000 Hz prior to being used as an excitation. The results have been very encouraging; the vibrator
compared favourably to both a 12-gauge shotgun and a hammer and plate as a source. The baseplate was found
to be the preferred coupler, although the long auger generated a correlation wavelet with a higher centre-frequency
than the baseplate on VSP recordings. The short auger produced relatively poor records. Analysis of the strain
gauge response suggests the short auger applied the least force to the earth. Very good correllograms were obtained
using: 8 second sweeps from 50-1000 Hz, stacked 16 times, and recorded at a sampling rate of 2 kHz. The filtered
MLS excitation was found to be just as effective as sweeps of similar duration, stack count and frequency content.
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High Resolution Seismic Reflection Profiling At Aberdeen Proving Grounds, Maryland
Authors Richard D. Miller, Jianghai Xia, Stanley Swartzel, Jose Llopis and Paul MillerThe effectiveness of shallow high resolution ‘seismic reflection (i.e., resolution potential) to image geologic
interfaces between about 70 and 750 ft at the Aberdeen Proving Grounds, Maryland (APG), appears to vary locally
with the geometric complexity of the unconsolidated sediments that overlay crystalline bedrock. The bedrock surface
(which represents the primary geologic target of this study) was imaged at each of three test areas on walkaway noise
tests and CDP (common depth point) stacked data. Proven high resolution techniques were used to design and acquire
data on this survey.
Feasibility of the technique and minimum acquisition requirements were determined through evaluation and
correlation of walkaway noise tests, CDP survey lines, and a downhole velocity check shot survey. Data processing
and analysis revealed several critical attributes of shallow seismic data from APG that need careful consideration and
compensation on reflection data sets. This survey determined: 1) the feasibility of the technique, 2) the resolution
potential (both horizontal and vertical) of the technique, 3) the optimum source for this site, 4) the optimum acquisition
geometries, 5) general processing flow, and 6) a basic idea of the acoustic variability across this site. Source
testing involved an accelerated weight drop, land air gun, downhole black powder charge, sledge hammer/plate, and
high frequency vibrator.
Shallow seismic reflection profiles provided for a more detailed picture of the geometric complexity and
variability of the distinct clay sequences (aquatards), previously inferred from drilling to be present, based on sparse
drill holes and basewide conceptual models. The seismic data also reveal a clear explanation for the difficulties previously
noted in correlating individual, borehole-identified sand or clay units over even short distances. Geologic
cross-sections derived from CDP stacked data and borehole logs suggest locally complex geometries and horizontally
variable geologic contacts.
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High Resolution Seismic Refection To Determine Thickness Of Alluvium And Faulting In Bedrock
Authors Bart Hoekstra, Gilein Steensma, Walt Grinyer and Norman HarthillA seismic reflection survey was conducted at March Air Force Base (MAFB) with the objective to map the depth to
bedrock, locate bedrock faulting and define the stratigraphy overlying bedrock. The survey was conducted for the
purpose of identifying possible groundwater and contaminant migration pathways or obstructions, as well as areas
of depressed basement where dense non-aqueous phase liquid (DNAPL) contaminants might concentrate. The
survey consisted of a coarse grid of seismic lines and was designed to get coverage over a large area in an attempt to
get a basewide basement picture and define potential fault systems crossing the base (Figure 1). The design has the
disadvantage that the data are aliased because the station spacing is much shorter than the line spacing. However,
the design allowed us to determine four very valuable pieces of information: a) anomalous trends in groundwater
movement, b) depth and general structure of the basement reflector, c) areas where more detailed work may be
necessary and d) input for the more judicious placement of future drilling locations.
Previous work conducted in specific areas of the base by Tetra Tech (1993) and IT (1994) indicated that it was
possible to delineate these targets with the seismic reflection method, by that the data quality was dependent on
local site conditions and survey design had to be adjusted for these conditions. The types of cultural noise present at
the site during the survey included aircraft and ground traffic, with the latter posing a particular challenge, and
being a significant contributor to the choice of source type, as discussed in the next section. Other sources of noise
included electrical noise and all motors, engines and other noise associated with normal base operations.
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