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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
81 - 100 of 111 results
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3-D Induced Polarization Inversion Using Complex Electrical Resistivit Ies
Authors Weiqun Shi, William Rodi and F. Dale MorganInduced polarization (IP) is a current-stimulated electrical phenomenon observed as a delayed
voltage response in earth materials. The basic mechanism of the IP response is mainly driven
by electro-chemical processes. It has been indicated that the IP method is an effective means of
detecting and mapping environmental contaminants in many situations where conventional d.c.
resistivity has been ineffective. In this paper we present an inversion technique for interpreting
frequency-domain IP data in terms of complex electrical resistivity in the subsurface. The amplitude
of the observed voltage and its phase shift from the injected current are inverted to recover a
3-D subsurface resistivity structure. Extended from a 3-D d.c. resistivity inversion technique,
the algorithm uses the complex bi-conjugate gradient method in both the forward modeling and
inversion, The IP inversion procedure we have developed can be applied to l-D, 2-D, or 3-D earth
models and to any configuration of current and potential electrodes. We illustrate our methods by
inverting synthetic data taken over a contaminated plume embedded in a homogeneous background
medium. The model parameters are based on laboratory measurements.
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Ip And Tem For Discrimination And Resolution In Mapping Groundwater Contamination At Monument Valley
Authors S.K. Sandberg, N.T. Rogers, K.E. Karp, C.S. Goodknight and L.F. SpencerAs part of characterization activities at the UMTRA Monument Valley Project Site in
Arizona, a surface geophysical survey was performed from May 6 to 15, 1997. One of
the primary objectives of this survey was to provide a more comprehensive understanding
of the extent of nitrate and sulfate contaminant plumes in the alluvial aquifer. An analysis
of IP, resistivity, and TEM datasets, in conjunction with lithologic and groundwater
sampling data horn existing wells, indicates that the groundwater contamination plume is
detectable as a low resistivity zone which is contained predominantly within the alluvial
sediments. IP soundings obtained directly over the low resistivity zone indicate that it is
a low chargeability horizon with higher chargeability units above and below. In a
comparison with data from monitoring wells, simultaneous resistivity/IP and
resistivity/IP/TEM modeling indicates the low resistivity/chargeability horizon coincides
in depth and thickness with the depth and thickness of saturated alluvial sediments above
the bedrock. In addition, the lateral extent of the low resistivity zone is consistent with
groundwater contamination observed in monitoring wells screened in the alluvial aquifer.
This alluvial groundwater contamination plume appears to extend from the former
tailings pile area in a northerly direction for at least 4500 ft, based on contoured apparent
resistivity from TEM soundings, using 20 ps sample data (285 Hz base frequency). The
width of the groundwater contamination plume 2700 feet down gradient from the former
evaporation pond is at least 500 feet wide. The vertical extent of the alluvial groundwater
contamination plume appears to be constrained from above by the top of the saturated
zone of the water table, and below by the top of the bedrock surface. Bedrock is
comprised of the Shinarump Member of the Chinle Formation, composed primarily of
firmly cemented conglomerate and sandstone.
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A Conceptual Model For The Geoelectrical Response Of Lnapl Plumes In Granular Sediments
More LessGeophysical site characterization investigations at fuel spill sites have been generally guided
by a working hypothesis which assumes that the Light Non-Aqueous Phase Liquids (LNAPL) are a
fully saturating phase of intrinsically very high electrical resistivity. Using observations from other
related sciences, and contrary geophysical observations, a different model is developed which treats
these spills as dynamic, changing systems dominated by surprisingly low resistivities. The major
geophysical response of a mature or established spill of this type is due to an anomalously low
resistivity zone in the lower vadose zone and upper portion of the aquifer. This zone is produced by
a high TDS leachate which is aperiodically flushed down from the volume of intimately mixed
hydrocarbon, water, oxygen and soil where microbial activity is a maximum. This leachate is a result
of acidification of the heterogeneous free/residual product levels by organic and carbonic acids and
is produced by the leaching and etching of the native mineral grains and grain coatings. This
conductive plume is generally coincident with the uppermost part of the anaerobic dissolved plume
as defined by hydrochemical studies, but is thin and most concentrated at the top of the aquifer. It
has been best detected and mapped by virtue of the amplitude shadow it causes on GPR profiles, and
more recently by direct measurement using Vertical Resistivity Probes (VRP) with readings every two
inches from the surface to more than 25 feet in depth. Other electrical geophysical methods function
only if conditions are optimal. The conductive zone has been known for some years by hydrochemists
and hydrogeologists, especially at sites where water samples are collected from short screens at multilevel
wells.
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Internal Structure Of The 800 Area Landfill, Argonne National Laboratory, From Integrated Geophysical Measurements
Authors Christopher J. Black and Philip J. CarpenterGeophysical techniques were employed in this investigation to determine refuse character
and thickness, to identify shallow buried metal objects, and to distinguish between leachate and
shallow buried metal objects within the 800 Area landfill at Argonne National Laboratory.
Resistivity soundings, electromagnetic (EM) and magnetic profiles were completed across the
northern portion of the landfill between 1990 and 1993 to achieve these goals.
Resistivity soundings were inverted for layered earth models consisting of three or four
layers. Simple layered models were applied to obtain one-dimensional structure at each sounding
position. A low-resistivity layer (5 10 ohm-m), approximately 8-35 ft (3- 12 m) thick, was
consistently seen in the soundings over the landfill and probably represents leachate-saturated fill.
Layers of significantly higher resistivity (30-90 ohm-m) were seen below the low-resistivity layers
and represent the underlying glacial till. The modeled depth to the base of refuse closely matched
depth to the refuse-till interface as determined by borings made through the landfill.
EM and magnetic profiles identified areas of high conductivity caused by metal debris or
accumulations of leachate across the northern portion of the landfill. A particularly large anomaly
represents a French drain pipe (used for disposal of liquid waste) which was abandoned in place.
EM anomalies near known leachate seeps on the landfill (with no corresponding magnetic
anomaly) probably represent elevated leachate levels.
Each of these assessed characteristics is useful. Refuse thickness estimates aid in
monitoring well screen placement. Determining the location of buried metal was important in
locating a French drain buried in the landfill, which was used for liquid waste disposal. Finally,
leachate levels must be monitored to properly manage the landfill’s water balance. Geophysics
provides a quick, noninvasive method of assessing these hydrologic and waste characteristics.
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The Use Of Ground-Penetrating Radar To Accurately Determine Water Table Depths
Authors N.M. Trenholm and L.R. BentleyWater table elevations are used to determine ground water flow directions in geotechnical and
contaminant site investigations. Sparsely distributed monitoring wells often provide the water
level measurements used to construct water table contour maps. The use of ground-penetrating
radar to improve interwell interpolations through such techniques as cokriging is being
investigated. The water table is often associated with a strong radar reflection which cuts across
other events from the subsurface. The effectiveness of using GPR in interwell interpolation is
dependent on the precision to which correlations with the water table can be developed. As GPR
is sensitive to changes in water content rather than pressure changes, the reflection associated
with the water table actually occurs at the top of the capillary fringe. The size and variability of
the capillary fringe affects the correlation between the reflection from the top of the capillary
fringe and the water table. This correlation is also reduced by errors in radar velocity, surface
elevations and measured water table elevations. Fixed offset profiles and common midpoint
(CMP) gathers were collected at an aggregate extraction site on the Red Deer River, Alberta,
Canada. The radar profiles were positioned near existing monitoring wells, and water levels
were routinely measured during GPR data acquisition. Velocity control was provided by
common midpoint gathers. Processed GPR profiles were then used to determine the depth to the
top of the capillary fringe. Analysis of the precision with which ground-penetrating radar
interpretations can be correlated with measured water levels is presented.
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Leachate Plume Investigation Using Mise-A-La-Masse Resistivity
Authors Seth R. Lemke and Charles T. YoungMise-a-la-masse resistivity is a surface resistivity method that utilizes a down-hole current
source in contact with a conductive body. The data obtained can be interpreted to determine the
long axis of the conductive body or can be modeled to help determine the direction and extent of
the body. Mise-a-la-masse was used to assist in the delineation of a contaminant plume emanating
from a landfill in Ishpeming, Michigan. The long axis and direction of migration of the
plume were interpreted and the data was modeled as a series of point sources and using the finite
element program MODFLOW. Mise-a-la-masse was also used to assist in determining the direction
of bedrock fracturing and groundwater flow in Rock, Michigan where petroleum contamination
had occurred.
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High Resolution Reflection Seismic And Resistivity Imaging Of Deep Regional Aquifers For Stratigraphic Mapping
Authors E.W. Gilson, R. Kellett, J.W. Fennell, P.D. Bauman and C.B. SikstromChloride contamination has been detected in several deep regional aquifers in the Cold Lake,
Alberta area. Mapping these regional aquifers to determine their stratigraphy and their structural
relationships with the shallower overburden and the underlying bedrock has been the primary
focus of the geophysical investigations. Reflection seismic data were acquired to determine the
stratigraphy of the overburden and to identify possible pathways for contaminant migration. The
data suggest that glacial thrusting and differential loading, as well as fluvial features such as
channels have played a major part in controlling the stratigraphy in the area. To gain an
understanding of the hetrogeneties of the overburden, deep penetrating electrical resistivity
tomography (ERT) data were acquired. Hardware modifications were performed specifically for
this project in order to provide an increased depth of investigation. The ERT data suggests that
the overburden formations are heterogeneous over an order of tens of meters. Channel like
features were also imaged with the ERT. These results correlated well with both the seismic and
geophysical well logs, thus strengthening the overall geological interpretation. As a result of the
geophysical investigation, drilling targets have been identified, and hydrogeological models have
been refined, producing cost savings in the overall site characterization program.
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The Influence Of Micro-Geometry On The Hydraulic Permeability And The Induced Polarization Response Of Sandstones
Authors John T. Sturrock, David P. Lesmes and F. Dale MorganThe transport properties of porous rocks and soils are of broad interest to many
disciplines. An elusive goal in rock physics has been to develop a method of estimating
permeability in-situ, and various models have been developed. These existing models characterize
the porous media with parameters including porosity, hydraulic radius, grain packing, tortuosity,
specific surface area, fractal surface dimension, grain size and grain size distribution. Induced
polarization (IP) measurements are sensitive to many of the same variables associated with
hydraulic permeability as well as with the pore fluid chemistry, and may be able to measure the
parameters which make up the permeability models.
A series of experiments were performed on five different sandstones. Hydraulic
permeability and broadband (spectral) induced polarization measurements were made on several
sandstone samples both parallel and perpendicular to bedding. lnteresting relationships are shown
for the anisotropy data as a function of frequency. Porosity values were also determined for all of
the sandstones. Permeability model parameters, including formation factor, porosity, specific
surface area and surface fractal dimension were calculated using the IP data. Empirical models
which relate the polarization/capacitance of the sample to the grain size, surface fractal dimension
and the specific surface area were investigated. The results indicate that the surface fractal
dimension is the key component of the model, and that the values determined from our
experiments align extremely well with previously published data. Further investigation is being
pursued in efforts to determine the fractal dimension solely from electrical measurements.
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Dc-Resistivity Multi-Electrode Profiling In Hydrogeological Investigations: A Comparative Study Of The Pulled Array Continuous Electrical Sounding Method And A Multi-Electrode Method With Fixed Electrodes
Authors Ingelise Møller, Kurt Sørensen and Niels B. ChristensenIn environmental and engineering problems 2D DC-resistivity data sets are now commonly
collected using computerised data acquisition systems of which several are commercially
available. At least two different approaches have been followed in the design of computerised
acquisition systems, one using many fixed electrodes (e.g. Griffiths and Turnbull 1985, van
Overmeeren and Ritsema 1988, Dahlin 1996) and another using electrode configurations
that are moved while the measurements are carried out (e.g. Hesse et al. 1986, Panissod et
al. 1997, Sorensen 1996, Sorensen and Sorensen 1995).
One system using moving electrodes, developed for the purpose of large-scale near-surface
mapping, is the Pulled Array Continuous Electrical Profiling, PACEP, system, (Sorensen
1996) where measurements are carried out continuously and simultaneously in 3 electrode
configurations. To enhance the vertical resolution the method is upgraded to carry out
measurements in 8 electrode configurations simultaneously, the Pulled Array Continuous
Electrical Sounding, PACES, system (Sorensen and Sorensen 1995).
In this paper data measured with the PACES method is compared with data measured
with a multi-electrode system with fixed electrodes. The ABEM Lund Imaging System will
represent the fixed electrode systems (Dahlin 1996).
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Groundpenetrating Radar And High-Resolution Reflection Seismic Surveys Of Subsidence Features Along Interstate 44 Springfield, Missouri
Authors Glendon Adams, Neil Anderson, Jesse Baker, Michael Shoemaker, Anthony Shaw and Alen HathewayIn May of 1997, the Department of Geology and Geophysics at the University of Missouri-Rolla
conducted a reflection seismic survey and a ground penetrating radar (GPR) survey for the Missouri
Department of Transportation (MoDOT) along and adjacent to a 300 meter section of Interstate 44 in
Springfield, Missouri. In October of 1997, a second GPR survey was conducted along the same section of
interstate. The site was located approximately 1.5 kilometers west of Missouri Highway 266. The section of
interstate studied overlies an active sinkhole and has experienced continued, localized subsidence.
Seven 12-fold reflection seismic profiles were acquired along or near Interstate 44, using a Bison 24-
channel seismograph and an EWG weight drop source. Forty-live GPR profiles were acquired along paved
sections of Interstate 44 during the first survey. During the second GPR survey, the survey area was
expanded to include a total of seventy ground penetrating radar profiles. A GSSI SIR-S GPR unit equipped
with a 500 MHZ (megahertz) monostatic antenna/receiver was used to acquire the data.
The geophysical surveys were successful. The reflection seismic data established that sinkholes,
both active and nonactive are prevalent in the area. The seismic data also supports the interpretation that a
sinkhole lies immediately beneath the interstate. The ground-penetrating radar data also proved to be of
significant utility. Anomalous areas interpreted as voids on the GPR data were drilled and significant volumes
of grout were injected. The second GPR survey established the success of the grouting program.
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Airborne Geophysics Helps Determine Route Of Proposed Railroad Tunnel
Authors Les P. Beard and Ole LutroAn airborne geophysical survey of about 830 line-kilometers was flown over an area of 180 sq
km in southern Norway to aid in determining which route a proposed railroad tunnel should
follow. The average flying height was 80 meters with a nominal line spacing of 200 meters.
Groundwater flow is fracture controlled in much of Norway, and failure to locate water-bearing
fractures can cause expensive delays in tunnel construction. A primary goal of the helicopter
survey was to locate any fractures or other structures using a combination of airborne
magnetometry, frequency-domain and very low frequency (VLF) electromagnetics, and
radiometrics, and to attempt to estimate via electromagnetics which fractures might be waterbearing.
Magnetic measurements proved to be the most useful geophysical tool for locating
faults, fractures, and dikes in the area. Several known faults appeared clearly in the data,
particularly on the western side of the survey area. Other lineaments also are presumed to be
fractures, faults, or dikes. Although magnetometry provided more detail than any other
individual data set, all the data sets proved valuable by adding new information and by
corroborating the findings from other data sets. From the geophysical data, it appears that the
western half of the survey area is more intensely fractured than the eastern half, although a few
large lineations, presumably fractures or faults, appear in both areas.
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Use Of Gpr In 2D And 3D Imaging Of Bridge Footings And Scour Studies
Authors Doria Kutrubes and Kenneth MaserGPR was used at two different Massachusetts bridges to help assess the stability of their foundations. At
one highway bridge, GPR was used to confirm the lateral extent of a bridge footing, while at another it was
used to help evaluate the extent of river scouring at the bridge’s piers.
During the bridge footing study, data were collected continuously along a 2-foot survey grid using a GSSI
SIR System-2 and monostatic 900, 400, and 200 MHz antennas. CMP gather measurements were made
using bistatic 400 MHz antennas with the intent of producing CMP stacked sections of each survey line in
the grid and imaging the footing using tomography.
Because of its unexpected shallow depth, the 900 MHz antenna provided the best 2D resolution of the
bridge footing’s transverse and longitudinal sections. 3D data imaging from the data collected continuously
failed to produce a clear image of the bridge footing, although this is partially attributed to a heterogeneous
mixture of metal scraps, boulders, and cobbles in the topsoil covering the bridge footing. The CMP gathers
helped establish the interval velocity through the topsoil, and hence give accurate depths to the footing.
However, because CMP gather measurements were very time-consuming using only two antennas and 3 cm
offsets, we were unable to collect a sufficient number of CMP gathers to produce a meaningful CMP
stacked section. Imaging the bridge footing using radar tomography in the future may provide the highest
resolution 3D data, but is currently impractical and costly using a two-antenna array.
A monostatic 200 MHz antenna was used to map sediments up to 3 feet below the Charles River waterbottom.
Data were collected continuously between, and upstream from, existing bridge piers along survey
lines spaced approximately lo-feet apart. The water velocity, and hence the depth to the river-bottom, was
derived from existing Cole-Cole Distribution Models (Cole and Cole, 1941) assuming a resistivity of 1,000
ohm-meters and a temperature of 55 F. Because of the strong water-bottom reflector and an adequate line
spacing, 3D images derived from compiling 2D data were adequate in defining at least one potential area
of scour immediately upstream from a bridge pier. Due to a lack of velocity information below the water-
‘bottom, however, other areas which had been potentially scoured and replaced with fine river-bottom
sediments may not have been identified. The application of radar tomography, with use of a multi-antenna
array, could enable the determination of river sediments velocities, and help more reliably identify those
areas which have been scoured and subsequently filled in by fine river sediments.
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High-Resolution Reflection Seismic Surveys Of Abandoned Lead And Zinc Mines Along A Proposed Interstate Route, Near Joplin, Missouri
In October and November of 1996, the department of Geology and Geophysics at the University
of Missouri-Rolla conducted a reflection seismic survey for the Missouri Department of
Transportation (MoDOT) along a proposed interstate route near Joplin, Missouri. In total, six,
24-fold reflection seismic profiles (6,000 m) were acquired across previously mined (lead/zinc)
ground. Checkshot data was acquired to correlate the reflection seismic data with the subsurface
geology. The principle objective was to provide MoDOT with interpreted seismic profiles (re:
abandoned open-pit mine workings) with a view in assisting MoDOT engineers with route
planning and site remediation.
The reflection seismic survey was successml. The survey established that the high-resolution
refection seismic method can be used to image Mississippian bedrock, paleosinkholes, and
abandoned mine access and ventilation shafts. Paleosinkholes, trending north-northeast, were
prospective sites for shallow mining activities.
The reflection seismic data effectively delineated old mine workings in the study area. The
interpretations of the seismic profiles (confirmed by borehole and checkshot survey data) were
transferred onto existing mine location maps as an aid to MoDOT engineers involved in hazard
assessment, route planning, and site remediation.
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Seismic Refraction And Gravity Investigation Of A Tunnel Route Near Columbus, Ohio
Authors Mark J. Howell, Paul J. Wolfe, Benjamin H. Richard, Ernest C. Hauser and Heather MarshA sewer tunnel is under construction in the northwest section of the Columbus, Ohio
metropolitan area. The tunnel is 10 kilometers long and 20 meters below the ground surface.
Bedrock is limestone with till overburden of variable thickness. During tunneling through limestone
an unexpected zone of soft material was encountered, resulting in a shutdown of the tunneling
machine. A long, expensive delay followed while a vertical shaft was excavated and the soft material
was tunneled by hand. Concern that other problem zones might be encountered led to contracting
a geophysical survey to identify additional problem areas in advance of tunneling.
The tunnel route is through an affluent residential area with extensive infrastructure
development. After evaluating the background noise that would affect various geophysical methods
and desiring to minimize the disruption to traffic flow and local residents, we decided that
microgravity and seismic refraction were the most appropriate methods. Station spacing was about
6 meters for both gravity stations and geophone locations. The gravity survey was reduced to the
Bouguer anomaly with modeling to interpret terrain effects in areas where significant valleys crossed
the profile or where the edge of a parallel valley came close to the line. The regional gravity trend
was removed graphically.
A strong, non-explosive, minimally-intrusive seismic source was required so we chose an
elastic wave generator. This provided enough energy to see first arrivals over above background
noise to offsets of 75 meters. We used 24 single-geophone traces for the seismic refraction. The
source points were typically one station off the ends of each spread. The line was advanced in 12
station steps to provide a continuous bedrock profile with some redundancy. We picked first breaks
‘and analyzed the data as a two-layer case with ray tracing software.
Gravity and seismic bedrock-elevation profiles generally show good correlation with the
results of the extensive geotechnical borings that had been done. By selecting the appropriate density
contrast between the overburden and bedrock, we constructed a bedrock elevation profile which
agreed well with boring information at most locations. The seismic refraction and boring logs were
used to determine the bedrock profile. The gravity profile was examined to identify zones where the
gravity was lower than expected on the basis of bedrock topography alone. These zones were
considered to be areas where the density of the material below the bedrock surface was abnormally
low, and were targeted as potential bad rock zones. Additional subsurface investigation was
recommended at these locations to allow advanced planning for tunnel problems in the area.
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A Ground-Penetrating Radar Study Of Quaternary Faulting In The Benton Hills Area, Southeast Missouri
Authors S. Oppert, N.L. Andersen, D. Hoffman, N. Fraser, M. Shoemaker and T. ShawTwo grids of ground-penetrating radar profiles were acquired in the Benton Hills area, southeast
Missouri. The first grid, consisting of six north-northeast trending GPR profiles, was acquired adjacent and
parallel to the Upper Rainbow Trench (MDNR-USGS trench), near the southern edge of the Benton Hills
Escarpment. The Upper Rainbow Trench crosses a section of the Commerce Geophysical Lineament, and
exposes several near-vertical deep-seated faults that displace juxtaposed Quaternary and Tertiary strata and
extend to the surface. These exposed faults and the shallow strata were imaged on the GPR data and correlated
across the grid of profiles. The interpretation of the GPR data confirms that the recent faults trend
predominantly west-southwest, essentially parallel to the Commerce Fault Lineament.
The second grid, consisting of nine north-northeast trending GPR profiles, was acquired to the westsouthwest
of the Benton Hills across the projected extension of the Commerce Geophysical Lineament. The
GPR grid was centered adjacent to a reflection seismic anomaly (interpreted deep-seated fault). The
interpretation of the GPR data correlates with the seismic data and suggests that the anomalous seismic feature
is indeed a shallow fault (that extends to the base of disturbed soil). These data support the thesis that the
Commerce Geophysical Linearnent extends to the west-southwest of the Benton Hills and into the lowlands of
the Mississippi Embayment.
The interpretations of the GPR data are significant for two reasons. First, these data indicate that GPR
can be used to detect shallow faults in southeast Missouri, even where such faults are overlain by a thin veneer
,of surficial soil, or by disturbed soil (agricultural areas). Second, the GPR data supports the thesis that the
Commerce Fault extends into the Mississippi Embayment. This indicates that significant tectonic activity has
occurred in southeast Missouri outside of the New Madrid area of active micro-seismicity, implying that the
earthquake risk in southeast Missouri is greater than generally thought. It also supports the thesis that tectonics
played an important role in the development in the Mississippi Embayment.
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Imaging And Charcacterizing Fractures In A Rock Slope
Authors Toshiyuki Kurahashi, Shiro Watanabe, Tomio Inazaki and Tomo OhtaniThis paper describes that a Seismic reflection survey and logging were quite useful to image and
characterize fractures inside of crystalline rock slope.
Until now, rock slope stability has been qualitatively evaluated from the surface observation data on a
rock slope. However, it is very diflicult to estimate fracture extension inside of a rock slope only from the
surface evidence in quantity. The main feature contmlling the slope stability should depend on the spatial
distribution of fractures in a rock mass. For the first step to delineate the fractures in a rock slope, we
performed a seismic reflection survey and leggings.
A test site is located on Shikoku Island, western part of Japan. The rock slope consisted of schist, which
was hctured with a large number of joints and fissures. It was predicted to cause rock slide.
We set receivers spaced 0.5 meters apart in length and width lines along the slope surface. For imaging
the fractures oblique to the survey lines, we adopted pre-stacked depth migration technique. Some fractures
were imaged and detected in the interior of the rock slope. Locations and directions of the fractures were
consistent with the surface observation data.
leggings were carried out in a borehole drilled from the top of the slope body into the back side of the
surface. Borehole camera mapped thirty-six fractures. Besides natural gamma spectrum, temperature and
VSP characterized and revealed that all fractures were not open and conductive. We interpreted that the slope
would be stable.
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Hysteresis In The Electrical Conductmty-Saturation Relationship Of A Sand
Authors S.D. Piggott, J.D. Redman and A.L. EndresThere is a growing interest in the use of electrical measurements for the detection,
mapping and monitoring of zones of non-aqueous phase liquid (NAPL) contamination and
for the measurement of soil moisture content in the unsaturated zone (for example,
electrical resistivity tomography during air sparging). The interpretation of these
measurements depends on an understanding of the conductivity-saturation relationship.
Previous laboratory experiments have shown that this relationship is hysteretic. This
paper discusses our laboratory investigation of the effect of fluid saturation and saturation
history on the electrical conductivity of partially saturated sand.
Our ongoing experiments involve 2- and 4-electrode measurements of the complex
electrical conductivity of a sand sample during drainage-imbibition cycles with water-air
and water-NAPL systems. This paper presents the results of a drainage-imbibition
experiment using air and water as the pore fluids. Capillary pressure measurements were
made concurrently with the electrical measurements. Hysteretic behaviour was observed
in both the conductivity-saturation and the capillary pressure-saturation relationships. In
particular, the conductivity at a given saturation was found to be significantly lower during
imbibition than during drainage over most of the saturation range. The results of these
studies suggest that the saturation history should be considered when interpreting
electrical measurements from the unsaturated zone and areas of NAPL contamination.
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Geophysics For Groundwater Prospecting In Republic Of Panama
Authors Abel Carrasquilla, Alberto Caballero and Moises OrtegaBetween 1985 and 1992, two institutions in the Republic of Panama, Physics Department of
University of Panama (DF/UP) and Groundwater Department of National Institute of Aqueducts
and Seawages (DFSIDAAN), employed geophysical methods to prospect groundwater resources.
The surveys were conducted in areas where residents had problems with water supply, over recent
sedimentary and igneous terrains, in localities of El Cristo, Capellania, Nuevo Peru, Cerro Morado,
Isla Verde and Chame-Bejuco. The geophysical data consisted of 110 vertical electrical soundings
and several profiles with spontaneous potential (10 km), WennerSchlumberger resistivity (4 km)
and magnetic (3 km) methods. All these data were complemented with lithological informations
obtained from 50 DFSIDAAN wells and geological maps. Despite the complexity of the geology,
interpretations of these data resulted in the determination of geological faults, water-table,
groundwater flow directions, bedrock depth, aquifers thickness, contaminated/uncontaminated
zones, etc. Also, this methodology permitted us the suggestion of more suitable places to drill
productive wells, justifying the use of geophysical methods, and saving important economical
resources, which would be wasted in fruitless drillings.
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Cross-Plotting Conventional Geophysical Well Logs To Identify Permeable Pathways In A Fractured Bedrock Aquifer
Authors James M. Cooper and Philip J. CarpenterFractures in the Galena-Platteville aquifer serve as conduits for the migration of
groundwater contaminants. Conventional geophysical well-log data from the Parson’s Casket
Hardware Superfund site, Beividere, IL were cross-plotted to test the hypothesis that permeable
pathways in Galena Group dolomite produce diagnostic clustering of log values. Bedding-plane
fractures, subvertical fractures, and zones of vuggy porosity were identified in several wells and
then geophysical well logs, including natural gamma, short-normal resistivity (SNR), and caliper
were examined for the same intervals. Log values were cross-plotted and visually inspected for
clustering in the vicinity of permeable intervals. A bedding-plane fracture produced two highresistivity,
low-gamma clusters and a low resistivity, high gamma “tail”. The two clusters
represent a difference in lithology on either side of the fracture and the “tail” represents a claychoked
bedding-plane fracture. The fracture produced clustering of log values in several adjacent
wells. Inconsistent clustering is observed for SNR versus caliper plots in subvertical fracture
zones. Low resistivities and enlarged borehole diameter appear to correlate with subvertical
fracturing; however, clustering signatures were not repeated between adjacent wells cut by steeply
dipping fractures. Cross-plots of SNR versus gamma and SNR versus caliper exhibited clustering
of log values for intervals of vuggy porosity. Logs from several wells produced bimodal clusters
which suggest lithology and/or porosity variations throughout the vuggy interval. This study
demonstrates that typical water-well logs exhibit clustering of log values associated with
permeable fractures and horizons; however, only those clusters associated with bedding-plane
fractures proved to be repeatable between wells.
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An Application Of Electrical Anisotropy In Hydrogeological Modeling
Authors Timothy Davis and Robert W. TaylorThe electrical anisotropy of near surface aquifers can be determined through azimuthal
resistivity surveys, which measure the apparent resistivity as a function of compass bearings.
Systematic azimuthal variations in the apparent resistivity may result from the presence of
preferred current flow through joints or fractures, which can also be paths for preferred
groundwater flow. Hence, electrical anisotropy should provide a geophysical means to improve
groundwater models. To establish an empirical relationship between measured electrical
anisotropy and the horizontal hydraulic anisotropy required for groundwater models, azimuthal
resistivity surveys were conducted at an established test site near Sturgeon Bay, Wisconsin. This
site is well documented by the Wisconsin Geological and Natural History Survey (WGNHS)
through extensive studies of groundwater flow in the fractured media of the area. Fracture
azimuths were found to be 75”, 115”, and 160”. The average electrical anisotropy was 1.2.
Groundwater modeling of a pump test conducted at the site indicated a hydraulic anisotropy of
2.6. The independent measurements of electrical and hydraulic anisotropy were used to establish
empirical relationships between the two quantities.
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