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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
81 - 100 of 134 results
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Complex Electrical Resistance Tomography Of A Subsurface Pce Plume
Authors Abelardo Ramirez, William Daily and Douglas LaBrecqueA controlled experiment was conducted to evaluate the performance of complex electrical
resistivity tomography (CERT) for detecting and delineating free product dense non
aqueous phase liquid (DNAPL) in the subsurface. One hundred ninety liters of PCE were
released at a rate of 2 liters per hour from a point 0.5 m below ground surface. The spill
was conducted within a double walled tank where saturated layers of sand, bentonite and
a sand/bentonite mixture were installed. Complex electrical resistance measurements
were performed from 4 vertical electrode arrays, each with 10 electrodes spaced between
3 m and 0.5 m depth. Data were taken before the release, several times during, and then
after the PCE was released. Magnitude and phase were measured at 1 and 64 Hz. Data
from before the release were compared with those during the release for the purpose of
imaging the changes in conductivity resulting from the plume. Conductivity difference
tomographs showed a decrease in electrical conductivity as the DNAPL penetrated the
soil. A pancake-shaped anomaly developed on the top of a bentonite layer at 2 m depth.
The anomaly grew in magnitude and extent during the release and borehole television
surveys data confirmed the anomaly to be free-product PCE whose downward migration
was stopped by the low permeability clay. The tomographs clearly delineated the plume
as a resistive anomaly. Images showing phase changes caused by the spill are also
presented. The phase changes at 64 Hz suggest that the DNAPL spill increased the
induced polarization (IP) effect of the clay layers.
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Tank Leak Detection Using Electrical Resistance Methods
Authors Abelardo Ramirez, William Daily, Andrew Binley and Douglas LaBrecqueTwo field experiments were performed to evaluate the performance of electrical
resistance tomography (ERT) as a leak detection method under metal underground
storage tanks (UST). This paper provides a summary of the field experiments performed
under a 15 m diameter steel tank mockup located at the Hanford Reservation,
Washington. Two different leak events were created. About 3800 liters of saline solution
were first released along a portion of the tank’s edge and another 1900 liters were later
released near the tank’s center. The release rate averaged about 26 liters/hour for the leak
on the tank’s side and about 3.0 liters/hour for the center leak. Two and three
dimensional tomographs were calculated using data collected before, during and after
each spill. The tomographs show that, as the solution penetrated the soil, readily
detectable resistivity decreases appeared where the associated plume was expected. The
results indicate that the plume associated with these releases could be reliably detected
after approximately 190 liters were released. Results are also shown where the metal
tank is used as a large electrode.
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Electrical Resistivity Tomography At The Doe Hanford Site
Authors S.M. Narbutovskih, W. Daily, A.L. Ramirez, T.D. Halter and M.D. SweeneyRecent work at the DOE Hanford site has established the potential of applying Electrical Resistivity Tomography
(ERT) for early leak detection under hazardous waste storage facilities. Several studies have been concluded to test
the capabilities and limitations of ERT for two different applications. First, field experiments have been conducted
to determine the utility of ERT to detect and map leaks from underground storage tanks during waste removal
processes. Second, the use of ERT for long term vadose zone monitoring has been tested under different field
conditions of depth, installation design, acquisition mode/equipment and infiltration chemistry. This work involves
transferring the technology from Lawrence Livermore National Laboratory (LLNL) to the Resource Conservation
and Recovery Act (RCRA) program at the DOE Hanford Site. This paper covers field training studies relevant to
the second application for long term vadose zone monitoring.
Electrical resistivity tomography is a cross-borehole, imaging technique for mapping subsurface resistivity
variations. Electrodes are placed at predetermined depths in an array of boreholes. Electrical current is introduced
into one electrode pair located in one borehole while the resulting voltage change is detected between electrode
pairs in other boreholes similar to a surface dipole-dipole array. These data are tomographically inverted to image
temporal resistivity contrasts associated with an infiltration event. Thus a dynamic plume is spatially mapped as a
function of time.
As a long-term vadose zone monitoring method, different field conditions and performance requirements exist than
those for short term tank leak detection. To test ERT under these conditions, two vertical electrode arrays were
constructed to a depth of 160 feet with a linear surface array between boreholes. The fielding was used to facilitate
the technology transfer from LLNL to the Hanford RCRA program. Installation methods, commercial equipment
and acquisition mode were evaluated to determine economic and technical feasibility to assist design of long-term
monitoring networks. Preliminary results of the training test are presented.
Until recently, vadose zone monitoring techniques could provide only local point or linear coverage for leak
detection and thus, are used primarily under liquid collection systems at land disposal units. As developed by
LLNL, ERT can provide area1 coverage under waste treatment and storage facilities given the right conditions.
Advantages of ERT to groundwater protection programs are explored along with suggestions for future uses where
ERT can be employed today.
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Applications Of Geophysical Logging For Dilution Control In Mining
Authors Richard G. McCreary and Stefan WänstedtIn mining, waste rock dilution occurs when uneconomic rock is mined and processed with economic mineralized
ore. A survey of selected stopes from various Noranda Group mines estimated that aggregate dilution and oreloss
stood at 20% and 8% respectively in 1994. Various technologies are being investigated by Noranda Group mines
in an ongoing effort to minimize dilution and oreloss. The following three potential applications of geophysical
logging have been identified for dilution and oreloss reduction through improved orebody delineation.
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Assessment Of Geotechnical Features Of Miocene Volcaniclastics Using Integrated Logging
Authors Toshiyuki Kurahashi, Tomio Inazaki and Yasuo NakamuraThis paper describes that integrated logging evaluates geotechnical conditions of fractures in Miocene
volcaniclastics, where the damsites are for proposed. The purpose of this study was to detect fractures for
seepage contols along borehole wall and characterize its geotechnical features by integrated logging. It is
first step to evaluate the continuity and permeability of fractures.
Integrated logging comprises of caliper, acoustic borehole televiewer, full-wave sonic, temperature,
borehole video logging, natural gamma spectrum, and magnetic loggings. Natural gamma spectrum and
magnetic logs would characterize litholofacies. Caliper, acoustic borehole televiewer and borehole video
loggings would identify the depth, the dip and the azimuth of fractures. Besides temperature logging would
determine the groudwater flow and suggest the permeable fracture.
Integrated loggings were performed at damsites for proposed in northern Kanto district, middle part
of Japan, where Miocene volcaniclastics are widely exposed. In Yunishigawa site, thermal jump was
remarkaby observed at 84 meters in depth from temperature logging. Its thermal gradient, differntiation of
temperature, was 30 “C/m. The farcture causing it was recognized by the borehole enlargemnts from caliper
logging, and had low slowness of full-wave sonic logging.
Integrated logging enabled us to identify the fractures in the order of a dozen centimeters along a
borehole wall. The identification would be useful for the geotechnical evaluation. Especially Differentiation
of temperature revealed the fracture for seepage controls and classified the fractures into three types, which
are (1) the permeable fracture circulating the runoff, (2) the permeable one controlling the groundwater
from the deep, (3) the non-permeable fracture. As the first and the second are fractures for the seepage
controls, they would involve the geotechnical problems.
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Application Of Borehole Geophysics At The Retsof Salt-Mine Collapse Site, Western New York
More LessBorehole-geophysical methods were used in the hydrogeologic characterization of the valley fill and bedrock
at the site of the 1994 Retsof salt-mine collapse in western New York. Collapse of the mine roof and fracturing
of the overlying bedrock in the southern downdip section of the IO-square-mile salt mine resulted in flow of
ground water into the historically dry mine at a rate of more than 15,000 gallons per minute. A suite ofboreholegeophysical
logs, including caliper, gamma, electromagnetic induction, specific conductance, temperature,
acoustic televiewer, video camera, and impeller flowmeter, was collected from 17 test wells. Most of the logged
test wells were 700 to 850 feet deep and were completed with steel casing through the valley fill and finished as
open hole in the bedrock.
The valley-till sequence at the mine-collapse site was about 500 feet thick and the bedrock sequence, from
top of bedrock to the salt mine, was about 600 feet thick. Analysis of the borehole-geophysical logs and correlation
with reported drilling logs delineated (1) glaciolacustrine fine-grained sediments that restricted downward
flow of surface water and ground water from the overlying alluvium; and (2) ground-water flow zones in
glaciofluvial sand and gravel in the mixed deposits of the lower valley fill, near the top of bedrock, and in fracture
zones at the limestone-dolomite contact and in the lower part of the dolomite. Fractures that provided the vertical
hydraulic connection between the ground-water flow system and the mine were penetrated by several test wells
drilled within the collapse.
Borehole geophysics proved to be a highly efficient method for obtaining information on the hydrogeology
of the valley fill and bedrock at the Retsof salt-mine collapse site. This information was critical in evaluating
potential remedial actions at the site and developing a plan of study to assess the effects of the mine collapse on
the water resources of the area.
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The Synthetic Roadcut -- A Nuherical Technique For The Multiple-Scale Integration Of Surface And Borebole Data
Authors Frederick Paillet and Colleen BartonIntegration of geophysical soundings or tomograms with geophysical logs
and hydraulic test data from boreholes is complicated by the very different
scales of investigation of each of these techniques. We present a numerical
approach designed to optimize the comparison of large-scale geophysical
measurements with data from borehole arrays. This technique addresses four
important requirements: 1) borehole data are projected into a single vertical
plane or “synthetic roadcut”; 2) projections between boreholes are adjusted in
the representation plane within specified limits to improve spatial
correlations between boreholes; 3) borehole data are subjected to a range of
spatial averaging and smoothing techniques; and 4) soundings or tomograms and
borehole data sets are kept rigorously separate until the processed data are
formally compared. The technique is illustrated using data from the FSE
borehole array at the Mirror Lake, New Hampshire fractured rock hydrology
research site. The Mirror Lake data show that comparisons of borehole logs
and borehole packer test data with tomograms can indicate how discontinuous
fractures are organized into larger scale conductive zones, and how the
distribution of those zones may be related to the lithologic fabric of the
bedrock.
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Use Of Directional Borehole Radar And Azimuthal Square-Array D.C. Resistivity Methods To Characterize A Crystalline-Bedrock Aquifer
Authors Melinda J. Chapman and John W. LaneDirectional borehole radar and azimuthal square-array resistivity surveys were used to determine
orientations of fracture zones in a crystalline-rock aquifer in the Piedmont Province of northern Georgia. The aquifer
media is a sheared and highly jointed amphibolite that is overlain by about 25 feet (ft) of regolith. A production well
tapping fracture zones in this aquifer supplies ground water to the City of Lawrenceville, Ga., at an average rate of
about 230 gallons per minute (gal/min). Four bedrock observation wells near the production well yield more than
100 galimin each, and respond quickly when the production well is pumped. An integrated suite of borehole
geophysical logs, including caliper, focused resistivity, long- and short-normal resistivity, gamma, acoustic
televiewer and velocity, temperature, spontaneous potential, fluid resistivity, deviation, heat-pulse flowmeter, and
video camera, and single-hole directional radar, were collected in the four observation wells to delineate and
characterize zones of fracture permeability. Orientations of highly permeable zones were determined by correlating
results from directional borehole radar with surface azimuthal square-array resistivity, standard borehole geophysics
data, and detailed surface geologic structural mapping.
Single-hole borehole radar reflection surveys were conducted using a 60-megahertz (MHz) directional
antenna that allowed interpretation of the orientation and projected borehole intersection depth of reflectors as far as
115 ft from the borehole. The borehole radar data include many reflectors; however, only reflectors that could be
correlated with other geophysical anomalies and production zones reported in drilling logs were selected for further
reflector orientation analyses. Stikes of many reflectors near production zones were determined to be NlO-20W, N40-
5OW, and E-W (N80E to N80W), dipping 24-88 degrees. Depths of these fracture zones ranged from about 30 to 290
ft below land surface. Caliper, video camera, and acoustic televiewer logs indicate that productive fracture zones are
associated with zones of borehole enlargement and intense fracturing. Anomalies interpreted from focused resistivity
and acoustic velocity logs also were associated with borehole fracturing.
Surface azimuthal square-array resistivity surveys were used to provide estimates of the orientation of
areally extensive fractures. Results of azimuthal square-array resistivity soundings indicated fractures oriented at
N30-60W, N15E, and E-W. Apparent resistivities of the bedrock are low, ranging from about 55 to 450 ohm-meters.
Using calculated anisotropy ratios of 1.35 to 1.75, the interpreted secondary fracture porosity in the bedrock aquifer
ranges from 13 to 25 percent. These values of fracture porosity are extremely high and most likely reflect the
influence of foliation and (or) sulfide mineralization on bedrock anisotropy and apparent resistivity data.
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Comparison Of Heat-Pulse Flow Measurements And Vertical Gradients In A Fractured Limestone Aquifer
Authors Larry L. Dearborn, Scott F. Calkin, R. Henry Andolsek and William S. AllisonEstablishing a site-specific relationship between heat-pulse flowmeter (HPFM) data and corresponding
vertical gradient data may allow prediction of potential vertical gradients through HPFM logging alone.
Vertical gradient and corresponding HPFM rates were determined for 117 test intervals in a fractured
limestone bedrock aquifer. From these data, it appears that HPFM data can be used in place of more labor
intensive borehole packer testing to provide estimates of vertical gradients in this type of hydrogeologic
system.
Groundwater conditions in the fractured bedrock were investigated through testing of 66 open boreholes,
as part of the hazardous waste remedial investigation at the former Loring Air Force Base (LAFB) in
northern Maine, USA. Borehole geophysical logging tools, including HPFM and acoustic televiewer
(ATV), in conjunction with air hammer drilling logs, were used to target specific fracture(s) to test using
conventional straddle packers. HPFM and head data from 41 boreholes met general requirements for
comparison purposes, and a linear correlation trend was identified.
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Clean Enough For Industry? An Airborne Geophysical Case Study
Authors Jonathan E. Nyquist and Les P. BeardData from two airborne geophysical surveys of the Department of Energy’s Oak Ridge Reservation (ORR) were
extremely valuable in deciding whether a lOOO-acre (400 hectare) parcel of the ORR should be leased to the City of
Oak Ridge for industrial development. Our findings, based on electromagnetic and magnetic data, were
incorporated in the federally mandated Environmental Assessment Statement (EAS), and in general supported
claims that this land was never used as a hazardous waste disposal site. We estimated the amount of iron required
to produce each anomaly using a simple dipole model. All anomalies with equivalent sources greater than
approximately 1000 kg of iron were checked in the field, and the source of all but one identified as either a bridge,
reinforced concrete debris, or a similarly benign object. Additionally, some smaller anomalies (equivalent sources
of roughly 500 kg) have been checked; thus far, these also have innocuous sources. Airborne video proved
invaluable in identifying logging equipment as the source of some of these anomalies. Geologic noise may
account for some of the remaining anomalies. Naturally occurring accumulations of magnetic minerals in the soil
on the ORR have been shown to produce anomalies which, at a sensor height of 30 m, are comparable to the
anomaly produced by about 500 kg of iron. By comparison, the electronic noise of the magnetic gradiometer, O.Ol-
0.02 nT/m, is equivalent to only about 50-100 kg of iron at a 30 m sensor height. The electromagnetic data,
combined with field mapping of karst structures, provided evidence of a northeast-southwest striking conduit
spanning the parcel. The possible existence of a karst conduit led the EAS authors to conclude that this is a
“sensitive hydrologic setting.” We conclude that aerial geophysics is an extremely cost-effective, and efficient
technique for screening large tracts of land for environmental characterization.
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Integrated Environmental Site Characterization Involving Geochemistry, Geophysics, And Geology: A Shortcut To Remediation
Authors James Viellenave, Gerald Church, Mark Slatten and Mike AndersonEnvironmental site characterization processes have evolved from simple drill-and-sample routines into more
sophisticated evaluations of increasingly complex problems involving a variety of contaminants. Strategic
integration of several geoscience tools into a more holistic approach benefits the site owner/operator by developing
a synoptic perspective of the site at the earliest possible time, allowing for more selective and focused use of the
expensive and invasive technologies. The ultimate effect is a better site characterization. including attention to
difficult PRP issues, lower liability, fewer risks of bypassing potentially hazardous contaminant accumulations, and
a result that is more targeted to environmental and human health risks.
An integrated site investigation system requires good geology and hydrology, but is properly augmented by use of
modem and sophisticated geochemical and geophysical tools, Establishing characterization objectives is critical in
deciding what geoscience tool(s) to deploy in any given situation, For each tool, critical criteria are identified that
will enable the user to best decide which to use for what purposes.
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Integration Of Geochemical And Geophysical Data For The Montana Mineral-Environmental Study
Authors A.E. McCafferty, D.H. Knepper and H.V. AlminasAeromagnetic, Landsat multispectral scanner (MSS), radiometric, and geochemical data covering the state
of Montana were digitally integrated to identify areas that may be negatively affected from surface-water
contamination by metals and acid run-off due to mining activity and natural processes. Two mineralenvironmental
models were constructed using a variety of statistical, image processing, and frequency
filtering techniques. The first model is a combination of relative magnetizations (calculated from the
aeromagnetic data) and two suites of geochemical elements. The geochemical distributions include one
based on a lanthanide association, which characterizes intermediate and alkalic intrusives; another
association characterizes sulfide mineralization. The second model is a mathematical combination of
relative magnetizations, iron oxide data (calculated from MSS data), and apparent potassium
concentrations (derived from airborne gamma-ray data). This model is useful for locating areas of possible
potassic alteration and leaching or weathering of exposed and shallowly buried iron-rich igneous rocks.
Magnetization domains associated with iron oxide and relatively high potassium concentrations can be
indicative of hydrothermal alteration environments that host metal sulfides. Despite sedimentary cover in
eastern Montana, both models reveal areas where magnetization sources are at depths shallow enough to
permit leaching, possible alteration, and the presence of heavy metals at the surface. Follow-up studies
and integration of these results with geologic and hydrologic information will provide a clearer
understanding of the possible environmental effects within the identified areas.
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The Use Of Multiple Geophysical Techniques For Site Assessment In Mine Reclamation
Authors L.E. Dalverny, K. Kluger Cohen, G.L. Mowrey and A.G. KimSite assessment at a burning coal waste bank in Midvale, Ohio was conducted by the U.S. Bureau of
Mines with five different geophysical techniques. The geophysical study was the first phase of a
project to test a control method for subsurface fires. The site characterization was performed using
terrain conductivity, magnetometry, ground penetrating radar and seismic surveys, as well as infrared
imagery. Results from these five geophysical techniques were integrated to obtain information on
the subsurface structure of the bank, the probable location of combustion zones, and the presence of
underground water. Magnetometry measured magnetic anomalies due to changes in the magnetic
properties of materials exposed to elevated temperatures in combustion zones. Electrical terrain
conductivity identified changes in ground conductivity of the waste bank; anomalies were believed
to be related to the presence of mineral-bearing water in fracture zones beneath the surface of the
bank. Both ground penetrating radar and seismic methods determined waste bank structure and
interfaces between dissimilar materials. The ground penetrating radar was effective to a maximum
depth of approximately 3 m (10 ft); the seismic technique provided information to a depth of 8.3 m
(27 ft). Infrared imagery detected areas of elevated temperature near the surface of the waste bank.
These combined results indicated that the bank consisted of a surface layer of coal waste that was
underlain by a core of another material. Several high temperature areas were identified on the eastern
slope of the bank. Drilling and temperature surveys provide supplemental information and verified
the results obtained with geophysical techniques. Site characterization that integrated the data from
multiple geophysical techniques resolved uncertainties, with respect to subsurface conditions and the
locations of heated zones and water saturated areas, that could not have been adequately explained
with any single technique.
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Case Histories Of Buried Borehole Detection: An Exercise In Flexibility
Authors Norman R. Carlson and Kenneth L. ZongeIt is sometimes difficult to determine in advance exactly which equipment or technique is best suited to a given project. Although the response of a particular target may be predictable, the local background response may be more difficult to assess until field data have actually been gathered. This problem becomes economically significant when the job site is relatively remote, requiring downtime and/or additional airfreight expenses when the actual field results don't match expectations and a change in equipment systems becomes necessary. An example of this is a recent series of surveys in Indiana and Ohio, in which the goal was to ensure that no abandoned, buried oil wells were present within in a prescribed radius of proposed injection wells. After local tests, both the physical survey layout and the data processing techniques were varied in order to detect the various possible targets in the different environments.
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Demonstration Of Geophysical Methods For Burial Ground Geophysical Characterization Study At The Doe Savannah River Site
Authors James C. Hasbrouck and H. David Mac LeanRust Geotech, operating contractor at the U.S. Department of Energy Grand Junction Projects
Office (DOE-GJPO), conducted a demonstration of the trench boundary and large-object
location capabilities of five nonintrusive geophysical methods in the Low-Level Radioactive
Waste Disposal Facility (LLRWDF) at the DOE Savannah River Site (SRS). The plan for
Resource Conservation and Recovery Act (RCRA) closure of the SRS LLRWDF specifies inplace
compaction of “B-25” metal boxes containing low-level radioactive wastes. The boxes
are buried in Engineered Low-Level Trenches (ELLTs) at the facility. To properly guide and
control the compaction operation, the coordinates of the trench boundaries must be
determined to an accuracy within 5 feet and the outer edges of the metal boxes in the
trenches must be determined to within 2 feet.
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Application Of Surface Geophysical Methods To Delineate Fracture Zones Associated With Photolinear Features In West-Central Florida
More LessThis study was designed to investigate the geophysical characteristics of fracture zones associated with
photolinear features in West-central Florida and to determine the most effective geophysical profiling methods for
rapid field characterization. Multiple surface-geophysical techniques were employed to verify the geologic
significance of two photolinear features at the study site. Interpretation of the geophysical data was constrained by
using multiple geophysical methods, site lithologic data and information obtained in previous studies of photolinear
features in West-central Florida.
Closely spaced horizontal-loop electromagnetic (HLEM) and very low frequency electromagnetic (VLF)
tilt-angle data were used to locate geophysical anomalies associated with the fracture zone. Resistivity profiling
data correlate well with the results of the HLEM and VLF methods. The optimum-offset, seismic-reflection
technique accurately delineates the large-scale fracture zone; however resolution is insufficient for detailed
stratigraphic interpretation. Self-potential data show a negative-streaming potential associated with the center of the
fracture zone. Microgravity data along a section of the fracture zone indicate lateral variations in density that
correlate well with anomalies identified with the other geophysical methods. Future studies of photolinears in
West-central Florida should use high-density VLF and HLEM surveys as rapid reconnaissance techniques to locate
anomalies associated with photolinears. High-resolution, shallow, seismic reflection, resistivity, SP and gravity
methods can be used to provide additional correlation.
Geophysical and lithologic data from the study site are incorporated into a geologic model of the fracture
zone. Data suggest that the two photolinears are related to a large-scale fracture zone in the Floridan Aquifer. The
fracture zone appears to be greater than 700 m wide with a 100 m wide sand-filled bedrock low. The bedrock low is
flanked by zones of higher bulk density approximately 200-400 m wide. The zones of higher bulk density are
interpreted as zones of recrystallized limestone caused by the precipitation of calcite along the fracture zone. The
recrystallized limestone zones appear to host multiple limestone pinnacles and clay-tilled fractures. The observed
photolinears correspond to a sand-filled bedrock low and a zone of dense, recrystallized limestone adjacent to the
bedrock low.
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The Effective Use Of Electromagnetic Methods To Delineate A Fluvial Paleochannel System Controlling Oil Migration Near Glenrock, Wyoming
Authors Noel T. Rogers, Stewart K. Sandberg and Greg PowellA combination of three different electromagnetic methods was effectively used to map the shallow lithologic
variation interpreted to control the subsurface migration pattern of hydrocarbons near Glenrock, Wyoming.
Petroleum hydrocarbons were seeping from the south bank of the North Platte River approximately one-half mile
west of the Township of Glenrock, Wyoming. Product was moving along the top of the water table through coarse
grained sediments and discharging into the river. Initial investigations, resulting in the excavation of three trenches,
were unable to determine the source and migration pathways that the hydrocarbons were following. A geophysical
survey was performed that provided a map of the subsurface geology that controlled free and dissolved product
migration.
The geophysical methods used included terrain conductivity, using two instruments that allowed two different coil
separations (the Geonics EM-31 and EM-34), and transient electromagnetics (TEM). The portability and data
collection efficiency of the EM-31 guided its use as a reconnaissance tool. Data from the EM-31 provided a map of
near-surface conductivity patterns indicative of shallow lithologic variation between coarse-grained paleochannel
deposits and tine-grained fluvial overbank sediments. Correlation of the EM-31 response with known geology
observed in test pit excavations resulted in some concern as to whether the mapped lithologic variation was too
shallow to have controlled groundwater migration. To address this, the EM-31 was operated at ground, and at 3 and
7 feet (ft) heights above ground over a high conductivity zone (interpreted to be fine-grained material). Data
obtained indicated that the high conductivity material was not a thin surficial veneer, but instead represented finegrained
deposits at depths of 0 to greater than 10 ft. The EM-34 instrument was also used at a lo-meter coil
separation along selected traverses to confirm the vertical extent of the interpreted lithologic variation. In addition, a
profile of central loop TEM sounding provided enough information to allow one-dimensional computer modeling,
which provided a quantitative depth extent of fine-grained (fluvial overbank) deposits. Therefore, preferential
groundwater migration pathways (fluvial paleochannels) were not only detected, but quantitatively delineated using
this effective combination of electromagnetic methods.
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Orientation And Characteristics Of Fractures In Crystalline Bedrock Determined By Surface And Borehole Geophysical Surveys, Millville And Uxbridge, Massachusetts
Authors Bruce P. Hansen and John W. LaneFour geophysical techniques were used to determine bedrock fracture orientation and other site characteristics
that can be used to determine ground-water flow and contaminant transport at a study area underlain by fractured
crystalline bedrock in Millville and Uxbridge, Massachusetts. In the study area, azimuthal seismic-refraction and
azimuthal square-array direct-current resistivity surveys were conducted at three sites, borehole-radar surveys were
conducted in a cluster of three wells, and ground-penetrating radar surveys were conducted along roads.
Azimuthal seismic-refraction data indicated a primary fracture strike ranging from 56 to 101 degrees at the
three sites. Graphical and analytical analysis of azimuthal square-array resistivity data indicated a primary fracture
strike ranging from 45 to 90 degrees at the same three sites, Directional borehole-radar data from three wells
indicated 46 fractures or fracture zones located as far as 147 feet from the surveyed wells. Patterns of low radar-wave
velocity and high radar-wave attenuation from cross-hole radar surveys of two well pairs were interpreted as a planai
fracture zone that strikes 297 degrees and dips 55 degrees south. Ground-penetrating radar surveys with IOO-MHz
antennas penetrated 5 to 50 feet of unconsolidated overburden and as much as 60 feet of bedrock where the bedrock
surface was at or near land surface. Horizontal and subhorizontal fractures were observed on the ground-penetrating
radar records at numerous locations. Comparison of results from the various geophysical techniques indicates good
agreement and indicates primary high-angle fracturing striking east-northeast. Square-array resistivity data yielded an
average secondary bedrock porosity of 0.0044 and an average aperture of 0.007 1 foot for high-angle fractures.
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Geophysical Methods To Identify Underground Erosion In Lateritic Soil
Authors Augusto F. Mendonça and Augusto C.B. PiresUnderground erosion is a serious problem to development of urban areas in
the neighborhoods of Brasilia-Brazil. This problem was first identified in 1986, when the
occurrence of numerous pseudo-sinkholes condemned an area of 300,000 square meters,
causing the demolition of 52 houses. Occurrence of pseudo-sinkholes in other places
around Brasilia led the University of Brasilia Institute of Geology to investigate the
problem. The research focused on the determination of the factors that generate the
underground erosion, and analyzed the geological, hydrogeological, and geotechnical
characteristics of the areas where the pseudo-sinkholes were identified.
One of the most serious aspects of underground erosion is that it is only
detected at the surface, in advanced stages of evolution. When the pseudo-sinkholes start
to develop, the geotechnical conditions of the area are already jeopardized, implying huge
remediation costs. Therefore, the location of underground cavities in early stages of the
underground erosion process was considered a key issue, and the selection of geophysical
methodologies to locate the cavities were one of the focus of the research.
Eletroresistivity, microgravimetry, and the vertical gravimetric gradient were the
geophysical methodologies tested in the areas with underground erosion. The use of
geophysical methodologies proved to be of great value. The interpretation of geophysical
data, supported by geotechnical, geological, and hydrogeological studies, allowed the
identification of areas with potential risk for underground erosion. Geophysics
employment also permitted the precise location of the underground cavities.
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Microgravity Survey For Shallow Subsurface Investigations
Authors Kyozo Nozaki and Takashi KanemoriRecently, the portability, repeatability, and accuracy of gravity meter measurements have improved
enough to be useful for shallow applications for engineering purposes. The authors have performed microgravity
surveys in Japan for the last several years with successful results. In this paper, the authors present three case
histories of typical microgravity applications.
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