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6th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 18 Apr 1993 - 22 Apr 1993
- Location: San Diego, California, USA
- Published: 18 April 1993
1 - 50 of 65 results
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Detection Of Subsurface Hydrocarbons In The Vadose Zone With Ground Penetrating Radar - A Case Study
Authors Wayne R. Saunders, Michael Wightman and Robert WindschauerOver the past 10 years numerous investigators have attempted to detect subsurface hydrocarbons with various geophysical methods and techniques. These investigations have met with a
range of successes and failures. It has not been clearly understood why a particular geophysical survey is either successful or unsuccessful in a particular geologic setting. This paper will provide a detailed review of a particular survey which was performed in a carbonate environment in the southeastern United States. While the main objective of the survey was not to detect and delineate hydrocarbons, this particular survey appears to have detected hydrocarbons within the vadose portion of the subsurface. In several areas of the survey, monitoring wells revealed no floating product within the wells. Nevertheless, the analysis of the GPR data indicated an effect from hydrocarbons detected by other investigators. A correlation between the presence of hydrocarbon vapors and analysis of the GPR was made and will be discussed, with a possible explanation for this apparent cause and effect given.
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The Use Of Time Domain Reflectometry To Detect The Movement Of Contaminants In A Simulated Aquifer
Authors Robert K. Mallan, D. Scott Worley and Robert S. KerrThe application of time domain reflectometry (TDR) to determine soil water content via soil dielectric measurements has been widely investigated [Topp et al., 1980; Topp et al., 1982, and
references therein]. These investigations have shown TDR to be a viable technique in determining soil water content.
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Geophysical Monitoring Of A Controlled Kerosene Spill
Authors S.M. DeRyck, J.D. Redman and A.P. AnnanIn order to minimize groundwater contamination from a hydrocarbon fuel spill, it is
necessary to locate and then remediate zones in the subsurface where the fuel has pooled.
Electrical property based geophysical surveys have met with mixed success in delineating
these contaminated zones. The objectives of our research were to provide a basis for
improved survey design and interpretation for accidental spill sites by testing several
geophysical methods on a controlled hydrocarbon fuel spill in a relatively simple
environment.
Monitoring instrumentation was installed in a 3.6 m diameter by 1.7 m deep polyethylene
tank. A system of well screens was placed on the bottom of the tank in a gravel layer and
the test cell was then packed with sand. The monitoring instrumentation included a vertical
geophysical access tube, glass observation wells, peizometers, soil gas access tubes,
dielectric permittivity, and resistivity monitoring probes.
Kerosene was injected near the centre of the test cell in five separate injections of varying
I quantities. A total of 343 litres of kerosene was injected. Ground penetrating radar, in situ
dielectric permittivity and resistivity, neutron logging, and radon soil gas surveys were
performed throughout the experiment. Surface EM, surface resistivity, EM induction
logging, and borehole television surveys were also performed, but the results of these
surveys are not be discussed in this paper as these were less definitive in understanding pool
development.
Following each kerosene injection, we observed changes in the responses of most of the
geophysical techniques which we attribute principally to the reduction in water content due to
the movement of kerosene into the capillary fringe and unsaturated zone.
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Modelling The Electrical Properties Of Porous Rocks And Soils Containing Immiscible Contaminants
Authors Anthony L. Endres and J. David RedmanInclusion-based models are used to describe porous rocks and soils containing
immiscible contaminants. The results of modelling show that the functional relationship
between dielectric properties and the level of fluid saturation can vary significantly as the
pore-scale fluid distribution changes in terms of both the geometrical configuration and the
choice of wetting phase. The magnitude of these variations increases with decreasing
measurement frequency, indicating that the location of the conductive water phase is
important. The low frequency electrical conductivity depends on the connectivity of the
water phase. An inclusion-based model is used to compute the electrical stratigraphy of an
immiscible contaminant pooled at the water table interface.
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The Use Of Neutron Logging Methods For The Detection And Monitoring Of Chlorinated Solvents: A Quantitative Study
Laboratory experiments have been performed to quantify the ability of neutron
logging techniques to detect and monitor chlorinated solvent contamination in aquifers.
The effects of chlorine concentration and the spatial distribution of chlorine-rich layers
were investigated. The thermal neutron device responds significantly ( 20% or more
reduction in neutron count) to chlorine concentrations corresponding to relatively low
levels of perchloroethylene saturation (- 10 %). The sensitivity of the device to variations
in chlorine concentration is greater at low levels of chlorine content. A chlorine-rich layer
of 0.63 cm (0.25 in) thickness is detectable; however, visual thickness estimates are
difficult for thicknesses less than the source-detector length. Visual resolution of
individual thin layers requires layer separations greater than the source-detector length.
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Electrical Resistance Tomography During In Situ Remediation Of A Tce Plume At The U.S. Department Of Energy Savannah River Site
Authors W. Daily and A. RamirezThe Savannah River Integrated Demonstration Project was undertaken by the Department of Energy (DOE) to demonstrate the remediation of volatile, organic compounds from saturated and
unsaturated soil horizons at the Savannah River Site, South Carolina. A dilute mixture of air and methane was pumped into a horizontal well that was drilled in the contaminated zone. Methane was intended as a nutrient for native populations of microorganisms whose metabolic products degrade the TCE. A horizontal vacuum extraction well was installed in the vadose zone to remove solvent vapors that had been stripped from the vadose zone. The experimental site consisted of alternating units of permeable sands with low fines content and significantly less permeable clayey sand and clay units. Generally, the clays tended to be thin and discontinuous. Cross borehole electrical resistance tomography was done in 4 planes
near the air-methane injection well to determine the extent and dynamics of the gas plume below the water table. Tomographs were done before gas injection began, then soon after injection began and again after gas had been flowing for four months.
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Review Of Using Acoustic And Seismic Waves For Stimulation Of Oil Reservoirs: Methods And Results
Authors I.A. Beresnev and P.A. JohnsonNumerous observations, accumulated during the development of various oil fields
show that seismic waves generated from earthquakes and even traffic noise may affect oil
production. Elastic waves are observed to exert an influence on the permeability of the
saturated porous medium. In some cases application of waves may appreciably increase
the permeability of reservoirs and the mobility of oil. The effect of elastic waves on the
permeability of saturated media has been confirmed in numerous laboratory experiments.
Two different kinds of applications have arisen from these findings. In the first, highpower
ultrasonic waves are applied for downhole cleaning of scales, precipitants, mud
penetration, etc. in the near well-bore. In many cases ultrasound effectively removes the
barriers to oil flow into the well. The ultrasonic method is reported to be successful in
40-50% of the cases studied. In the case of successful treatment, the effect of improved
permeability may last for several months. However, this method has a very local effect.
A second method is used to stimulate the reservoir as a whole. Here seismic frequency
waves are applied at the earth surface by arrays of Vibroseis-type sources. This method
has produced promising results; however, further testing and understanding of the
mechanisms is necessary. An advantage of using elastic waves for stimulation of oil and
gas reservoirs is the ecological safety of this method.
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Crosswell Seismic Imaging Of An In-Situ Air Stripping Waste Remediation Process
More LessThe restoration of environmentally contaminated sites at DOE facilities has become a major
effort in the past several years. The variety of wastes involved and the differing characteristics have
driven the development of new restoration and monitoring technologies. One of the new remediation
technologies is being demonstrated at the Savannah River Site near Aiken, South Carolina. In
conjunction with this demonstration, a new technology for site characterization and monitoring of the
remediation process has been applied by Sandia National Laboratories.
The remediation technology being demonstrated involves the in-situ air stripping of
contaminated soils and groundwater. The geology at the site consists of horizontally-layered
unconsolidated sands, clayey sands, sandy clays, and clays. A leaking process sewer line contaminated
the area under study with chlorinated volatile organic compounds (CVOC’s), primarily
trichloroethylene (TCE) and tetrachloroethylene (PCE). To remove the contaminants, two horizontal
wells were drilled: a vapor extraction well at depths from 9 m to 26 m (AMH-2), and an injection well
at depths from 36 m to 56 m (AMH-1). Figure 1 shows the projection of these horizontal wells on the
surface. Air was injected into the lower well beneath the water table depth of 42 m, and a vacuum
was applied to the upper well to extract the injected air. As the air passed through the subsurface, the
CVOC’s were dissolved into the gas phase and brought out the extraction well.
Little was known about the distribution of the air injected into the earth. This air distribution
has a direct effect on which areas are remediated and also on the flow regimes set up during the air
injection process. A method for imaging the air distribution was developed at Sandia using crosshole
seismic data and based on the changes in seismic velocities as a result of changes in saturation due to
the injected air.
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Preliminary Results Of An Integrated Geophysics Program For Nonintrusive Characterization Of Mixed-Waste Landfill Sites
More LessChem-Nuclear Geotech, Inc. (Geotech), conducted surface geophysical surveys as part of the Mixed-
Waste Landfill Integrated Demonstration (MWLID) for the U.S. Department of Energy Office of
Technology Development . The survey objective was to demonstrate that an integrated program of
surface geophysics could effectively and nonintrusively characterize mixed-waste landfill sites. To
accomplish this objective, integrated field demonstrations were conducted over two areas (designated
Areas A and B) within the MWLID test site at the Chemical Waste Landfill, Technical Area 3, at the
Sandia National Laboratory in Albuquerque, New Mexico. Pit locations were known in Area A and
suspected in Area B. Preliminary displays and analyses of the data from the field demonstrations
completed in 1992 will be presented in this paper.
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Large Scale, High Resolution Survey For Burial Pit And Trench Mapping
Authors G.S. Carpenter and T.M. DeignanA large scale, high resolution geophysical survey was performed at the Idaho
National Engineering Laboratory, a Department of Energy facility. This facility
has served as a test site for the development of nuclear power production since
the early 1950's. Beginning in the early days of operation at the site, a
variety of waste products from industrial and nuclear processes have been
disposed of primarily at one approximately 88 acre site at the Subsurface
Disposal Area within the Radioactive Waste Management Complex.
In support of the CERCLA process, a chemical sampling program is being designed
to characterize the migration of hazardous products away from the burial pits and
trenches into the surrounding environment. In order to safely and effectively
remove soil samples for chemical analysis from the areas immediately surrounding
the burial sites without encountering the waste pits an integrated geophysical
survey was proposed. The purpose of this geophysical survey was to map the
location of the burial pits and trenches and identify unrecorded burial sites
within the survey area.
A geophysical survey consisting of over 50,000 magnetic field measurements and
nearly 50,000 induced electromagnetic readings was conducted over approximately
50 acres of the site. Within the area, 9 large pits and more than 25 closely
spaced narrow trenches were mapped. In addition several areas of potential
contamination were identified that can now be addressed in the chemical sampling
survey. This paper will discuss the design considerations and results of this
unusual survey.
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Shear-Wave Investigations In Poorly Consolidated Materials
Authors David Butler and Edward P. PedersenShear-wave (S-wave) refraction is a powerful tool for investigation
of the shallow subsurface. Compressional-wave (P-wave) refraction,
while very successful for bedrock mapping, is not successful at
differentiating bedding within the alluvial section. In poorly
consolidated and saturated materials P-wave velocities are
determined by the speed of sound in water (about 5200 feet/second).
S-wave propagation is not greatly affected by water content, thus
layering within the alluvial section can often be mapped with Swave
refraction.
Travel-time curves for models illustrating these points include a
P-wave model with two alluvial layers over bedrock. The P-wave
velocities of lOOO'/s, 3OOO'/s and 7OOO'/s are shown to be masked
by saturation within the alluvial section. Modeled S-wave
velocities for the same section predict clear detection of the Swave
first arrivals. The acoustic (through the air) arrival can
complicate the S-wave results.
A landfill investigation case-history is an example of the use of
S-wave refraction. A suite of geophysical tools were proposed to
assist in the sitting of monitor wells. The geologic setting is
lo-70 feet of loess over 20-100 feet of till over limestone
bedrock. Water table was within the loess but the loess-till
interface (a potential aguiclude) was the primary target. GPR, DC
resistivity, EM-34, P-wave refraction, P-wave reflection, and Swave
refraction were tested. GPR penetration was less than 15 feet
and the resistivity contrasts between the loess and till were less
than 30% thus the electromagnetic methods were discarded. P-wave
refraction mapped the top of the water table, a secondary target.
P-wave reflection mapped the top of bedrock (also of secondary
interest). S-wave refraction produced excellent data and mapped
the horizon of interest.
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Cross Borehole Electromagnetic Imaging Of Chemical And Mixed Waste Landfills
Authors David J. Borns, Gregory Newman, Larry Stolarczyk and William MondtThe Mixed Waste Landfill integrated Demonstration (MWLID) is testing noninvasive
site characterization methods at several locations, including the Chemical Waste
Landfill (CWL) at Sandia National Laboratories. The CWL comprises shallow, unlined
pits that were used for the disposal of acids, oils, solvents, and inorganic compounds
from 1962 until the CWL was closed in 1985. The soils of the landfills are alluvial,
predominantly sand, gravels and cobbles with small quantities of silts and clays. The
focus of this study is an unlined chromic acid pit (LEAP). The UCAP pit is rectangular
(approximately 35 by 12 feet), which reported/y received unknown volumes of
chromium in the form of chromic acid (liquid) and other hazardous materials. At this
location, we have demonstrated a continuous waveform (CW) system for site charac-
terization. During this year, we will also utilize a crossborehole pulsed radar system.
Both methods are sensitive to variations in either electrical conductivity or dielectric
constant in the soils or host rock at a waste site. These earth properties are some of
the most responsive geophysical indicators of metallic, acidic and water-based subsurface
contaminants.
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A Geophysical Program To Aid In The Environmental Investigation Of The U.S. Army Base At Fort Ord, California
By E.J. TickenGeophysical methods have been used during environmental studies conducted for the Corps of Engineers,
Sacramento District, to characterize potential sources of contamination at the U. S. Army base at Fort Ord,
California. Small-scale industrial operations have been ongoing since the base was established in 1917.
The most environmentally significant operations have been associated with military motor pools, landfills,
sewage plants, and bum pits. Fort Ord was added to the National Priorities List (Superfund) in February
1990 and has been targeted for closure. More than 50 site-specific or basewide investigations have been
conducted at the base, which covers over 23,000 acres on the Pacific Coast.
Geophysical methods have been used extensively for investigations at the base since 1985. Ground
penetrating radar and electromagnetic profiling were used to map the extent of subsurface debris during a
landfill investigation and to map underground storage tanks and associated utilities for the Underground
Storage Tank Management Program. Magnetometry was used to locate buried wellheads, and both
natural gamma and video logs were collected in old water supply wells prior to their destruction as part of
the Well Management Program.
For the Superfund investigations, an integrated program of geology, hydrogeology, and geophysics was
developed by Harding Lawson Associates, the Corps of Engineers, Fort Ord personnel, and the regulatory
agencies. Geophysical methods used in the investigations included high resolution seismic reflection
profiling, ground penetrating radar, electromagnetic profiling, magnetics, and geophysical borehole
logging. One or more of these methods was used for the following applications during the Superfund
investigations:
. Delineate suspected landfill areas
. Locate underground storage tanks and former tank locations
. Provide stratigraphic correlation and identify saturated sections in monitoring wells
. Clear borehole locations of subsurface utilities and obstructions, including unexploded ordnance
. Map subsurface geologic features such as an aquiclude layer, a fault, and a bedrock high.
Using geophysics at Fort Ord has reduced both cost and the number of intrusive activities and has aided
in the understanding of the subsurface geology and potential sources of contamination.
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Landfill Mapping Using Multi-Disciplinary Geophysical Techniques At The U.S. Air Force Academy, Colorado Springs, Co
Authors Robert J. Horton, John W. Busby, Michael H. Powers and Ronald N. KnoshaugThis paper describes a multi-disciplinary geophysical survey conducted over a
landfill on the U.S. Air Force Academy grounds near Colorado Springs, Colorado.
The landfill is known to contain waste generated during the construction of the
Academy and reportedly contains buried steel drums. The purpose of the
geophysical surveys was to determine the subsurface distribution of buried
metallic objects within the landfill.
Different geophysical techniques were evaluated along a test line to determine
their relative effectiveness at this site. The geophysical methods included
total magnetic field, vertical magnetic gradient, VLF, horizontal and vertical
coplanar electromagnetic, GPR and seismic refraction.
Magnetic and coplanar electromagnetic (EM) methods were chosen to survey the
entire landfill because they easily detected magnetic and conductive sources and
have better anomaly resolution than other methods evaluated, as demonstrated by
the test line results. In addition, these methods are rapid and cost effective
for surveys involving a large number of measurements.
Surveys of the landfill identified numerous magnetic and conductive anomalies
indicating the presence of buried metallic objects. The vertical gradient and
EM measurements indicate that several of the large total field anomalies are
produced by groups of smaller objects rather than by single, large buried
sources. Many of the smaller anomalies are associated with the position of a
recently dismantled railroad track and result from iron and steel parts buried
along the abandoned grade.
Two long, narrow conductive anomalies were identified by the electromagnetic
surveys. These conductive features have no surface expression and apparently run
the length of the landfill. The EM data indicates these conductors are narrow
and relatively shallow. One conductor is relatively magnetic, the other
conductor has no magnetic signature suggesting a different composition.
The geophysical surveys determined that large areas of the landfill are
relatively free of buried metal due to the lack of observed magnetic or
conductive anomalies. The geophysical data also suggests the landfill may be
larger than originally thought. Numerous magnetic and conductive responses were
observed beyond the eastern edge of the present landfill in an area thought to
be natural terrain.
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Evaluation Of Infrared Thermographic Imaging For Environmental Applications
Authors Jeffrey A. Havlena and Robert G. KnowltonHigh-resolution infrared thermography is a fairly recent technique that has been
developed to investigate a wide number of mechanical, electrical, and structural
defects in equipment, structures, and soil. The equipment and technique have more
recently been modified by the engineering community to detect subsurface voids,
pipeline leaks, and buried utility line leaks. These methods have been successfully
used to determine the location of leaks, cracks, subsurface voids, and elevated
moisture content at a number of sites under a variety of conditions. However, this
technique has seen little use in the detection of buried waste, underground tanks, and
burial trenches in the environmental arena.
Sandia National Laboratories (SNL) recently conducted an environmental
characterization program using infrared thermography and other surface geophysical
techniques. The primary objective of the SNL testing and evaluation program was to
determine the efficacy of high-resolution infrared thermography to identify buried
wastes and objects under the real-world, field-scale conditions which may be
expected at hazardous, mixed, and radioactive waste landfill sites at SNL, and
elsewhere through the DOE complex. SNL completed an evaluation of this method at
a chemical and mixed waste landfill site. Results of the infrared thermography method
compare favorably with the known characteristics of the site and other non-invasive
geophysical techniques. Several undocumented burial sites were discovered during
the course of the infrared thermography investigation.
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The Application Of Automated High Resolution DC Resistivity In Monitoring Hydrogeological Field Experiments
Authors George W. Schneider, S.M. De Ryck and P.A. FerreOver the past several years, an automated high resolution DC resistivity system
has been developed and used at the University of Waterloo as a spatial and temporal
monitoring technique in hydrogeological field experiments. This paper focuses on the
technology and presents three case histories of its application.
Measuring DC resistivity in situ can be an effective technique with which to
monitor hydrogeological field experiments, and it can pose some interesting challenges
to those wishing to employ the method.
During the summer of 1991, in situ probes and surface arrays were used to
monitor the migration of perchloroethylene in a saturated sandy aquifer. PC based
automated DC resistivity scanning equipment was developed in order to determine the
utility of resistivity to detect PCE in situ. The probes used were a casing with embedded
electrodes and a series of removable probes which contained all cabling, making them
transparent to ground penetrating radar when the probes are temporarily removed.
Conventional, removable, surface arrays were also employed.
In September of 1992, a series of kerosene injections were completed in a 3.6
metre diameter by 1.7 metre deep tank packed with a medium sand. A current booster
was used in conjunction with the same apparatus developed for the PCE experiment to
monitor the migration of kerosene in the unsaturated zone using both an in situ probe
and surface arrays.
During a soil flushing field experiment the system was used to monitor a fresh
water infiltration event, salt plume emplacement event and a drainage event. In this
case, more economical in situ probes were constructed (since GPR was not an issue),
and in greater quantity, to monitor at a higher spatial density. The kerosene and the soil
flushing experiments were conducted concurrently using a serial network of distributed
multiplexers linked to a common data acquisition system.
The technique has shown to be able to acquire a data set sampled very frequently
in time and space, during parts of experiments when time constraints may not allow such
data to have been gathered by any other means.
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Application Of Two Ground Electromagnetic Systems In Environmental Investigations
More LessGround electromagnetic (EM) methods have, in the last few years,
proved effective in environmental investigations, such as detecting
and monitoring of groundwater contaminated with chemical and/or
municipal wastes. Groundwater pollution can result from the
movement of acid mine drainage from tailing ponds, from municipal
or chemical waste dumps, or saline water invasion in coastal areas.
Contaminant l@plumesll in those cases can be mapped and monitored
using inductive terrain conductivity meters, VLF-EM resistivity
techniques, and transient and multifrequency EM systems.
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Geophysical Data Processing Techniques As Related To Groundwater Contamination Studies
Authors Paul Bauman, Janan Sallomy, Ted Wong and Paul HardistyShallow geophysical surveys are regularly applied to site investigations at some of the hundreds
of oil and gas facilities located in western Canada. Terrain conductivity surveys are frequently
used to map produced saline water plumes, abandoned flare pits, drilling sumps, and sulphate
plumes. Magnetometer surveys are regularly used to delineate landfills and identify pipeline
locations. Often, raw data accompanied by minimal data enhancement are sufficient to provide
the required information. On other occasions, however, simple but powerful data processing
techniques are necessary to give meaningful sense to the acquired data. In this paper, four case
studies are presented which illustrate the application of simple data processing routines to
overcome:
1. The influence of steeply sloping topography on terrain conductivity data,
2. The effects of very high soil salinities which may mask inorganic groundwater
contaminant mapping,
3. Difficulties in identifying relatively subtle, but discrete fractures in Tertiary
siltstone/sandstone bedrock, and
4. The challenge of electrically mapping complex structure at the subcrop level of the
bedrock/water table interface.
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Seismic Reflection Survey At Douglas County Landfill, Nevada
Authors Mary-Linda Adams and Brent LewisIn October 1992, a shallow high-resolution seismic reflection survey was performed at the Douglas County
Landfill near Gardnerville, Nevada as part of the site investigation (SI). The closed portion of the landfill
operated between 1963 and 1987. Presently five (5) monitor wells exist, two of which, including the
production well, are contaminated with solvents.
The site is located within the Basin and Range Province in the Carson Valley, a graben bounded by northsouth
trending mountains. The site is overlain by Tertiary sedimentary rocks and sediments of lacustrine and
fluvial origin. North-south trending normal faults are predominant, with a set of less prominent roughly eastwest
trending faults that present additional possible conduits for contaminant transport. The monitor wells
are typically 360 feet deep with the static water level about 200 feet beneath the surface. The stratigraphy,
according to the well logs, consists of older alluvium overlain by younger alluvium. Within the older alluvium
are thin flows of basalt and volcanic tuff. The older alluvium is layered sand, silty gravel, sandy silt, and silty
sand that is partially consolidated. The younger alluvium is unsaturated and consists of unconsolidated
coarse sand and gravel.
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Microgravity Method For The Detection Of Abandoned Shafts Of The Schuyler Copper Mine In Northeastern New Jersey
More LessThe microgravity method has been used to detect subsurface
abandoned shafts of the Schuyler Copper Mine in the Borough of
North Arlington, northeastern New Jersey. The Schuyler Mine, a
potential hazard because of its highly urbanized location, was
known to have more than 40 mine shafts, all covered by
unconsolidated sediments. The applicability of microgravity for
detection of abandoned mines had previously been demonstrated at
the Lawrence Iron Mine in northern New Jersey. Synthetic
computer models were used to simulate the gravity anomalies
expected from shallow shafts in a sandstone bedrock.
Microgravity surveys at the Schuyler Mine were conducted at
21 possible shaft locations. Data were collected at 686 stations
along 38 profile lines. The stations on each profile were spaced
5 feet (1.52 m) apart. Special field procedures were followed to
minimize errors due to instrument drift, station elevation and
location. The resulting gravity maps and profiles showed
anomalous gravity lows ranging from 0.02 to 0.05 mGa1 which were
interpreted to be due to buried shafts. Some gravity profiles
were modeled using 2.5-dimensional gravity inversion software.
The models showed about 10 feet (3.05 m) of unconsolidated
sediments over a shaft and bedrock. The model5d shaft dimensions
ranged from 8 ft2 (2.44 m2)to 15 ft2 (4.57 m ). Data from 2 of
the 21 sites surveyed did not indicate shafts. Of the remaining
19 sites that indicated the presence of shafts, 15 were
subsequently confirmed and delineated by drilling. The newly
located shafts are to be filled with stone and capped.
The success of the microgravity method is attributed to the
shallow depth of the mine shafts, instrument accuracy, closelyspaced
stations, accurate station location and accurate station
elevation control.
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Characterization Of Karst Features Using Electromagnetics And Microgravity: A Strategic Approach
Authors Lynn Yuhr, Richard Benson and Dwain ButlerThe flat-lying limestones of western Texas are naturally jointed with preferential
dissolution occurring along joints and bedding planes. This area has some of the
largest air-filled open caves in West Texas and large paleokarst collapse features
have been identified throughout a wide region. Aerial photos indicate the
presence of extensive joints, lineaments and paleokarst. Site characterization to
detect and map such subsurface features using borings alone is inadequate to
produce a reasonable level of spatial sampling. This paper addresses the
application of surface geophysical techniques, and an assessment of the spatial
sampling and instrument sensitivities necessary to define the karst features of
interest in this geologic setting.
Electromagnetic measurements using a Geonics EM34 were selected because
the measurements provide an excellent means of locating dissolution-enlarged
joints. Microgravity was selected because it is the only surface geophysical
method that will provide the location of karst feature regardless of their shape or
fill material as long as there is a sufficient density contrast. While both methods
have limitations, as do all methods of site characterization, the combination of
gravity and EM measurements are complementary in this application.
A known cave system was used to establish the spatial sampling criteria for the
detection and characterization of such features. Field tests were then run over
two known karst sites; a paleokarst collapse; and a localized doline to provide
anomaly signatures in this geologic setting.
It is clear that the EM and the gravity techniques are appropriate for detecting and
characterizing karst features in this geologic setting. However, one of the key
issues in planning and carrying out a geophysical survey, is developing a spatial
sampling criteria. This criteria should be based upon an understanding of project
objectives, a conceptual geologic model of site conditions and, if possible,
existing data from the area of interest.
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Geophysical Investigation For Buried Drums: A Case Study
Authors Don Pierce and John DeReamerA vertical gradient magnetic survey was implemented over a 100 acre site to
locate isolated buried drums. Clusters of buried drums at the site had been
discovered and excavated during the course of previous investigations. The
survey was designed and field tested for the ability to detect a single drum
buried at a depth of up to 10 feet below the ground surface. Field tests
demonstrated that vertical gradient magnetic data collected on five-foot
centers would provide the best means for detecting isolated drums. High clay
content soils severely limited both electrical and ground penetrating radar
methods. Based on the field results and theoretical calculations, a minimum
anomaly criterion were established for deciding which vertical gradient
anomalies would be investigated. The vertical gradient magnetic contour maps
were then inspected and anomalies exceeding these criterion were targeted for
investigation. More than 1,000 anomalies were investigatedbytrenching with
a backhoe. Most of the trenched anomalies resulted in the discovery of
miscellaneous magnetic debris. Thirty-six of the trenched anomalies resulted
in the discovery of drums or drum fragments at depths of up to nine feet. The
results of this survey show that isolated drums were easily detectable at
depths of 10 feet. Careful management of this investigation was essential due
to the large number of anomalies and the complexity of coordinating heavy
equipment operations and the magnetometer surveys.
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Using Surface Geophysics To Locate Buried Drums At An Industrialized Superfund Site In New Jersey
Authors Mary Kate Dwyer and Donald JohnsonObjectives of a surface geophysics investigation at an SO-acre Superfund site in New
Jersey were to locate potential source areas of contamination leaking from buried
drums and underground tanks. Surface geophysics was chosen as a cost-effective, nonintrusive
method to accurately identify potential source areas and reduce risk by
locating areas of buried metal to be avoided during a subsequent phase of drilling.
Areas of buried metal identified using geophysics were investigated during a later phase
of test-pitting to characterize the type of materials buried at the site. A
magnetometer/gradiometer was selected as the best method to conduct the study, since
the investigation target was buried metal. The vertical gradient often provides higher
resolution of magnetic anomalies and may allow the collection of useful data closer to
buildings than do total field measurements. The gradiometer was critical to the
investigation, since many magnetic interferences such as buildings, overhead process
lines, underground utilities, and fences were present at the site. Since the potential
target could be as small as a single drum, the survey was performed on a 10 by 10 foot
grid spacing.
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Some Applications Of Frequency Domain Electromagnetic Induction Surveys For Landfill Characterization Studies
Authors John Jansen, Michael Pencak, Richard Gnat and Bassem HaddadFrequency domain electromagnetic induction surveys (FDEM) can provide useful information for
landfill site characterization studies. This paper presents three examples of FDEM surveys
conducted on landfills which illustrate some of the applications of this method.
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Em Delta: A Techni Ue For Rapid High-Resolution Reconnaissa R Ce In The Inphase Mode
More LessWhat is described here is a method for carrying out
rapid high-resolution terrain conductivity (EM) surveys for
buried metal by using a Geonics EM31-D in the inphase mode.
Surveys that are carried out on 3m parallels can actually
attain resolution closer to 1.5m spacing, thus providing
extraordinary control when looking for isolated drums or
metallic targets. Scans were run on 3m parallels, and at
each 3m station along the transect, two readings were taken;
one with the boom running longitudinally, and again
transversely with the boom rotated 90 degrees in the
horizontal plane. Rather than average out the two readings,
as is often done in quadrature phase surveys to reduce
background noise, the value of the difference between the 2
readings (hence the term EM delta,&) was plotted on a contour
map, amplifying the differences in conductivity between the
two readings.
If a buried metal drum exists below the surface, the
ratio between the secondary and primary magnetic fields will
fluctuate widely as the boom is rotated in the horizontal
plane over or near the drum. By obtaining the value of the
difference, the possibility of detecting isolated and/or
deeply buried drums is increased. When a GPR-produced
bedrock and utility map was superimposed over the EM contour
map, one could correlate between EM anomalies and known
underground or surface targets. In the completed EM map, all
the prominent EM anomalies could be attributed to surface or
near-surface metallic materials, such as vehicles, fences,
drains, and monitoring well covers. We concluded that the
area was free of any buried metal.
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Preliminary Results Of A Helicopter Electromagnetic And Magneztic Survey Of The Oak Ridge Reservation, Tiwnessee For Environmental And Geologic Site Characterization
Authors W.E. Doll, J.E. Nyquist, J.S. Holladay, V.F. Labson and L. PellerinThe 35,252 -acre Oak Ridge Reservation (ORR), in the western portion of the
Appalachian Valley and Ridge province in Tennessee, has been a nuclear production and
development facility for 50 years. Contaminants in the many waste sites on the ORR
include heavy radioactive isotopes as well as many organic and inorganic compounds.
The locations, geometry, and contents of many of these waste sites are reasonably well
known, while others are poorly known, and some may be unknown. To better
characterize the known sites and locate and characterize additional environmentally
hazardous sites, a two-phase aerial survey of the ORR was begun in April, 1992. Phase
I, which began in April, 1992, consisted of aerial radiation, multispectral scanner, and
photographic surveys. Phase II, which began in November, 1992 and is described in this
presentation, consisted of a helicopter electromagnetic (HEM), magnetic, and radiation
survey. Targets of the survey were both man-made (drums, trench boundaries, burn pits,
well heads) and geologic (fractures, faults, karst features, geologic contacts).
The Phase II survey has three components: testing, reconnaissance, and high-resolution
data acquisition. To date, the testing and reconnaissance data acquisition have been
completed, and part of the data have been processed. They indicate that: 1) magnetic
and HEM data are complementary, and do not always highlight the same anomaly; 2)
under favorable circumstances, helicopter magnetometer systems are capable of detecting
groups of 4 or more 55-gallon drums at detector altitudes of 15 m or less, 3) HEM data
provide data which compare favorably with surface data collected over burial trenches, 4)
well casings seem to be related to magnetic monopole anomalies, as would be expected,
5) Changes in EM and magnetic anomaly character are related to lithologic changes and
might be used to track contacts between known outcrops.
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First Arrival Inversion Of Crosswell Radar Data By Finite-Difference Solutions To Maxwell’s Equations
Authors Wenying Cai, Fuhao Qin and Gerard T. SchusterThe wave equation traveltime inversion (WT) method is adapted to the reconstruction
of the dielectric distribution from first arrival traveltime radar data. A gradient
optimization algorithm is used and the gradient function is computed from finitedifference
solutions to the 2-D Maxwell’s equations. The key advantage of the radar
WT method over conventional ray tracing radar tomography is that it accounts for
scattering and diffusion effects in the data and works well in both highly resistive and
moderately conductive rocks. This technique is successfully applied to both synthetic
and real radar data. Comparisons with a ray tracing (RT) tomography scheme show
that the radar WT method is more reliable and accurate than the RT method when rock
conductivity is larger than .002 S/m. The WT and RT methods are about equally
effective when conductivity is less than or equal to .OOl S/m. The disadvantage of the
WT scheme is that it generally demands an order of magnitude more computational
time than the RT method.
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The Use Of In Situ Permeable Flow Sensors To Monitor An Air-Stripping Waste Remediation Experiment At The Savannah River Site, Sc
More LessIn Situ Permeable Flow Sensors, new instruments which use a thermal perturbation technique
to directly measure the direction and magnitude of the 3-dimensional groundwater flow velocity
vector at a point in a permeable medium, are being used to monitor changes in the groundwater
flow regime in the immediate vicinity of an underground air-stripping waste remediation
experiment underway at the Savannah River Site, SC. Two horizontal boreholes were drilled into
the sandy sediments with interbedded clay lenses beneath the site, which is contaminated mainly
with trichloroethylene and perchloroethylene. The two holes are at depths of approximately 20
and 50 meters. The water table is at a depth of 42 meters, in between the two horizontal wells.
Air is injected into the deeper hole, flows upward through both the saturated and unsaturated
sediments and is extracted from the shallower hole. As it travels from the injection to the
extraction well, the air volatilizes the contaminants which are then removed from the extraction
well along with the injected air.
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A New Application For Geophysics: Monitoring At The Dig-Face During Waste Site Excavation
By N.E. JostenThere are many hazardous waste sites throughout the United States that have been mandated by
law to undergo remediation. That process has been painfully slow and expensive, partially due
to the inadequacy of the waste characterization schemes that have been counted on to define the
extent and nature of site contamination. Conventional characterization efforts simply fall short
of providing the level of information on buried waste that is consistent with human safety during
clean-up operations. Preliminary design has been completed for a new technology, called Safe
Step Remediation, that addresses this problem. The key component of the Safe Step approach is
a Dig-face Monitoring System. The monitoring system produces waste characterization data in
small, careful increments. Each new characterization increment drives a new increment of
excavation.
Dig-face monitoring poses some new challenges for geophysicists. One challenge is to meet
performance requirements for quantitative, exact interpretations that go well beyond those that
apply to most conventional geophysics. In dig-face monitoring, interpretations directly and
continuously protect site workers by guiding the removal and handling of dangerous materials.
Several unique aspects of the dig-face monitoring application should enhance capabilities for
making these accurate interpretations. First, within the Safe Step Remediation approach, the
monitoring system will be able to make an extraordinary set of measurements. These
measurements will be made on multiple planes as the excavation progresses and will include
close-up measurements made in the immediate vicinity of hazards. Second, and even more
important, the physical retrieval of targets following each increment of characterization generates
a unique opportunity to validate interpretations continually. This provides a basis for steadily
improving the quality and accuracy of interpretations over time.
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High Resolution In Saturated Sediments - A Case For Shear Wave Reflection
More Less“High Resolution Reflection” all too often carries with it the implied meaning of “High Frequency
Reflection.” As the resolving power of a reflected wave is a function of wavelength and not merely frequency, high
frequency in and of itself does not exactly define high resolution. While Poisson Ratio (u) in sedimentary and hard
rock masses hovers in the 0.25-0.33 range, within saturated sediments, (T approaches 0.45-0.49. What this means,
in effect, is that the ratio of compressional (P) to shear (S) wave velocity in such loose media is in the range of 5: 1
up to 10: 1. In terms of wavelengths for the w frequency input wavelet, an S wave will have wavelengths of
from l/10 - l/5 that of a P wave in the same medium. This is a direct increase in resolution by a factor of 5-10.
Under many circumstances, time is better spent using shear waves for reflection than in developing and employing
exotic higher frequency compressional sources which may only increase resolution a factor of two over more
standard seismic sources.
Example S wave reflection studies relating to stratigraphic delineation and growth fault studies from the
US Gulf Coast, Southeast US, and Eastern US areas illustrate the type of resolution obtained with minimal,
additional field effort.
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Near Surface S-Velocity Profiles At 30 Salt Lake Valley Sites From Inversion Of Surface Wave Dispersion And Analysis Of S-Wave Refraction Data
Authors Yonghe Sun, Wenying Cai, Greg Andrews, Jeff Wolfe, Peter Fivas, Jinlong Xu, Changxi Zhou, Gerard T. Schuster and Todd AtkinsSurface wave and shear refraction data were collected at thirty Salt Lake Valley
sites using a Bison 9000 seismic recorder. Each experiment used 10 Hz vertical and 4
Hz horizontal geophones at group intervals of 5 feet, with spread lengths up to 1000
feet. Both horizontal and vertical 100 lb weight drop sources were used, as well as the
Bison EWG 500 lb source. Rayleigh waves, Love waves and shear refraction waves
were recorded and inverted for the shallow S-wave velocity structure. Results show
that the Rayleigh wave and refracted shear wave data were the most reliable and generally
provided similar velocity profiles; the Love wave data proved to be the least reliable,
although over half the sites showed that the Rayleigh, Love and refraction wave
velocity profiles agreed to a depth of about 30 meters.
These shallow S-velocity profiles were used to simulate the seismic amplification
from earthquake waves impinging on the basin floor. For all sites, it is shown that the
deep (about 1 km thickness) 2-D basin structure causes about 2-3 times more
amplification than the shallow S-velocity structure for frequencies from O-4 Hz. The
shallow velocity structure is important for amplification at frequencies greater than
about 5 Hz.
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Observation Of Borehole Mach Waves And Implications For Improved In-Situ Physical Property Measurements And Subsurface Imaging In Soils And Low-Velocity Rocks.
Authors J.T. Rutledge, J.N. Albright and J.A. MeredithA crosswell seismic survey was conducted in the McKittrick oil field, California in which
no direct body waves were observed. However, strong arrivals were detected as a result
of Mach waves radiating from the source borehole. The phenomenon of borehole
seismic sources generating secondary body waves, or Mach waves, has only recently
been recognized and understood. The McKittrick data demonstrate that secondary body
waves provide a means to efficiently transmit seismic energy from a borehole penetrating
low-velocity rocks or soils. Exploitation of Mach waves may enable the measurements
of in-situ physical properties and imaging of the subsurface in environments where
conventional seismic techniques fail.
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Designing Geophysical Instrumentation Specifically For Environmental Applications
By H.O. SeigelThe application of geophysical methods to the solution of problems in the environmental field
has been facilitated by the availability of full-blown technology in the mining and groundwater
explorations field. There are, however, basic differences between these two fields and the
environmental field, which demand a fresh approach to the design of instrumentation for
environmental applications.
The station density (stations per km2) for environmental-type surveys may be 1000 times higher
than in resource-type surveys. Thus, the time required for a single measurement becomes the
prime factor in the environmental survey cost. The time to move between stations, which
constitutes a large component of the cost of resource surveys, is no longer consequential. Thus,
individual measurement times of 0.5 seconds or less, are required in order to permit surveys to
be made at a walking pace.
Ease of use by non-geophysical personnel is a second requirement. Whereas most resource
geophysical surveys are done under the supervision of, if not operation by, a trained geophysicist,
the environmental field will have large numbers of practitioners who do not have this
background. Thus, geophysical instruments for environmental purposes must be very simple to
operate and require little prior instruction.
At the same time, however, high sensitivity and high precision are often not very important in
the environmental field, as the physical property contrasts of the targets involved are generally
larger and their depth below surface generally smaller than in the resource field. This facilitates
a reduction in weight, size, complexity and cost in environmental instrumentation.
Lower capital cost is an important consideration in the environmental field, as individual projects
tend to be small in size and of lower revenue than in the resource exploration field. In addition,
environmental contractors are likely to belong to small groups with low capitalization.
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Practical Considerations For Grm Refraction Surveys In Glacial Terrains
Authors Dirk Kassenaar and John LuttingerGeneralized Reciprocal Method (GRM) refraction surveys in glacial terrains frequently encounter
complex geologic and hydrogeologic conditions. The complexity frequently centres around the
shallow water table and low velocity of the unsaturated zone. Inaccurate determination of the
velocity and thickness of this low velocity layer can have a significant effect on the depth
estimate of lower layers. This paper discusses methods to optimize field data collection and
processing to ensure proper analysis in these situations.
A general rule of GRM surveys is that the geophone spacing should be less than one third of the
XY distance of the shallowest layer. Since the XY of a shallow water table layer is often less
than 2 metres, geophone spacings of less than 1 metre may be necessary. These spacings are
frequently not cost effective, and alternative approaches may be required. Alternatives include;
1. using the GRM average velocity method and an estimate of the XY based on modelling, 2.
combining the upper two layers, and 3. using time intercept methods to interpret short spreads
collected at various points along the line.
The sensitivity of the GRM is evaluated using a simple model. Random pick errors are
introduced into the model, and the merit of arrival averaging is studied. Similarly, layer
velocity errors, XY analysis errors and hidden layer problems are reviewed.
Practical suggestions for GRM surveys in glacial terrains are offered. An approach for
performing cost effective GRM surveys is presented. The approach is based on high redundancy
data collection (multiple mid shots) for water table velocity analysis and duplicate arrival
averaging. Finally, the importance of XY analysis for detecting hidden layers and bedrock
surface features is discussed.
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Integrated, Flexible, And Rapid Geophysical Surveying
Authors S.F. Miller, L.D. McGinnis, M.D. Thompson and C. TomeDetailed environmental studies associated with landfills, burial pits, vaults,
underground storage tanks, contaminant plumes, and unidentified contaminant
sources adjacent to buildings at Aberdeen Proving Ground, Maryland, are being
conducted. Efficient and innovative data-acquisition procedures are imperative in
order to provide complete coverage at a large number of small-sized sites.
Because APG is a chemical weapons research and development facility,
noninvasive geophysical techniques are a necessity. Real-time data processing
and interpretation using computers in a field setting permit rapid changes in the
design of the survey and in decision making.
Magnetic and electrical interference caused by metal buildings, power
lines, and buried utilities limit applicable geophysical techniques. A pilot study to
test a variety of techniques resulted in the selection of horizontal electrical
resistivity profiling, magnetic gradiometry, total field magnetics, and groundpenetrating
radar.
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Seismic Refraction And Gravity With Drilling Verification
Authors Paul J. Wolfe and Benjamin H. RichardSeismic refraction and gravity surveys have been used to guide a drilling crew evaluating
water table and bedrock depths in an environmentally sensitive area. The Ohio Department
of Transportation plans to construct a freeway past the Cedar Bog nature preserve. To
evaluate the potential impact of the freeway construction on the ground water flow that
sustains the bog, an extensive geophysical and drilling program is being conducted by
Wright State University.
Cedar Bog is located at the edge of a major preglacial river valley that was buried by
Pleistocene glacial deposits. From previous geophysical studies we believe that water flows
underground through gravel-ftied, buried tributary channels in the bedrock and then moves
to the surface emerging in artesian springs near the bog. Determination of the hydrological
regime and the details of the buried channel locations is necessary for designing the highway
so that the bog will maintain its cold water supply. Two proposed routes for the road total 8
miles long: We conducted seismic refraction and gravity surveys continuously along the
routes to determine the water table depths, the bedrock depths, and any major changes in the
glacial material that covers the bedrock. We interpreted the geophysical data and drew cross
sections to show the locations and depths of channels in the bedrock surface that were likely
to act as ground water flow zones. Holes were drilled at selected locations along the
proposed route to verify the geophysical interpretations and to examine the detailed glacial
stratigraphy. The large number of holes drilled along the lines allowed us to evaluate the
precision of the seismic refraction surveys.
We found good qualitative agreement between the seismic and drilling results and applied
several statistical measures to quantify the relationship.
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High Resolution Seismic Reflection For Characterizing Longwall Coal Mine Subsidence
Authors James A. Jessop, Calvin L. Cumerlato, Kevin M. O‘Connor and John A. SiekmeierAs part of an Illinois mine subsidence initiative, the U. S. Bureau of
Mines has conducted high resolution surface seismic surveys at two longwall
coal mine sites in southern Illinois to help characterize subsurface
subsidence. Premine and postmine surveys conducted above 183 meter (600 foot)
deep longwall panels consisted of 12-fold common depth point (CDP) data
collection, using a 24-channel engineering seismograph, 60 Hz land geophones
and a nondestructive surface energy source. Source offset ranged from 91.4
meters (300 feet) to 152.4 meters (500 feet) and a station interval of 3.1
meters (10 feet) were determined from noise test data at each site. Data from
these surveys were processed into seismic sections using commercially
available microcomputer-based software. Sonic logs from a nearby borehole and
mine maps showing mined and unmined areas were used in the processing and
interpretation of the data. The sonic logs were used to generate a plot of
the two-way traveltime through each formation and a cumulative plot of
traveltimes through progressive layers to the coal seam. A coaxial cable was
grouted into the borehole and Time Domain Reflectometry (TDR) was used to
monitor overburden movement during mining. This data was compared with the
seismic sections and cumulative traveltimes plots to identify bridging within
the overburden. Bieniawski's Rock Mass Rating system (RMR) was used to
characterize the overburden. The RMR was computed for each formation and is
used to determine the deformation modulus, then the modulus is combined with
parameters based on mine geometry and overburden properties to compute a
bending stiffness and bridging potential for each bed.
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Fracturing Of Glacial Drift And Bedrock Over Longwall Mine Panels: Integrated Geophysical And Hydrological Measurements
Authors Matthew A. Johnston and Philip J. CarpenterP- and S-wave refraction profiles were combined with
resistivity soundings, resistivity and electromagnetic profiles,
azimuthal surveys and hydrological data to characterize fracturing
and associated hydrogeological changes over three subsided longwall
coal mine panels in the southern Illinois basin. Approximately 6 ft
of Herrin #6 coal was mined from the base of a 260-320 ft section of
Pennsylvanian rocks capped by 40-90 ft of unconsolidated glaciolacustrine
deposits. Subsidence-induced fractures significantly
altered the mechanical and hydrogeological properties of the
overburden.
Fracturing of the drift over one panel to at least the depth of
the water table (lo-20 ft) was marked by decreases in shallow SH-wave
velocity and a non-uniform pre- to post-subsidence drop in the water
table. Apparent resistivity also increased in the dynamic tension
zone where surface fractures opened just behind the mine face.
Inversion of resistivity soundings could not be used to constrain the
depth of fracturing in this area, however. Increases in earth
conductivity, decreases in apparent resistivity, and the development
of azimuthal resistivity variations suggest long-term fracturing of
drift along panel margins, and, in some cases over barrier pillars.
Fracturing of the upper bedrock (to 80 ft below the bedrock
surface) was indicated by reduced P- and SH-wave velocities, sharp
potentiometric declines, and post-subsidence increases in hydraulic
conductivity. SV- head waves from the bedrock surface, however,
showed no such reduction in velocity, suggesting bedrock Vsv is
controlled primarily by bedding anisotropy.
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“A Study Of Cryoseisms (“Frostquakes”) In The Sebago Lake Region, Maine
More LessDuring the winter of 1990-91, residents in the Sebago Lake Region of
southwestern Maine became alarmed by a noise phenomenon which they were convinced was
caused by a natural gas transmission pipeline in the area. The unexplained noises caused
residents to suspect the nearby transmission line. In response to these concerns, the pipeline
operator thoroughly checked the pipeline for potential safety problems. No leaks were detected
and all other safety checks had revealed that nothing was amiss.
An investigation was undertaken to study the source of the noise phenomenon on behalf of the
pipeline company. Local residents were interviewed and a network of 3-component
seismographs was installed to establish background vibration levels and to identify the source
of the disturbances. The seismographs used in this study can be set to record events over a wide
range of vibration levels. Area residents were given log books in which to describe any unusual
noise events and to record the time of occurrence.
Based on approximately two months of continuous monitoring in the study area, personal
interviews, a literature search for related vibration phenomena, and first-hand observations of
the noise phenomena by the author, we concluded that the loud noises were the result of
cryoseisms (‘lffostquakes”) or frost-induced microearthquake activity.
Cryoseisms are an unusual phenomena which occur under limited winter weather conditions in
northern climes. They can locally produce loud noises such as those described by residents in
this study as ‘a slamming door, ” “an automobile running into the side of the garage’: “a furnace
blowing up’: and “a rifle shot’: Cryoseisms are also frequently accompanied by minor fissures
which may be as wide as several millimeters at the ground surface and which re-heal with time.
Examples of fissures ji-om the current study will be presented. The mechanisms that produce
cryoseisms and the climatic conditions under which they occur will be described.
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Rock Mass Characterization Using Geophysics For Stope I,Eaching
Authors J.J. Snodgrass and D. BoreckThe Bureau of Mines is researching the feasibility of in situ stope
leaching for active underground metal mines. A test facility was
developed in an underground mine near Idaho Springs, CO, to
investigate the processes of leaching solution control. An
important aspect of the research is the characterization of the
rock mass in terms of discontinuities which will determine fluid
flow and affect fragmentation from blasting. Geologic mapping and
core logging documented the fracture and joint systems in the
simulated stope and adjacent areas of the underground facility.
Slimline borehole geophysical logs were used to verify lithologic
changes and indicate locations of fracture zones. Conventional
and full wave form sonic (FWS) logs were obtained, where
possible, to interpret fracture zones and define optimum
locations for permeability tests. In addition, sonic shear wave
profiles were obtained with a wall-clamping probe in the
boreholes drilled at the site. Shear wave measurements were not
as sensitive to fracturing as the conventional travel time P-wave
logs, but the capability for dry hole operation provided data in
zones of the rock mass that would otherwise be unavailable.
Integrated interpretation of the geophysical logs enhanced the
geological characterization of the in situ rock mass by
distinguishing open fractures (possible fluid conduits) from the
closed fractures. Estimates of the dynamic elastic moduli
provided a baseline for comparison of post-blast measurements to
evaluate overbreak and control solution loss during the leaching
phase of the experiment. Cross-borehole sonic surveys and
tomographic imaging were also demonstrated effective in
correlating the fracture zones between drillholes.
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Geophysical Characterization Of Mineral Waste Sites
Authors J.J. Snodgrass and C.M. LepperSurface geophysical methods including electromagnetic conductivity,
spontaneous potential, seismic reflection and refraction, ground
penetrating radar (GPR), and magnetometry were evaluated at mineral
waste sites to characterize subsurface geology and hydrology.
Three metal mine waste sites were investigated: (1) an open pit
operation with extensive spoils and sub-ore grade materials
distributed over an area approximately one square mile; (2) a
typical mill waste dump site, a few acres in size; and (3) a small
mill waste tailings pond. At the open pit mine, four different
locations were investigated to determine pre-mining topography,
characterize the subsurface groundwater flowpaths, and design the
most efficient drilling program for monitor wells. Interpretation
of the geophysical surveys at the waste dump site provided a model
of the ground water conditions which was crucial to the development
of a remediation plan. Results of the geophysical surveys at the
tailings pond provided information on the thickness of tailings and
water channelling for subsequent modeling of the hydrology, in
addition to determining feasibility of new methods for
characterization of mine wastes. The work to date has emphasized
the need for a multi-disciplinary approach to the characterization
of mineral waste sites with geological, hydrological, and
historical input to determination of the best technical evaluation
of site conditions. An integrated approach using multiple
techniques provided a high degree of confidence in interpretation
of conditions that were relevant to design of drilling plans,
development of groundwater models, and remediation efforts.
Efforts at two mill tailings piles, and at the abandoned open pit
mine indicate that each investigation must necessarily be
considered an applied research study and that the best methods will
evolve from ongoing surveys.
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Assessment Of Mississippi River Revetments Using The Self-Potential Method
Authors Keith J. Sjostrom, Dwain K. Butler and Robert F. CorwinTwo of the U.S. Army Corps of Engineers (USACE) primary mission
areas are the maintenance of the nation's navigable waterways and
development of flood control measures. These tasks are most evident
along the Mississippi River where revetments are used to maintain the
river channel and prevent riverbank failure. Revetments are structures
placed parallel to the river current for the purpose of stabilizing the
concave banks in river bends and to form a smooth bank line. Along the
lower Mississippi River, articulated concrete mattresses (ACM's) are
used exclusively for river bank protection (see Figure 1). An ACM
consists of an assemblage of concrete blocks reinforced and held
together by a metal fabric to form a flexible, protective blanket over
the river bottom (Petersen, 1986). A section of ACM is typically 150 ft
wide and extends from the river bank to the center of the navigable
channel. Adjacent ACM sections are installed by overlapping the
downstream mattress by five feet. The ACM are secured to the river bank
with launching cables anchored into the bank material.
Over time, ACM's are subjected to numerous cycles of high and low
water levels, changing river velocities and currents, and structural
fatigue. These factors may cause the ACM to become damaged, displaced
by the undermining of streambed material, or buried by sediment. The
condition of ACM's must be routinely monitored to determine whether or
not remedial or replacement measures need to be undertaken.
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Neural Networks For Highway Maintenance Investigations Using Ground Penetrating Radar*
Authors Dorothy V. Minior and Stanley SmithThe early detection of potential subsurface erosion in highway systems allows for the application of appropriate
maintenance rather than more expensive rehabilitation or reconstruction. Currently, there are few practical test
procedures to assist a highway maintenance engineer in detecting these problems or in evaluating how effective
remedial measures were in repairing the problem. Existing procedures rely on complex algorithms that are
computationally expensive and are not amenable to real time applications. Our research evaluates the synergy
between Neural Networks (NNs) and Ground Penetrating Radar (GPR) as tools for early detection of subsurface
highway problems. We trained NNs to identify the following pavement characteristics in GPR SC~IIS:
(i) pavement thickness (ii) the degree of moisture in the surface layer, (iii) the degree of moisture in the base
layer, (iv) voids or loss of support beneath slabs, and (v) overlay delamination. Undesirable conditions can then
be treated by maintenance, and major structural problems requiring extensive repair can be avoided. Our research
shows that the NN/GPR approach is a viable solution. The NNs extrapolate over noisy or incomplete data, and
they provide faster responses than existing methods.
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Gas-Rich Sediment And Coastal Wetland Loss In Louisiana
Authors M.D. Thompson, L.D. McGinnis, P.L. Wilkey and S.F. MillerHigh rates of wetland loss in southern Louisiana provide the impetus for
examining the role that trapped, biogenic gases play in regulating subsidence of
coastal areas. A significant cause for wetland loss in this region is relative sea-level
rise produced by sediment-volume reduction. Dewatering, grain reorientation and
packing, and oxidation of organic-rich sediments are thought to be the main
processes for volume loss. It is argued that natural and anthropogenic causes for
sediment degasitication play a critical role in sediment-volume reduction.
Compressional wave velocities were measured at 34 sites in both the
abandoned (Holocene) and modem parts of the Mississippi Delta. A low-frequency
source (~200 Hz) was used to maximize sound-wave dispersion caused by
interstitial gas bubbles. Compressional wave velocities measured at low
frequencies relative to the gas-bubble resonant-frequency undergo maximum
change from the velocity for a gas-free sediment.
Seismic refraction studies and velocity measurements using standard
reverse-spread profiling indicate that the upper 40-70 m of the sedimentary section
has sound velocities significantly less than the speed of sound through water
(1,500 m/s), despite the fact that the water table is within 3 m of the ground
surface. The low-velocity zone has average velocities ranging from 800 to 1,150
m/s in the Holocene delta and from 900 to 1,300 m/s in the modem delta. Uphole
traveltimes obtained from approximately 90 km of continuous-coverage, seismic
reflection profiles yield velocity estimates that range from 900 to 1,300 m/s. First
and secondary arrivals determined from the reflection profiles are currently being
used to model the low-velocity zone.
Theoretical velocity modeling, using material properties consistent with
observed sediment types, suggests that approximately 6-20s of the available pore
space must be occupied by gas to account for the observed velocities. This gas is
believed to be trapped in unconnected pore spaces to exist in the form of methane
gas bubbles within the interstitial pore waters.
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Mapping Fracture Connectivity Between Boreholes Using High Resolution Temperature Logs
More LessThe use of temperature logs in detecting fracture zones within a borehole is
sufficiently documented in the literature. This is an indirect technique and has been
used primarily for single-hole fracture characterization. Drilling fluid circulation and/or
groundwater movements induce temperature anomalies that are interpreted to indicate
permeable fractured zones. The use of temperature logs to map fracture
interconnectivity between boreholes is not well documented. This paper presents an
application of high resolution temperature logs in detecting fractured zones, mapping
fracture connectivity between holes and in monitoring groundwater movements. A
well system consisting of five vertical boreholes was drilled through sandstone to a
depth of approximately 55 m, one hole at each corner of a 7.62 m (25 ft) square and
one in the centre. These boreholes were drilled for experiments in connection with
mine technology development conducted by Centaur Mining Exploration Limited. High
resolution temperature logs and several other geophysical logs were acquired in these
boreholes with the Geological Survey of Canada R&D logging system. Borehole fluid
temperature logs recorded immediately after drilling and well development located
several water-producing fracture zones by their characteristic temperature signatures.
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Integrating Geophysical Well Logs, Surface Geophysics, And Hydraulic Test And Geologic Data In Ground-Water Studies - Theory And Case Histories
By F.L. PailletIntegration of surface geophysical surveys, borehole geophysics,
hydraulic tests, and geologic data has the potential to: 1) relate detailed
aquifer properties at specific well locations to large-scale structure mapped
across an investigation site; and 2) calibrate geophysical measurements in
terms of hydraulic variables of direct interest in most contaminant dispersion
studies. This paper reviews the physical and mathematical constraints on the
formal inversion of such data sets as illustrated by a number of case studies.
Our analysis demonstrates that the coupled inversion of these data depends on
1) formulation and verification of a specific site model, 2) statistical
sampling of the distribution of formation properties which depends mostly on
the number of boreholes, 3) specific location of boreholes with respect to
contacts and boundaries given in the model and interpretation. The importance
of these considerations is demonstrated by the synthesis of various field
studies including photo lineament analyses, magnetic surveys, surface seismic
reflection profiles and electrical resistivity soundings with well logs, and
with the results of various hydraulic tests. The case histories illustrate
the importance of logs in formulating and testing specific site models. These
results repeatedly demonstrate the need for obtaining logs from enough
boreholes to relate the boundaries and contacts inferred from surface surveys
to the much greater vertical spatial resolution given by the logs.
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Shallow Seismic Studies Of A Large Playa Basin Near Amarillo, Texas
Authors J.G. Paine, A.R. Standen, G.T. Jones, L.B. Kelley and S.M. RooksSevenmile Basin is a playa basin in the Texas Panhandle that is being studied as
part of a geological and hydrological characterization of the region surrounding
the U. S. Department of Energy’s Pantex Plant near Amarillo. Shallow seismic
data were collected in an attempt to understand the origin, development, and
hydrogeological framework of playas, which are recharge points for the
economically important Ogallala aquifer. This exceptionally large playa basin,
which is 5.5 km across in an east-west direction and 3.6 km across in a northsouth
direction, is less than 0.5 km from the southern boundary of the Pantex
Plant.
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High Resolution, Shallow 3-D Seismic Survey For Environmetnal And Geotechnical Sites
Authors Dr. James Rector and Paul MilliganThe University of California at Berkeley has a multiple test well site in Richmond, that is a
prime candidate for high resolution 3-D surface reflection seismic survey techniques. More than ten
wells, covering an area of 100m by 100m, have been drilled into looseIy consolidated sand and gravel
units seperated by clay layers, with a basement unit of a hard shale at an approximate depth of 37m.
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Hydrogeologic Characterization Using A Shallow Seismic Reflection Survey At Fort Ord, California
Authors Carlene Merey, Richard D. Miller, Edward J. Ticken and J. Scott LewisShallow seismic reflection profiling was successful in delineating a shallow impermeable layer acting to perch fluids at depths ranging from 125 to 175 ft. The near-surface material was dry dune sands with varying grades of sands and gravels within the upper 20 ft. The seismic data possessed dominant frequencies in excess of 150 Hz resulting in bed resolution on the order of 3
ft at depths less than 150 ft. Several key hydrogeologic features were identified and their significance more completely ascertained using the reflection data.
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Mapping Hydrogeologic Properties Using Ground Penetrating Radar
Authors Michael D. Knoll and Rosemary KnightGround penetrating radar (GPR) images provide information about the location, scale
and geometry of subsurface heterogeneities. With well control or geologic intuition, this
information can be used by hydrogeologists to fix the location of boundaries in fluid flow
and contaminant transport models. While the location of boundaries is important, the
hydrogeologist also requires estimates of the physical properties within the boundaries, for
instance the hydraulic conductivity and porosity of different stratigraphic units. In essence,
what the hydrogeologist really wants are the input parameters to his or her model. If the
model is deterministic, these inputs are a discretized map of hydrogeologic properties. If
the model is stochastic, these inputs are geostatistical parameters such as the correlation
length, distribution, mean, variance and trend of hydrogeologic properties. With our goal
being to help the hydrogeologist, we are investigating the capabilities of GPR to map
hydrogeologic properties and characterize the heterogeneity of shallow aquifers.
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