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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 - 20 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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