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4th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 11 Mar 1991 - 14 Mar 1991
- Location: Knoxville, Tennessee, USA
- Published: 11 March 1991
30 results
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Ground Water Geophysics
More LessIn the exploration for, and the extraction and management of
ground water resources, surface geophysics is used for three
general objectives. These are:
(1) Delineation of aquifers and aquitards, and mapping
geologic features impacting movement and storage of
water in such strata (e.g., fracture and shear zones,
intrusive dikes).
(2) Mapping water quality. The economic value of ground
water resource depends on the concentration of the
dissolved solids in the water. In many coastal
aquifers fresh water resources of high value are
underlain by brackish and saline ground water of lower value.
(3) Wellhead protection, such as delineation of prime
recharge areas.
More than 90 percent of practicing exploration geophysicists
are employed by the oil and gas industry in the search for
hydrocarbons, and by far the dominant geophysical methods
employed is marine and land reflection seismics. The application
of geophysics to ground water is more balanced in terms of
techniques utilized, mainly because seismic properties are not
much influenced by water quality. On the other hand, electrical
resistivity is highly influenced by TDS, and electrical and
electromagnetic methods compete on equal footing with seismics
for their fair share of geophysical surveys in ground water
investigations. Case histories will illustrate the contribution
of surface geophysics to various ground water studies.
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Geophysicists, Gods And Graves
More LessThe traditional use of geophysical surveys to provide design parameters for civil engineers,
aquifer parameters for hydrogeologists, and geologic information for mineral and petroleum
explorationists has been well documented. The non-traditional uses of geophysical surveys have
intrigued a small number of investigators over the years, such that geophysicists are becoming
involved in some rather unique investigations.
Borehole geophysical logging techniques have been utilized to map the conditions of and
delineate anomolous areas surrounding a horizontal drain system along the toe of a dam. These
same techniques played a major role in determining in-situ density changes in mine tailings
during dynamic compaction, and providing input for tailings compaction without the use of
explosives.
Electrical resistivity surveys are being used to monitor fresh water encroachment upon mines and
well fields, while self potential surveys have been used to map seepage paths and monitor
seepage from impoundment dams. Seismic refraction surveys are being used to investigate
embankment integrity of earthen dams and airborne geophysical surveys are being used to
characterize hazardous waste sites.
Just as the mineral and petroleum explorationist is often seeking a needle in a haystack the
archaeologist is forced to work with a similar exploration philosophy. Geophysical surveys are
being increasingly used in archaeological investigations from searching for the remnants of the
Spanish Plate Fleet to delineating anomolous areas around the Pyramid of the Sun in Mexico, a
temple built to the Gods.
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Seismic Surveying For The Non-Geophysicist
By R. HugginsSeismic surveying is a geophysical technique that employs sound
waves to image subsurface structures. In suitable environments, seismic
methods yield useful data on bedrock topography, soil layering, aquifers
and aquitards, faulting, buried channels and lateral changes in rock and soil.
Acoustic energy is introduced into the ground by means of an
impact, explosion or vibratory source. Diverse types of waves result in
differing modes of vibration and velocities. Measurement of transit
times and apparent velocities of some of these waves yield data on
subsurface properties. Refraction surveying uses the patterns of first
arrival of waves to determine depth and velocity of the interfaces.
Reflection surveying measures energy arriving later in the seismogram
that has been echoed from interfaces with contrasting acoustical
properties. And boreholes are often employed to measure sound waves
introduced either at the surface or in another borehole. All techniques
have pitfalls and limitations that should be understood and accounted for
when interpreting the data.
Inexpensive personal computers, advances in acquisition
instrumentation and processing software have made seismic surveying
less costly, less time consuming and more effective. In conjunction with
ground truth from geophysical techniques or limited drilling, seismic
surveying can reduce contaminant cleanup costs, assist road construction
and help find water. These times and cost savings make seismic
surveying a sensible tool for problems in hydrogeology, construction, waste management and resource exploration.
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Application Of Borehole Geophysical Logs To Water Resources Investigations
More LessBorehole geophysics have been around since the late 1800's when U.S.G.S. personnel used a wireline to take temperature measurements in a borehole. Up until the early mid 1970's or SO
the use of borehole geophysics were largely confined to the petroleum industry. The advancement of microelectronics has brought borehole geophysics to the water well industry by allowing the downhole probes to be smaller in diameter, lighter weight and more affordable. Borehole geophysical logs compliment other types of data collected during a site investigation such as
cuttings, cores and surface geophysical data. Borehole geophysical logs have several unique features in that they can be run quickly, in small diameter mudded and/or cased holes (almost all logs can now be run in two-inch casing). Many logs can ilsee" through casing, but more importantly they provide a continuous record of various borehole parameters of interest to a hydrogeologist, particularly: bulk porosity, resistivity, density, and clay content. An experienced log analyst can take this information much in the same manner as an experienced hydrogeologist observes a core and analyzes what he sees. From this information the hydrogeologic characteristics can be used to map hydrostatigraphic units across the study area and develop a better understanding of vertical lithologic gradations and lateral facies changes essential to the development of a good understanding of the site hydrogeology. Currently, most borehole geophysical logging is used in a semiquantitative manner to assess lateral and vertical variations. Calibrated and compensated geophysical logs are available and could be used more effectively to more precisely determine: formation bulk porosity, matrix density, formation resistivity, clay content, carbonate content, water quality and even permeability and some cases vertical migration of contaminants. In summary, logs are capable of providing a continuous record of numerous parameters in a small diameter hole and in a timely manner, hydrogeologists take this data, fit it into a working hydrological model of the site to better understand the groundwater flow system.
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Application Of Electromagnetic Methods For Groundwater Studies
By J.D. McNeilMeasurement of the electrical resistivity of the earth has been
used for groundwater exploration for many years (Zohdy et al.,
1974). Earth resistivity measurements were made using an array of
grounded electrodes (Wenner, Schlumberger, dipole-dipole, etc.)
with which to inject current into the ground and to measure the
resulting potential difference. However, use of grounded
electrodes can provide problems in areas of high surface
resistivity, where obtaining useful levels of current flow can be
difficult. Considerable effort is usually required to locate and
move the array so that resistivity surveys tend to be expensive.
More generally, the intercoil spacing used in many electromagnetic
(EM) methods is shorter than the array length used for conventional
resistivity surveys, resulting in improved spatial resolution when
used in the profiling mode.
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Tutorial -- Engineering And Environmental Applications Of Microgravimetry
More LessThis tutorial covers the engineering and environmental
applications of gravity surveying. Applications of this type
require a high precision, high accuracy, and high resolution
gravity measurement program. The term "microgravimetry" is applied
to this type gravity surveying. The material which follows
provides information on the gravity method, field procedures, data
processing and interpretation, case histories, and copies of most
of the visuals used in the tutorial presentation, and is grouped in
three sections.
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Seismic Shear Wave Anisotropy Used To Locate F’Racture Zones In Limestone
Authors Benjamin H. Richard, Stephanie A. Clasen and Paul J. WolfePrimary and shear wave studies were conducted over fractured and solutioned limestone
to determine the value of using these waves to find the orientation of fractures and solution
cavities. Data were analyzed to ascertain the directional velocity and attenuation anisotropy.
Transmission data were gathered in a N-S, E-W, N45”E and N45”W direction and analyzed for
velocity and attenuation differences. These data were then compared to a previous reflection
test across two oil-producing wells. This previous test located two nearly vertical faults trending
N6O”W. The faults were the producing zones. The study suggested that this fault trend should
be reflected as a fracture trend in the near surface limestone. If so, shear waves should
determine the orientation by velocity and attenuation anisotropy.
The primary wave transmission showed little velocity and attenuation anisotropy. On the
other hand, the shear waves did have significantly different velocities and attenuation. The line
oriented N45”E had the lowest bedrock velocity while the line oriented N45”W had the highest.
The N-S and E-W lines had intermediate velocities which support fracture paths in a N6O”W
direction. The velocities should be lowest perpendicular to the fractures and highest parallel to
the fractures. The highest rate of amplitude attenuation was in the N45”E and the N-S direction
whereas the N45 “W and the E-W direction had the lowest rate of attenuation. These data
demonstrate that shear waves transmitted across fractures will have lower velocity and a higher
rate of attenuation. As the transmission direction becomes parallel to the fractures the velocity
increases and the attenuation reduces. Therefore, shear wave studies can greatly improve the
delineation of faults, fractures and solutional zones.
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Application Of Geotomography At The Monks Hollow Damsite, Utah
By Phil SirlesAn integrated seismic investigation at the Monks hollow damsite demonstrated the
importance of concomitant field engineering studies and wide-aperture geophysical testing
to assess, define, and quantify the lateral and vertical extent of variable rock strength
present in an abutment for a proposed arch dam. Seismic common offset, refraction and
tomography surveys were conducted within the right abutment adit and on the ground
surface. The surveys defined the lateral extent of low moduli rock, associated with
jointing, faulting, and poor cementation; and also identified the presence of high moduli
material, indicative of well cemented and unfractured rock.
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Subsurface Investigation For A Housing Complex In Norton, Virgin-Ia Using Seismic Refraction Methods
More LessMuch of the level land which is suitable for building on in the coal
fields of Virginia is the result of previous strip or surface mining activities.
In this process, the overburden on a hill or mountain is removed until the
desired coal seam is reached and mined. The end result of this is a
relatively level surface and an abundance of waste-rock. It is a common
practice to stockpile the waste material and backfill the area with it at the
completion of mining. The objective of this investigation was to determine
the depth and extent of the backfile underneath a proposed housing site in
Norton, Virginia.
In order to determine the depth to bedrock, a seismic refraction
survey was conducted. Also, numerous test pits were excavated to rock
throughout the site to verify the results of the survey. The results of the
seismic survey and the depths to bedrock encountered in the test pits
correlated extremely well in areas that had been stripped during mining.
Areas near the edge of the site which still contained natural soil were not
as well correlated since the stiff natural clay had a similar compressional
wave velocity to that of the extremely weathered bedrock. Additionally,
the test pits revealed an apparent rock ledge running parallel to the site
which the geophysical survey did not clearly show. It was concluded that
seismic refraction surveys could be successfully used at similar sites where
the depth of fill to bedrock was required.
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Integrated Seismic Site Characterization Of The Paducah, Kentucky, Area: Preliminary Results
Authors J.B. Harris and R.L. StreetIt is generally accepted that local site geology can play a significant role in the
spectral content, amplitudes, and duration of ground motions that are observed as a
result of an earthquake. Situated along the lower Ohio river valley are several
communities underlain, in part, by poorly consolidated, water saturated sediments. These
communities are particularly vulnerable to earthquake damage. One of the largest and
most vulnerable cities in the region is Paducah, Kentucky, which is located near the
northern extent of the New Madrid seismic zone.
Preliminary seismic studies have been performed at several sites in the Paducah area
with the intent of determining: 1) the shear wave velocity structure of the thick, poorly
consolidated overburden; and 2) depths to bedrock. Conventional P- and SH-wave
refraction analysis has given near surface velocities and common-offset reflection profiles
have identified shallow subsurface structure. Modified expanding spreads have produced
P- and SH-wave top-of-bedrock reflections from depths exceeding 120 m and stacking
velocities to major reflectors have been calculated. Downhole seismic data has been
used to constrain the velocity models developed using surface seismic methods.
From the shear wave velocity and overburden thickness data, site response modeling
was performed to determine expected ground motions as a result of an earthquake.
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Transverse Isotropy In Seuimentary Sequences
More LessOver the last ten years geophysicists have begun to
recognize the frequent occurrence and importance of non-isotropic
earth sections. In particular, many sedimentary sequences
exhibit a transverse isotropy in properties typified by a
horizontal preferential oriented fabric which can have a profound
effect on the strength of a material and fluid migration through it.
A field technique to measure the transverse isotropy of a
sedimentary sequence utilizing 3-component seismic shear and
compressional wave records from boreholes is described. A
maximum shear wave anisotropy of 50% was recorded for a clay
sequence with up to 20% compressional wave velocity anisotropy.
Laboratory studies including ultrasonic measurements and scanning
electron microscope imaging are compared to the field data. The
results are explained using a model based on the structural
geologists card deck. The ground section at this site can be
adequately described as transverse isotropic in velocities with
implications to the 2D dynamic moduli characterization.
As the engineering industry progresses towards a more
comprehensive range of site testing for marginal land
construction and with the growing concerns over hazardous waste
migration in the ground water the necessity to consider nonisotropic
ground sections is becoming more apparent. This work
represents one of the many recent attempts to address this
complex problem which could add significant information
especially in conjunction with other survey techniques, such as
borehole seismic tomography.
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Application Of Full Waveform Borehole Sonic Logs To Environmental And Subsurface Engineering Investigations
Authors R.E. Crowder, J.J. LoCoco and E.N. YearsleyRecent experience with small diameter borehole sonic probes and digital full waveform
acquisition and processing capabilities has demonstrated the practical value of full waveform
sonic (FWS) logs in environmental and engineering investigations. Applications of FWS include
evaluation of lithology and permeability, delineation of geologic contacts, fracture analysis,
and calculation of engineering properties.
Four field examples presented in this paper document these applications. The first example
illustrates the characterization of lithology in a 4” diameter borehole drilled in Tertiary fluvial
sediments and presents a permeability profile derived from the tube wave portion of the full
waveform that shows the qualitative permeability variation between the sandstones,
siltstones, and claystones. The second example highlights the full waveform display,
commonly referred to as the variable density log (or VDL), as a means to identify geologic
contacts. The third example compares tube wave derived permeability with field tests and
core data from a 4” borehole in fractured welded tuff. The last example examines the
engineering analysis from a 3” corehole in fractured granite that includes the calculation of
engineering properties (Young’s modulus, shear modulus, and Poisson’s ratio) from
compression and shear velocities.
The analyses and interpretations illustrated by these field examples are made possible by
the recent development of digital FWS acquisition software for small diameter sonic probes
and advanced interactive FWS post processing. The post processing includes options to
examine individual waveforms, interactively repick travel time, and calculate the amplitude of
any portion of the waveform. Also, the technique of plotting vertically stacked waveforms as
a variable density log (VDL) is utilized here to enhance and contribute to the interpretation
power of FWS logs.
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Geophysical Surveys For Mapping Migration Of Brines From Evaporation Pits And Ponds
Authors Raye M. Lahti and Pieter HoekstraPractices of disposal of liquid oil field wastes are now
regulated to prevent contamination of soils and ground water.
Current disposal often is by means of deep injection wells.
However, a legacy of poorly designed disposal sites of the past
require evaluation of the extent and pathways of contaminant
migration, so that remediation efforts to contain further
migration can be designed, and the effectiveness of removal of
contamination from soil and ground water can be monitored.
Surface geophysical surveys were undertaken on three oil
field brine disposal sites on lands in southwest Texas. The
objectives of the surveys were to determine the extent of brine
migration emanating from pits used for disposal of oil field
brines. These pits were devised to evaporate water leaving a
solid (salt) residue. This process, however, also resulted in
seepage of liquid brine into the subsurface.
The technical approach utilized is a combination of
frequency domain electromagnetic profiling with the Geonics EM-
34 to quickly determine lateral extent of brine migration, and
time domain EM soundings with the Geonics EM-47 to determine the
vertical distribution of brine.
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Shallow High-Resolution Seismic Reflection Program To Characterize Hydrogeology
Authors Larry Irons, Brent Lewis and Michael McGuireA shallow high-resolution (HR) seismic reflection program was conducted at the Department
of Energy’s Rocky Plats Plant near Denver, Colorado. The program was initiated to
characterize the hydrogeology of the site. The variability of the geology over relatively small
distances prompted the use of this method to enhance the borehole investigation program.
The HR seismic program consisted of three phases:
o seismic reflection modeling
o field tests consisting of approximately 375 records (1,500 feet)
o production phase consisting of 3,000 records (12,000 feet)
All data were collected using the common depth point (CDP) method. The seismic data were
processed using a rigorous set of algorithms. The results of each processing step were
carefully reviewed before proceeding to the next step. Interpretation of the seismic profiles
involved the integration of borehole lithologic and borehole geophysical data. Synthetic
seismograms were generated and seismic events were correlated to known, identifiable
geologic beds. The data from a vertical seismic profile were also used in the interpretation.
Seismic events corresponding to channel &posits were identified and mapped across a grid
The channel deposits consist of fluvial sandstones, siltstones, and claystones deposited in a
very low energy environment. These channel deposits provide a greater potential for the
migration of contaminants than the hosting claystone.
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Seismic Transmission Tomography
By J. WongSeismic transmission tomography is a high-resolution method for characterizing
subterranean geology. By recording many seismograms with sources and
detectors located on the boundary of a rock mass (for example, along co-planar
boreholes, or on two levels of a mine, or around a rock pillar), information
is obtained which enables one to construct a cross-sectional map of seismic and
mechanical properties, In the most common form of seismic tomography, the acoustic
velocity (i.e., the speed of sound) is the physical property which is mapped.
One begins by measuring the time it takes for a seismic pulse to travel along a
path through the rock section under study. This measurement is repeated for
hundreds or even thousands of paths with as many different directions through
the rock section as possible, The travel times, the path lengths, and the source
and detector locations on the boundary of the section are then used to create a
detailed two-dimensional image of the acoustic velocity within the rock section.
Image creation is done using computer-assisted tomographic (CAT) techniques
similar to those first developed for radiological imaging in diagnostic medicine.
The final tomographic image is best displayed as a coloured-coded map on which
different acoustic velocities are represented by different colours.
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Seismic Reflection/Refraction Reconnaissance Survey To Measure Pleistocene Sand Aquifer Thickness Along The Lower Wisconsin River Valley
Authors William A. Sauck and John W. AttigIn response to a request for a rapid, inexpensive assessment of the depth to pre-
Pleistocene rock along an approximately 160 km (100 mile) length of the Lower Wisconsin
River valley, a field program was devised to make six transects at approximately 32 km (20
mile) intervals with the hammer seismic technique. The work was done in 3 weeks of field
time and with a similar amount of time for analysis and interpretation. Each transect
consisted of a number of 24-channel spreads spaced approximately 0.8 to 1.6 km (0.5 to 1
mile) from each other, as budgetary and time conditions precluded continuous profiling.
Each spread was a composite of two 12-channel spreads using an older signal-enhancement
seismograph and was shot from both ends and from the center. A 6 m (19.6 ft) geophone
interval was used, which did not always permit refracted first arrivals to be seen at far
geophones, but showed clearly the prominent bedrock reflector even at the maximum offset
of 141 m (462 ft). The good signal quality and success of the survey were attributed to good
coupling due to the presence of a surface loess or compact soil layer; sufficient rainfall to
maintain the surface layer moisture; the homogeneous nature of the underlying san section;
and the level sites free of static effects. Refraction analysis gave depth to water table and
the sonic velocities of the upper two layers. Reflection analysis of the analog records was
done simply by the T2-X2 method and resulted in average velocities and depths to the first
prominent reflector. Depths were generally 130 - 200 ft (40 - 61 m) across the entire valley,
although one transect did confirm a shallow bedrock terrace extending to one-third of the
width of the modern valley. These results are being used to guide further hydrogeological
studies and to assist in groundwater flow modeling. This study shows that for certain very
special conditions, such shallow noncontinuous seismic profiling can be an extremely rapid
and cost effective method of orienting further hydrogeologic investigations.
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An Electrical Technique For The Determination Of Groundwater Flow Parameters
Authors Keith J. Sjostrom and Dr. William R. SillHydrogeologic investigations to determine the flow direction and velocity of
groundwater movement are crucial to the development and assessment of groundwater
resources. Information from monitoring wells, tracer tests, and flow meters are often used
to determine the direction and rate of groundwater flow. Surface and borehole-to-surface
electrical measurements can also be used to determine this information using conductive
plumes within the aquifer. The direction of the detected plume with respect to the source
indicates the direction of groundwater flow and the change in length with respect to time
provides information about the flow velocity.
Initial methods of determining groundwater flow paramters were through the
application of surface geophysical techniques. Glaccum et al. (1982) and White (1988)
discuss experiments in which a conductive plume was introduced into an aquifer and
mapped by surface electrical methods. The results of these tests were used to demonstrate
the methodology in determining flow directions and estimates of groundwater velocity.
However, one problem associated with using surface geophysical techniques is that if the
conductive plume is located too deep below the surface, surface-only methods become
relatively insensitive to variations in potential (Wilt et al., 1983). Another factor is the
inhomogeneity of the near-surface material or presence of cultural features may produce
strong variations on the measured potentials such that the effects may mask electrical
signals from deeper conductive anomalies of interest such as an induced plume.
By placing one or more electrodes beneath the near-surface layer or in contact
with a conductive region improves the detection sensitivity of the electrical survey. Thus
the borehole-to-surface electrical technique has an advantage over surface-only methods
in that it is possible to resolve conductors which are deep in the subsurface or whose
contrast is so small that they cannot be detected by a surface survey. Wilt and Tsang
(1985) further demonstrated that a subsurface current electrode in contact with a conductive
medium can improve measurement sensitivity by an order of magnitude over surface
measurements alone.
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Conjunctive Use Of Geophysical And Geological Data In The Construction Of A Broundwater-Flow Model
More LessGeophysical survey information combined with test-hole logs
were used as the data base for the construction of a numerical
model to simulate groundwater flow and the impact of pumping on
part of the lower Platte River alluvial aquifer located in
eastern Nebraska. The hydrogeologic framework of the study area
was determined from the interpretation of seismic-refraction
data, shallow seismic-reflection profiles, and vertical
electrical soundings. Seismic-survey results calibrated to testhole
logs provided the basis to map the areal extent of the
complex bedrock surface beneath the study area. Results of the
geoelectric survey were used to map the area1 extent and
thickness of a near-surface, low-permeability clay unit along the
western margin of the Platte Valley floodplain. Geophysical data
were also used to estimate the bulk density and porosity of the
aquifer material. Utilizing all of the field data available, a
three-dimensional groundwater-flow model was designed to simulate
a two-layer system in which part of the aquifer is under confined
flow conditions. The model was then applied to the study area to
evaluate the impact of seasonal pumping schedules of a well field
that provides municipal water to the City of Lincoln.
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Tri-Potential Resistivity Applications At A Leaky Underground Storage Tank Site
Authors Albert E. Ogden, James T. Taylor and M.O. SmithThe tri-potential method of earth resistivity was used to locate fractures and caves in a karstic
limestone terrane where kerosene had leaked from the lines of an underground storage tank. For air-filled
fractures, the apparent resistivity values increased for the CPPC and CPCP configurations of the standard
Wenner Array, but decreasedin the CCPP configuration. As the resistivity traverse crosseda known airfilled
cave, all configurations of the array increased. If the kerosene migrates, its detection will be
insured by placing future monitoring wells on the fractures where most of the ground water movement
occurs.
Data from four initial monitoring wells showed that the bedrock-soil contract slopes in a
southwest direction, similar to surface topography. A dye trace of surface drainage showed water from
the sinking stream off-site emerged at a spring in just six hours. The static water level measurements
from the wells indicate a northeast direction of ground water flow, opposite to surface drainage. Depth
to water averages 30 ft. below land surface, but fluctuates over 15 ft. between wet and dry seasons.
Contaminated soil and the tanks were removed. The soil was placed in lined and bermed pits and
treated by passive volatilization and enhanced biodegradation. Soil borings were made by hollow stem
augering with split spoon sampling for TPHC to define the extent of the plume. Migration of the
kerosene was primarily along gravel-fill beds along the water line around the building. Levels of TPHC
in the ground water are relatively low as are hydraulic conductivity values of the soil. Therefore, no
threat to human health and the environment is anticipated.
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An Electromagnetic Volumetric Analysis Of Pitch Ponds
By Mark TuckerThis case history describes a unique geophysical engineering project that
utilised remote sensing Electromagnetic (EM) equipment to determine an
approximate volume for some of the world’s largest Pitch Ponds, located in the
Middle East. Several traces were made across these man-made ponds at critical
points, to assess the geological parameters that would give a volumetric
analysis.
The paper discusses the physical difficulties experienced and the preferred
solutions amongst the numerous alternatives. Although reasoning for the analysis
is offered, comprehensive details are omitted to preserve client confidentiality.
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Interactive Image Analysis Of Borehole Televiewer Data
Authors Colleen A. Barton, Lawrence G. Tesler and Mark D. ZobackThis chapter describes an interactive graphics system designed for borehole
televiewer (BHTV) image analysis. The software provides one of the fust comprehensive
tools for borehole image data analysis available to exploration and research scientists on a
low cost and easy-to-use personal computer, the Apple Macintosh II.
The program, called BHTVImage, provides an integrated environment for analyzing
borehole shape and features. Images of BHTV data are displayed in false color on a
graphics screen and are manipulated with a mouse pointing device. A variety of two and
three-dimensional displays of borehole radius and acoustic reflectivity are used to display
the data. Tens of meters of borehole wall can be rapidly viewed by scrolling through the
data within a graphics window. Interactive measuring tools are provided to quickly
measure and record wellbore features. The values of the scales, units, and grid intervals
can be modified interactively during program execution. Gross scale features can be easily
extracted from the images through interactive thresholding to produce a scaled comparison
with complementary log data such as resistivity or sonic recordings. Alternatively, fulI
resolution images can be analyzed to investigate the fme details of fractures or crossbedding
intersecting the well.
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Geophysical Surveys For Underground Storage Tank Locations Hickam Air Force Base, Hawaii: A Case Study
Authors H.D. MacLean, J.W. Dickerson, J.C. Hasbrouck and E.T. CotterChem-Nuclear Geotech, Inc. (Geotech), personnel performed geophysical surveys
at Hickam Air Force Base (HAFB), Hawaii, to determine the presence or
absence of underground storage tanks. Geophysical survey techniques included
magnetic total field, magnetic gradient, and horizontal-loop electromagnetic
methods. This case study presents validated data from five sites.
Twenty-three sites at Hickam Air Force Base were designated for investigation
following interviews with HAFB personnel and examination of base records and
historical land use. Geophysical investigations were conducted at 10 sites to
confirm the removal of tanks and at 3 sites to establish the location of tanks
presumed abandoned in place. Tests were conducted at an additional 10 sites,
for which no historical tank disposition records were available, to determine
whether tanks were present.
Magnetic and electromagnetic (EM) instruments respond characteristically
to steel underground storage tanks because the tanks are ferromagnetic
and good electrical conductors. Determinations were made of an equivalent
magnetic susceptibility and a conductor EM-response parameter range
characteristic of steel storage tanks. The presence of other metal objects
within the surveyed areas affected the instrument readings, but these objects
usually produced significantly different response parameters.
In six areas, our interpretation of the geophysical data confirmed the
presence of 14 tanks and the probability of an additional 5 tanks.
Interpretation of the data indicated that no tanks were present at 16 of the
investigated areas. Survey results from one area were indeterminant because
of the limited scope of the measurements.
Exploratory excavations and augering were conducted at 9 of the 23 sites to
provide ground-truth for the geophysical interpretation of the collected
survey data. Conclusions derived from the geophysical data were validated at
eight of the nine sites.
This case study presents data from five of the investigated sites.
Underground storage tanks were uncovered at three of the sites, and the
absence of storage tanks was verified at one site. The fifth site illustrates
the need for an integrated, multiple-method approach for proper geophysical
investigations.
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Geophysical Methods Used To Investigate A &Jdioac!I’Ivb Waste Burial Site
Authors Jo-Ann Sherwin and Alan WittenA suspected radioactive waste burial site was investigated using geophysical
surveys, performed in sequence to learn about the site without intruding into it. The
site is described as active in the 1950s and the location where low-level
radioactive materials such as cathode ray tubes, scopes and radium dials were
disposed of in a 15-foot deep trench approximately forty feet in length. The waste
materials were covered with general refuse and local soil material. Examination of 1988 aerial photographs detected a 4-acre scar of disturbed earth that may be the location of the burial site.
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Cavern Detection In The Presence Of A High-Contrast Surface Layer
More LessVarious geophysical methods for cavern detection were tested over a known shallow
cavern (Cave of the Mounds in S. Central WI) where the geologic section was particularly
problematic for geophysical work. The physical properties contrasts of the weathered
shale/shale, and massive dolomite section were so great that the conventional use of several
geophysical methods failed. Seismic refraction, however, was useful for defining the shale
thickness and also showed that the shale-dolomite contact was irregular, with several meters
of local relief. A detailed gravity survey showed numerous small features which were
apparently due to the variability in the thickness of the low-density surface layer. No seismic
Head waves passed through the cave level because the cave is located well within the thick
dolomite unit; hence arrival time delays were not significant. Shallow reflection records
were overwhelmed by reverberations due to the 1:10 velocity contrast. GPR was unable to
reach the dolomite because of the 20 Ohm-m surface resistivity. Slant resistivity soundings
using the Bristow technique (pole-dipole array) was the only technique which showed the
presence of the 30 foot deep cave, but even this required drastic modification of the
reduction and plotting methods to accommodate the 1:40 contrast in layer resistivities. This
example illustrates how crucial it is to obtain physical properties information early, to assist
in the choice of the appropriate geophysical techniques or their modifications.
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Environmental Geophysics - Breaking The Cost Barrier
Authors James R. Rossiter and J. Les DavisThe number of environmental site assessmentsc onducted in North America has grown rapidly
over the past decade. This growth is expected to continue, driven by tighter environmental
regulations in almost all jurisdictions and concerns about environmental liability. However, the
use of non-invasive, geophysical techniques for environmental work has not kept pace with this
growth generally. We find this relative reluctance to use geophysics for environmental
investigation curious, particularly since it would appear that there is a need for the kind of rapid,
non-invasive information that geophysics has been providing steadily to the explorationist for
several decades (Barinaga, 1990; Won, 1990).
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Electromagnetic-Terrain-Conductive And Ground-Penetrating-Radar Investigation At And Near The Ciba-Geigy Superfund Site, Ocean County, New Jersey: Quality Controland Quality Assurance Plan And Results
Authors Gary J. Barton and Tamara IvahnenkoGround water is the principal source of drinking water in the
vicinity of the Ciba-Geigy Superfund site near Toms River, Ocean County,
New Jersey. The presence of earlier identified point sources of
organic-compound and, to a lesser extent, metals contamination dt the
Ciba-Geigy Toms River Chemical Company Plant has resulted-in severe
degradation of ground-water quality and has increased the potentiil for
water-supply problems (NUS Corporation, 1988). The point sources of
contamination include a manufacturing area, a backfilled-lagoons area, a
former fire-prevention training area, several sludge-disposal areas, and
a drum-disposal area. A borrow area also is considered a potential
source of contamination (Camp Dresser 6 McKee, Inc., 1989).
The U.S. Environmental Protection Agency requested that the U.S.
Geological Survey evaluate the hydrogeologic conditions in the Kirkwood-
Cohansey aquifer system (Zapecza, 1989) and the extent of ground-water
contamination on the property of the plant (which includes the Superfund
site) and in Winding River Park, which borders the Toms River
immediately to the east of the Superfund site (Barton, 1989). This
investigation included an electromagnetic-induction survey covering
45 line miles throughout the site and a ground-penetrating-radar survey
in part of the borrow area.
The quality assurance/quality control plan (QA/QC) for the
electromagnetic-induction survey established guidelines for performance,
system audits, and data validation, and set control limits for
instrument and procedural precision. The QA/QC plan for the groundpenetrating-
radar survey sets guidelines for performance and system
audits.
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Low And High Frequency Electromagnetics In Landfill Investigations
More LessLow frequency induced electromagnetic (EM) fields (< 1 megahertz (MHz)) primarily respond to
the earth’s conductivity, whereas high frequency EM fields (propagated as in ground penetrating
radar (GPR)) are a complex function of the earth’s dielectric permittivity, conductivity and
magnetic permeability.
Low frequency (c 40 kilohertz &Hz)) EM data can be utilized to approximate a shallow (< 6
meters (m)) multi-layered earth model provided different instruments and intercoil spacings are
used. The constructed model can be used as a starting model for higher frequency EM
techniques such as GPR, which operate at radio frequencies and provide increased resolution of
the geologic system.
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The Determination Of Physical Properties Of Groundwater Aquifers Using Shallow Seismic Reflection Methods
Authors P.T. Miller, J.A. Madsen and S. McGearyA land-based shallow seismic reflection data set was
acquired along the barrier island complex of coastal New Jersey
for the purpose of understanding the physical properties and two
dimensional organization of four Tertiary hydrogeologic units:
the Kirkwood-Cohansey Aquifer System, the Rio Grande Water
Bearing Zone, the Atlantic City 800 Foot Sand, and the Piney
Point Aquifer. A series of 6-fold common depth point seismic
sections covering 5.0 km. was obtained. Data were acquired on
sandy beach areas, oceanward of the central island axis, to
depths of 600 feet. A detailed velocity analysis (5 m intervals)
revealed subtle vertical and lateral velocity variations
(detectable to 10 m/s) within the sediments. Dominant reflector
frequencies of ,200-250 Hz were observed leading to a theoretical
vertical resolution of 1.0 to 2.0 m. The seismic results were
combined with local well log information to make important
inferences regarding porosity, permeability, and aquifer
geometry. A synthetic seismogram generated from the Island Beach
State Park Test Well No.1 (sonic log) correlates well with field
sections, enabling a more precise definition of bed boundaries
and aquifer wavelet responses.
A linear multiple regression model was developed which
relates acoustic traveltime to relative gamma ray and relative
resistivity readings at the test well. The relation was then
applied to nearby wells possessing gamma ray and resistivity
logs, yet devoid of acoustic information. Pseudo-synthetic
seismograms were generated based upon the model and relative
gamma ray and relative resistivity readings from the new wells.
Comparison of the pseudo-synthetic seismograms with field seismic
sections shows good agreement, thereby enabling estimation of
lithology, velocity, and porosity. A characteristic seismic
signature for the aquifers of interest has been constructed from
the above information.
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Shallow Seismic Reflection Applied To Quaternary Paleochannels
Authors R.B. Genau, J.A. Madsen and S. McGearyA high resolution seismic reflection survey was conducted
at Taylors Island, Maryland with the objective of identifing buried
paleochannels of the Susguehanna River. One 2.5 km six-fold CDP
reflection profile was acquired using a portable Bison 8012A
seismigraph, a 10 lb. sledge hammer/aluminum plate source, and 12
single accelerometers as receivers. Reflections from interfaces
to depths of 100 m were desired in order to observe the
relationship between incised Quaternary paleochannels and the
underlying Tertiary marine sediments. The data were collected
using an end-on spread configuration with a 25 m source to first
receiver offset and a 5 m receiver spacing. Profiles obtained from
processing of the data clearly indicate the presence of one or more
paleochannels in the subsurface that were cut during a low stand
of sea level. Channel margins are apparent due to the undulating
nature of the unconformable reflecting interface that is observed
to truncate older concordant sub-horizontal reflectors below.
Channel fill reflectors representing sediments deposited during a
subsequent high stand of sea level are generally horizontal and
concordant with underlying seismic reflectors except where they
onlap at the channel margins. The shallowest fill reflector
detected is at 10 m depth, whereas a channel depth of 53 m is
calculated assuming a velocity of 1420 m/s. A spectral analysis
of selected traces indicate a dominant frequency of approximately
125 Hz, with frequencies of up to 200 Hz not uncommon. The
theoretical vertical resolution of 1.8 m is possible using the l/4
wavelength criterion.
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A Shallow Seismic Reflection Study Of The Water Table
Authors P.T. Miller and R.B. GenauA shallow seismic reflection survey was conducted
approximately one mile southeast of Newark, Delaware to determine
the depth and geometry of the water table. The site is located
0.25 miles from a landfill where this method may be used in a
pollution remediation effort. Of particular interest in this
study are the optimum in-field seismograph recording parameters
and geophone array spacings, the adequacy of a hammer/aluminum
plate seismic source in this environment, the vertical and
horizontal resolution of the water table reflector given an
unsaturated overburden, the seismic facies characteristics of the
water table reflector (reflector frequency, amplitude, coherency,
interval velocity, and configuration), and the three dimensional
geometry of the water table surface. A series of walkaway noise
tests were performed to (a) determine the feasibility of
accurately recording a reflection from the water table, (b)
define the ,low cut filter which maximizes the water table
reflection signal while simultaneously suppressing ground roll
effects, (c) to ascertain the nature of all coherent and
incoherent seismic noise (ground roll, ground coupled air wave,
multiple reflections, wind, and electrical), Cd) to observe
possible phase and amplitude distortions in the water table
reflector as predicted by the Zoeppritz equations, and (e) to
compare calculated reflection and refraction depth and velocity results.
All data were recorded with severe, low cut pre-emphasis
filters (350 Hz) to attenuate the high amplitude, low frequency
ground roll component which dominated the early portions of the
record. Reflection signals from the water table interface
exhibited dominant frequencies of loo-150 Hz and stacking
velocities of 1420 feet per second, leading to a theoretical
vertical resolution of 2.4 feet (based on l/4 wavelength
criterion). Reflection depths and velocities were 34.8 feet and
1420 feet per second as compared to refraction depths and
velocities of 35.5 feet and 1434 feet per second.
Two parallel six-fold common depth point seismic sections
and 5 common offset seismic sections comprised a three
dimensional seismic grid from which the geometry of the water
table was determined. The CDP seismic sections exhibited a
coherent, high amplitude reflection at 50 ms. Using a stacking
velocity of 1420 feet per second, a reflector depth of 35 feet
was calculated. Later drilling confirmed the water table depth at
35 feet.
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