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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
1 - 20 of 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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