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10th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 23 Mar 1997 - 26 Mar 1997
- Location: Reno, Nevada, USA
- Published: 23 March 1997
1 - 50 of 106 results
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History Of Geophysical Studies At The Waste Isolation Pilot Plant (Wipp), Southeastern New Mexico*
More LessA variety of geophysical methods including the spectrum of seismic, electrical, electromagnetic and potential field
techniques have used support characterization, monitoring and experimental studies at the Waste Isolation Pilot
Plant (WIPP). The geophysical studies have provided significant understanding of the nature of site deformation,
tectonics and stability. Geophysical methods have delineated possible brine reservoirs beneath the underground
facility and have defined the disturbed rock zone that forms around underground excavations. The role of
geophysics in the WIPP project has evolved with the project. The early uses were for site characterization to satisfy
site selection criteria or factors. As the regulatory framework for WIPP grew since 1980, the geophysics program
was focused on support of experimental and field programs such as Salado hydrogeology and underground room
systems and excavations. In summary, the major types of issues that geophysical studies addressed for WIPP are:
l Issue I: Site Characterization
l Issue 2: Castile Brine Reservoirs
l Issue 3: Rustler /Dewey Lake Hydrogeology
l Issue 4: Salado Hydrogeology
l Issue 5: Excavation Effects
The nature of geophysics program for WIPP has been to support investigation rather than being the principal
investigation itself The geophysics program has been used to define conceptual models (e.g., the Disturbed Rock
Zone-DRZ) or to test conceptual models (e.g., high transmissivity zones in the Rustler Formation). An effect of
being a support program is that as new project priorities arose the funding for the geophysics program was limited
and withdrawn. An outcome is that much of the geophysics survey information resides in contractor reports since
final interpretation reports were not funded.
*This work was supported by the United States Department of Energy under contract DE-AC04-94AL85000
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United Sates Department of Energy
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Electrical Resistivity Monitoring Of The Thermomechanical Heater Test In Yucca Mountain
Authors A. Ramirez, W. Daily, M. Buettner and D. LaBrecqueA test is being conducted in the densely welded Jopopah Springs tuff
within Yucca Mountain, Nevada to study the thermomechanical and
hydrological behavior of this horizon when it is headed. A single 4 kW heater,
placed in a horizontal borehole, was turned on August, 1996 and will continue
to heat the rockmass until April 1997. Of the several thermal, mechanical and
hydrological measurements being used to monitor the rockmass response,
electrical resistance tomography (ERT) is being used to monitor the movement
of liquid water with a special interest in the movement of condensate out of the
system. Four boreholes, containing a total of 30 ERT electrodes, were drilled to
form the sides of a 30 foot square with the heater at the center and
perpendicular to the plane. Images of resistivity change were calculated using
data collected before and during the heating episode. The changes recovered
show a region of decreasing resistivity approximately centered around the
heater. The size this region grows with time and the resistivity decreases
become stronger. The changes in resistivity are caused by both temperature
and saturation changes. The observed resistivity changes suggest that the rock
adjacent to the heater dries as heating progresses. This dry region is
surrounded by a region of increased saturation where steam recondenses and
imbibes into the rock.
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Test Results Of Cpt-Deployed Vertical Electrode Arrays At The Doe Hanford Site
Authors S.M. Narbutovskih, W. Daily, A.L. Ramirez and R.M. MoreyField studies were conducted at the DOE Hanford Site to test cone penetrometer installation of
vertical electrode arrays (VEA) for use with Electrical Resistivity Tomography (ERT). Most
VEA installation methods in current use are not economic for environmental applications. The
cone penetrometer technology (CPT) can provide an economic and relatively non-intrusive
installation method. However, a VEA with deployable and properly functioning electrodes was
required. Results of the design, installation and testing of CPT VEAs are reported in this paper.
Several designs were developed and bench tested for use with the CPT. After initial field
installation studies, one design was chosen for further testing at the DOE Hanford Site. Four
VEAs were each pushed to 100 feet in 4 days. To test the CPT VEAs, an infiltration experiment
was conducted with cross VEA tomographic data collected for three vertical planes. These data
were processed using the electrical resistivity tomography code developed by Lawrence
I,ivermore National Laboratory (LLNL). Tomographic images for each vertical plane tracked
the subsurface resistivity changes associated with the migrating fluid.
It is concluded from these test results that the CPT is a viable method for installing VEAs. The
VEAs were rapidly and economically installed to the maximum depth required, data of adequate
quality were obtained and tomographic images from the infiltration experiment verified that the
CPT VEAs provide viable ERT data.
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Focusing Transformations - The Reliable Way To Search For Safe Location For Nuclear Waste Disposal
Authors V.A. Pozdniakov, V.A. Tcheverda and D.V. SafonovAt present pre-stack migration is widely used in seismic data processing in order to get wave image of
a medium under investigation. Of course it is rather time-consuming procedure but its application is
justified by necessity to deal with complicated geological structures and to recover them as completely
as possible. The paper is devoted to the approach to pre-stack migration of multicoverage seismic data
by means of diffraction and focusing transformation in application to the problem of careful geological
investigation of consolidated rocky blocks in order to choose as safety as possible location of nuclear waste
deposit. Results of some real seismic data processing gathered at Nijnekanskaja area near Krasnoyarsk
city, where a granite block is searching for as a suitable candidate for disposal of radioactive waste, are
presented and discussed.
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Locating Buried Trenches Using An Integrated Geophysical Study At The Parks Shallow Land Disposal Area
Authors Robert J. Withers, John E. Scaife and Steffan HelbigAt the Parks site, Western Pennsylvania, a geophysical survey was undertaken to
determine the size and location of buried trenches containing radiological waste. Several
geophysical techniques were used to non-intrusively locate the trench boundaries. The
trenches outlines could be mapped from surface conductivity profiles. Magnetometer
surveys identified much of the ferrous material buried within the trenches, and further
defined the location of the trenches.
Several vintages of ground penetrating radar were used, with the need for low frequency
antenna demonstrated by testing. A 50 MHz GPR antenna was elected for a series of
profiles across the trenches. By integrating interpretations from the total magnetic field,
magnetic gradients, conductivity data and radar profiles it was possible to provide an
accurate map of the trenches verified by subsequent drilling. Seismic and microgravity
techniques were not able to verify the location of mine workings beneath the area of
investigation.
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Practical Geophysical Applications For Everyday Operational And Engineering Problems At Newmont Gold Company
Authors Kevin Freeman, Jim Wright and Nigel PhillipsNewmont Exploration Limited’s Carlin based geophysics team primarily concerns itself with
gold exploration along the 101 Moz Carlin Trend. While most of this activity takes place off the
mine sites, occasions arise where the team is called upon or recognizes an opportunity to perform
tasks for mine operations and engineering.
Various electrical and potential field methods have been used in different applications with
mixed results. Past efforts have included electrical potential mapping for leak detection in leach
solution collection ponds and bio-leach culture tanks; time domain EM was utilized in leach pad
solution saturation studies; and highly detailed gravity has been used in void detection in the
Rain Pit. Brief case histories of each of these are presented. Included are logistical scenarios and
methodology of select applications and their results.
Results of past geophysical efforts by NEL have proved highly successful, resulting in quick,
inexpensive, and definitive answers for mine operations and engineering problems.
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The Development And Testing Of A Geophysical Blast Movement Measurement Technique For Surface Mines
Authors Geraint W. Harris, Pierre Mousset-Jones, Jaak Daemen and Robert KarlinIn order to minimize the ore dilution induced by blast movement in surface mines it is necessary to quantify
such movement. This will make it possible to more accurately locate the grade boundaries on the surface of a
blasted rockpile prior to excavation. This paper describes the development and testing of a method to accurately
measure the direction and magnitude of sub-surface blast movement. The technique involves the use of magnetic
gradiometry, using a cesium vapor gradiometer and real-time differentially corrected, GPS system, to locate the
post-blast position of magnetically enhanced target objects, whose pre-blast locations are known, without
disturbance of the blasted rock. The objective is to design an accurate and eflicient technique which can be
integrated into a mine’s production cycle.
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Case Histories Of Electrical And Electromagnetic Geophysics For Environmental Applications At Active Mines
Authors Norman R. Carlson and Kenneth L. ZongeAlthough some aspects of the environmental service industry appear to be leveling off or
declining, as a contractor primarily serving the minerals exploration industry, we continue to see
a slow but steady increase in the use of geophysics for enviromnental and engineering problems
at active mines. We attribute this to several factors. One reason is that mine geologists and
engineers are typically more familiar with geophysics (its applications and limitations) than
many of our environmental clients, and they are more familiar with a broader range of
geophysics. Other important factors are the technological advances (both hardware and
software) that allow data acquisition in mine enviromnents that were once too noisy or
complicated. In this paper, we discuss several examples of environmental and engineering
projects at active mines, with an emphasis on how these projects differ from our “normal”
enviromnental projects.
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Tests Of Ground Penetrating Radar And Induced Polarization For Mapping Flwial Mine Tailings On The Floor Of Coeur D’Alene River, Idaho
In order to investigate sequences of mine tailings that have settled in the bed of the Coeur
d’Alene River, we improvised ways to make geophysical measurements on the river floor. To
make ground penetrating radar (GPR) profiles, we mounted borehole antennas on a skid that was
towed along the river bottom. To make induced polarization (IP) profiles, we devised a bottom
streamer from a garden hose, Pb strips, and insulated wire. Both expedients worked well. GPR
showed shallow stratigraphy, but did not directly detect the presence of contaminating metals. IP
showed a zone of high chargeability that is probably due to pockets of relatively higher metal
content. Neither method was able to define the base of the fluvial tailings section.
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Downhole Ip Measurements Ulth A Piezocone At Mine Tailings Impoundment
Authors H. Kristiansen, R.G. Campanella, M.P. Davies and C. DanielThe last decade’s environmental concerns have resulted in additions to the Cone
Penetration Test piezocone making it capable of measuring geophysical parameters and therefore
effective at identifying anomalies. This paper describes one of the most recently developed
additions, which measures the induced polarization (IP) effects.
The testing site used to assess the capability of the IP-piezocone consisted of very
conductive mine tailings impoundments. The piezocone was used to study the detailed
stratigraphy, measure hydrogeological and geotechnical parameters and therefore provided
ground truthing.
The IP measuring system is described in detail. A typical IP trace is presented and
discussed in combination with the resistivity piezocone profiles and parameters.
Factors affecting results and application of IP measured with the piezocone are discussed.
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Surface And Downhole Em Investigations At Potash Mine Sites In Saskatchewan, Canada: Case Histories
Authors G. Phillips and H. MaathuisWaste products from the potash industry in Saskatchewan are stored in surface tailings
management areas (TMA) which commonly consist of a salt tailings pile and a brine pond,
surrounded by dykes. Because the fluid level within the TMA is higher than its surrounding, and
because of density effects, surface brine migration occurs in varying degrees.
The high electrical conductivity of the brine compared to normal groundwater make geo-electrical
methods well suited to measure and monitor both lateral and vertical brine migration originating
from TMA’s. Historically, shallow lateral brine migration was measured by monitor wells or soil extract
analyses, but in recent years surface EM measurements increasingly have been used for mapping,
and for monitoring of the rate of migration and impact of remedial measures.
Vertical brine migration occurs both within the TMA as well as outside the perimeter dykes, in
areas affected by shallow lateral brine migration. In the past, detailed data on vertical brine
migration could only be obtained by means of soil extract data. Downhole EM measurements
have shown that vertical brine migration occurs typically to depths of 5 to 10 m. The transition
zone from full brine concentration to background values commonly is in the order of 2 to 3 rn,
consistent with diffusion-controlled vertical brine migration.
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The Use Of Geophysical Methods For Assessment Of The Fracture Process Due To Mining Subsidence
More LessThe space distribution and time advance of subsidence of rock overburden in exploited
areas depends on the volume and shape of the exploited area.
Three causes of subsidence are discussed in the paper; tunnels with elliptical crosssection,
upward moving elongated disturbance of rock properties, and exploited areas beyond
long-wall exploitation. In each case, seismic and geoelectric methods may be used to assess
the fracture zone shape and location or predict the appearance of fracturing.
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Seismic Mapping Of An Abandoned Lead-Zinc Mine, Joplin, Missouri
More LessIn the mining of lead and zinc in the Joplin Missouri area, the open-pit method of mining was used if the ore extended
close to the surface or following the collapse of the underground operations. A regional hospital in Joplin Missouri
owns property in this mining region that in places overlies an infilled and abandoned open-pit lead-zinc mine. Much
of the infilled pit area has been covered by asphalt and is currently used as an open parking lot (about 62,500 ff). The
southern edge of the parking lot abuts a wing of the hospital complex; the eastern edge abuts a multi-storied parking
facility.
The hospital administration is considering erecting a multi-storied annex to the hospital wing on the asphalt parking lot
site. Recent subsidence of the asphalt paving occurred in parts of the parking lot which did not correlate with existing
maps that outlined the old open-pit mine location. To obtain a more detailed structural control at bedrock level, drilling
to bedrock and the acquisition of 25 parallel high-resolution reflection seismic profiles were undertaken. On the seismic
data, the bedrock reflector can be interpreted and spatially mapped across the parking lot from one line to the next. The
interpretation of the reflection seismic data was constrained and validated by the test boreholes within the parking lot
area.
This case study is presented as an example of the utility of the seismic technique for mapping bedrock in a structurally
complex area, and in a noisy urban environment with adverse surface conditions.
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Reflection Seismic Mapping Of An Abandoned Coal Mine, Belleville, Illinois
Authors Neil Anderson, Ronald Hinds, Doug Lambert, Mike Roark, Mike Shoemaker and Jesse BakerOld mine location maps (1958 vintage) indicate that the northwestern part of an undeveloped property near the town of
Belleville, St. Clair County, Illinois, is situated above an abandoned and now water-filled, room-and-pillar type coal mine.
The central and southeast parts of the Belleville property are shown as overlying intact (non-mined) coal. The coal unit
mined at the Belleville site, the Herrin #6 is Pennsylvanian in age and about 2.5 m thick at a depth of around 40 m.
The current owners of the BelleviIle property want to construct a large building on the central and southeast parts of the site,
but have been concerned about the accuracy of the old mine location maps because of recent mine-related surface subsidence
in areas designated on the maps as not mined. To ensure that the proposed new development is located on structurally stable
ground, a grid of ten high-resolution reflection seismic lines was acquired on-site. On these reflection seismic data, minedout
areas can be visually identitied and d&rentiated Tom non-mined areas. The interpretation of the reflection seismic data
was constrained and validated by 15 test boreholes. These seismic and borehole data confirm that the central and southeast
parts of the property have not been mined extensively. Development of the Belleville site has proceeded with confidence.
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Seismic Exploration For Cretaceous Kaolin Deposits In Glaciated Terrains: Example From Nova Scotia, Canada
Authors S.E. Pullan, R.R. Stea, P.W. Finck, R.A. Bums, M. Douma and R.L. GoodThe Geological Survey of Canada and the Nova Scotia Department of Natural Resources have
recently completed a joint project which has led to a major discovery of commercially viable
deposits of kaolin in the Shubenacadie and Musquodoboit Valleys in central Nova Scotia. The aim
of the project was to delineate the three-dimensional distribution of Cretaceous and Quatemary
unconsolidated sediments in the study area using shallow seismic reflection surveys, drilling, and
borehole geophysical logging. During the three year project a total of 65 shallow seismic test sites
and 10 line-km of CDP (12-fold) shallow seismic reflection profiles were acquired, and over 20 new
boreholes were drilled within the survey area. Initial seismic test results suggested that there were
areas in the Shubenacadie basin where bedrock was at depths greater than 100 m below surface.
This was subsequently confirmed by NSDNR drilling. The test sites were used to determine the
optimum locations for followup seismic profiling in the Shubenacadie valley (1994) and the
Musquodoboit valley (1995). The seismic sections and drilling results have clearly delineated
Cretaceous sediments in basins buried beneath glacial cover and have substantially increased the
known area1 extent of buried Cretaceous basins (from ~1 km2 to >57 km* in the Shubenacadie and
Musquodoboit valleys alone). Analyses of core samples have shown that these basins may harbour
economic quantities of kaolin suitable for use in the paper industry. Presentation of preliminary
results of the work in November 1995 resulted in the immediate staking of over 80,000 acres (2,000
claims) in the Shubenacadie, Musquodoboit and Antigonish valleys.
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Borehole Vector Magnetics: The Mcconnell Deposit, Sudbury, Canada
Authors Edna L. Mueller, William A. Morris, W. Hamilton, Patrick G. Killeen and Steve BalchIn this paper we investigate the potential advantages of employing vector, or 3-component
magnetic data. Vector magnetics have been applied in previous studies in a variety of
scenarios: land-based, borehole, and marine. The present study focuses upon vector
magnetic data from a series of boreholes around the McConnell nickel deposit located
within the Sudbury Basin, Canada.
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Borehole Correlation In Mineral Deposits Using Geophysical Parameters: Duck Pond, Newfoundland
Authors William A. Morris and Sean E. WalkerCorrelation of geophysical well logs is an exploration technique that has been employed
for many years in oil exploration. Over the past decade, a number of new slimline tools
have been introduced advancing the application of logging to the smaller diameter mineral
exploration boreholes. Interpretation of this data may follow a number of paths. Through
the collaborative use of procedures such as: a) univariate, bivariate, or multivariate
statistical methods it is often possible to characterize some lithologies that are defined by
distinctive physical property signatures; and b) the correlation of distinctive geophysical
signatures between adjacent boreholes can be used to establish structural features of the
subsurface. In this paper we show that gridding of geophysical parameters as
conventionally applied to both ground and airborne surveys can also be applied to a suite
of borehole data to produce an image of the subsurface which contains information on
both structure and lithology. In this example we present results from a multi-well and
multi-parameter survey of the volcanogenic massive sulphide deposit from Duck Pond,
Newfoundland.
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The Application Of Borehole Geophysics To The Delineation Of Leachate Contamination At The Trail Road Landfill Site: Nepean, Ontario
Authors Daron G. Abbey, C. Jonathan Mwenifumbo and Patrick G. KilleenThe sanitary landfill site poses many monitoring problems for the environmental scientist. An understanding of
the materials and processes controlling the distribution of leachate in the groundwater at a landfill site is an
essential component of landfill management. The Trail Road landfill site in Nepean, Ontario, is perched upon a
sand and gravel ridge which is part of a lacustrine silty clay plain. The delineation and monitoring of leachate
contamination, created by the decomposing municipal solid waste mixing with groundwater, is presently carried
out through chemical sampling of monitoring wells. Such monitoring of a complex and hydraulically conductive
environment, is expensive and time consuming because water samples must be analyzed in a laboratory.
Borehole geophysical logging of eight of these monitoring wells showed that the delineation of leachate plumes
could be accomplished through a full suite of borehole geophysical logs. The logs acquired included: natural
gamma ray, density, total magnetic field, magnetic susceptibility, electrical conductivity, and temperature. The
use of a full suite of logs provided a measure of the in-situ physical properties of all the components of the
subsurface, including air, water, and sediments. Chemical sampling provided only an inventory of ions present in
the groundwater. The geophysical logs in this study were acquired, processed and interpreted, in a shorter period
than the chemical sampling, and provided information allowing for a more refined geological interpretation, than
from drilling alone. Anomalous physical properties, interpreted to be the result of the presence of leachate
contamination, correlated well with data from chemical sampling which showed elevated levels of leachate
indicator ions, such as, calcium, magnesium, chloride, sulphate, potassium, ammonia, dissolved organic carbon
(DOC), phenols and iron. It was concluded that an interpretation of borehole geophysical logs produced a more
accurate delineation of areas of leachate contamination at the Trail Road landfill site, more directly (i.e. in situ).
and with greater efficiency and simplicity than chemical sampling.
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Geophysical Borehole Logging In Massive Carbonates Whitmoyer Laboratories Superfund Site Meyerstown, Pa
Authors Bill Bour and Craig B. ClemensCorrelation within massive carbonate sequences like those present in central Petmsylvania’s
Lebanon Valley is often difficult because the units contain few distinct marker beds and the
contacts between units are frequently gradational. This project demonstrated that correlations
among boreholes can successfully be made using a combination of geophysical logs,
particularly natural gamma, high resolution density and sonic. The technique involves
positioning logs from several of the deepest holes on cross sections at the best estimated
structural elevation (correcting for regional dip), comparing the general shape of the natural
gamma curve and adjusting the logs positions for a best fit. Once a datum has been
hypothesized, (all) the logs are examined for anomalies that might be traced through. The high
resolution density proved capable of identifying small fractures which appeared to correlate
along strata lines with the gamma. These hypothetical “marker beds” are projected onto as
many logs as possible to verify the anomaly. Once an anomaly (bed) is identitied on several
logs over a sutXciently wide area, three point strike and dip calculations are performed to verify
that the correlations fit the regional pattern.
This technique is not particularly new, it has been used in subsurface investigations for
decades. The application to massive units with only the most subtle marker beds, however, has
not been particularly successful. It proved successful at this site because of the use of sensitive
gamma and density sondes, and by a large number (62) of boreholes, a third of which penetrate
over 400 feet of section. (Although only two wells intersected the Ontelaunee-Amiville contact.)
Only very slight density (lower) and velocity (faster) differences were noted between the
dolomitic limestone (bioherm) of the Ontelaunee Formation and the limestones of the Amwille
and Epler Formations. These differences were so slight as to not be noticed until the logs were
positioned correctly on the sections by gamma and density log correlations.
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Borehole Geophysics Used To Characterize Vertical Fractures And Their Connections To Bedding Plane Aquifers In Dolomite
More LessGeophysical logs in an array of six boreholes in argillaceous dolomite at a contamination site in northern Illinois
intersect four aquifers: a shallow bedding plane fracture, a vertical fracture, and two deeper permeable beds. The
vertical fracture aquifer appears to be representative of a set of such fractures dipping at about 85 degrees and
striking approximately northeast. Cross-borehole flow measurement techniques are used to investigate the
properties of the three horizontal aquifers, and to infer the character of the connection between the vertical fracture
and the deeper horizontal aquifers. The data indicate that the vertical fracture and the shallow bedding plane have
transmissivities nearly an order of magnitude greater than those of the two deeper horizontal aquifers in the
vicinity of the point where they intersect boreholes. The data also indicate that the deeper aquifers are
characterized by a storage coefficient of about 2x10”. The bedding plane and vertical fracture aquifers cannot be
modeled as confined aquifers, although relatively large storage coefficient values characterize models that most
closely approximate the variation of flow in the shallow bedding plane aquifer. The cross-borehole flow
experiments suggest that the vertical fracture is hydraulically connected with the deeper horizontal fractures, but
that fracture aperture decreases somewhat with depth.
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Deconvolution Of High Density Electrical Well Logs
Authors Flemming Effersø and Bo Holm JacobsenDetailed information on vertical resistivity variations is relevant as an indicator of change in
lithology, porefluid chemistry, porosity and permeability. Traditionally the formation resistivity
is estimated solely on curves of the observed data, which are plotted as apparent resistivity data.
Apparent resistivity data may be a poor approximation to the local formation resistivity, and it
can be impossible to determine the resistivity levels and the location of the layer-interfaces. Jn
this paper we will present results of deconvolution of normal log apparent resistivity data.
Deconvolution of data using dual electrode offsets gives a significantly enhanced resolution of
the formation resistivity when moderate resistivity contrasts are considered.
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Advances In Instrumentation And Data Processing For Multi-Spaced Resistmty Measurements: Hydrogeologic P Arameters And Lithostratigraphic Delineation
By Charles OdenRecent advances in portable borehole geophysical instrumentation and processing
software allow hydrogeologic characterization of sandstone aquifers. Lithostratigraphic
delineation is also performed. A slimline logging probe allows logging of 4 normal
resistivity measurements (8, 16, 32, and 64 inch), as well as single point resistance,
spontaneous potential, natural gamma, fluid temperature and fluid resistivity. The data
can be quickly and easily processed on site to provide derived parameters and properties.
Hydrogeologic information can be easily calculated using processing software. Water
resistivity is indicated using shallow and deep resistivity measurements along with the
resistivity of the mud filtrate. Total dissolved solids can be found fi-om the water
resistivity. Porosity can be determined from Archie’s Law if a fresh water correction is
used. Hydraulic conductivity is indicated using depth of mud filtrate invasion. Examples
illustrate that calculated parameters agree with measured or otherwise indicated values.
Lithostratigraphic units can be delineated by synthesizing an approximated Laterolog7 log.
The synthesized focusing currents provide good a indication of bed boundaries. The
models generated from data sets obtained with the new multi-spaced normal resistivity
probe provides better vertical resolution due to the 8 inch normal resistivity measurement.
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Neural Network Interpretation Of Em39 Well Log Data
Authors Lin Zhang and Mary M. PoultonWe used a Modular Neural Network(MNN) to invert well logging curves from a Geonics EM39
induction logger In the interpretation scheme, there are several subsets of networks that depend on the
relative resistivities of adjacent layers, e.g. Rl>R2, RlR3, etc. The well logging curves are
subdivided into several pieces and run through each sub-network. The results are estimates of resistivity
and thickness of every layer. Using synthetic data, several training sets were made. The networks were
also tested on field data. The networks were examined for their ability to compute the right output patterns
for the corresponding input patterns of the training set and the ability to interpret the new patterns that are
not present in the training sets.
The results show that neural networks do facilitate interpretation of well logging data. When tested on
data from shallow wells 4-5m deep representing uniform material, the trained networks had an overall
accuracy of about 90% for both resistivity and thickness. When tested on a multi-layer case, the networks
gave reasonable estimates for each layer’s thickness and resistivity, although a shift in depth was
observed for some layers. Recently, we generated more training patterns for thinner layers and
incorporated a new way to pick data points for the input patterns. We found the results more satisfactory.
For two complicated multi-layer field cases, the networks had an overall accuracy of 92.1% for the
resistivity and thicknesses of the layers.
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The Interest Of Geophysical Methods For Locating Hydromorphic Horizons In Acid Pedological Systems
Authors V. Chaplot, P. Curmi, A. Hollier-Larousse, J. Pellerin and C. WalterThe aim of this study is to determine the interest of some geophysical methods, in particular Radio&IT in
locating hydromorphic horizons for acid pedological systems. Two areas were studied, one under forest, another in
an intensive agricultural site. A direct relationship between apparent resistivity and horizon type distribution was
established on the first site with a relative uniform soil depth. On the second site, soil depth and topsoil water
content explained most of the variations of the apparent resistivity, the influence of which was modelized by
multiple regression. Deviations from the regression model were then interpreted by taking into account the
succession of soil horizons. Results of Ground Penetrating Radar and Electrostatic Quadripole confirmed the
position of some of the limits between soils suspected by Radio-MT data. Finally, the prediction of hydromorphic
soil systems with the help of geophysical techniques is discussed.
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Gpr Attenuation Tomography For Detecting Dnapls
Authors Lanbo Liu and Youli QuanUsing the ground penetrating radar (GPR) to detect organic contaminants in the geological
formations is a relatively new technique for environmental site characterization. We present here an
attenuation tomography method that could be used to image subsurface organic contaminants like
DNAPLs. This method is based on centroid frequency shift when the electromagnetic (EM) waves
propagate in subsurface media. In most natural materials, EM wave attenuation increases with
frequency. As a result, the centroid of the signal’s spectrum experiences a downshift during
propagation. This downshift is proportional to a path integral through the attenuation distribution
and can be applied to the GPR data to tomographically reconstruct the attenuation distribution. The
frequency shift method is applicable to any data when the signal bandwidth is broad enough and
the attenuation is high enough to cause noticeable losses of high frequencies during propagation. In
comparison to some other methods of estimating attenuation, the frequency shift method is
relatively insensitive to geometric spreading, reflection effect, source and receiver coupling,
radiation patterns, and instrument responses. High attenuation has been regarded as the major
hurdle for using GPR in areas with high electric conductivity, this method provides an alternative
view to the attenuation.
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An Evaluation Of Geophysical Techniques To Measure Sub-Surface Hydrocarbon Contamination At A Disused Fuel Distribution Depot
Authors V.T. Nguyen, M.J. Quick, A.S. Eriksen, S. Booth, D. van der Roest, A. Wagebert, W. Al-Nuamy, M. Nakhkash, D. Zhang and Y. HuangAs part of a continuing programme to evaluate the potentials of non-invasive methods for rapid site assessment,
measurements have been made at a disused fuel distribution depot using geophysics techniques with particular
emphasis on the detection of sub-surface contamination. These include GPR, EM conductivity and 2D resistivity
imaging. The data have been analysed by a number of approaches and the results are compared with traditionally
acquired and chemically analysed soil samples.
The analysis of the data highlights the need for more work to be directed to the understanding of the physical processes
that give rise to the observed features, but encouragingly the comparison between the interpreted geophysics results
and the soil analysis has indicated that the non-invasive methods have potential to be developed into useful cost
effective screening tools. The direction for future work is discussed.
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Case Studies Of 2D Resistivity Surveying For Soils, Waste Management, Geotechnical, And Groundwater Contaminant Investigations
Authors Paul D. Bauman, Mike Lockhard, Anil Sharma and Richard KellettThe direct-current (DC) resistivity method has seen wide use in mining, groundwater, contaminant, and
geotechnical applications since its first use in the early 1920’s. More recently, however, fixed frequency and
transient electromagnetic (EM) methods have supplanted many applications traditionally addressed by resistivity
techniques. In particular, the application of resistivity profiling and sounding has suffered from the time intensive
nature of these surveys as compared to using EM equipment.
The advent of the automation of 2D resistivity surveys (Dahlin. 1993) and the ability to quickly invert relatively
large 2D data sets (Loke and Barker, 1996) promises to re-introduce the DC resistivity method into a wide variety
of applications in the geosciences. This paper presents four case studies where 2D resistivity surveying was
applied. The technique was used in various geological terrains, extremes of weather, studies requiring varying
depths of investigation, and with varying degrees of cultural interference. The accuracy of the method is evaluated.
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Electromagnetic Terrain Conductivity Surveys Used To Screen Swine Confinement Facilities For Groundwater Contamination
Authors T.H. Larson, I.G. Krapac, W.S. Dey and C.J. SuchomskiElectromagnetic terrain conductivity (EM) surveys were conducted at two swine confinement
facilities in Illinois to assess the presence and extent of groundwater contamination and to guide
the installation of monitoring wells. At Site A, the waste lagoon was on a glacial outwash terrace
near an alluvial valley. At Site C, the waste lagoon was on glacial till overlying shale bedrock.
Measurements were made with a GEONICS EM-34 terrain conductivity meter on 10-m grids
surrounding the waste lagoons. Wells were installed in areas where EM surveys suggested that
contamination may and may not be occurring. Two shallow wells at Site A and one well at Site
C yielded water samples with electrical conductivity values 6 times higher than background,
suggesting contamination. Each of these wells was in areas of high EM conductivity. None of the
deep wells in areas of high EM conductivity or shallow wells in areas of low EM conductivity at
Site A or Site C yielded water samples with electrical conductivity values much higher than
background. It appeared that a terrain conductivity survey is a useful tool for for delineating
areas of groundwater contamination.
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High Resolution Seismic Reflection Survey To Map Bedrock And Glacial/Fluvial Layers In Fridley, Minnesota
Authors Richard D. Miller and Jianghai XiaShallow seismic reflection in conjunction with uphole velocity profiles and local borehole
data allowed delineation of discrete laying within the alluvial materials and glacial drift overlying
the St. Peters Sandstone and/or the Prairie du Chien dolomites at an average depth of around 120 ft
along the western perimeter of the Naval Industrial Reserve Ordnance Plant (NIROP) in Fridley,
Minnesota. The primary goals of this study were to determine the feasibility and limitations of the
technique and develop a continuous subsurface image of as many geologic/hydrologic contrasts as
possible that can be characterized by changes in acoustic impedance. Establishing feasibility includes
determining the horizontal and vertical resolution potential, optimum acquisition geometries
and parameters, best suited equipment for surface and subsurface conditions, level of effort necessary
to delineate the geologic/hydrologic features of interest, and to establish a reasonable set of
expectations for the technique at various unique near-surface settings across the entire facility.
Shallow seismic reflection profiles allowed delineation of potentially discontinuous confining units
within the glacial drift at this site. Incorporation of a high confidence reflection profile with existing
hydrologic models should greatly improve the understanding and therefore prediction of contaminant
transport and fate models in areas with significant lateral variability. The glacial drift that
covers the entire site consists of till, outwash, valley train, and lake deposits.
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Seismic Reflection Processing For Characterization Of A Hazardous Waste Site
Authors Zhi-Ming Liu and William E. DollSeismic reflection data have been acquired by the Kansas Geological Survey near the Oak Ridge
K-25 Plant on the Oak Ridge Reservation, Tennessee, to assist in the selection of ground water
monitoring well locations. The data were recorded in uncorrelated format to allow flexibility in
enhancement of stacked images.
During the summer of 1996, five of the thirteen seismic reflection lines acquired were
processed. An unconventional correlation procedure, “Vibroseis Whitening” (VSW) (Coruh and
Costain, 1983) has been applied to produce improved seismic sections. Refraction statics
corrections, which remove the detrimental effect of an irregular weathered layer, have also been
utilized to improve the seismic sections. The seismic data were stacked using the velocities
obtained from a standard semblance velocity analysis tool. Locations and orientations of faults
or fault zones can be interpreted from these stacked sections, and they are in agreement with the
interpretations of the surface mapping in the area.
This paper concludes that VSW and refraction statics can be important to near-surface
swept source seismic data processing.
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Dynamic Properties Of Municipal Solid Waste Landfills From Surface Wave Tests
Authors Christopher D. Haker, Glenn J. Rix and Carlo G. LaiThe seismic stability of municipal solid waste (MSW) landfills is often a significant consideration in landfill
design. However, until recently, the dynamic properties of the waste material itself, which govern the seismic
response of MSW landfills, have often been approximated or assumed. Tests to determine the dynamic properties
of the material directly have been limited. Measurements of seismic surface waves were used to determine the
dynamic properties of MSW, which are the initial tangent shear modulus and low-strain hysteretic damping ratio.
Surface wave tests were performed at three MSW landfills to determine their shear modulus and damping ratio
profiles. Surface wave tests are ideal for measuring the near-surface shear modulus and damping profiles of MSW
landflls because the tests are non-invasive, an advantage for testing environmentally sensitive waste material.
Factors which influence the dynamic properties of waste including density, confinement, age, and placement
techniques are used to interpret the measured shear modulus and damping ratio profiles.
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Evaluation Of Earth-Fill Embankments In Romania Using Electrical Methods And Ground Penetrating Radar
Authors Jean Christophe Gourry and Traian MoldoveanuDuring the 1970s embankments were constructed along several hundred kilometres of
rivers in Romania In recent years, several leaks have developed along the berms.
The aim of this study is to check whether GPR and electrical methods are efficient for
characterizing anomalies above the known leaks and, if so, to extrapolate the results into
unknown areas for systematic surveillance. GPR revealed variations of 5 to 10% in the
apparent water level, whereas piezometric measurements showed variations of less than
5%. Such large variations in the apparent water level may be due to variations in EMwave
velocity. CMP results confirmed a lower EM-wave velocity in the landslip area than
in the safe zone. The apparent water level drops in the area affected by the landslip.
Electrical soundings revealed that resistivity is higher in the anomalous crest areas than in
the secure parts of the embankment. One striking feature recorded in the anomalous zones
was that resistivity anomalies were positive along the crest, yet negative along the berm,
i.e. high resistivity along the crest but low along the berm for the landslip area. One
explanation is that the weak crest areas are made up of porous material, whereas those
along the berm consist of saturated material.
This study has demonstrated the correlation between known leakage areas and geophysical
anomalies. Furthermore, the coupling of GPR and electrical soundings proved efficient
for detecting most of the known anomalies. Due to its good spatial resolution and high
acquisition speed, GPR is perfectly adapted to this type of application.
2-D GPR tomography along the dams also provided detailed and interesting results.
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Seepage Investigation Using Geophysical Techniques At Coursier Lake Dam, B.C., Canada
By Phil SirlesSubsurface seepage flow at Coursier Lake Dam was identified by onshore
and offshore self-potential surveys, and electrical resistivity
profiles and soundings during a Deficiency Investigation by BChydro.
For typical seepage investigations baseline geophysical data are
collected at @llow-pool I1 level and the measurements are repeated when
high hydraulic gradient conditions exist. At Coursier Lake Dam a
rather unanticipated outcome of the low-no01 surveys was that
significant seepage beneath the structure'was detected. The low-pool
results were conclusive enough that, when combined with visual
inspection and observation of sinkholes on the embankment, an
immediate restriction was placed on the pool elevation. Thus, because
of the identified potential hazard, the remaining geophysical
investigations were conducted under a "minimum-pool" reservoir level
in order to complete the comparative study. Therefore, the dam was
studied under low- and minimum-pool reservoir conditions in the spring
and fall of 1993, respectively.
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Seepage Characterization During Dewatering Activities - Bradbury Dam, California
Authors Robert Furgerson, Robert Corwin and Jerry WrightA geophysical investigations program was performed at Bradbury Dam near Santa Ynez, California. The purpose of
the investigation was to 1) provide data on possible seepage paths into the dewatering area downstream of Bradbury
Dam, and 2) determine lithologies and stratigraphic correlation within the alluvium. The geophysical investigations
included surface self potential (SP) surveys along the centerline and offsets right and left of the newly constructed
cutoff wall, SP surveys within the dewatering area and stilling basin, borehole induction conductivity, borehole
temperature logs and other borehole geophysical logging.
Anomalies from the SP data were indicated along the downstream cutoff wall at several wall stations . SP data
collected within the dewatering area confirmed these anomalies No significant SP anomalies were noted in the
spillway stilling basin.
Correlation of borehole induction conductivity logs run in 34 observation wells and 8 geophysical shear wave holes
indicated the existence of two correlatable main units and four sub-units within the foundation alluvium. Lithologic
descriptions of these units are based on variations in electrical conductivity with supporting data Ii-om SPT samples,
core samples, grab samples, and corrected Becker blow counts. These lithologic units were used to aid in the
interpretation of the borehole temperature data, SPT and Becker results.
Borehole temperature logging was conducted on four separate dates in the observation wells. Temperatures in the
spillway stilling basin were also measured. Average temperatures in each borehole were calculated at the water
table, the base of the alluvium, and within bedrock where available. In addition the maximum variation of
temperature within the alluvium was also calculated.
A geologic model was constructed from correlation of induction conductivity logs , Becker blows, and analysis of
grab samples obtained during excavation of the downstream cutoff wall. SP and temperature data provided seepage
and hydrologic information within the dewatering area.
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Seismic Refraction And Downhole Velocity Surveys For Investigation Of The Tinemaha Dam Inyo County, California
Authors Mark Olson, Yogesh Prashar and Sinan InelSeismic refraction and downhole velocity surveys were conducted as part of a
geotechnical investigation of the Tinemaha Dam. Thirteen P and S-wave velocity surveys
were conducted concurrently with cone penetration testing along the crest and
downstream toe of this dam. One additional velocity survey was conducted from a
borehole on the crest of the dam using a downhole geophone array. The shear wave
velocities estimated for the dam and its underlying alluvium ranged from 345 to 2008
ft/sec. A depth profile of these velocities indicates areas within the dam and its
foundation where the shear wave velocities are less than 600 ft/sec. These velocity
estimates were used with other geotechnical data, to calculate the dynamic elastic moduli
for the dam and its foundation, to evaluate the stability of the dam in the event of a local
earthquake on the Owens Valley fault. To further investigate the location of this active
strike-slip fault, seismic refraction surveys were conducted at two locations. The first
refraction survey was positioned south of the dam across the expected trend of the Owens
Valley fault. A velocity analysis of the upper refracting horizon was conducted using the
generalized reciprocal method. This revealed changes in lateral velocity that were
interpreted as due to a fault-transition from saturated alluvium to volcanic bedrock. The
second survey was positioned along the dam’s crest at the west abutment to help locate a
possible fault beneath this area. The interpreted depth section from this survey locates a
point where the deeper bedrock unit appears to be truncated by faulting. This information
will help locate additional boreholes to evaluate lithologic conditions.
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Multi-Frequency Gpr Surveys
Authors Jeffrey E. Lucius and Michael H. PowersGround penetrating radar (GPR) antenna selection must be based on specific survey objectives. The choice of
antenna is primarily a compromise between resolution and depth of penetration. This paper briefly presents three
case histories of multi-frequency GPR surveys that explain and illustrate how the antenna and the electromagnetic
(EM) properties of the ground affect wavelength, loss mechanisms, far-field radiation, and the near-field. Changes in
these parameters are important because they affect vertical and horizontal resolution, and depth of penetration. In
general, when the depth of penetration is limited by conductive losses instead of scattering losses, changing to a
lower-frequency antenna will not help in detecting shallow features, and may not help in detecting deeper features.
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Locating Salinity Sources In West Texas With Airborne And Ground-Based Geophysical Methods And Gis
Authors Jeffrey G. Paine, Erika M. Boghici, Alan R. Dutton and Steven W. TweedySalinization of soil, surface water, and ground water is a chronic environmental and
agricultural concern in the southwestern U.S. In this study of a 91 km2 area near Ballinger in
Runnels County, Texas, we used a geographic information system (GIS) to integrate results from
airborne and ground-based geophysical surveys, water and soil sampling, and chemical analyses
to locate areas of brine concentration and determine their origin. Potential salinity sources in this
area are (a) subsurface brine moving upward along natural conduits (faults, fractures, joints, and
permeable stratigraphic units), (b) brine migrating downward from surface disposal pits,
(c) leaking oil and gas wells, and (d) evaporating shallow ground water as a result of agricultural
landscaping. A prime goal of the study was to determine the effectiveness of the method in
distinguishing potentially leaking wells from other salinity sources.
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Mapping Bedrock Topography With Seismic Refraction At An Oil And Gas Field: A Comparison Of Engineering And Petroleum Exploration Methods At A Site In The Wind River Basin Of Wyoming.
Authors David Phillips, Robert Grimm and Bart Hoekstra“Weathered-layer” corrections to seismic-reflection travel times for deep hydrocarbon exploration are often made
using shallow refracted arrivals on the reflection records. There has been recent interest in using these “refractionstatics”
solutions to provide depth-to-bedrock maps for environmental or engineering applications as a by-product of
the deep seismic survey. We show that the depth to bedrock and bedrock velocities produced by automatic
refraction-statics processing of a three-dimensional seismic survey of a gas field in Wyoming differ significantly
from those determined from an engineering survey. Manual reprocessing of the refraction statics produced some
improvement, but is still inferior to refraction data taken for the specific purpose of determining depth to bedrock.
Automatic refraction statics provide adequate travel-time adjustments to reflection records due to a shallow lowvelocity
layer, but such solutions should not be taken to represent explicit depth to bedrock.
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Neural Network Interpretation Of High-Frequency Electromagnetic Ellipticity Data
Authors RaIf A. Birken and Mary M. PoultonThis paper focuses on the detection of three dimensional (3D) small conductive targets from high-frequency
electromagnetic elliptic@ data using neural networks. For environmental investigations it is necessary to provide
as much information on the location of shallow buried conductive objects or the electrical properties of possible
contaminants. The networks are trained with one-dimensional (ID) forward models to estimate the resistivity and
dielectric constant structure of the ground. The input is given by elliptic@ sounding curves from eleven discrete
frequencies in binary steps in a range from 32 kHz to 32 MHz. Halfspace and layered earth neural networks will
provide reasonable fit to sounding curves even if they are influenced by shallow conductive 3D objects. We show
that a detailed inspection of elliptic@ profiles over targets such as a 5 m by 3 m aluminum sheet (depth of 1 m), a
5%gallon barrel (depth of 0.63 m), and two metal desk (depth of approximately 1 m) can help to detect these
anomalies. Piecewise halfspace neural network are capable of enhancing the anomalies in resistivity depth sections
and provide additional information for the detection and possible localization of the object. The visualization of
the results is very important since small targets will show up as subtle anomalies. Based on observations of
elliptic@ sounding curves and profiles we can train a neural network to classify target responses versus
background responses for specific sites, assuming that enough soundings are available to train the neural networks.
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Joint Seismic And Electrical Tomography
Authors Jie Zhang and F. Dale MorganSeismic and electrical properties in the earth are not directly associated with each other. If a large seismic
velocity contrast occurs across a geological interface, certain change of the corresponding electrical properties
may also take place, but not necessarily. To seek solutions with maximum structure coherency between
the two physical parameters in the earth, we develop a joint seismic and electrical tomography method
that simultaneously reconstructs seismic velocities and electrical resistivities with cross constraints on the
normalized structure curvatures.
The joint approach applies a 2D nonlinear refraction traveltime tomography method and a 2.5D d.c.
resistivity tomography method in a joint process. It includes a wavefront raytracing method for rapid
traveltime calculation, and a transmission-network method for electrical potential calculation. A Conjugate
Gradient (CG) approach is applied to solve an inverse problem that minimizes the seismic traveltime and
the electrical potential misfits, and meanwhile maximizes coherency of their normalized structure curvatures
by the way of Tikhonov regularization.
We demonstrate the performance of the joint tomography technique with a numerical example. A geological
structure is characterized with three zones in terms of seismic and electrical properties. Applying
the joint inversion method takes advantages of both geophysical approaches and automatically reconstructs
consistent structures.
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Two-Dimensional Imaging Of Transient Electromagnetic Soundings
More LessTransient electromagnetic (TEM) soundings have been used extensively in geophysical
investigations in connection with environmental problems (Christensen and Sørensen 1994, 1995).
Within the geophysical community there is a growing realization that dense measurements
over large areas are needed to produce the reliability and detail demanded by the severity of many
of the problems addressed in environmental geophysics (Sørensen et al 1995, Sørensen 1996).
This calls for new efficient methods of data collection and fast interpretation procedures.
Consequently much interest has been directed towards rapid approximate interpretation
algorithms - imaging procedures - which will allow a quick overview of the data collected and
provide good starting models for rigorous, iterative inversion procedures.
A number of imaging methods for transient electromagnetic soundings have been suggested
based on the variation of the diffusion velocity with conductivity, e.g. Eaton and Hohmann (1989).
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3D Electromagnetic Inversion For Environmental Site Characterization
Authors David L. Alumbaugh and Gregory A. NewmanA 3-D non-linear electromagnetic inversion scheme has been developed to produce images
of subsurface conductivity structure from electromagnetic geophysical data. The solution is
obtained by successive linearized model updates where full forward modeling is employed
at each iteration to compute model sensitivities and predicted data. Regularization is applied
to the problem to provide stability. Because the inverse part of the problem requires the
solution of 10’s to 100’s of thousands of unknowns, and because each inverse iteration
requires many forward models to be computed, the code has been implemented on
massively parallel computer platforms. The use of the inversion code to image
environmental sites is demonstrated on a data set collected with the Apex Parametrics
‘MaxMin 1-8s’ over a section of stacked barrels and metal filled boxes at the Idaho
National Laboratory’s ‘Cold Test Pit’. The MaxMin is a loop-loop frequency domain
system which operates from 440 Hz up to 56 kHz using various coil separations; for this
survey coil separations of 15, 30 and 60 feet were employed. The out-of phase data are
shown to be of very good quality while the in-phase am rather noisy due to slight
mispositioning errors which cause improper cancellation of the primary free space field in
the receiver. Weighting the data appropriately by the estimated noise and applying the
inversion scheme is demonstrated to better define the structure of the pit. In addition,
comparisons are given for single coil separations and multiple separations to show the
benefits of using multiple offset data.
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3D Ert Applied To The Monitoring Of Joule Heating
More LessThis paper describes the application of three-dimensional Electrical Resistivity Tomography (ERT) to monitor
in-situ soil six-phase ohmic-heating (SPOH) at the Integrated Demonstration for VOCs in Soils and Groundwater at
Non Arid Sites. The ERT data sets, collected between five pairs of adjacent wells using an array of electrodes
located in 4 boreholes, have been previously interpreted with a 2D algorithm (Ramirez and Daily, 1994) to map the
changes in conductivity in a clay layer located in the vadose zone over a period of approximately 2 months (before,
during and after electrical heating). The objective of this work is to show the usefulness of a new 3D ERT
interpretation technique for a complete monitoring of a real 3D underground process. Heating causes moisture
content, temperature, and fluid conductivity changes, all of which affect electrical resistivity, and the successful
application of ERT provides a three-dimensional view of the subsurface resistivity or conductivity distribution.
This information helps us to understand the interaction between heating and the heterogeneous subsurface, the
effects of heating on fluid flow and on contaminant removal. The reconstruction of both the background resistivity
and of the changes over time, caused by the combined effects of moisture redistribution and heating caused by
SPOH and vapor extraction, is enhanced by the 3D interpretation.
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Effective Design Of A Horizontal Air Sparging Well
Authors Doug LaBrecque, Gianfranco Morelli and Paul D. LundegardIn air sparging, in-situ remediation is accelerated by injecting air in the saturated zone. The
success of air sparging depends on the size and geometry of the region of air flow. This study
used ERT surveys to observe changes in saturation, and thus air flow for a horizontal sparge
well. The site is in relatively homogeneous, well sorted fine to medium grained dune sands at
the Guadalupe Gil field. The horizontal sparge well was 4.7 m below the surface. The well was
constructed of stainless steel and perforated along 15 m interval. ERT images were made along
a pair of east-west trending planes near the north and south ends the well.
ERT images showed that the zone of influence of the horizontal sparge well is relatively small.
During the early times, the width of the zone of influence is between 1.5 and 2.5 m. Later, the
width expands slightly but never exceeds 3.3 m.
There were some differences between the north and south planes. The north plane had a slightly
narrower zone of influence but a higher peak saturation, exceeding 50% during the test. An
increase in pressure created a small increase in the saturation and size of the zone of influence.
The ERT images could not be reconstructed until the metal well was included in the FEM mesh.
The well was found to be located slightly further from the center ERT cables than expected.
The background resistivities are consistent with those of clean sands. Although there is some
variability in the resistivity of the background, it is small compared to the variability of the
image reconstructions and can be ignored.
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Three-Dimensional Georadar Studies Of Deformation In Deltaic Sediments
Authors Milan Beres, Andre Pugin and Alan GreenThree-dimensional ground-penetrating radar (georadar) data collected across a 4-m-high hill in the Rhone
River delta (western Switzerland) contain prominent reflections from faulted and folded bedding. Crosssections
and time slices extracted from the georadar data reveal a number of faults that strike parallel to the hill
slopes. Several circular-shaped uplifted blocks together with intervening graben-like features are observed at
various depths. A similar-sized graben-like structure with step faults that converge with depth has been mapped
at a nearby outcrop. Thin layers of silt and fine sand that were intersected in boreholes correlate with strong
georadar reflections and are interpreted as elastic intrusions. Our results are consistent with the hypothesis that
the hill and its internal structures were formed by a large rockfall.
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High Resolution 3-D Seismic Experiment Across The Oquirrh Fault
Authors David M. Morey, Chengjun Wu, Yue Wang, Zhaojun Liu, Jing Chen and Jerry SchusterA 3-D seismic data set was recorded across the Oquirrh fault in May of 1996
by University of Utah personnel. The data (112,896 traces) were collected on a
fine grid in order to image the shallow (SO’-100’) detailed features of this fault.
Previous trenching studies to a depth of about 20 feet along this fault reveal
offset bedding caused by large magnitude earthquakes. The goal of this study is
to explore the potential for complementing trenching studies with 3-D seismic
imaging, with the hope of correlating earthquake recurrence intervals to bed
offsets in the seismic image.
The processing of this data set includes 3-D tomography and 3-D prestack
migration. Results from 2-D tomography show a clear imaging of the shallow
subsurface fault to a resolution of about 10 feet.
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Characterization Of The Hydrogeologic Framework At Aberdeen Proving Ground, Maryland, Using Marine Seismic Reflection Profiling
More LessHigh-resolution, single-channel, marine seismic reflection profiles were collected in the shallow waters of
the Chesapeake Bay estuarine system surrounding the Edgewood Area of Aberdeen Proving Ground, MD. This
data set furthers our understanding of the hydrogeologic framework by identifying paleochannel locations,
delineating paleochannel geometries, and characterizing relationships between multiple generations of
paleochannels.
Fine-grained bottom sediment resting in deeper, low-energy waters were found to block high-frequency
acoustic wave transmission; therefore, data acquisition is limited to areas near-shore were water depths are less
than three meters. Approximately 120 km of shoreline were surveyed yielding 50 km of high quality data. Three
major reflectors are identified as erosional unconformities, and are interpreted to be Pleistocene paleochannel
surfaces. Paleochannels are found to be spatially and physiographically associated with all modern subaerial
lowlands and their local drainage systems. The marine seismic reflection data provide strong evidence that
present-day lowlands are underlain by an aquifer dominated by paleochannels. Paleochannel configurations,
together with paleochannel-fill models, indicate these paleochannels have the potential support vertical and
horizontal contaminant migration.
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Mapping A Near Surface Variable Geologic Regime Using An Integrated Geophysical Approach
Authors Noel T. Rogers, Stewart K. Sandberg, Peter Miller and Greg PowellAn integrated geophysical approach involving seismic, electromagnetic, and electrical methods was employed to
map fluvial, colluvial and bedrock geology, to delineate bedrock channels, and to determine fracture and joint
orientations that may influence migration of petroleum hydrocarbons at the Glenrock Oil Seep. Both P (primary)-
wave and S (shear)-wave seismic refraction techniques were used to map the bedrock surface topography, bedrock
minima, stratigraphic boundaries, and possible structure. S-wave data were preferred because of better vertical
resolution due to the combination of slower velocities and lower frequency wave train. Azimuthal resistivity/IP
(induced polarization) and azimuthal electromagnetics were used to determine fracture orientations and
groundwater flow directions. Terrain conductivity was used to map the fluvial sedimentary sequences (e.g.,
paleochannel and overbank deposits) in the North Platte River floodplain. Conductivity measurements were also
used to estimate bedrock depth and to assist in the placement and recording parameters of the azimuthal
soundings.
The geophysical investigation indicated that groundwater flow pathways were controlled by the fluvial
paleochannels and bedrock erosional features. Primary groundwater flow direction in the bedrock and collwial
sediments was determined from the azimuthal measurements and confirmed by drilling to be N20-4OW along the
measured strike of the bedrock and joint orientations. Joint/fracture orientations were measured at N20-4OW and NlO-
30E from the azimuthal data and confirmed from measurements at a bedrock outcrop south of the site. The bedrock has
an apparent NlOE anisotropy in the seismic velocity profiles on the old refinery property that closely match that of
measured joint/fracture orientations and may have a minor effect on groundwater flow.
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Application Of Electromagnetic Methods To Locate Shallow Water Well Sites In A Fractured Aquifer
Authors Kendrick Taylor, Matt Chesley, Korblaah Matanawi and Tim MinorLow permeability fractured bedrock makes it difficult to select productive locations
for shallow water wells in the Atebubu district of Ghana, West Africa. Drilling has
demonstrated that productive and nonproductive sites are frequently separated by less than
50 meters. The drilling success rate is approximately 25 percent when experienced
hydrogeologists use only surface visual observations to select drilling sites. A combination
of remote sensing and geophysics was used to improve the success rate. SPOT satellite
imagery and stereo aerial photography were used to identify areas where suspected
fractures cross drainages. These areas were typically 100 to 150 meters in diameter. The
frequency domain electromagnetic method was used to locate zones in these areas where
the electrical conductivity increased with depth. These were interpreted as zones of deeper
weathering and fractured bedrock, and were selected as favorable drill sites. Use of this
exploration strategy increased the success rate 35 f 10 percent when the best site within
2 kilometers of the target community was selected. This doubled the success rate and
reduced the number of holes drilled to provide wells to these community by more than a
factor of two. When SPOT satellite imagery and aerial photography were used without the
frequency domain electromagnetics, the success rate only increased 8 f 8 percent. The
methods are cost effective when they are implemented by Ghanaian personnel and in areas
that have a low drilling success rate.
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Monitoring Of A Pumping Test In An Unconfined Aquifer With Ground Penetrating Radar
Authors Anthony L. Endres, David L. Rudolph and William P. ClementThe temporal and spatial response of an unconfined aquifer at CFB Borden to a pumping
test was monitored by means of ground-penetrating radar (GPR) profiling. The reflection
caused by changing water content in the transition zone between the overlying residually
saturated material and the water saturated aquifer below was used to estimate the location of
the top of the water saturated zone (TWSZ). Observed depth variations in the TWSZ
reflection differed significantly from the water table drawdown measured in the piezometers.
The GPR results are consistent with the in-situ water content measurements of Nwankwor et
al. (1992) and support their proposed delayed drainage mechanism. Diffractions along the
TWSZ reflection indicate that differential drainage due to small scale variations in aquifer
properties occurs. Arrival time variations of reflections from underlying stratigraphic
horizons are accounted for by changes in the overlying velocity structure caused by the
lowering of the TWSZ.
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