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14th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 04 Mar 2001 - 07 Mar 2001
- Location: Denver, Colorado, USA
- Published: 04 March 2001
1 - 50 of 129 results
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Mapping Grain Size Facies For The Hydrogeologic Model Of The Middle Rio Grande Basin, New Mexico Using Airborne Time-Domain Electromagnetic Data
The Santa Fe Group aquifer in the Middle Rio Grande Basin is the main source of municipal water for
the greater Albuquerque metropolitan area. One objective of U.S. Geological Survey studies is to improve the
hydrogeologic models of the Middle Rio Grande Basin, so as to help land managers plan and develop water
supplies. Airborne time domain electromagnetic (TEM) data near the town of Rio Rancho, northwest of
Albuquerque, New Mexico, provided a three-dimensional depiction of the electrical resistivity distribution that
was used to infer the extent of geologic units within the Santa Fe Group aquifer. The correlation between
resistivity and grain size was based on lithologic and induction resistivity logs, which showed that the bulk
average resistivity in the saturated zone correlate with grain size as follows: Coarse (40-70 ohm-m), Medium
(20-40 ohm-m), Fine (10-20 ohm-m). These resistivities were used to infer likely grain sizes from the TEM
inversions. From lithologic well data, axial river-channel deposits are primarily composed of coarse grain size
facies, fluvial sand deposits are primarily composed of medium grain size facies, and fluvial silt and clay deposits
are primarily composed of fine grain size facies. Significant changes in the TEM response occur at several
mapped faults and at faults inferred from a high-resolution airborne magnetic survey. Coarse-grained sediments
that exist at 1500 m above sea level appear to be less than 50 m thick below the water table (apparently
pinched-out at the 1450 m level). At the 1350 m level, the lateral extent of axial-channel sand deposits appears
to be much reduced, suggesting that they are pinching out. The resistivity model provides a framework for
forecasting hydrologic conditions in areas less explored by drilling. Our interpretation of grain size facies
distribution provides direct input to ground-water flow models that are critical to water management agencies.
Introduction
The Albuquerque-Santa Fe region is rapidly growing. The Santa Fe Group aquifer in the Middle Rio
Grande Basin
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Identification Of Saltwater Intrusions And Coastal Aquifers Using The Bgr Helicopter-Borne Geophysical System
Authors Detlef G. Eberle and Bernhard SiemonAs part of a research programme, a helicopter-borne survey was conducted by the Federal
Institute for Geosciences and Natural Resources (BGR) of Germany covering the Coastal Aquifer Test
Field situated in NW-Germany between the estuaries of the Elbe and Weser rivers into the North Sea.
Electromagnetic data were collected in five frequencies and inverted to apparent resistivity/centroid
depth data sets for each frequency. These served as starting models to determine 1-D stratified n-layer
models for each data point using a Marquardt inversion. Results are presented as resistivity maps for
selected depth levels and as vertical sections depicting the variations of the true resistivity with depth
and distance along each flight line. Saltwater intrusions, seaside and inland freshwater aquifers and
glacial meltwater channels have been clearly identified and mapped. Airborne geophysics have proven a
fast and cost-effective tool for natural resource management.
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Using Airborne And Ground Electromagnetic Data To Map Hydrologic Features In Everglades National Park
Authors David V. Fitterman and Maryla Deszcz-PanGround-water flow requires the development of a three-dimensional model of aquifer properties
and boundaries. This task has been traditionally accomplished through drilling and water-quality
sampling in wells. While the data obtained by these means are highly accurate, they represent only a
very small fraction of the total model volume. Furthermore, in areas where drill sites are limited due to
difficult access, model geometry obtained by interpolating between widely spaced wells may be
somewhat inaccurate.
Helicopter electromagnetic (HEM) resistivity mapping provides high density data coverage over
large areas, including those where access is difficult. Interpretation of these data poses other problems
due to noise and errors in the HEM data. However, when combined with ground electromagnetic
soundings and limited well information, hydrologic features can be mapped with more certainty than
possible by interpolating between widely spaced wells.
As an example, we present a study from Everglades National Park, Florida. Data consist of an
HEM survey, time-domain electromagnetic soundings, well logs, and water quality samples. The
interpretation provides information on the depth to the base of the aquifer, the extent of saltwater
intrusion, and a three-dimensional picture of water quality in the aquifer.
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Tools And Techniques For The Application Of Highresolution Aeromagnetic Surveys To Engineering Problems
More LessModern high-resolution airborne magnetic surveys can be a practical tool for engineering and
environmental applications. Surveys conducted by fixed-wing aircraft over gentle topography can
detect magnetic point sources separated by 150 meters or linear magnetic sources that extend 150 meters
in at least one direction; examples include well casings, geologic unit boundaries, faults, and pipelines.
For smaller targets, and in areas of rugged topography, helicopter surveys are required.
Careful planning is the key to a successful high-resolution aeromagnetic survey. Flight-line
spacing should be no greater than the minimum terrain clearance in order to avoid undersampling of the
magnetic anomalies. Flight lines should be flown perpendicular to the expected strike of sources of
interest.
Even after standard data corrections are applied, variations in terrain clearance can result in
uneven resolution of features across the survey area. Equivalent source continuation of the data can be
used to evaluate the magnetic field on a surface that is more reasonable for interpretation than the
surface on which the data were collected. This is demonstrated on helicopter magnetic data for a
hazardous waste site in Tennessee. Phase-shifts of magnetic anomalies due to the local direction of the
geomagnetic field vector can be corrected using a reduction-to-the-pole filtering operation. In many
cases, this has the effect of moving the anomaly peaks directly over the sources, as is demonstrated
using aeromagnetic data over an oil field in Oklahoma.
At the interpretation stage, various techniques and software tools are available for extracting
geologic and cultural information from the data. The magnetic fields of shallow sources can be
separated from those of deeper sources using a process known as matched bandpass filtering. Three
methods for locating magnetic sources in three dimensions and identifying properties of the sources are
demonstrated on aeromagnetic data for Tucson, Arizona.
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Aeromagnetic Mapping Of Hydrologically Important Faults, Albuquerque Basin, New Mexico
More LessData from high-resolution aeromagnetic surveys over the Albuquerque basin show expressions
of numerous hydrologically important faults that offset basin fill or volcanic rocks, many of which were
previously unknown. The faults generally strike northerly and are commonly sinuous and linearly
extensive, up to 50 km in length.
Aeromagnetic profiles across the faults reveal a range of signatures, from symmetric curves with
one inflection point to asymmetric curves with multiple inflection points. All the fault signatures can be
explained by the juxtaposition of lithologic layers having different magnetic properties. The asymmetric
signatures are distinctive and indicate a thin magnetic layer on the upthrown side and a thick magnetic
layer on the downthrown side of the fault, called the "thin-thick layers" model. The thicker,
downthrown layer may have resulted from sedimentation related to growth faulting. The model may
indicate a concentration of possibly high-permeability, coarse-grained material on the downthrown side
of the fault. This is corroborated by a comparison of water-level data to aeromagnetic signatures over
two splays of the hydrologically important Hubbell Springs fault system. Therefore, aeromagnetic data
have potential for providing information on the causes of subtle changes in water level that are important
for siting individual water wells.
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Aerogeophysical Measurements Reveal Collapseprone Hydrothermally Altered Zones At Mount Rainier Volcano, Washington
Authors Carol A. Finn, Thomas W. Sisson and Maryla Deszcz-PanNew helicopter-borne electromagnetic and magnetic data, combined with detailed
geologic mapping, reveal that appreciable thicknesses of mostly buried altered rock lie
mainly in the upper west flank of Mount Rainier identifying this as the most likely source
for future large debris flows capable of reaching now densely-populated areas. Most of the
summit, central core and upper east side of the volcano are free of significant volumes of
weak, highly-altered rock and are at relatively lower risk of collapse. In addition, the lack
of highly altered rock lies in the volcano's core may impede collapse retrogression and limit
volumes and inundation areas of future debris flows. High-resolution geophysical and
geologic observations yield an unprecedented view of the 3-D distribution of collapse-prone
altered rock at Mount Rainier, and have potential application to hazardous volcanoes worldwide.
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Developing Geophysical Signatures To Constrain Geologic Mapping
Authors Jeff Wynn, Sue Karl, Bruce Smith and Anne McCaffertyThe Greens Creek ore deposit is one of the largest producing volcanogenic massive sulfide
(VMS) deposits in southeast Alaska, and is found on north-central Admiralty Island in southeastern
Alaska. This deposit and all other known VMS prospects in the region are found only in Triassic Hyd
Group volcanic rocks. Future success of mineral exploration in the region is thus highly dependent on
accurate geologic maps that accurately portray where these Hyd Group rocks are located in a region
that is largely covered by dense vegetation, water, and tidal mud-flats. Because of this extensive cover,
we have used ground and airborne geophysical information to constrain the geologic mapping. A large
airborne geophysical survey, incorporating both electromagnetic (EM) and magnetic sensors was flown,
supplemented by ground magnetic and VLF-EM resistivity profiles over key areas where geology
could be unequivocally mapped. From these, we developed a matrix of geophysical signatures of key
rock units that was then used in predictive geophysical models (essentially pseudo-geology maps)
derived from the airborne data. Geologic ground control was then used to convert these models into a
substantially revised geologic map of the region that now shows more than a 30% increase in Triassic
Hyd Group rocks.
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Spectral Induced Polarization Studies Of Mine Waste Piles In Colorado And New Mexico
Authors David L. Campbell and Robert J. HortonTo help interpret spectral induced polarization (SIP) data collected in the field on waste piles
from sulfide mineral-bearing mines in Colorado and New Mexico, the piles were extensively sampled
and the SIP properties of the samples were measured in the laboratory. The laboratory spectra were fit
using 2-term Cole-Cole relaxations. Simple IP indices which can readily be compared with field data
were also defined and calculated from the laboratory results.
Samples containing unaltered sulfide minerals had strong relaxation features in the frequency
range from about 0.01-100 Hz. These relaxation features consist of resistivity values that start out low
and that continue to drop with increasing frequency, and phase values that are generally negative and
large and whose curves are concave down, over that frequency range. In most western U.S. mine waste
piles, however, the original sulfide minerals have been weathered and chemically altered to an extent,
changing to sulfate and oxide minerals such as jarosite, anglesite, and goethite. As this happens the
sulfide relaxation features on the corresponding spectra fade and disappear. The resistivity curves rise
and flatten over that frequency range, whereas the phase curves drop in absolute value and flatten or
even become concave up.
As water is added to dried samples, they become more conductive and their spectra change
markedly, typically changing from spectra like those of weathered mine waste to spectra that
superficially resemble those of material containing unaltered sulfide minerals.
Unfortunately, laboratory spectra generally fail to resemble spectra measured in the field. This
might be because of varying water content, or because of disturbances resulting from the sampling
process. We suspect, however, that inherent scaling factors are responsible for the differences between
spectra measured on cm-size laboratory samples and those measured on m-size field blocks.
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Using Resistivity To Map Acidic Waters At The May Day Mine Dump, Silverton, Colorado
Authors Bruce D. Smith, David L. Campbell and Winfield G. WrightThe May Day Mine dump is located on a mountain side above Cement Creek, north of Silverton,
Colorado. As Cement Creek flows past the May Day Mine, dissolved metal loads increase in the
stream, although there are no tributaries entering the stream reach. It had been thought that these metals
were being leached from the dump by snowmelt and rainwater. Induced polarization studies show local
pockets in the upper bench of the May Day Mine dump with acidic pore waters. The pore water appears
to get less acidic as it migrates through the dump. Nevertheless, water samples from a well located at
the toe of the dump are highly acidic, and they contain large concentrations of dissolved metals. An
airborne EM survey was recently flown over the area, which shows a linear conducting feature that
enters Cement Creek just south of the May Day Mine dump. The airborne survey was followed up with
ground geoelectrical surveys, which confirm the existence of the conducting feature and suggest that it
dips steeply and extends to a depth of at least a few tens of meters. We speculate that this conductor
may reflect a fissure zone that carries water. Such a fissure zone could be the source of some of the
water that enters Cement Creek near the May Day Mine. The metals dissolved in that water might
originate through natural processes, rather than human-caused ones.
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Using A Vernacular Language To Present Unfamiliar Lexicons In Mining And Reclamation
More LessThere are different contexts to describe man-made and natural elements in mining and its
associated landforms. All of them are important in perceiving the process of surface mining and in
making land-use decisions. The study of mining and reclamation must bridge the chasm between
science and art; one way is to use a common language. Too often, one discipline undervalues the other’s
expertise. Scientists, designers, and citizens can be aware of a wider mutual understanding in their
different perspectives of mining and landscape through the use of a vernacular language (i.e. the
normal/common spoken form) rather than filling sentences with their unique subject vocabulary
(lexicon). The public, governmental agencies, and industry can then more effectively communicate with
one another.
In sharing data and presenting site information at public meetings, it behooves industry and
consultants to speak with citizens and each other in easily understood forms or to explain specialized
expressions indigenous to a particular field. We speak of and view mining from four fundamentally
different disciplines:
1) Architectural—spoken by the “designer” (including land planner or landscape architect)
2) Natural context—the “science speak” of the geologist, hydrologist, or engineer
3) Social—regulated by health, safety, or transportation departments
4) Cultural—relating to aesthetics and customary beliefs
The author presents a spreadsheet of the four perspectives and includes citizen reaction to
permitting through newspaper articles. Regulatory consideration is taken from a State and Federal law
standpoint. People are more willing to accept the presence of mining if they understand the science and
if the industry is responsive to aesthetic and sociocultural factors.
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Integrating Electrical Geophysical Surveys And Subsurface Probing To Locate Karst Aquifer Recharge Features, Northern Illinois
Authors Shawkat Ahmed and Philip J. CarpenterSinkholes, solutionally enlarged bedrock fractures, soil pipes, and swallow holes are important
recharge features for shallow karst aquifers. These features also may transmit significant quantities of soil
water, sediment and waste material within the vadose zone. In many cases, however, sinkholes are filled
with sediment or debris and have no topographic expression. In this study, geophysical methods are
employed to noninvasively identify and characterize buried sinkholes associated with soil pipes, enlarged
bedrock fractures and conduits. Geophysical surveys suggest a 500-m long soil pipe, or networks of pipes
underlie a portion of the Perry Farm Park in Bourbonnais, Illinois. Boring and electrical resistivity
soundings suggest Silurian dolomite bedrock lies at a depth of approximately 4 m across most of the site,
and soil pipes are probably related to hydraulically active fractures within the bedrock. Electrical resistivity
pseudosections suggest an undulating bedrock surface. Buried sinkholes appear in inverted pseudosections
as 4-8 m wide low conductive zones. Electromagnetic conductivity profiling identified high conductivity
anomalies over the locations of buried sinkholes, over soil pipes and topographically low areas. Values as
high as 33 mS/m were recorded over the sinkholes and over portions of the pipe system, whereas the average
background conductivity is about 22 mS/m. A linear zone of elevated conductivity trends WSW from the
sinkholes, and may indicate a soil pipe linked to other sinkholes in a ravine. Some reaches of the pipe
system also exhibit small sand "volcanoes", suggesting subsurface liquefaction and upward movement of
sediment. Probing data indicate drill stem drops of 0.3 m close to a known buried sinkhole. These drops
may be due to cavities in the soil or weathered dolomite bedrock.
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Mapping A Paleochannel System Controlling Contaminant Migration At A Wood-Treating Facility Using Electromagnetics
Authors Stewart K. Sandberg, William Corso, Jessica R. Levine, Gary Newhart and Greg PowellA wood-treating facility, located near Sheridan, Oregon, has been the focus of a groundwater
contamination investigation. A geophysical survey was conducted in April, 2000. Objectives of this
geophysical survey included detection and delineation of coarse-grained channel-type deposits in the
unconsolidated section above bedrock. These deposits constitute preferential pathways for groundwater
flow, and hence contaminant migration through the subsurface. Contaminants consist of petroleumbased
creosote and pentachlorophenol (PCP) solutions. Dissolved phases of these contaminants
comprise the groundwater contamination expected, and the delineation of the extent of this
contamination was the overall objective of the investigation. In addition, dense non-aqueous phase
liquid (DNAPL) contamination was expected in accumulations in topographic lows of the bedrock
surface near the source area. Therefore, another objective of the geophysical survey was to investigate
any topography on the bedrock surface.
In this preliminary phase of the investigation, geophysical methods used consisted of transient
electromagnetic (TEM) soundings, a resistivity sounding, and an extensive terrain conductivity (EM-31)
survey. Due to the extensive sources of cultural interference at the site (buildings, fences, railroad
tracks, etc.), the geophysical survey was limited. Three profiles of 20-m central-loop TEM soundings
were obtained, along with some isolated soundings where it was thought that cultural interference could
be avoided. TEM approximate depth sections clearly identify the paleochannel system in cross-section.
Correlation with terrain conductivity response provides confidence in the terrain conductivity
interpretation of the paleochannel system in areas where only that type of data were obtained. One
resistivity sounding, and an adjacent TEM sounding, were modeled simultaneously to the same layeredearth
in order to test the idea of improving the resolution of the interpretation. This proved valuable in
delineating an electrical equivalent of the working model of the hydrogeological section based on nearby
drilling information.
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Supervised Classification Of An Arid Groundwater Flow System
Authors Alberto Barud-Zubillaga and Dirk Schulze-MakuchThis investigation addresses the hydrogeology of the White Sands National Monument (WSNM)
to determine the characteristics of an arid groundwater flow system using remote sensing methods. A
supervised classification using Landsat-7 data sets were used to identify the different terrains leading to
a better understanding of the White Sands, where depth to water table is closer in areas with lower
elevations and resulting in wetter or moister regions. Results showed that the ancient Lake Otero floor
has been carved deeper and deeper in the western margin of the White Sands area by water and wind
since the Pleistocene. The Alkali Flat is now exposing the ancient deposits of the Lake Otero floor. The
entire White Sands Dune Field today is increasing in size as the Alkali Flats and Lake Lucero shrink due
to dryer conditions as time goes on. The active dune field is growing on top of the ancient Lake Otero
floor to the east. Groundwater flow is to the west and discharges in the hydraulic sink of Lake Lucero, a
highly saline playa ephemeral lake.
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Detection Of Shallow Water Table Fluctuation Using The Spectral Analysis Of Surface Waves (Sasw) Technique
Authors Jaime O. Hincapie, Diane I. Doser, Deren Yuan and Mark R. BakerThe spectral analysis of surface waves (SASW) technique uses measurements of surface wave
dispersion to construct shear velocity versus depth profiles. Although the method is commonly used in
geotechnical investigations, applications to environmental studies have been limited. We have evaluated
the potential of SASW to monitor water table fluctuation at shallow depths (< 2 m) at two sites located
near the Rio Grande northwest of El Paso, Texas. The shallow depth of the water table and presence of
low velocity zones make use of the seismic refraction method difficult at these sites. The
inhomogeneity of sediment grain size and seasonal variations in sediment moisture and salinity also
make use of electrical techniques problematic. The first study site was located on an earth-fill levee ~20
m west of the Rio Grande. An irrigation ditch was located immediately west of the levee. We
successfully detected water infiltration from the ditch into the levee by comparing shear velocity profiles
from surveys taken before and after irrigation. The second site was located on homogeneous soils of a
historical channel of the Rio Grande (~10 m east of the present Rio Grande). Seasonal fluctuations of
the water table related to changes in river level were detected at this site and confirmed with borehole
information. Our results suggest the SASW method is effective for detecting the depth and fluctuation of
the water table and provides additional valuable information (e.g. shear velocity, shear modulus) when
coupled with other types of seismic surveys.
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Studies Of Grain Size Variations In Fluvial Sediments Of The Rio Grande Using Magnetic Techniques
Authors Diane I. Doser, Richard P. Langford, Mark R. Baker and Oscar Dena-OrnelasCharacterizing the heterogeneity of fluvial sediments in the Rio Grande Valley near El Paso,
Texas is important to understanding shallow groundwater flow, groundwater/river exchange and the
build-up of soil salinity. We have found the magnetic technique an effective way to rapidly evaluate
grain size changes within the river soils. This is in contrast to electrical techniques (resistivity,
conductivity) that are strongly affected by seasonal variations in soil moisture and salinity. Medium to
coarse-grained sands of the ancestral Rio Grande contain significant (up to 10% by weight) amounts of
magnetite, and thus appear as magnetic highs. We have conducted magnetic surveys at four study areas
within the Mesilla Valley northwest of El Paso. The sites have a range of sizes (10’s to 100’s of meters
in dimension) and geologic complexity (homogeneous channel sands to complex interfingered crevasse
splay and flood plain deposits). Borehole grain size analysis, as well as surface geophysical studies (e.g.
conductivity, resistivity, seismic, GPR) were conducted at the sites. Our results indicate we can detect
small crevasse splay (~2-3 m wide) channels buried at depths of 1-2 m, as well as larger channels (~20
m) at depths of ~20-30 m, through simple analysis of magnetic contour maps and magnetic gradients.
We believe the technique has the potential for rapidly evaluating the location of channels within the Rio
Grande valley, for corroborating surface soil maps and determining potential contaminant flow
pathways.
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Estimating Petrophysical Data From Borehole Geophysics
Authors M. Pokar, L.J. West, P. Winship and A.M. BinleyA study of the ability of ground penetrating radar and natural gamma logging to indicate
porosity, lithology and permeability in the unsaturated zone of a sandstone aquifer is described. The
aquifer (Sherwood Sandstone, UK) consists of a series of fluvially derived sequences fining upwards
from medium to fine sandstone and siltstone. Laboratory measurements on core samples of porosity,
grain size, mineralogy and hydraulic conductivity are reported. Vertical hydraulic conductivity is lower
than horizontal hydraulic conductivity, probably as a result of preferential sand grain orientation and
fine/medium sand lamination. Clay is the primary influence on hydraulic conductivity, although porosity
also has some effect.
Data for hydraulic conductivity, porosity and clay content are compared to dielectric constant
values from zero-offset 50 MHz cross borehole radar profiles and natural gamma activity. Natural
gamma activity increases with increasing clay content and reducing hydraulic conductivity.
Dielectric constant averaged over six months also correlates with clay content, despite scatter due to
variations in the moisture content during this period. Dielectric constant (unlike gamma activity) is very
strongly influenced by moisture content because of the very high dielectric constant of water compared
with those of mineral solids and air. However its correlation with clay content is preserved in the
unsaturated zone because clay rich layers also have high moisture retention. Dielectric constant
increases with reducing hydraulic conductivity when the profile is relatively wet, but not when it is drier.
However low hydraulic conductivity layers may be detectable using ground penetrating radar because
they show large seasonal fluctuations in dielectric constant.
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Application And Comparison Of Methods For Horizontal Flow Measurements In Fractured Bedrock
Authors William H. Pedler and Wayne MandellThe ability to measure ambient groundwater flow conditions is a valuable tool in the evaluation
of hydrologic properties in environmental and geotechnical studies. Vertical flow characterization
methods (spinner flow meter, USGS heat pulse, EM flow meter and Hydrophysical logging) are
commonly applied and readily accepted for hydrogeologic and hydraulic evaluation in both fractured
and porous mediums. The ability to detect and measure ambient horizontal flow in a wellbore provides
a means with which to further define the ambient flow conditions in an aquifer. A family of downhole
methods may provide the ability to highly resolve the location of horizontally flowing cells; measure the
velocity and direction; and possibly, evaluate the degree to which a fractured flow system behaves in a
porous and/or plate/channel flow manner. This enhanced definition could present a means to
dramatically improve our understanding of the subsurface flow system and remediation efforts.
To evaluate their appropriateness and accuracy, the US Army Environmental Center is presently
sponsoring a comparative study of methods to characterize horizontal flow in boreholes. As part of this
study, a preliminary field comparison was conducted at two US Army sites (Fort Campbell and
Jefferson Proving Grounds) both sited in fractured karst hydrogeologic settings. The methods applied
were;
1) Hydrophysical logging (HPL)
2) Colloidal Borescope Flow Meter (CBFM)
3) Acoustic Doppler Velocimeter (ADV)
4) K-V heat pulse flow meter (KVFM)
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Condition Risk Assessment Of Underground Utilities With Sewreel Seismic Imaging
More LessAmerica has a major investment in underground utilities beneath its cities, principally sewers and
pipelines, and there is widespread concern that this urban infrastructure has not been adequately
maintained to meet needs. Managers of these utilities rely on condition assessments based on internal
inspection and limited geotechnical information. These assessments are inadequate as they fail to
effectively sample, test and account for external ground conditions. Unexpected catastrophic ground
collapse due to pipe collapse is a relatively common occurrence. Excessive expenditure on new
construction, rehabilitation and foreshortened repair life are also consequences of the failure fully
understand and evaluate the influence of ground conditions above and around these utilities.
Recently, a specially developed seismic imaging technology called SEWREEL has demonstrated
the ability to locate significant voids or weak ground around underground utilities and to prevent
collapses. SEWREEL is similar to crosshole seismic tomography and provides detailed information
between the pipe and the ground surface. This method represents a significant improvement over
surface seismic methods. Its use to prevent collapses around a gravity main is illustrated with a recent
field example from Orlando, Florida.
The application of SEWREEL within a condition assessment program provides the means for
establishing the subsurface condition of the soils around the pipe in a reproducible, timely, and efficient
manner. Any comprehensive program of condition risk assessments for underground utilities should
include seismic imaging as a key component in a preventive engineering and risk assessment and
reduction strategy.
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The Use Of A Giant Magneto-Resistance (Gmr) Based Magnetometer For Differentiation Of Subsurface Electrical And Non-Electrical Materials
Authors T. David McGlone and Doria L. KutrubesGround penetrating radar (GPR) has been used with moderate success to locate electrical
conduits buried within reinforced concrete slabs. However, when conduits are located very near walls or
directly beneath rebar of similar size, GPR as a methodology of investigation alone has been inadequate.
X-ray techniques are also not useful as they emit dangerous gamma radiation, and both X-ray and EM
induction techniques penetrate only a few inches and cannot differentiate between rebar and conduits
cased in steel or aluminum. Therefore, the ability of a magnetometer with a wide frequency response
bandwidth is of interest for these situations.
This experiment uses Smoke Creek Instrument's IMAG01 magnetometer to explore the
frequency responses of several common construction materials such as pipes, conduits, and spikes. The
material samples are placed beneath 0.15 m of a natural, decomposed granitic soil with a consistency of
coarse sand similar to that used in concrete. The magnetometer is used to measure each situation and the
results presented herein. Measurements are first made with no artificial external field applied, and then
with a controlled source magnetic field.
The IMAG01 is a single-axis magnetometer based on the giant magneto-resistance (GMR) effect
and detects magnetic field strength of both static and electromagnetic fields. This version of the
instrument has a frequency response of DC – 100 kHz with a sensitivity of approximately 0.02 nT. As
such, it is expected to respond to the potential magnetic field of the ferrous material and the magnetic
component of the 60 Hz electromagnetic field of electrically-active wire without the controlled source
transmitter. With the transmitter, the instrument is expected to respond to the induced magnetic field of
the metallic materials as well as the potential and 60 Hz fields of the previous situation. Of particular
interest is the ability to differentiate between metallic, non-electrical materials and electrically-active
conduits when placed in close proximity.
The use of wide bandwidth receivers in electromagnetic geophysical surveys can provide
information that limited bandwidth instruments can not. A magnetic field receiver with a small physical
size allows array measurements of electromagnetic fields in a manner similar to arrays used for seismic
measurements over relatively small surface areas. This makes feasible 2D and 3D electromagnetic
imaging, although for purposes of this experiment, only 1D measurements were made.
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Ground-Penetrating Radar Amplitude Analysis For Archaeological Applications
More LessThe primary goal of most GPR surveys is to identify the size, shape, depth and
location of buried remains and related stratigraphy. The most straightforward way to
accomplish this is by identifying and correlating important reflections within twodimensional
reflection profiles. These reflections can often be correlated from profile to
profile throughout a grid, which can be very time consuming. Another more sophisticated
type of GPR data manipulation is amplitude slice-map analysis that creates maps of
reflected wave amplitude differences within a grid. The result can be a series of maps that
illustrate the three-dimensional location of reflection anomalies derived from a computer
analysis of the two-dimensional profiles. This method of data processing can only be
accomplished with a computer using GPR data that are stored digitally.
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Integrated Use Of Geophysics, Gis And Archeological Records To Locate Seneca Village
Authors Roelof Versteeg, Nan Rothschild, Diana Wall, Cynthia Copeland, Herbert Seignoret and Koray ErgunSeneca Village was an African American - Irish village located in what is now New York City's
Central Park. As part of the construction of Central Park the village was razed in the 1850s and no
surficial remnants remain. Increasing interest in the history of Seneca Village led to an effort to use a
combination of geophysics, archeology and GIS to locate possible remnants of the village. This effort is
complicated due to possible intense (but unknown) site modification during the construction of the park
and post park infrastructure construction. Using a range of geophysical tools (GPR, resistivity and
EM31) a suite of layers of geophysical data was collected. These were integrated in a GIS with a high
quality basemap (which contains the current park layout), the subsurface infrastructure layout and
orthorectified and digitized historical maps. By interpreting these results jointly a number of more and
less promising areas were identified which possibly can contain undisturbed remnants of Seneca Village.
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Numerical Inversion Of Borehole Flow Logs
More LessBorehole flow logs are most effectively interpreted in conjunction with other borehole data.
Other geophysical logs are needed to help identify the hydraulically active zones within the scatter of
flow measurement data produced by borehole conditions. Ambient hydraulic-head conditions also need
to be taken into account in flow-log analysis. Hydraulic-head differences between zones intersecting the
borehole can be eliminated from the interpretation by subtracting a flow profile obtained under ambient
conditions from another flow profile obtained under stressed (pumping or injection) conditions. Fitting
of hydraulic flow model predictions to the flowmeter data set insures that flow interpretations are
physically possible and yields quantitative estimates of both zone transmissivity and zone hydraulic
head. Applications of flowmeter log interpretation methods are illustrated using data obtained in
fractured sedimentary and crystalline bedrock aquifers in Arizona, Wisconsin, Kentucky, and New York.
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Borehole-Surface And Cross-Borehole Mise-Á-La-Masse Delineation Of A Radial Injection Tracer Experiment In Partially Saturated Fractured Basalt
Authors Robin E. Nimmer, James L. Osiensky and Kenneth F. SprenkeBorehole-surface and cross-borehole mise-à-la-masse (MALM) measurements were taken over
time during an eight-well, radial injection, tracer experiment in partially saturated, fractured, Columbia
River basalt. In this experiment, an enhanced conductivity tracer stream was energized directly through
a current electrode placed in the bottom of the injection well. A constant concentration tracer solution of
potassium chloride was injected continuously above a perched water table at an average rate of 10
liters/day under a constant hydraulic head for 34 days. An asymmetrical ground water mound developed
over time during which borehole-surface and cross-borehole MALM measurements were taken to
delineate migration of the tracer. A 15 x 15 array of porous pot electrodes (copper sulfate), located
symmetrically about the centrally located injection well, was used for the borehole-surface MALM. The
cross-borehole MALM utilized 66 downhole electrodes in the eight wells combined with seven transect
arrays each comprised of six land surface, porous pot electrodes. Changes in the electrical potential
distributions over time were contoured to delineate anomalies caused by the presence of tracer solution
in the fractured basalt. Borehole-surface measurements delineated the lateral migration of tracer over
time. The cross-borehole measurements helped delineate the depth of a possible preferential pathway in
the fractured basalt.
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A High-Resolution, Short-Range, Directional Borehole Radar
Authors David L. Wright, Jared D. Abraham, David VonG. Smith and S. Raymond HuttonThe U.S. Geological Survey (USGS) has recently developed a prototype directional
borehole radar system designed to complement a commercial system that the USGS has used for
several years for fracture mapping in crystalline rock. The new prototype is designed for higher
resolution, shorter range operation. We have numerically modeled several types of antennas
using finite difference time domain (FDTD) code. Based on the FDTD numerical modeling, we
designed, fabricated, and physically tested a pair of cavity-backed monopole antennas. The
antennas were then incorporated into a pulsed time-domain radar system. Tests of the system in
a laboratory sand pack and an outdoor pit have demonstrated antenna directionality, good
detection of small metal and dielectric targets, and radial resolution of a few centimeters. Such a
radar system, with some modifications, might find use in utility and other obstacle avoidance in
horizontal drilling operations.
An important part of the development of the directional radar is data processing and
visualization. We can process our data in a number of ways, including average waveform
removal, filtering, and range-gain compensation. After processing, radar data can be displayed
in three dimensions using a graphical user interface (GUI). Received waveforms are mapped
into a cylindrical volume, where each waveform fills a cylindrical wedge. The angle subtended
by each wedge depends on the total number of waveforms acquired azimuthally in the borehole
at each depth. The length of the image cylinder is a function of the number of stations occupied
by the radar in the borehole. The GUI allows the operator to visualize the data using a variety of
tools, including: rotate, zoom, annulus view, slicing, and control of the color table into which
data values are mapped.
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Airborne And Ground Geophysical Surveys For Locating And Mapping Underground Storage Tanks At Bellows Air Force Station, Hawaii: An Integrated Approach
Authors Sandra A. Takata, Jeffery B. Hackworth and Doug McConnellAirborne and ground based geophysical surveys were conducted at Bellows Air Force
Station (BAFS), Oahu, Hawaii to locate buried metal potentially representing underground
storage tanks (USTs) and other buried debris of environmental concern. The airborne survey
was used for reconnaissance to screen approximately 3,000 acres of land and shallow offshore
areas at BAFS for buried debris. Once the airborne anomalies were evaluated for their potential
to have a UST source, focussed ground surveys were conducted to confirm the location and to
more accurately characterize anomaly source objects.
BAFS was established in 1917 as the Waimanalo Military Reservation and was
deactivated in 1970. Throughout BAFS’ history, USTs along with their associated pipelines
were installed to maintain base activities. Over the years, structures associated with the USTs
have been demolished resulting in the uncertainty of the UST locations.
Over 650 geophysical anomalies were identified in the airborne survey (both
electromagnetic and magnetic data). Following review of the flight path videos and further
scrutinizing of the data, most of the anomalies were attributed to surface cultural features or
geology and were eliminated from further investigation. The remaining 73 anomalies were
located on the ground and visual field reconnaissance at these locations eliminated some
anomalies from further investigation based on the terrain being unsuitable for construction and
surface metal hidden under thick canopy. Eighteen of the 73 anomalies were selected for
detailed ground surveys. The follow-up ground-based geophysical surveys were conducted
using frequency- and time-domain EM, magnetics and ground penetrating radar. Select
anomalies identified in the ground geophysical data were trenched and the sources excavated.
The technical approach of integrating site knowledge with airborne geophysical targets to
select ground geophysical survey locations proved to be efficient and successful in locating and
characterizing buried debris at Bellows Air Force Station.
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Sh-Wave Seismic Reflection Imaging Of Anomalous Foundation Conditions—Mississinewa Dam, Indiana
Authors Edward W. Woolery, Ron L. Street and Stephen T. HornbeckThe Mississinewa Dam in northern Indiana is a 2.4-km-long, 43-m-high, compacted earth-fill
embankment founded on approximately 25 m of glacio-alluvial sediment that overlies Mississippian
limestone bedrock. Differential settlement at the crest manifested in 1988; instrumentation indicates that
the increased rate of vertical deformation initiated in 1982, and has been approximately constant since
that time. As part of an effort to understand the cause for the irregular settlement, a noninvasive
seismic-reflection survey was performed in the general area of the crest deformation. The objectives
were to acquire a high-resolution image of the bedrock surface beneath the structure, and to identify
signal anomalies in the data that may be indicative of severely weathered rock or karst morphology. The
SH-wave reflection profile successfully imaged a coherent, relatively continuous signal consistent with
known points of top-of-rock elevation. Depth to bedrock along the profile varied from 20 m near the
right abutment, to 52 m near the outlet works at the western terminus. A sharp amplitude anomaly in the
bedrock reflector directly below the settlement area was imaged, and interpreted as a karst structure.
Subsequent geotechnical drilling of the anomaly found a 6.5 m discrepancy in the expected top-of-rock
elevation, as well as soft foundation fill, thus confirming the geophysical interpretation.
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Use Of Seismic Reflection Amplitude Versus Offset (Avo) Techniques To Image Dense Nonaqueous Phase Liquids (Dnapl)
Authors Michael G. Waddell, William J. Domoracki and Tom J. TemplesField experiments were conducted to determine the location and distribution of subsurface DNAPL
contamination at two DOE sites by use of two-dimensional, high-resolution seismic reflection surveys and
borehole geophysical data. These studies make use of seismic reflection amplitude versus offset (AVO)
technology to directly detect the presence of subsurface free-phase DNAPL.
The research was conducted at Savannah River Site, SC and Hanford Site, WA. At each of these sites
research consisted of site evaluation, seismic model studies, seismic acquisition, processing, and interpretation.
The seismic model studies were undertaken prior to field acquisition to determine the likelihood of an AVO
response from the DNAPL. The full Zoeppritz equations were used to create the model responses.
At M-Area, Savannah River Site the model data show that at the base of the DNAPL plume the
seismic amplitude should flip polarity on the far offsets (Class 2 AVO). The 2D seismic reflection field data
processed into near and far offset stacks indicate such anomalies occur at the depth where the known DNAPL
occurs. At the 200 West Area, Hanford Site the DNAPL is suspected to pool at two different stratigraphic
intervals. The model data show that at the upper interval, the Hanford Fine/Plio-Pleistocene boundary,
amplitude should decrease along the boundary if DNAPL is present (Class 1 AVO anomaly dim-out). At the
lower boundary the models show that, if DNAPL is present, the seismic amplitude should become very negative
(Class 3 AVO bright spot). A series of 2D seismic lines were acquired and processed. In the areas where
DNAPL was suspected to occur (high concentrations measured in wells) seismic amplitude was found to
decrease dramatically along the Hanford Fine/Plio-Pleistocene contact and along the Plio-Pleistocene/caliche
contact the seismic amplitudes became even more negative consistent with the model study.
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Investigation Of Seepage Through An Earthen Dam Using Ground Penetrating Radar
Authors Mark J. Howell and Scott J. LudlowA GPR survey was conducted during a seepage investigation of an earth dam. The survey
successfully imaged areas where perched water breached a geotextile drain fabric, delineated mounded
groundwater, and revealed structural features within the dam. This information, in conjunction with an
array of detailed geotechnical data assimilated by others, was used to evaluate seepage conditions and
develop remedial procedures. This paper presents images of selected features within the dam and
documents the methodologies used to for data acquisition and processing.
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A Gpr Mapping Survey On Jacui River
Authors Jandyr. M. Travassos and Adekunle. A. AdepelumiA combined marine and land GPR survey was conducted on the Jacui river, southern Brazil,
where it flows close to an abrupt end of the Serra Geral Cretaceous magmatic cover. The extruded basalt
overlays sandstone of the Triassic/Jurassic period. The survey covered an area on the river and on its
bank opposite to the basalt. The land survey was conducted on medium to coarser sand with occurrences
of gravel. About half of the area was forested displaying variable topography. The field configuration
allowed depths of penetration in excess of 15 m on land. Most lines averaged 200 m in length covering a
total area of 90,000 m2. Landlines successfully mapped the top of the sandstone unit below the soil
cover. A previously unknown landfill was also mapped. River lines revealed a rocky bottom with strong
and chaotic reflections reaching the end of the data window.
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Integrating Geophysical Methods In A Hydrogeological Pilot Study In The Northern Coastal Plain Of Rio De Janeiro State - Brazil
Authors Abel Carrasquilla, Luiz Geraldo Loures and Sergio L. FontesON/MCT and LENEP/UENF undertook a geophysical pilot study in the Northern
Coastal Plain of Rio de Janeiro State, near Rio das Ostras Town, 200 km NE from Rio de Janeiro
City. This region is characterized by both scarcity of drinking water for human consumption
and existence of saline intrusions, this last caused mainly by the presence of a complex
quaternary geology and excessive exploitation of aquifers. The research was performed by
employing magnetic and electromagnetic methods (time (TEM) and frequency (FEM) domains),
in order to evaluate the effectiveness of these methods in this particular geological
condition to allow the formulation of a more comprehensive hydrogeological research in the
region in near future. FEM profiles and one-dimensional interpretation of TEM data were capable
of depicting different geological formations, to locate the main aquifer and to reveal the
fresh-saline water contact. On the other hand, the magnetic method showed the presence of
a NE-SW direction geological fault, which is located just in the geological contact between
sand and clay, coincidentally with the fresh-saline water contact. Our results evince a promising
future for using electromagnetic geophysical methods in this region in small scale surveys,
as well as it confirmed that a precursory regional survey, chiefly using the magnetic method,
is essential in the indication of most suitable places to locate productive fresh-water wells.
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An Empirical Approach To Interpretation Of A Challenging Helicopter Electromagnetic Dataset
Authors Louise Pellerin, Victor F. Labson and Victor F. LabsonA helicopter electromagnetic survey acquired at the U.S. Deptartment of Energy Idaho National
Engineering and Environmental Laboratory used a traditional mining airborne method flown at low
levels for detailed characterization of shallow waste sites. The low sensor height, used to increase
resolution, invalidates most assumptions traditionally used in processing HEM data. Although the survey
design strategy was sound, interpretation techniques routinely used in industry, proved ineffective.
Apparent resistivity maps were severely distorted, and hence unusable, due to low flight height effects,
high magnetic permeability of the basalt host, and the conductive, highly three-dimensional nature of the
waste site targets.
To accommodate these interpretational challenges we modified a one-dimensional inversion
routine to include a linear term in the objective function to allow for the magnetic and three-dimensional
responses in the in-phase data. Although somewhat of an ad hoc approach the use of this term in the
inverse routine, referred to as the shift factor, was successful in defining the waste sites and reducing
problems due to the low flight height and magnetic characteristics of the host rock. Data from three areas
processed with this scheme highlight different interpretational aspects of the survey. Wastes sites were
delineated with the shift terms in two of the areas, allowing for separation of the anthropomorphic
targets from the natural one-dimensional host. In the other area, the estimated resistivity and the shift
factor were used for geological mapping. This was particularly interesting in that the disturbance of the
soil in the near surface was mapped with the shift term, because of the high magnetic content of the
native soil.
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Monitoring Of An In-Situ Air Sparging Experiment Using Electrical Resistivity Tomography
Electrical resistivity tomography (ERT) was successfully used for monitoring the air distribution
in an in-situ air sparging (IAS) experiment at the Oregon Graduate Institute of Science and Technology.
The experiment evaluated the removal of residual gasoline from the source zone of a simulated gasoline
spill in a tank filled with water-saturated sands The percent difference in resistivity provided a
quantitative view of air saturation in the sand tank. The initial background resistivity image suggests the
tank filled with water-saturated sands is geoelectrically non-homogenous though the sands in tank are
homogeneous in the sense of grain sizes. The background resistivity images also showed the evidence of
trapped residual gasoline around the water table.
The in-situ air sparging (IAS) in three different modes was carried out. The IAS at a flow rate of
20 cubic feet per minute (cfm) resulted in a larger radius of influence than the IAS at 5 cfm. At the same
air flow rate, the IAS in the pulse mode created a larger affected zone than the IAS in the continuous
mode. Two weeks was not long enough for the trapped air to escape from the tank. Unlike the
continuous IAS operation, the pulse operation produced a more complex air distribution in the tank.
Skewed airflow at both 5 cfm and 20 cfm flow rates indicated that there are preferential pathways in the
tank.
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Conductivity And Susceptibility Mapping Using Broadband Electromagnetic Sensors
Authors Haoping Huang and I.J. WonThe apparent conductivity and apparent magnetic permeability are derived from broadband
electromagnetic data using a conductive and magnetic half-space model. The apparent permeability is
first estimated from the inphase and quadrature (or amplitude and phase) data at a low frequency. This is
followed by the computation of the apparent conductivity at each frequency, using the pre-computed
apparent permeability. The apparent conductivity may be computed from broadband data using five
different algorithms. In general, three of the five methods yield a unique solution; the three yielding a
unique solution are the apparent conductivity defined from inphase, phase, or phase-amplitude.
A suite of synthetic data based on two-layer models is presented and it is concluded that the
phase-amplitude approach is preferred method. The apparent permeability is virtually independent of the
conductivity contrast at low induction numbers. However, the permeability contrast affects the apparent
conductivity, especially when a resistive layer overlays a conductive basement. The field data examples
show the usefulness of the broadband electromagnetic data and the resultant frequency-dependent
conductivity-permeability maps for characterizing complex environmental sites.
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Transient Controlled-Source Electromagnetic Induction In A Uniaxial Half-Space
Authors Mansour A. Al-Garni and Mark E. EverettAnisotropy in Earth materials often leads to unexpected geophysical responses that can not be
anticipated from simple isotropic models. In this paper, we investigate the effects of lateral anisotropy on
the time-domain, controlled-source electromagnetic (CSEM) response of a uniaxial conducting half-space
excited by a horizontal loop. In the theoretical results, a “paradox of anisotropy” is observed in which the
response is more pronounced along the strike of relatively conductive fractures. This is opposite to what
one might expect based on the isotropic theory. The results presented here have applications to fracture
mapping in bedrock.
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Inversion Of Multifrequency Electromagnetic Data To A Conductive And Permeable Sphere
Authors Haoping Huang and I.J. WonWe have developed an algorithm for inverting broadband electromagnetic (EM) data to
determine its electrical conductivity, magnetic permeability, radius, and depth using a sphere model. The
algorithm is based on traditional non-linear inverse methods with singular value decomposition. Studies
using synthetic GEM-3 broadband EM data indicate that the technique is dependable and produces fastconverging
solutions.
The geometrical parameters obtained through this technique are very close to true values, and the
electric parameters for noise-free data are correct to within a few percent. When a 10 percent noise is
added to the data, the geometric parameters are still well determined, but the electric parameters are
poorly resolved, particularly for a sphere that is both conductive and permeable. We also present an
analysis of the relative importance and reliability of the model parameters based on their partial
derivatives with respect to EM response.
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Quantitative Geoelectric Mapping Of The Hydraulic Conductivity Field
More LessThis paper describes in a stochastic framework how geoelectrics can be used to quantitatively
map the hydraulic conductivity field. This is achieved by describing both a microscale and a macroscale
model of aquifer electrical and hydraulic flow. The microscale model reveals that electrical and
hydraulic conductivities are a function of two common microscopic parameters. They are connected
pore volumes and connected pore surface areas, both assumed to be spatial random variables. Assuming
either a predominating pore-volume or pore-surface electrical flow environment, the self-similar scaling
of pore sizes, resulting in skewed pore size distributions, produces power-law relationships between
electrical and hydraulic conductivities and porosity and specific surface area. In turn these power-law
relationships imply a log-log linear relationship between electrical and hydraulic conductivities. The
coefficients of this log-log linear relationship are a function of several unique aquifer properties.
However, they can be field calibrated by linearly regressing coincident and equal-scale field
measurements of electrical and hydraulic conductivity. This electrical-hydraulic conductivity
relationship offers the quantitative physical rationale for converting inexpensive field measurements of
electrical conductivity into hydraulic conductivity.
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Constrained Time-Lapse Resistivity Imaging Inversion
By M.H. LokeChanges in the subsurface resistivity with time are frequently measured by conducting 2D or 3D
resistivity imaging surveys over the same area at different times. Examples of such studies include
mapping the flow of water through the unsaturated zone, changes in the aquifer saturation due to water
extraction, subsurface flow of chemical pollutants and leakage from dams. One common technique to
interpret the data from such surveys is carry the inversion of each data set individually and then
determine the relative changes in the subsurface resistivity from the differences in the resulting models.
This approach can lead to artifacts in the sections showing the relative changes in the subsurface
resistivity. In order to reduce such artifacts, a joint inversion technique that uses the results from the
inversion of the first data set as a reference model to constrain the inversion of the later time data sets is
used. Several types of cross-model constrains are examined. They are the simple damped or Marquardt
constrain, the l2 norm least-squares smoothness constrain and the l1 norm or robust constrain. By using
the appropriate cross-model constrain, artifacts in the sections showing the relative changes in the
subsurface resistivity can be significantly reduced. Where the changes in the model resistivity values are
expected to vary spatially in a gradual manner, the l2 norm constrain is more appropriate. When the
changes have sharp boundaries, the l1 norm constrain gives better results. In cases where the relationship
between the resistivity of the subsurface material and water content follows Archie’s Law, the change in
the water saturation can be estimated from the change in the model resistivity.
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Use Of Electromagnetic Induction Tools In Salinity Assessment/Appraisals In Eastern Colorado
More LessElectromagnetic induction (EMI) is a relatively low-cost and rapid method for measuring,
assessing spatially soil salinity. The two studies were conducted to evaluate data obtained with a
single-frequency (EM38 meter) and multi-frequency (GEM300 sensor) EMI instruments and to relate
apparent conductivity measured by these instruments with the more conventional conductivity of the
saturated soil extract. These two studies were geo-referenced for soil correlation sampling, salinity
mapping and future monitoring of salinization and/or degradation. The one study area (54 hectares)
utilized a subset sample area for the comparison procedure; the other site (323 hectares) was conducted
solely with the EM38 instrument. Data was incorporated into modeling programs and further used into
mapping software to develop two-dimensional maps.
Correlation coefficients between the two instruments ranged from 0.8039 to 0.8617. Although
the GEM300 sensor predicted somewhat less accurately the conductivity of the soil samples collected
and also produced higher apparent conductivity measurements, spatial patterns of apparent and
electrical conductivity produced by the two instruments were similar, reasonable and practical for the
end user, the agricultural producer.
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A New Fast Method To Measure Azimuthal Resistivity Anisotropy
More LessApparent electric anisotropy can be estimated with an electrode configuration formed as the letter
X. One electrode is placed at the end of each branch of the X and one is placed at about 1/3 of the
distance from the center. Together with one electrode at the center, by the instrument, this makes a total
of nine electrodes. By combining different electrodes it is now possible to measure the potential
difference to current ratio in a large number of ways. Wenner arrays can e.g. be formed along the two
diagonals of the X and square array measurements can be made with the four outermost electrodes. It is
convenient to use multi-channel equipment since many electrode combinations can be measured quickly.
For single reading methods we usually normalize the data by calculating an apparent resistivity.
The X-configuration measurements are also normalized, but in this case we use a homogeneous
anisotropic half-space for the normalization. There is of course no explicit formula for this conversion.
The data are fitted to the model in a least-squares sense by Monte-Carlo inversion. The output of the
inversion is apparent resistivity, apparent anisotropy direction and apparent anisotropy coefficient. The
apparent resistivity is defined as the geometric mean of the horizontal resistivities and it is usually a
good estimate of the bulk resistivity of the investigated rock volume. The influence of any local
heterogeneity is small due to the large number of measurements with different geometry and the rather
large electrode separations.
Examples are shown where X-configuration measurements have been performed on Precambrian
rocks in Sweden. The apparent anisotropy direction correlates well with direction of bedding in folded
supracrustal rocks and with the direction of foliation in metamorphic rocks.
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Problem Of Inductive Interference Between Current And Measuring Lines At Electrical Sounding
Authors Abbas Mohammed Abbas, Alexey Bobachev, Alexandr Karinski and Vladimir ShevninVertical electrical sounding (VES) is rather simple and very popular geophysical method with
wide spectrum of application for the decision of shallow depth hydrogeological, engineering geological
and ecological problems. During the last few years VES method has been transformed to a very
powerful technology, called electrical imaging or electrical resistivity tomography (ERT). At the field
registration of electrical signals in measuring line, some problems can arise and produce distortions,
which sometimes can spoil the data obtained. For realization of great amount of measurements at ERT,
pulsed current with short impulses (0.5-2 sec) is used. Such current is very close to AC. An example of
field data, highly distorted at large distances was the main cause of this investigation. In this report the
origin of induction is considered. This inductive interference is more dangerous in the case of low
ground resistivity. Calculations for Schlumberger array and dipole axial array were made. Criteria for
checking inductive interference are formulated. Knowledge of this phenomenon can help in avoiding
distortions and receiving high quality field data. Some other sources of distortions are also discussed.
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Anomalies From Horizontal Metal Pipes In Resistivity And Ip Fields
Authors Albert Ryjov and Vladimir ShevninIn urban and industrial areas geophysical methods are frequently applied for investigation of
metal pipes in ground or for other problems decision in their vicinity. Study of such pipes’ influence can
include: 1 - detection of a pipe (its position, orientation, depth), 2 - estimation of pipe technical
condition (corrosion, quality of isolation), 3 - distorting influence of a pipe on the fields of electrical and
EM methods at the decision of various geological problems. I.e. the pipes can be both object of study
and noise. For resistivity and IP methods pipes more often appear as noise, though from these methods
some pipes' parameters can be received.
It is possible to apply numerical methods to model pipes influence on apparent resistivity and IP
fields, though for a simple problem - rectilinear indefinitely long pipe - the application of analytical
method based on the strict account of boundary and starting conditions at the decision of the Laplace
equation is possible. The analytical decision allows estimating influence of a pipe in a wide range of
pipe and environment parameters. The problem is decided for horizontal cylinder with a covering in a
field of a point current source.
Influence of several pipe's parameters was investigated. Among these are depth and orientation
of pipe, pipe and covering resistivity, influence of array's and current electrodes' removal from the pipe
axis. In all cases IP values are more sensitive to pipe's parameters changes in comparison with apparent
resistivity. Pipe influence grows with current electrode approaching to the pipe axis and with lowering
the resistivity of covering.
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Transmission Line Approximation Of Pipelines With Cathodic Protection
Authors A. Mousatov and E. NakamuraThe effective and reliable pipeline exploitation requires regular control of their technical
condition. Now electromagnetic geophysical methods are widely applied to determine a position and
depth of the pipeline and to estimate a state of its electrical insulation and cathodic protection operation.
One of most effective methods for the solution of the specified problems is the non-contact
measurement of magnetic field created by the current, flowing in the pipeline. The cathodic protection
station or external generator connected directly to the pipeline and grounded in infinity can be used as
such current source.
The forward problem solution for the pipeline with variable insulation resistance and the pipe
conductivity requires application of numerical mathematical methods for 3D medium. Even in the
simplified 2D statement, assuming axial symmetry, the problem remains enough difficult, taking into
account specific geometry of model and very high contrast of cylindrical layers’ resistivities.
In this paper the approximation method to calculate the magnetic field above the pipeline is
considered. The transmission line approximation is used to substitute the pipeline with variable electrical
resistance of insulation. In this case the current distribution along the pipeline is obtained. Then the
magnetic field from the current variating along the pipeline is calculated. The dimension of a problem is
lowered and the analytical solution can be received with the considered approach.
On the basis of the described approximation the models of pipelines with zones of a various
extent of electrical insulation damages were accounted. The derivative of a magnetic field along an axis
of the pipeline corresponds to a current outflow, which is, in this case, quantitative characteristic of the
insulation condition.
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Time Domain Em Profiling Over A Shallow Conducting Target
By J.B. MerriamAn em 37-47 pro le over a known target at one to three m depth is described here. The response
of the highly conductive target is easily separated from the background response of the moderately
conductive till and is traced from the earliest gate of the em-47 ultrahigh range (7 s after turn o ),
to the latest gates of the em-37 medium range (28ms after turn o ). Three separate responses are
identi ed on the basis of their decay characteristics and spatial variation. They are interpreted to
be: a galvanic response, a vortex response, and a viscous magnetic remanence (VRM) response.
In time and frequency domain electromagnetics the vortex response is often expected to be the
dominant response, indeed, the galvanic response has only recently been added to many modeling
packages. In this environment - a very good conductor, with high permeability, at shallow depth in
a moderately conducting host, the galvanic response is the largest, followed by the viscous magnetic
remanence. The vortex, or inductive, response is only briefly above the noise level.
Many other em surveys have been performed over this target, including VLF, em-31 and
maxmin. These show either no response, or a response that is barely above the noise level. The
frequencies used in these techniques (less than 56 kHz) are probably too low to strongly excite the
galvanic or VRM modes, and it is possible that they are only exciting the relatively weak vortex
mode. Thus, time domain em is not only e ective in this environment, it is the only one of the
common em techniques that is e ective.
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Joint Influence Of Resistivity Anisotropy And Inhomogeneity For A Single Dipping Interface Between Isotropic Overburden And Anisotropic Basement
Authors Evgueni Pervago, Aleksandr Mousatov and Vladimir ShevninThe media with joint influence of anisotropy and inhomogeneity have large practical interest.
The difficulty of practical anisotropy studying with collinear arrays results from the fact, that anisotropy
exhibits itself weaker, than inhomogeneity at equal resistivity contrasts. The relative anisotropy and
inhomogeneity influence is considered with the help of mathematical modeling for gentle dipping
interface with anisotropic basement. The algorithm is based on the integral equations’ method for 3D
models with anisotropy.
This model is considered for three different directions of strike of dipping interface and strike of
anisotropic basement, in comparison with isotropic model of dipping interface and horizontally - layered
model with the anisotropic basement. The modeling data are submitted as azimuthal diagrams and
results of their spectral analysis. Spectral analysis helps to receive some diagnostic parameters for
anisotropic - inhomogeneous media.
All resistivity arrays on sensitivity to anisotropy are divided into two groups: collinear arrays
(Schlumberger, pole-pole, pole-dipole, dipole axial) with the axes ratio equal λ, and non-collinear arrays
(dipole equatorial, T-array, etc.) with sensitivity up to λ5. Most sensitive to inclined contact is dipole
axial array, and it is the least sensitive to anisotropy.
The inhomogeneity influence is displayed in the first harmonic of azimuthal diagrams’s
spectrum, and anisotropy - in the second harmonic. The inhomogeneity also influences on the second
harmonic. The absence of the first harmonic (and following odd ones) testifies to absence of
inhomogeneity influence. For comparison of relative influence of anisotropy and inhomogeneity, it is
necessary to consider the ratio of the sum odd to the sum of even harmonics. When O/E ratio is >1, the
inhomogeneity influence prevails, and when <1 - the anisotropy influence prevails. At joint influence of
basement anisotropy (λ = 2) and dipping interface (dip is 5°), the anisotropy influence prevails only for
non-collinear arrays.
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Application Of Geophysical Methods To Delineate Contamination In Fractured Rock At The University Of Connecticut Landfill, Storrs, Connecticut
Authors Carole D. Johnson, John W. Lane Jr., John H. Williams and F.P. HaeniAn integrated suite of geophysical methods was used to characterize the hydrogeology of a
fractured-rock aquifer to identify contamination or pathways for contaminant migration near a former
landfill at the University of Connecticut, Storrs, Connecticut. Surface-geophysical methods were used to
identify the dominant direction of fracture orientation and to locate potential leachate plumes. Two
shallow, electrically conductive anomalies near surface-water discharge areas north and south of the
landfill were interpreted as leachate plumes. Two other sheet-like electromagnetic (EM) anomalies were
identified and targeted for drilling and borehole-geophysical investigation. These methods were used to
determine the location, orientation, and lateral continuity of fractures and to quantify the hydraulic
properties of the transmissive fractures.
One borehole was located to intersect an anomaly observed at a depth of about 18 meters. The
EM-conductivity log measured a high electrical conductivity anomaly at a depth of 21 meters, which
coincides with a layer observed to contain sulfide mineralization. The lack of high conductivity fluids in
the borehole supports the interpretation that this anomaly is caused by a lithologic change rather than by
fractures that contain conductive leachate.
The second borehole was positioned to intersect a conductive feature at a depth of about 18
meters. A fracture with similar strike and dip was observed at a depth of about 22 meters in optical- and
acoustic-televiewer images, in borehole-radar surveys, and was determined to be hydraulically
conductive during heat-pulse flowmeter tests. This feature was also characterized by a high-conductivity
spike in the EM log. Although the specific conductance of the fluid in this depth zone was high, it could
not account for the spike in the EM log. The anomaly is interpreted to be caused by a combination of
conductive fluids in the fracture and of conductive minerals in the rock. Water-quality samples from a
discretely isolated zone near a depth of 22 meters indicated the presence of landfill leachate. This
investigation illustrates the effectiveness of the use of combined geophysical methods for identification
and evaluation of electrically conductive contaminant plumes.
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Deterministic Deconvolution Of Ground-Penetrating-Radar Data At A Limestone Quarry
Authors Jianghai Xia, Tom Weis, Evan Franseen and Richard MillerA 30 m by 30 m two-dimensional grid was designed on a flat bench behind a quarry face of
predominantly limestone with thin shale layers located at a Bonner Springs, Kansas site to test the utility
of ground-penetrating radar (GPR) for stratigraphic studies. GPR data were collected along seven lines
parallel to the quarry face and seven lines perpendicular to the quarry face, each separated by 5 m, using
50 MHz, 100MHz, 200 MHz, and 400 MHz antennas. As a part of the project, confirmation of reflection
events, ringing suppression, and velocities of electromagnetic (EM) wave propagation in the limestone
were studied. GPR instrument wavelets were successfully collected in the air. With a known GPR
instrument wavelet, ringing in GPR data was significantly suppressed by a deterministic deconvolution.
The validity of using a wavelet acquired in air as the operator for deterministic deconvolution was
shown in the real-world application of a radar system with 400 MHz antennas to a quarry site consisting
of interbedded limestones and shale partings. A total of 78 horizontal holes were drilled in key locations
on three exposed quarry faces where data were acquired before and after conductive steel rods, 1.5 m in
length, were placed in the holes. Diagnostic GPR responses from the horizontal steel rods serve as
known reflectors. The steel rods provide critical information for: 1) confirmation and nature of specific
geologic reflection events in the GPR data, 2) GPR resolution limits, 3) accuracy of velocities calculated
from common-midpoint data, and 4) identification of multiples. The effectiveness of suppressing ringing
waveforms suggests that the deterministic deconvolution of GPR data with the GPR instrument wavelet
in the air should be included as a standard GPR data processing step.
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Application Of A Geophysical “Tool-Box” Approach To Characterization Of Fractured-Rock Aquifers: A Case Study From Norwalk, Connecticut
The U.S. Geological Survey conducted a geophysical investigation at a site in Norwalk,
Connecticut where solvents have contaminated a fractured-rock aquifer. Borehole, borehole-toborehole,
and surface geophysical methods were used to characterize the bedrock fractures, lithologic
structure, and transmissive zone hydraulic properties in 11 boreholes and their vicinity. The
geophysical methods included conventional logs, borehole imagery, borehole radar, flowmeter, and
azimuthal square-array dc resistivity soundings.
Integrated interpretation of geophysical logs at borehole and borehole-to-borehole scales
indicates that the bedrock foliation strikes northwest, dips northeast, and strikes north-northeast to
northeast, dips both southeast and northwest. Although steeply dipping fractures that cross-cut foliation
are observed, most fractures are parallel or sub-parallel to foliation. Steeply dipping reflectors
observed in the radar reflection data from three boreholes near the main facility building delineate a
north-northeast trending feature. Results of radar tomography conducted close to a suspected
contaminant source area indicate that a zone of low velocity and high attenuation exists above 50 feet
in depth - the region containing the highest density of fractures. Flowmeter logging was used to
estimate hydraulic properties in each of the boreholes. Thirty-three transmissive zones were identified
in 10 of the boreholes. The vertical separation of the transmissive zones in a borehole typically is 10 to
20 feet.
Open-hole and discrete-zone transmissivity was estimated from flowmeter data acquired under
ambient and pumping conditions. The open-hole transmissivity ranges from 2 to 86 feet squared per
day (ft2/d). The estimated transmissivity of individual transmissive zones ranges from 0.5 to 70 ft2/d.
Draw down monitoring in nearby boreholes under pumping conditions identified hydraulic connections
along a northeast-southwest trend between boreholes as far as 560 feet apart. The vertical distributions
of open fractures can be described by power law functions, which suggest that the fracture network
contains transmissive zones consisting of closely spaced fractures surrounded by a less fractured and
much less permeable rock mass.
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Lateral And Vertical Delineation Of Water Producing Fractures And Zone Specific Water Quality Data With Csamt And Hydrophysical Logging
Authors Martin J. Miele, Tony Morgan and Greg BauerFracture zones and faults are typical targets for groundwater production in areas characterized by
hard bedrock. Groundwater production and water quality may vary within those aquifers. The vertical
and lateral interconnection between fracture sets is a major consideration for the suitability of a
sustained water producer. The site for this investigation is a tree nursery facility in Southern California,
in San Diego County. The operation needs added water supply from additional well(s). The area
consists of rolling hills that abut the San Luis Rey River. The geology in the area is characterized by
granitic bedrock reported to be tonalite.
One of the existing and favorable wells on the facility (favorable well) was drilled to a depth of
approximately 1,000 feet. The well had an artesian flow of 55 gpm. A widely spaced CSAMT (MT)
survey was conducted along a traverse located adjacent to the well and across the local valley. The
geologic or geoelectric strike of the area was known, therefore, the MT data was processed in transverse
magnetic mode. The CSAMT traverse indicated that the well was drilled into a system with two
anomalous zones interpreted to be major fracture zones. One extended down to a depth slightly greater
than 1,000 feet. A second anomalous zone interpreted to be fractures existed at depths greater than
2,000 feet. The two systems are apparently unconnected in the MT data. The shallow fracture zone was
logged with HydroPhysicalä testing which indicated four major water bearing fracture systems with
varying water quality conditions. The deeper fracture system correlates with another local well reported
to be completed at depths greater than 2,000 feet. Water quality varied between the wells.
Three additional CSAMT traverses were conducted in the valley and local hills. A shallow
fracture system and deeper fracture system occurred in the data. The fracture systems correlate between
traverses.
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3-D Gpr Imaging Of The Neodani Fault, Central Japan
Authors Ernest C. Hauser, Daiei Inoue, Shintaro Abe, K. Kusunoki and Yoshida OdaGPR data collected across a segment of the Neodani Fault in the Tokai region of central Japan
represents the first 3-D GPR data successfully collected across a major seismogenic fault in Japan.
Despite the inherent difficulty of GPR to significantly penetrate wet, clay-rich soils, a 3 meter bedrock
offset across the fault was imaged through 3-6 meters of saturated unconsolidated material. The GPR
data were collected astride the known trace of the fault in a 12.5 x 12.5 m area adjacent to an earlier
excavated trench, which provided constraints on depth and overburden stratigraphy.
These GPR data were collected using a stepped-frequency (1-45 MHz) GPR system in bistatic
mode using a constant antenna center point separation of 2.5 m. The data comprise a series of 23
parallel 2-D profiles spaced 0.5 m apart, with GPR soundings along each profile collected at a 0.5 m
spacing.
On October 28, 1891, the 80 km long Neodani Fault and surrounding region experienced one of
the largest historic earthquakes in Japan, the Nobi earthquake, which registered an estimated magnitude
8.0. This study is near the epicenter of that earthquake, and not far from the Earthquake Fault
Observation Museum, which is designated a “National Monument of Special Interest.” Significant
variation of vertical offset occurred along the largely strike-slip Neodani Fault and this museum
preserves a spectacular excavation of the fault exhibiting 6 m of vertical offset.
These results indicate that 3-D GPR profiling is a viable method for locating or observing
bedrock fault offsets concealed beneath unconsolidated material, even in the generally GPR-challenging
ground conditions found in Japan, and could be used to nondestructively identify or locate optimal sites
for trenching.
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Use Of Ground Penetrating Radar For The Determination Of Soil Profiles In Slopes Of Rio De Janeiro
Authors Patricia L. Grazinoli, Euripedes A. Vargas Jr. and Franklin AntunesThe present paper describes the experience in the use of the Ground Penetrating Radar (GPR) in
the determination of soil-rock interfaces and weathering profiles in natural slopes in the City of
Rio de Janeiro, Brazil. A number of slides and other stability problems occurred in natural slopes
during the summer of 1996, when heavy rains affected the degree of saturation of the soils above
the rock, mostly colluvial and residual soils of gnaissic origin. In two such slopes, described in
the present paper, a slide occurred at the soil-rock interface.
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