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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 - 20 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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