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6th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 18 Apr 1993 - 22 Apr 1993
- Location: San Diego, California, USA
- Published: 18 April 1993
21 - 40 of 65 results
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Characterization Of Karst Features Using Electromagnetics And Microgravity: A Strategic Approach
Authors Lynn Yuhr, Richard Benson and Dwain ButlerThe flat-lying limestones of western Texas are naturally jointed with preferential
dissolution occurring along joints and bedding planes. This area has some of the
largest air-filled open caves in West Texas and large paleokarst collapse features
have been identified throughout a wide region. Aerial photos indicate the
presence of extensive joints, lineaments and paleokarst. Site characterization to
detect and map such subsurface features using borings alone is inadequate to
produce a reasonable level of spatial sampling. This paper addresses the
application of surface geophysical techniques, and an assessment of the spatial
sampling and instrument sensitivities necessary to define the karst features of
interest in this geologic setting.
Electromagnetic measurements using a Geonics EM34 were selected because
the measurements provide an excellent means of locating dissolution-enlarged
joints. Microgravity was selected because it is the only surface geophysical
method that will provide the location of karst feature regardless of their shape or
fill material as long as there is a sufficient density contrast. While both methods
have limitations, as do all methods of site characterization, the combination of
gravity and EM measurements are complementary in this application.
A known cave system was used to establish the spatial sampling criteria for the
detection and characterization of such features. Field tests were then run over
two known karst sites; a paleokarst collapse; and a localized doline to provide
anomaly signatures in this geologic setting.
It is clear that the EM and the gravity techniques are appropriate for detecting and
characterizing karst features in this geologic setting. However, one of the key
issues in planning and carrying out a geophysical survey, is developing a spatial
sampling criteria. This criteria should be based upon an understanding of project
objectives, a conceptual geologic model of site conditions and, if possible,
existing data from the area of interest.
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Geophysical Investigation For Buried Drums: A Case Study
Authors Don Pierce and John DeReamerA vertical gradient magnetic survey was implemented over a 100 acre site to
locate isolated buried drums. Clusters of buried drums at the site had been
discovered and excavated during the course of previous investigations. The
survey was designed and field tested for the ability to detect a single drum
buried at a depth of up to 10 feet below the ground surface. Field tests
demonstrated that vertical gradient magnetic data collected on five-foot
centers would provide the best means for detecting isolated drums. High clay
content soils severely limited both electrical and ground penetrating radar
methods. Based on the field results and theoretical calculations, a minimum
anomaly criterion were established for deciding which vertical gradient
anomalies would be investigated. The vertical gradient magnetic contour maps
were then inspected and anomalies exceeding these criterion were targeted for
investigation. More than 1,000 anomalies were investigatedbytrenching with
a backhoe. Most of the trenched anomalies resulted in the discovery of
miscellaneous magnetic debris. Thirty-six of the trenched anomalies resulted
in the discovery of drums or drum fragments at depths of up to nine feet. The
results of this survey show that isolated drums were easily detectable at
depths of 10 feet. Careful management of this investigation was essential due
to the large number of anomalies and the complexity of coordinating heavy
equipment operations and the magnetometer surveys.
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Using Surface Geophysics To Locate Buried Drums At An Industrialized Superfund Site In New Jersey
Authors Mary Kate Dwyer and Donald JohnsonObjectives of a surface geophysics investigation at an SO-acre Superfund site in New
Jersey were to locate potential source areas of contamination leaking from buried
drums and underground tanks. Surface geophysics was chosen as a cost-effective, nonintrusive
method to accurately identify potential source areas and reduce risk by
locating areas of buried metal to be avoided during a subsequent phase of drilling.
Areas of buried metal identified using geophysics were investigated during a later phase
of test-pitting to characterize the type of materials buried at the site. A
magnetometer/gradiometer was selected as the best method to conduct the study, since
the investigation target was buried metal. The vertical gradient often provides higher
resolution of magnetic anomalies and may allow the collection of useful data closer to
buildings than do total field measurements. The gradiometer was critical to the
investigation, since many magnetic interferences such as buildings, overhead process
lines, underground utilities, and fences were present at the site. Since the potential
target could be as small as a single drum, the survey was performed on a 10 by 10 foot
grid spacing.
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Some Applications Of Frequency Domain Electromagnetic Induction Surveys For Landfill Characterization Studies
Authors John Jansen, Michael Pencak, Richard Gnat and Bassem HaddadFrequency domain electromagnetic induction surveys (FDEM) can provide useful information for
landfill site characterization studies. This paper presents three examples of FDEM surveys
conducted on landfills which illustrate some of the applications of this method.
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Em Delta: A Techni Ue For Rapid High-Resolution Reconnaissa R Ce In The Inphase Mode
More LessWhat is described here is a method for carrying out
rapid high-resolution terrain conductivity (EM) surveys for
buried metal by using a Geonics EM31-D in the inphase mode.
Surveys that are carried out on 3m parallels can actually
attain resolution closer to 1.5m spacing, thus providing
extraordinary control when looking for isolated drums or
metallic targets. Scans were run on 3m parallels, and at
each 3m station along the transect, two readings were taken;
one with the boom running longitudinally, and again
transversely with the boom rotated 90 degrees in the
horizontal plane. Rather than average out the two readings,
as is often done in quadrature phase surveys to reduce
background noise, the value of the difference between the 2
readings (hence the term EM delta,&) was plotted on a contour
map, amplifying the differences in conductivity between the
two readings.
If a buried metal drum exists below the surface, the
ratio between the secondary and primary magnetic fields will
fluctuate widely as the boom is rotated in the horizontal
plane over or near the drum. By obtaining the value of the
difference, the possibility of detecting isolated and/or
deeply buried drums is increased. When a GPR-produced
bedrock and utility map was superimposed over the EM contour
map, one could correlate between EM anomalies and known
underground or surface targets. In the completed EM map, all
the prominent EM anomalies could be attributed to surface or
near-surface metallic materials, such as vehicles, fences,
drains, and monitoring well covers. We concluded that the
area was free of any buried metal.
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Preliminary Results Of A Helicopter Electromagnetic And Magneztic Survey Of The Oak Ridge Reservation, Tiwnessee For Environmental And Geologic Site Characterization
Authors W.E. Doll, J.E. Nyquist, J.S. Holladay, V.F. Labson and L. PellerinThe 35,252 -acre Oak Ridge Reservation (ORR), in the western portion of the
Appalachian Valley and Ridge province in Tennessee, has been a nuclear production and
development facility for 50 years. Contaminants in the many waste sites on the ORR
include heavy radioactive isotopes as well as many organic and inorganic compounds.
The locations, geometry, and contents of many of these waste sites are reasonably well
known, while others are poorly known, and some may be unknown. To better
characterize the known sites and locate and characterize additional environmentally
hazardous sites, a two-phase aerial survey of the ORR was begun in April, 1992. Phase
I, which began in April, 1992, consisted of aerial radiation, multispectral scanner, and
photographic surveys. Phase II, which began in November, 1992 and is described in this
presentation, consisted of a helicopter electromagnetic (HEM), magnetic, and radiation
survey. Targets of the survey were both man-made (drums, trench boundaries, burn pits,
well heads) and geologic (fractures, faults, karst features, geologic contacts).
The Phase II survey has three components: testing, reconnaissance, and high-resolution
data acquisition. To date, the testing and reconnaissance data acquisition have been
completed, and part of the data have been processed. They indicate that: 1) magnetic
and HEM data are complementary, and do not always highlight the same anomaly; 2)
under favorable circumstances, helicopter magnetometer systems are capable of detecting
groups of 4 or more 55-gallon drums at detector altitudes of 15 m or less, 3) HEM data
provide data which compare favorably with surface data collected over burial trenches, 4)
well casings seem to be related to magnetic monopole anomalies, as would be expected,
5) Changes in EM and magnetic anomaly character are related to lithologic changes and
might be used to track contacts between known outcrops.
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First Arrival Inversion Of Crosswell Radar Data By Finite-Difference Solutions To Maxwell’s Equations
Authors Wenying Cai, Fuhao Qin and Gerard T. SchusterThe wave equation traveltime inversion (WT) method is adapted to the reconstruction
of the dielectric distribution from first arrival traveltime radar data. A gradient
optimization algorithm is used and the gradient function is computed from finitedifference
solutions to the 2-D Maxwell’s equations. The key advantage of the radar
WT method over conventional ray tracing radar tomography is that it accounts for
scattering and diffusion effects in the data and works well in both highly resistive and
moderately conductive rocks. This technique is successfully applied to both synthetic
and real radar data. Comparisons with a ray tracing (RT) tomography scheme show
that the radar WT method is more reliable and accurate than the RT method when rock
conductivity is larger than .002 S/m. The WT and RT methods are about equally
effective when conductivity is less than or equal to .OOl S/m. The disadvantage of the
WT scheme is that it generally demands an order of magnitude more computational
time than the RT method.
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The Use Of In Situ Permeable Flow Sensors To Monitor An Air-Stripping Waste Remediation Experiment At The Savannah River Site, Sc
More LessIn Situ Permeable Flow Sensors, new instruments which use a thermal perturbation technique
to directly measure the direction and magnitude of the 3-dimensional groundwater flow velocity
vector at a point in a permeable medium, are being used to monitor changes in the groundwater
flow regime in the immediate vicinity of an underground air-stripping waste remediation
experiment underway at the Savannah River Site, SC. Two horizontal boreholes were drilled into
the sandy sediments with interbedded clay lenses beneath the site, which is contaminated mainly
with trichloroethylene and perchloroethylene. The two holes are at depths of approximately 20
and 50 meters. The water table is at a depth of 42 meters, in between the two horizontal wells.
Air is injected into the deeper hole, flows upward through both the saturated and unsaturated
sediments and is extracted from the shallower hole. As it travels from the injection to the
extraction well, the air volatilizes the contaminants which are then removed from the extraction
well along with the injected air.
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A New Application For Geophysics: Monitoring At The Dig-Face During Waste Site Excavation
By N.E. JostenThere are many hazardous waste sites throughout the United States that have been mandated by
law to undergo remediation. That process has been painfully slow and expensive, partially due
to the inadequacy of the waste characterization schemes that have been counted on to define the
extent and nature of site contamination. Conventional characterization efforts simply fall short
of providing the level of information on buried waste that is consistent with human safety during
clean-up operations. Preliminary design has been completed for a new technology, called Safe
Step Remediation, that addresses this problem. The key component of the Safe Step approach is
a Dig-face Monitoring System. The monitoring system produces waste characterization data in
small, careful increments. Each new characterization increment drives a new increment of
excavation.
Dig-face monitoring poses some new challenges for geophysicists. One challenge is to meet
performance requirements for quantitative, exact interpretations that go well beyond those that
apply to most conventional geophysics. In dig-face monitoring, interpretations directly and
continuously protect site workers by guiding the removal and handling of dangerous materials.
Several unique aspects of the dig-face monitoring application should enhance capabilities for
making these accurate interpretations. First, within the Safe Step Remediation approach, the
monitoring system will be able to make an extraordinary set of measurements. These
measurements will be made on multiple planes as the excavation progresses and will include
close-up measurements made in the immediate vicinity of hazards. Second, and even more
important, the physical retrieval of targets following each increment of characterization generates
a unique opportunity to validate interpretations continually. This provides a basis for steadily
improving the quality and accuracy of interpretations over time.
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High Resolution In Saturated Sediments - A Case For Shear Wave Reflection
More Less“High Resolution Reflection” all too often carries with it the implied meaning of “High Frequency
Reflection.” As the resolving power of a reflected wave is a function of wavelength and not merely frequency, high
frequency in and of itself does not exactly define high resolution. While Poisson Ratio (u) in sedimentary and hard
rock masses hovers in the 0.25-0.33 range, within saturated sediments, (T approaches 0.45-0.49. What this means,
in effect, is that the ratio of compressional (P) to shear (S) wave velocity in such loose media is in the range of 5: 1
up to 10: 1. In terms of wavelengths for the w frequency input wavelet, an S wave will have wavelengths of
from l/10 - l/5 that of a P wave in the same medium. This is a direct increase in resolution by a factor of 5-10.
Under many circumstances, time is better spent using shear waves for reflection than in developing and employing
exotic higher frequency compressional sources which may only increase resolution a factor of two over more
standard seismic sources.
Example S wave reflection studies relating to stratigraphic delineation and growth fault studies from the
US Gulf Coast, Southeast US, and Eastern US areas illustrate the type of resolution obtained with minimal,
additional field effort.
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Near Surface S-Velocity Profiles At 30 Salt Lake Valley Sites From Inversion Of Surface Wave Dispersion And Analysis Of S-Wave Refraction Data
Authors Yonghe Sun, Wenying Cai, Greg Andrews, Jeff Wolfe, Peter Fivas, Jinlong Xu, Changxi Zhou, Gerard T. Schuster and Todd AtkinsSurface wave and shear refraction data were collected at thirty Salt Lake Valley
sites using a Bison 9000 seismic recorder. Each experiment used 10 Hz vertical and 4
Hz horizontal geophones at group intervals of 5 feet, with spread lengths up to 1000
feet. Both horizontal and vertical 100 lb weight drop sources were used, as well as the
Bison EWG 500 lb source. Rayleigh waves, Love waves and shear refraction waves
were recorded and inverted for the shallow S-wave velocity structure. Results show
that the Rayleigh wave and refracted shear wave data were the most reliable and generally
provided similar velocity profiles; the Love wave data proved to be the least reliable,
although over half the sites showed that the Rayleigh, Love and refraction wave
velocity profiles agreed to a depth of about 30 meters.
These shallow S-velocity profiles were used to simulate the seismic amplification
from earthquake waves impinging on the basin floor. For all sites, it is shown that the
deep (about 1 km thickness) 2-D basin structure causes about 2-3 times more
amplification than the shallow S-velocity structure for frequencies from O-4 Hz. The
shallow velocity structure is important for amplification at frequencies greater than
about 5 Hz.
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Observation Of Borehole Mach Waves And Implications For Improved In-Situ Physical Property Measurements And Subsurface Imaging In Soils And Low-Velocity Rocks.
Authors J.T. Rutledge, J.N. Albright and J.A. MeredithA crosswell seismic survey was conducted in the McKittrick oil field, California in which
no direct body waves were observed. However, strong arrivals were detected as a result
of Mach waves radiating from the source borehole. The phenomenon of borehole
seismic sources generating secondary body waves, or Mach waves, has only recently
been recognized and understood. The McKittrick data demonstrate that secondary body
waves provide a means to efficiently transmit seismic energy from a borehole penetrating
low-velocity rocks or soils. Exploitation of Mach waves may enable the measurements
of in-situ physical properties and imaging of the subsurface in environments where
conventional seismic techniques fail.
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Designing Geophysical Instrumentation Specifically For Environmental Applications
By H.O. SeigelThe application of geophysical methods to the solution of problems in the environmental field
has been facilitated by the availability of full-blown technology in the mining and groundwater
explorations field. There are, however, basic differences between these two fields and the
environmental field, which demand a fresh approach to the design of instrumentation for
environmental applications.
The station density (stations per km2) for environmental-type surveys may be 1000 times higher
than in resource-type surveys. Thus, the time required for a single measurement becomes the
prime factor in the environmental survey cost. The time to move between stations, which
constitutes a large component of the cost of resource surveys, is no longer consequential. Thus,
individual measurement times of 0.5 seconds or less, are required in order to permit surveys to
be made at a walking pace.
Ease of use by non-geophysical personnel is a second requirement. Whereas most resource
geophysical surveys are done under the supervision of, if not operation by, a trained geophysicist,
the environmental field will have large numbers of practitioners who do not have this
background. Thus, geophysical instruments for environmental purposes must be very simple to
operate and require little prior instruction.
At the same time, however, high sensitivity and high precision are often not very important in
the environmental field, as the physical property contrasts of the targets involved are generally
larger and their depth below surface generally smaller than in the resource field. This facilitates
a reduction in weight, size, complexity and cost in environmental instrumentation.
Lower capital cost is an important consideration in the environmental field, as individual projects
tend to be small in size and of lower revenue than in the resource exploration field. In addition,
environmental contractors are likely to belong to small groups with low capitalization.
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Practical Considerations For Grm Refraction Surveys In Glacial Terrains
Authors Dirk Kassenaar and John LuttingerGeneralized Reciprocal Method (GRM) refraction surveys in glacial terrains frequently encounter
complex geologic and hydrogeologic conditions. The complexity frequently centres around the
shallow water table and low velocity of the unsaturated zone. Inaccurate determination of the
velocity and thickness of this low velocity layer can have a significant effect on the depth
estimate of lower layers. This paper discusses methods to optimize field data collection and
processing to ensure proper analysis in these situations.
A general rule of GRM surveys is that the geophone spacing should be less than one third of the
XY distance of the shallowest layer. Since the XY of a shallow water table layer is often less
than 2 metres, geophone spacings of less than 1 metre may be necessary. These spacings are
frequently not cost effective, and alternative approaches may be required. Alternatives include;
1. using the GRM average velocity method and an estimate of the XY based on modelling, 2.
combining the upper two layers, and 3. using time intercept methods to interpret short spreads
collected at various points along the line.
The sensitivity of the GRM is evaluated using a simple model. Random pick errors are
introduced into the model, and the merit of arrival averaging is studied. Similarly, layer
velocity errors, XY analysis errors and hidden layer problems are reviewed.
Practical suggestions for GRM surveys in glacial terrains are offered. An approach for
performing cost effective GRM surveys is presented. The approach is based on high redundancy
data collection (multiple mid shots) for water table velocity analysis and duplicate arrival
averaging. Finally, the importance of XY analysis for detecting hidden layers and bedrock
surface features is discussed.
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Integrated, Flexible, And Rapid Geophysical Surveying
Authors S.F. Miller, L.D. McGinnis, M.D. Thompson and C. TomeDetailed environmental studies associated with landfills, burial pits, vaults,
underground storage tanks, contaminant plumes, and unidentified contaminant
sources adjacent to buildings at Aberdeen Proving Ground, Maryland, are being
conducted. Efficient and innovative data-acquisition procedures are imperative in
order to provide complete coverage at a large number of small-sized sites.
Because APG is a chemical weapons research and development facility,
noninvasive geophysical techniques are a necessity. Real-time data processing
and interpretation using computers in a field setting permit rapid changes in the
design of the survey and in decision making.
Magnetic and electrical interference caused by metal buildings, power
lines, and buried utilities limit applicable geophysical techniques. A pilot study to
test a variety of techniques resulted in the selection of horizontal electrical
resistivity profiling, magnetic gradiometry, total field magnetics, and groundpenetrating
radar.
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Seismic Refraction And Gravity With Drilling Verification
Authors Paul J. Wolfe and Benjamin H. RichardSeismic refraction and gravity surveys have been used to guide a drilling crew evaluating
water table and bedrock depths in an environmentally sensitive area. The Ohio Department
of Transportation plans to construct a freeway past the Cedar Bog nature preserve. To
evaluate the potential impact of the freeway construction on the ground water flow that
sustains the bog, an extensive geophysical and drilling program is being conducted by
Wright State University.
Cedar Bog is located at the edge of a major preglacial river valley that was buried by
Pleistocene glacial deposits. From previous geophysical studies we believe that water flows
underground through gravel-ftied, buried tributary channels in the bedrock and then moves
to the surface emerging in artesian springs near the bog. Determination of the hydrological
regime and the details of the buried channel locations is necessary for designing the highway
so that the bog will maintain its cold water supply. Two proposed routes for the road total 8
miles long: We conducted seismic refraction and gravity surveys continuously along the
routes to determine the water table depths, the bedrock depths, and any major changes in the
glacial material that covers the bedrock. We interpreted the geophysical data and drew cross
sections to show the locations and depths of channels in the bedrock surface that were likely
to act as ground water flow zones. Holes were drilled at selected locations along the
proposed route to verify the geophysical interpretations and to examine the detailed glacial
stratigraphy. The large number of holes drilled along the lines allowed us to evaluate the
precision of the seismic refraction surveys.
We found good qualitative agreement between the seismic and drilling results and applied
several statistical measures to quantify the relationship.
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High Resolution Seismic Reflection For Characterizing Longwall Coal Mine Subsidence
Authors James A. Jessop, Calvin L. Cumerlato, Kevin M. O‘Connor and John A. SiekmeierAs part of an Illinois mine subsidence initiative, the U. S. Bureau of
Mines has conducted high resolution surface seismic surveys at two longwall
coal mine sites in southern Illinois to help characterize subsurface
subsidence. Premine and postmine surveys conducted above 183 meter (600 foot)
deep longwall panels consisted of 12-fold common depth point (CDP) data
collection, using a 24-channel engineering seismograph, 60 Hz land geophones
and a nondestructive surface energy source. Source offset ranged from 91.4
meters (300 feet) to 152.4 meters (500 feet) and a station interval of 3.1
meters (10 feet) were determined from noise test data at each site. Data from
these surveys were processed into seismic sections using commercially
available microcomputer-based software. Sonic logs from a nearby borehole and
mine maps showing mined and unmined areas were used in the processing and
interpretation of the data. The sonic logs were used to generate a plot of
the two-way traveltime through each formation and a cumulative plot of
traveltimes through progressive layers to the coal seam. A coaxial cable was
grouted into the borehole and Time Domain Reflectometry (TDR) was used to
monitor overburden movement during mining. This data was compared with the
seismic sections and cumulative traveltimes plots to identify bridging within
the overburden. Bieniawski's Rock Mass Rating system (RMR) was used to
characterize the overburden. The RMR was computed for each formation and is
used to determine the deformation modulus, then the modulus is combined with
parameters based on mine geometry and overburden properties to compute a
bending stiffness and bridging potential for each bed.
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Fracturing Of Glacial Drift And Bedrock Over Longwall Mine Panels: Integrated Geophysical And Hydrological Measurements
Authors Matthew A. Johnston and Philip J. CarpenterP- and S-wave refraction profiles were combined with
resistivity soundings, resistivity and electromagnetic profiles,
azimuthal surveys and hydrological data to characterize fracturing
and associated hydrogeological changes over three subsided longwall
coal mine panels in the southern Illinois basin. Approximately 6 ft
of Herrin #6 coal was mined from the base of a 260-320 ft section of
Pennsylvanian rocks capped by 40-90 ft of unconsolidated glaciolacustrine
deposits. Subsidence-induced fractures significantly
altered the mechanical and hydrogeological properties of the
overburden.
Fracturing of the drift over one panel to at least the depth of
the water table (lo-20 ft) was marked by decreases in shallow SH-wave
velocity and a non-uniform pre- to post-subsidence drop in the water
table. Apparent resistivity also increased in the dynamic tension
zone where surface fractures opened just behind the mine face.
Inversion of resistivity soundings could not be used to constrain the
depth of fracturing in this area, however. Increases in earth
conductivity, decreases in apparent resistivity, and the development
of azimuthal resistivity variations suggest long-term fracturing of
drift along panel margins, and, in some cases over barrier pillars.
Fracturing of the upper bedrock (to 80 ft below the bedrock
surface) was indicated by reduced P- and SH-wave velocities, sharp
potentiometric declines, and post-subsidence increases in hydraulic
conductivity. SV- head waves from the bedrock surface, however,
showed no such reduction in velocity, suggesting bedrock Vsv is
controlled primarily by bedding anisotropy.
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“A Study Of Cryoseisms (“Frostquakes”) In The Sebago Lake Region, Maine
More LessDuring the winter of 1990-91, residents in the Sebago Lake Region of
southwestern Maine became alarmed by a noise phenomenon which they were convinced was
caused by a natural gas transmission pipeline in the area. The unexplained noises caused
residents to suspect the nearby transmission line. In response to these concerns, the pipeline
operator thoroughly checked the pipeline for potential safety problems. No leaks were detected
and all other safety checks had revealed that nothing was amiss.
An investigation was undertaken to study the source of the noise phenomenon on behalf of the
pipeline company. Local residents were interviewed and a network of 3-component
seismographs was installed to establish background vibration levels and to identify the source
of the disturbances. The seismographs used in this study can be set to record events over a wide
range of vibration levels. Area residents were given log books in which to describe any unusual
noise events and to record the time of occurrence.
Based on approximately two months of continuous monitoring in the study area, personal
interviews, a literature search for related vibration phenomena, and first-hand observations of
the noise phenomena by the author, we concluded that the loud noises were the result of
cryoseisms (‘lffostquakes”) or frost-induced microearthquake activity.
Cryoseisms are an unusual phenomena which occur under limited winter weather conditions in
northern climes. They can locally produce loud noises such as those described by residents in
this study as ‘a slamming door, ” “an automobile running into the side of the garage’: “a furnace
blowing up’: and “a rifle shot’: Cryoseisms are also frequently accompanied by minor fissures
which may be as wide as several millimeters at the ground surface and which re-heal with time.
Examples of fissures ji-om the current study will be presented. The mechanisms that produce
cryoseisms and the climatic conditions under which they occur will be described.
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Rock Mass Characterization Using Geophysics For Stope I,Eaching
Authors J.J. Snodgrass and D. BoreckThe Bureau of Mines is researching the feasibility of in situ stope
leaching for active underground metal mines. A test facility was
developed in an underground mine near Idaho Springs, CO, to
investigate the processes of leaching solution control. An
important aspect of the research is the characterization of the
rock mass in terms of discontinuities which will determine fluid
flow and affect fragmentation from blasting. Geologic mapping and
core logging documented the fracture and joint systems in the
simulated stope and adjacent areas of the underground facility.
Slimline borehole geophysical logs were used to verify lithologic
changes and indicate locations of fracture zones. Conventional
and full wave form sonic (FWS) logs were obtained, where
possible, to interpret fracture zones and define optimum
locations for permeability tests. In addition, sonic shear wave
profiles were obtained with a wall-clamping probe in the
boreholes drilled at the site. Shear wave measurements were not
as sensitive to fracturing as the conventional travel time P-wave
logs, but the capability for dry hole operation provided data in
zones of the rock mass that would otherwise be unavailable.
Integrated interpretation of the geophysical logs enhanced the
geological characterization of the in situ rock mass by
distinguishing open fractures (possible fluid conduits) from the
closed fractures. Estimates of the dynamic elastic moduli
provided a baseline for comparison of post-blast measurements to
evaluate overbreak and control solution loss during the leaching
phase of the experiment. Cross-borehole sonic surveys and
tomographic imaging were also demonstrated effective in
correlating the fracture zones between drillholes.
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