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18th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 03 Apr 2005 - 07 Apr 2005
- Location: Atlanta, Georgia, USA
- Published: 03 April 2005
141 - 146 of 146 results
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Time-Lapse High Resolution Seismic Imaging of A Catastrophic Salt Dissolution Sinkhole In Central Kansas
Authors Jamie L. Lambrecht, Richard D. Miller and Steve DurrantTime-lapse 2-D high-resolution seismic reflection surveys successfully imaged and mapped
changes in structural features associated with a sinkhole that formed catastrophically after dissolution of
the Permian-age Hutchinson Salt in central Kansas (Figure 1). Symmetric steep sided cone structure
defined by reverse faults formed instantaneously continuing over time to gradually enlarge and elongate
along normal fault planes. Two orthogonal high-resolution seismic lines were acquired, in 1998 and
2004, to map the upper thousand feet and resolve structural features controlling development subsidence
rate preferential dissolution mechanism for fluid movement and to predict future growth. High
frequency vibrator and high-resolution seismic methods were used to acquire data from a 204 channel
fixed spread. Stations used in 1998 were as close as possible reoccupied on the 2004 survey. Common
mid-point stacked sections depict a very disturbed subsurface that has undergone multiple discrete
phases of loading, failure, and subsidence. Stress built up in roof rock overlying the salt void until its
strength is surpassed when failure, and subsidence occurs (Figure 2 (a), (b), and (c)). Strain associated
with layers bridging voids outside the tensional dome appears to be controlled by normal faults. Oilfield
disposal practices provided original fluids and pathway that initiated the dissolution process.
Currently shallower groundwater fuels the dissolution process with little or no contribution from
oil-field brines. Growth appears to be controlled by geologic trends with failure occurring much more
plastically during the later years, which initially growth was characterized by rapid brittle
deformation.
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Observation of Shear-Wave Splitting In Quaternary Sediments of The New Madrid Seismic Zone: An Indicator of Insitu Stress Conditions?
More LessShear-wave splitting, induced by stress-aligned inclusions (pore spaces, fractures, faults), causes
shear waves to exhibit directional polarizations in response to propagation through azimuthally
anisotropic media. This phenomenon is manifested by differences in shear-wave velocity between
waves traveling parallel (S1 - fast shear wave) and perpendicular (S2 - slow shear wave) to the trend of
the inclusions. This investigation provides further evidence of shear-wave splitting in Quaternary
sediments of the Lake County uplift (New Madrid seismic zone; central United States). Preliminary
analysis of a multicomponent downhole data set from Ridgely, Tennessee, recorded using hammer and
mass sources, indicates azimuthally-controlled shear-wave splitting related to in-situ tensional stresses
developed across the crest of the Lake County uplift. Orientation of the split shear waves, determined
from particle motion plots of the three-component downhole records, shows alignment of the S1
polarization with the major axis of the topographic uplift, and analysis of travel-time delays between S1
and S2 suggests anisotropy values of 10 to 12% for the upper ~30 m of the subsurface.
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Near-Surface Cavity Detection By High-Resolution Seismic Reflection Methods Using Short-Spacing Type Land Streamer
Authors Tomio Inazaki, Shigeki Kawamura, Oshie Tazawa, Yoshihiro Yamanaka and Naomi KanoHigh-resolution seismic reflection surveying by means of short-spacing type Land Streamers was
conducted to detect near-surface cavities. The target cavities were inferred to have high potentiality of
collapsing because they had been excavated about 60 years ago as air-raid shelter tunnels or drifts of
lignite mines. Until now, several surface geophysical methods have been applied to the cavity detection,
however, they only provided blurry images for cavities due to the poorness of spatial resolution or
insufficiency in obtaining information at the target depths. In contrast, our short-spacing type Land
Streamer tools, which were originally developed by the first author, can provide high-resolution images
up to 30 m in depth.
We applied three types of Land Streamer tools to the cavity detection: a horizontal geophone
array at 30 cm spacing, the same array but 20 cm spacing, and an accelerometer array at 20 cm spacing.
Each array has 48 channel sensors mounted on a woven belt which is easy to tow by hand. The targets of
this study were abandoned drifts of lignite mine at Nagoya, central Japan, and air-raid shelter tunnels
excavated in pyroclastic flow deposits at Kanoya, southern Kyushu.
As a result, distinct diffraction anomalies were imaged in unmigrated stacked sections at the just
points where actual abandoned cavities were checked by sounding or drilling from the surface. Some
anomalies occurred at depths shallower than the inferred horizons of shelter tunnels or lignite mines,
which indicates roof falling and upward growing of cavities. It is concluded that our Land Streamer tools
have high capability of detecting cavities smaller than 2 m in diameter located 5 to 10 m in depth.
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A Land Streamer Aided, Three-Dimensional (3-D) Seismic Reflection Survey, Belt, Montana
More LessThe Three-Dimensional (3-D) seismic reflection method has not been extensively utilized for
shallow subsurface investigations because of the relatively high cost of performing 3-D surveys. We
have designed and constructed a system that makes 3-D seismic reflection an affordable option for
shallow subsurface exploration by significantly reducing time and manpower. In a fashion similar to
marine work, an All Terrain Vehicle (ATV) tows an array of four parallel seismic cables, or land
streamers. Each streamer consists of 24 gimbaled geophones. The ATV drags the array from station to
station and shots are taken while the array is stationary. We tested our system near Belt, Montana to
help locate abandoned subsurface coal mines. For this survey, our receiver, receiver line, source, and
source line spacings were all 1 m. In total, we covered a surface area of 100 m by 34 m and achieved a
nominal fold of 24. Typical combined advance and occupation times for each station were less than 30
seconds using a crew of three people. A brute stack shows horizontal layering of the local geology.
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High-Resolution Seismic Reflection Survey Using Land Streamer In A Large Urban Area
More LessHigh-resolution shallow seismic reflection surveying using SH-wave type Land Streamer was
conducted in the Tokyo Metropolitan area, where more than 25 million inhabitants concentrate in small
but highly developed region of 3,500 square km. While seismic reflection surveys have been frequently
conducted in the area to delineate relatively deep geologic structure, shallow reflection surveys have
been exceptional due to the difficulty in surveying at such a high land-use area as well as under high
noise level condition caused by heavy traffic and industrial activities. In contrast, the Land Streamer
makes it easy to set a survey line and acquire high-quality data even in urban area because it was
originally developed to be utilized on a paved area and to sense SH waves instead of P wave signals
which are prone to be affected with ground noises. We applied it in combination with borehole logging
and passive surface wave method to field surveys in the Tokyo Metropolitan area. The objectives of the
field survey were to clarify the detailed structure in Holocene soft sediments, and to characterize them
based on the S-wave velocity and other physical properties. As a result, the seismic reflection surveys
helped to interpret the sedimentary sequence of Holocene and to assess the geotechnical properties of the
surface sediments in the area.
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Detecting Cavities With Seismic Refraction Tomography: Can It Be Done?
Authors Jacob R. Sheehan, William E. Doll, David B. Watson and Wayne MandelConventional seismic refraction analyses (e.g. delay-time or generalized reciprocal method)
generally perform poorly for imaging many karst-related features because first-arriving seismic waves
can generally circumvent the low-velocity target of interest without a major impact on travel time,
particularly where the target is three-dimensional. We are using synthetic models and field data to
evaluate the performance of seismic refraction tomography codes for karst detection. Two-dimensional
karst models have been created and used to generate synthetic travel time data for tomography analysis.
The results of this analysis suggest that the presence of a feature can sometimes be determined, but the
velocity is not accurately represented. In no synthetic case that we have tested has the velocity of the
cavity been an accurate approximation to the true low velocity. In contrast to the inconclusive synthetic
results, we have refraction tomography data that clearly indicate karst features. A refraction tomography
data set collected on the Oak Ridge Reservation shows a well-defined low velocity feature that crosses
three parallel lines along an axis that extends more than 200 meters. The feature that was detected has
velocities of about 1500-2000 m/s in a matrix of 3000-4000 m/s, reasonable velocities for a mud filled
void in saprolite at these depths, rather than having a velocity slightly lower than the matrix velocity, as
would be expected from our synthetic study. This suggests that the synthetic models are incomplete or
inaccurate. In one case, matrix smoothing of the synthetic model before generation of synthetic
traveltimes generates synthetic results that better match field results.
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