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(e.g., Goforth and Hayward, 1992; Hasbrouck, 1991), but separating the S- wave
reflections from the surface waves that often appear on seismograms at the
same time is a problem. Seismic shear waves may be useful for cavity detection
because they will not propagate through voids or water-filled cavities. Shear
wave reflections have also been used to evaluate the resources of a shallow coal
seam (Hasbrouck and Padget, 1982).
Geophysical tomography is conceptually and mathematically identical to
medical tomography in which three-dimensional X-ray imaging from within the
human body is accomplished by computed axial tomography (CAT scan). The
technique uses measured travel times or signal strength of many geophysical
ray paths through a volume of earth material. Seismic tomography has been
used to examine Earth's interior from scales of a tens of feet to thousands of
miles (e.g., Clayton and Stolt,1981; Humphreys et al., 1984).
Future seismic applications that merge P - and S -wave refraction
information may be useful (Hasbrouck, 1987). By combining P -wave and S-
wave velocities with density readings obtained from gravity surveys or borehole
density logs, one can measure the elastic parameters of rocks. When densities
and velocities are known, Poisson's ratio, Young's modulus, and the shear
modulus can be calculated. When these elastic constants are known, rock types
can often be identified and an estimate of pore-space fluid content usually is
possible (Domenico and Danbom, 1987).
New opportunities for three-component recording and multimode analysis
are a result of decreasing cost and increasing capabilities of seismic hardware
designed to collect and process high-resolution, near-surface seismic data. The
seismic wave types, generally discarded by classical seismic reflection
surveyors during the processing, analysis, and interpretation of data, contain
information about the upper tens of feet of the Earth. The capabilities of seismic
methods involving depths shallower than 100 feet can be extended by analyzing
the near-surface broadband seismic wavefield using three vector components
rather than one and by examining multiple types (modes) of seismic waves
instead of just P -waves.
The principles of in-seam seismic transmission and reflection surveys can
be applied to estimate the presence and location of faults (Buchanan et al.,
1981; Greenhalgh et al., 1986) and air- or water-filled or collapsed mine
workings (Mason, 1981). In-seam seismic surveys are typically performed in
panels surrounding blocks of coal prior to long-wall mining operations. Seismic-
wave transmission surveys are set up to test the transmissivity of the coal seam
by deploying seismic sources along one face of a coal panel and placing
geophones along the opposing face. If disturbances are inferred from the
transmission experiment, a seismic reflection survey may be used to
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