Geophysical Techniques - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 207

IN-SEAM SEISMIC TECHNIQUES
Data Acquisition
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 estimate their locations. Small
explosive charges deployed in horizontal holes drilled about 3 feet into the face
of the coal seam are used to generate elastic waves within the coal seam
(Dresen and Reuter, 1994). Geophones are routinely wedged into horizontal
holes similar to the source holes. The geophones are sensitive to motion in the
plane of the coal seam, and are designed to record channel waves of Love type
(see below). Although coal has a relatively high rate of seismic-wave
attenuation (Q factors range from 20 to 50), divergence of in-seam waves is two-
dimensional; thus propagation distances as long as a 11/4 mile have been
reported (Greenhalgh et al., 1986).
Channel Waves
A coal seam is a low-velocity channel for elastic waves. If a seismic source
is triggered in the middle of the coal seam, elastic waves propagate in all
directions throughout the coal. Wave motion encountering the coal-rock
interface along the top and bottom of the coal seam is constructively reflected
back into the seam at various angles and at different phase velocities. This
constructive-interference system is a channel wave that propagates within the
coal seam without radiating significant wave energy into the surrounding
bedrock. The two types of seismic waves, commonly interpreted as part of in-
seam seismic surveys, are Rayleigh waves comprised of body waves of the P
and SV type and Love waves comprised of SH waves only. Seismic-wave
phases created at various angles of reflection at the coal-bedrock interfaces
cause dispersion of the channel waves, which means that the channel waves
propagate at frequency-dependent speeds. Hence, at longer travel distances the
wave phases get separated and are recorded as a time- elongated arrival in the
seismogram.