Technologies for Locating Mine Workings - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 82

estimate their locations. Brentrup (1970) summarizes more than 200 in-seam
seismic surveys, reporting a success rate of 66 percent for reflection and 83
percent for transmission surveys. Since the 1960s, in-seam seismic methods
have been applied in coal mines to find disturbances in the coal seams that may
pose problems for mining (Schwaetzer, 1965). Disturbances include faults that
offset coal seams and reduce production rates of longwall mining operations,
abandoned mine workings that pose a general risk to underground activities,
and methane zones in which mining activity may trigger explosions. A more
extensive discussion of in-seam seismic methods is included in Appendix E .
Faults with vertical displacement of one or more seam thicknesses are
good reflectors of in-seam seismic waves because of the contrast between the
coal and the bedrock material. Water- or air-filled cavities are even better
reflectors. In the case of collapsed mine workings, in-seam seismic waves may
be scattered rather than reflected, so the reflected waves may not be as coherent
as those reflected from a fault or a cavity. Nevertheless, the amplitude of the
high-frequency transmitted in-seam seismic waves could still be sufficiently
decreased to infer the presence of collapsed workings. In-seam seismic
techniques can be applied to existing and planned coal-waste impoundments as
long as the appropriate coal seam is accessible underground. This includes
cross-hole tomographic methods, which have proven successful in other
applications. The overburden material and surface topography will have no
effect on the success of the experiment. Many improvements in seismic
equipment and analysis techniques introduced in the last two decades enable a
success rate greater than that noted by Brentrup (1970).
The advantages of in-seam seismic technology lie in the two-dimensional
propagation of seismic waves in coal seams, and often in accessibility to a coal
seam on both sides of an assumed disturbance. In-seam seismic methods allow
the use of higher frequencies and broader seismic bandwidths than surface
seismic methods, and these offer better resolution of features of interest. The
high-frequency waves are concentrated within the coal seam whereas lower-
frequency waves are present within as well as outside the seam (Gritto and
Dresen, 1992). The advantage of absence of surface seismic noise such as wind
and road traffic may be countered by mining activity, which might have to be
interrupted during the seismic survey within a distance of 0.5 to 1 mile from the
survey. However, the problems associated with surface topography are mostly
negated.