Technologies for Locating Mine Workings - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 80

FIGURE 5.2 Calculated theoretical gravity anomaly for three air-filled 20-foot
diameter horizontal cylindrical openings. The three curves represent
calculations for depths of 26, 52, and 78 feet to the center of the opening, with
the largest negative anomaly associated with the shallowest opening. The
background and instrumental noise for microgravity methods limit the absolute
value of useful anomalies to about 0.01 mGal. If the cavity were water-filled
instead of air-filled, the anomalies would only be one third as large.
1988; Watkins et al., 1967). In addition, some success in locating water-
filled coal-mine cavities at depths of less than 50 feet has been reported using
high-resolution P -wave (congressional, i.e., sound waves) reflection seismology
techniques (Branham and Steeples, 1988; Miller and Steeples, 1991) in which
absence of a normally strong coal-bed seismic reflection indicates a mined-out
coal bed.
Cook (1965) found that seismic energy transmitted through a cavity and
reflected from a deeper horizon gives rise to a seismic amplitude “shadow.”
Most of the seismic research on coal-mine detection has involved P -wave
refraction seismology or S -wave (distortional, i.e., shear waves) reflection
seismology. Significant improvements have been made since 1980 in near-
surface P -wave seismic-reflection techniques (Hunter et al., 1984; Steeples and
Miller, 1990), shallow-seismic refraction interpretation (Lankston and
Lankston, 1986; Palmer, 1980), and surface-wave methods (Park et al., 1999;
Stokoe et al., 1994; Xia et al., 1999). Surface-wave phase anomalies might be
employed to detect near-surface voids (Rechtien and Stewart, 1975). Shallow S -
wave reflection survey results are reported in the literature