Abstract

The presence of a void or cavity in the vicinity of a seismic source will modify the radiated signal from the classic solution for a point force in an infinite medium. To study this effect, solutions were obtained for the seismic fields from a point force applied to the surface of a cylindrical cavity in an elastic medium. The solutions were evaluated to give P- and S-wave frequency domain radiation patterns. For P wavelengths less than about 3 times the cavity diameter, the cavity acts to decrease the P-wave amplitude going outward in the direction opposite the source. Data taken in two coal mines show this shielding effect. High-frequency energy was observed, with surface seismometers, for signals generated by hitting the mine roof, whereas the high-frequency energy was much smaller on signals generated by hitting the floor. Time domain calculations show that the P-wave signal is delayed by approximately the time it takes an S-wave to propagate around the cylinder.

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