Rayleigh Waves Generated by Mining Explosions and Upper Crustal Structure around the Powder River Basin, Wyoming

by Rong-Mao Zhou and Brian W. Stump

Abstract

Fundamental-mode, intermediate period Rayleigh waves generated by mining explosions are utilized to constrain the crustal structure of Wyoming. Broadband seismic stations recorded data during two regional seismic experiments conducted in Wyoming in 1996 and 1997. The stations were deployed in a ring surrounding the Powder River Basin (PRB) at ranges 100–360 km from the Black Thunder Coal Mine located in the northeast corner of Wyoming. Signals generated by four explosions with total explosive weight from 1.1 kiloton to 2.7 kiloton are analyzed with the goal of resolving the crustal structure in northeastern Wyoming. Data from the experiments show evidence that kiloton-sized millisecond-delay-fired cast blasts produce strong 1- to 15-sec period surface waves, some extending to 20-sec period. Group velocities of fundamental-mode Rayleigh waves were estimated using the Multiple Filter Analysis (MFA) technique and refined with Phase Matched Filtering (PMF). The surface waves are dependent on propagation direction and source orientation. The northwest–southeast trend of the PRB means that some propagation paths are parallel to the basin orientation and others perpendicular. Group velocities exhibit normal dispersion and range from 0.97 (1 sec) to 3.1 (20 sec) km/sec. A least square inversion technique was used to invert group velocity dispersion curves for the shallow shear-wave velocity structure. The average model consists of nine layers in the upper 20 km of the crust. The two top layers are thin (0.35 and 0.45 km) with slow velocities (1.0 to 1.6 km/sec) as a result of the basin sediments. At depths below 15 km, the shear velocity is nearly the same for paths parallel and perpendicular to the PRB axis. A possible low-velocity zone is found along the PRB axis with a velocity contrast of 0.7 km/sec at depth of 9.8 km to 14.8 km. These detailed models give a much better fit to dispersion curves than the global CRUST2.0, particularly for periods below 5 sec, which indicates the need for path-specific models when analyzing mid-period surface waves. Comparison of theoretical seismograms using the empirically derived structure with the observed seismograms provides a basis for constraining the source processes of these mining explosions. The generation of the intermediate-period surface waves is directly related to the total source time duration that is on the order of several seconds for these cast blasts. Observational data suggest that source orientation is important as well, resulting from some combination of mine free face orientation, blasting direction, and material casting.

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