Analyses of Near-Field and Near-Regional Signals from the Black Thunder Mine

In this report we present analyses of seismic data recorded at the Black Thunder coal strip mine in the Powder River Basin of Wyoming. The data collected by Stump et al, 1996 are a set of recordings at distances of a few kilometers from mining activity at that mine. Our objective was to determine whether these near-field data revealed features of quarry blasting that were unique to that source type. In our previous annual report on this research contract Barker et al, 1997 and in Bonner et al, 1997, we described analyses of near-regional surface waves from a quarry in Texas. Their analyses used numerical simulations and predictions of the kinematic model in Barker et al, 1993 to interpret the dependence of amplitudes on azimuth in terms of quarry geometry and mass movement. Our objective in the current study was to apply these results to the recorded explosions at the Black Thunder mine. We find the following 1 Ripple firing effectively low pass filters the signals, in agreement with previous studies. Since for the near-field signals at Black Thunder, the observed high frequency components of the signals are associated with P waves and the lower frequency signals are associated with surface waves dominated by shear motion, ripple firing has the indirect effect of increasing the SP ratio in the seismograms. 2 We observe no consistent differences between motions from cast blasts and coal shots, nor between cast shots in which the throw or strike of the bench is different. This is due to the dominance of ground motion due to explosions relative to that caused by mass movement, which is in turn is due to relative source coupling or propagation. Since differences in mass movement are the primary differences in the source mechanisms between coal and cast blasts, the dominance of the explosion component obscures the source mechanism.