Quantification and characterization of regional seismic signals from cast blasting in mines: A linear elastic model

Cast blasts in coal mines, designed to move large volumes of overburden, are a source of large (1-5kt), frequent explosions in parts of the world with near-surface coal resources. Mining events of this source type are triggering the prototype International Monitoring System for the Comprehensive Test Ban Treaty being tested under the Group of Scientific Experts Technical Test 3 (GSETT-3). We wish to develop techniques to distinguish between the seismic signals produced by these explosions and equivalent size single (or point) sources. To that end, we have developed a linear elastic model to simulate regional-distance seismograms from mining cast blasts. Cast blasting involves a shot-array with delayed detonations casting rock horizontally into a pit. We model the effects of the millisecond-delay-firing pattern, the depth of the pit and the cast of material into the pit. We attempt to separate the effects due to the explosion, the vertical movement of mass and the horizontal movement of mass in order to produce a physical understanding of the resulting waveforms, which can be used to assess potential discriminants for these types of explosions. These physical models of source processes are constrained by near-surface measurements of cast blasts in NE Wyoming that are triggering GSETT-3. Two observational results at regional distances that are replicated by these models are the excitation of 8-12 s surface waves by the cast blasts and the insensitivity of peak amplitude to total explosive size for normal blasting practices. The insensitivity of peak amplitudes from the cast blast to total source size is a consequence of the delay-firing practice under normal procedures. This practice was initiated to reduce ground motions in the near-source region around the mine and it appears that it is also successful in controlling peak amplitudes at regional distances. The mass transfer into the pit has both a vertical and a horizontal force component, each of which contribute to the final seismogram, the latter being azimuthally dependent. Assuming maximum coupling, the contribution to the seismogram due to the vertical force component is approximately equal to the explosion contribution for pit depths of about 10 m, but dominates for pit depths of 20 m or greater. The contribution due to the horizontal force component is mainly in the enhanced Rayleigh waves. Comparison of high-frequency seismic radiation from the single shot and cast blast shows little qualitative difference in the regional waveforms.