Empirical Calibration of Small Explosion Seismic And Acoustic Phenomenology in New England
Final rept. 30 Aug 2005-30 Sep 2008
WESTON GEOPHYSICAL CORP LEXINGTON MA
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Sparse station distribution, broad areas with few prior calibration events, complex paths, and a variety of seismic sources, both man-made and natural, are problems plaguing seismic monitoring in many regions of Europe and Asia. Added to these complexities are a lack of a physical understanding and empirical calibration data for small explosion sources in a variety of different crystalline media, including granitic and metamorphic terranes. We conducted the experimental phase of the New England Damage Experiment in a granite quarry near Barre, VT. The goal of this experiment was to characterize the rock damage from an explosive source and to identify the sources of shear wave generation. We quantified crack nucleation and growth as a possible S-wave generation mechanism in the far field and mapped the cone of damage above a source, modeled by a compensated linear vector dipole. The velocity of explosive detonation VOD influences the amount of damage. A faster VOD generates higher pressures that crush the rock into a powder, which inhibits the explosive gasses from driving long cracks. We detonated black powder, ANFOEmulsion, and COMP B, which have significantly different VOD so we could compare and contrast the damage from each source. Five shots were detonated ranging in size from 134 to 270 lbs of explosives. Over 140 seismic sensors were installed from less than 5 m to 30 km from the blasts specifically to record this experiment. Pre- and post-blast studies of the source rock properties were conducted using acoustic and optical borehole televiewers, coring, laboratory measurements for rock core properties, and cross-hole tomography.