Abstract

We studied seismic‐wave generation from five small (60–122 kg) fully contained explosions detonated in Barre Granite as a part of the New England Damage Experiment (NEDE). The explosions were conducted using three types of explosives with different velocities of detonation (VOD): black powder, ammonium nitrate fuel oil (ANFO) emulsion, and composition B (COMP‐B). Empirical evidence suggests that the low VOD explosives produce more shear‐wave energy than high VOD explosives. The proposed mechanisms to explain this effect include: (a) inhibition of gas‐driven fracture propagation by thicker pulverized zone for high VOD explosions, and (b) fracture toughness increase at higher loading rate. The main objective of the experiment was to study differences in shear‐wave generation between different types of explosives, and to determine the likely mechanism responsible for these differences.

Seismic amplitude analysis revealed that COMP‐B releases more energy and larger amplitude P waves for the same weight of explosives, while producing smaller amplitude S waves. Furthermore, large radial cracks were observed on the surface after the ANFO and black powder shots, while there was no surface fracturing after the COMP‐B shots. Thus, longer fractures correlate with higher S‐wave amplitudes. However, drilling into the source region indicates that high VOD explosions may actually produce a smaller pulverized zone, which means that fracture inhibition is a less plausible explanation. Therefore we hypothesize that increase in loading rate, in combination with shorter impulse duration for high VOD explosives, inhibits fracture processes, and subsequently reduced S‐wave radiation.

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