ABSTRACT

We analyzed acoustic overpressure signals generated by overburied underground chemical explosions conducted in hard rock in New Hampshire in 2018. The explosions had comparable yields between 62.8 and 82.6 kg trinitrotoluene equivalent and were buried at depths between 12 and 13 m. Two explosions resulted in crater formation and gas venting, whereas the remaining explosions were fully confined and did not result in ground failure. Acoustic signals from the confined explosions were produced by the ground shock near ground zero. Acoustic signals from cratered explosions represent a combination of a ground shock signal and a time‐delayed high‐amplitude signal generated by gas venting. The cratering and venting occurred during the free‐fall phase observed on the near‐source accelerograms. We argue that the main reason for the cratering in this experiment is the low‐rock porosity, preventing postexplosion pressure relief in the cavity and promoting long fracture formation during the unloading phase and subsequent containment failure. The ground‐shock‐induced signals were modeled using the Rayleigh integral of the near‐source ground acceleration. The equations of nonlinear acoustics were used to model the observed gas venting signals produced by the gas flow from the explosion cavity to the surface. By comparing the near‐source signals produced by venting to theoretical signals from surface blasts we have shown that the venting signals have significantly lower peak pressures and longer signal durations compared to surface blasts of the equivalent impulse. The observed amplitudes of the acoustic signals produced by the venting are significantly higher than the ground‐shock related signals expected from overburied explosions. This is important to consider because higher acoustic amplitudes may potentially lead to errors in yield estimation.

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