The effect of the medium damage on seismic waves generated by underground chemical explosions in hard rock is investigated. An explosion experiment conducted in New Hampshire in 2016 has demonstrated that the amount of explosive gas products released in the cavity improves seismic coupling, which is manifested in higher seismic amplitudes. It has also shown that detonating explosions in water‐filled boreholes has similar effects on the spectra as increasing the amount of the gaseous detonation products by changing the explosive type. A postexplosion well logging survey revealed that using explosives releasing higher amount of gaseous products results in an increase in length and aperture of the explosion generated macrofractures. Placing the charges into water‐filled rather than drained boreholes results in a similar increase in fracture dimensions. Thus, the extent and intensity of postexplosion macrofracturing correlates with improved seismic coupling expressed as and amplitude increase, particularly in the low‐frequency range.
The radiation patterns of the P waves are different for the waveforms bandpassed in a low‐frequency range (1–15 Hz) and a high‐frequency range (15–100 Hz). The symmetry of the radiation patterns indicates the presence of nonzero terms associated with the off‐diagonal moment tensor terms ( and ). The amplitude of the seismic component attributed to the off‐diagonal moment tensor elements is significant and can be as large as 15%–16% of the isotropic moment. The observed P‐wave radiation pattern is consistent with either sliding along the pre‐existing fractures and zones of weakness or shear failure along the high‐angle borehole parallel fractures during the explosions.