Seismic coupling of chemical explosions in intact and fractured granite in Barre, Vermont

In this article, we investigate seismic amplitude reduction for the chemical explosions conducted in low-coupling fractured rocks. The explosions used in this study were detonated in the fracture zones left by earlier explosions (repeat shots) in granitic rocks in Barre, Vermont. The seismic amplitudes from the repeat shots were compared with the amplitudes from the same-yield explosion conducted in the intact rock in the same area.The results of the experiments show seismic amplitude reduction for the repeat shots by a factor of 1.5-2 in the entire frequency range. The amplitude reduction observed for the fractured rock explosions can be explained by compaction due to pore collapse, other inelastic losses, and by reduced elastic moduli in a localized damage zone around the charge. We used the Mueller and Murphy (1971) model to calculate spectra of the first and repeat explosions. To account for the reduction of the elastic moduli in the damage zone, we propose a hybrid medium model with the intact medium outside the elastic radius and the medium with reduced elastic moduli inside the elastic radius. The contribution from the pore collapse and other inelastic losses can be modeled by changing the compaction parameter d. Both models (higher compaction and the hybrid elastic moduli) predict reduced amplitudes at all frequencies and qualitatively agree with the observations. Because of the complexities in the explosion processes and the uncertainties in the damage zone configuration, the proposed model may not exactly reproduce the explosion source; however, using the models with a combination of the parameters can provide bounds on the resulting spectral ratios.