Abstract

A series of single-fired (simultaneously detonated) explosions were conducted in an Arizona copper mine. The explosions spanned yields from 773 to 6182 kg and were all detonated in an approximately 100 by 100 m area. The individual explosions were also detonated under three different emplacement conditions including at the free face of the mine under normal burden (distance from the free face) resulting in failure and movement of the free face, at twice-normal burden with free surface failure and cratering, and at twice-normal depth, which fully contained the explosions. The purpose of this analysis was to investigate the relationship between explosive weight and confinement on the amplitude and frequency content of seismic waves. Empirical scaling relations for the different shots in this test series were developed using common receivers in order to quantify the effects of explosive weight and confinement. Spectral ratios between observations at the same stations for the different explosions were used to remove common instrument and propagation path effects from the data set. Empirical scaling functions estimated from data at ranges from a few hundred meters to several tens of kilometers produce similar results. The Mueller–Murphy source model has been used to assess the effects of explosion depth and material in which the explosions are detonated. Material properties were measured for the porphyry granite at the mine and were used in forward models for the yield scaling of the fully contained explosions. The forward models replicate the empirical scaling functions at low frequency (<8 Hz) for the contained explosions illustrating the ability to appropriately account for source corner and relative sizes of the absolute energy coupled into the ground. Enhanced coupling near 10 Hz may be related to near-surface spallation or the cylindrical shape of the borehole containing the explosives. The empirical scaling relations suggest that the free-face explosions result in a factor of 2–4 smaller amplitudes below ∼3 Hz compared to the fully contained explosions with the spectral differences decreasing at higher frequencies as illustrated by time domain comparisons dominated by higher frequencies.

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