Moment tensor inversion of single-hole mining cast blasts

Abstract

We carried out a controlled field experiment to characterize the single-hole shots typically used in multishot mining blasts. Eight single-hole mining shots were detonated on an overburden bench, and near-source (49–54 m) ground accelerations were recorded. The sources were characterized by their cylindrical geometry, proximity (6 m) to an 11-m vertical free face and the Earth’s free surface, and lack of confinement at the horizontal free surface and the vertical free face. Charge sizes ranged from 59 to 296 kg. The ground motion data, supplemented with standard refraction data, were used to constrain the local velocity model. The resultant Green’s functions and the observational data were used to invert for the second-order, time-dependent source moment tensors with a frequency domain method. Despite the large isotropic component (~80 per cent), the source moment tensors show a significant degree of asymmetry among the diagonal components. The first peak of the vertical component M33 is 39 per cent (±15 per cent) larger than the first peaks of the two horizontal components, which have similar amplitudes. This observation cannot be explained by the cylindrical geometry of the source or the presence of the vertical free face in the source region. The complex source time history, an associated spectral peak and the increase of the degree of asymmetry with time among diagonal moment tensor components are indicative of secondary source effects. Together with the source asymmetry, they could be explained by the effects of a vertical spall source that includes both the vertical cast of the burden and, more importantly, the vertical spallation of the strata around the source. The horizontal cast of the burden was not resolved in the inverted source moment tensors, possibly due to their long period and low seismic efficiency. Although the off-diagonal moment tensor components are much smaller than the diagonal components, they contribute significantly to the seismic wave generation. Further studies are required to interpret their generating mechanisms.