Moment-Tensor Inversions of Single Cast Mining Cast Blasts

LA-UR-98-3881
Xiaoning Yang, Brian W. Stump*, and D. Craig Pearson
Geophysics Group (EES-11), Los Alamos National Laboratory and ^*Southern Methodist University

Introduction

Multi-shot mining and quarry explosions have been studied by verification seismologists for the purpose of monitoring undgerground nuclear explosions. In this study, we characterized single cast-shots that comprise these multi-shot explosions in hope of providing physical constraints on the source mechanisms of these explosions. We collected near-source, broad-band data from eight single-hole, mining-shot sources. The shots were characterized by their cylindrical source geometry, proximity to a vertical free face and the Earth's free surface, and the material cast during the detonation.

We used a frequency-domain linear inversion method to invert for the time-dependent source moment tensors of these shots. In order to calculate proper Green's functions for the inversion, the velocity structure of the propagation medium was constrained by a refraction survey and forward waveform modeling.

The estimated source moment tensors of the single mining cast-shots seem to be more complex than that of a simple isotropic explosion. Despite the dominance of the isotropic component, the moment tensors show apparent asymmetry among diagonal components with a larger M33 in the vertical direction. The complex source time history and spectrum indicate the effects of secondary sources such as material spallation induced by the explosion. Although much smaller compared with the diagonal components, synthetic tests indicate that off-diagonal moment-tensor components contribute significantly to the seismic-wave generation.

Figure 1 - Ground motion data were collected in a controlled field experiment consisting of eight single-hole, mining cast-shots. The charge sizes range from 59 kg to 296 kg. Test bed was a shale overburden bench of about 11 m high. Terra Technology SSA-302 accelerometers were deployed on the bench in the near-source region behind the sources.

Figure 2 - This depicts the vertical cross section of the source configuration

Figure 3 - Key source parameters are listed.

CLICK ON IMAGES TO ENLARGE.

Introduction | Data | Site Characterization | Moment-Tensor Inversion | Discussion

Please direct questions or comments to the principal author.