Application of Moment Tensor Inversion to Source Physics Experiments: A Method Accommodating Space-Time Distributed Explosion Sources

Three of the seven planned chemical explosions making up the U. S. Department of Energy's Source Physics Experiments (SPEs) have now been successfully conducted in the granites of Climax Stock on the Nevada National Security Site. These tests, the first with 100 kg yield and two with 1000 kg yield, were detonated in the same emplacement hole and recorded on the same networks of ground motion sensors in order to study the details of seismic wave generation and the role of source medium damage exciting S waves from explosion sources. The first two tests and a recently-conducted third test, based on preliminary reports, exhibit excellent coupling of down-going energy recorded in the far field and up-going energy resulting in strong spallation even though all tests were over-buried. Several lines of evidence suggest a significant source of seismic waves near the free surface in addition to the explosion centroid at depth and highlight the space-time distributed nature of the explosion source. Such a source poses difficulties interpreting moment tensor inversion results for a single centroid of energy release. In this paper, we explore a moment tensor method allowing two centroids, the first at the detonation point and the second at a depth to be determined and constrained to be a vertical dipole force model delayed in time. This model has shown promise as an elasto-dynamic representation of "late-time" damage related to shock-wave rebound and interactions with the free surface. We investigate the new inversion method with synthetic data to evaluate its performance (resolution) with frequency- and network-limited data sets and to gain understanding of the impact on inversion results for various space-time realizations of the explosion source. With the experience gained from synthetic trials, we expect to apply with confidence the inversion method to data recorded on SPEs for improved characterizations of explosion and damage sources.