Analysis and Modeling of Shear Waves Generated by Explosions at the San Andreas Fault Observatory at Depth

Using multiple deployments of an 80-element, three-component borehole seismic array stretching from the surface to 2.3 km depth, we examine recordings of chemical explosions to better understand the generation of shear waves by explosive sources. The well is near vertical in the upper 1.5 km and gradually transitions to a dip of 38 degrees at the deepest recording location. The chemical shots are high-velocity chemical shots buried 3–4 m and fired electrically, ranging in size from 5 to 10 lb. The shotpoints are offset from the wellhead by approximately 40 m. Analysis of the recordings gives a velocity structure ranging from 1500 m/s in the upper 50 m to 5000 m/s at the bottom of the well. We also determine a Q structure using the spectral ratio method (e.g., Bath, 1974). Q(P) varies between 11 in the uppermost 50 m to 45 between 1000 m and 2350 m depth. The recordings have a strong, impulsive P arrival on the vertical channels. Additional, coherent phases arrive later on the vertical channel propagating at the P velocity. At the very near surface, there is evidence for a weak S arrival on the horizontal channels. We also observe S in the form of P to S conversions at layer boundaries further downhole. We compute synthetic waveforms using the Direct Radial Integration method of Friederich and Dalkolmo (1995), which handles a layered transversely isotropic medium with anelasticity. We use the one-dimensional structure determined from the observations described above. For a zero-offset, shallow-burial purely explosive source the synthetics yield a highly impulsive P in addition to a small S, agreeing with our observations that the S is weak at vertical incidence, strengthening as the incidence angle decreases. The weak, near-surface S is generated from the surface reflection and subsequent conversion of the explosion generated P. We are presently investigating the source characteristics of the chemical shots, testing whether the waveforms can be completely matched using a purely explosive source or if there is a significant compensated linear vector dipole (CLVD) or secondary deformation (e.g., cavity collapse, distributed secondary shear dislocations) as well. This should shed light on the source radiation generated by chemical explosions and the interaction of shallow explosive sources with the free surface. Preliminary analysis indicates that a purely explosive source augmented by secondary distributed vertical shear (equivalent to a second-order moment tensor) directly above the explosive source is necessary to match our observations. 2011 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies