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

摘要

Using a deep deployment of an 80-element, 3-component borehole seismic array stretching from 1.5 to 2.3 kilometer (km) depth at the San Andreas Fault Observatory at Depth (SAFOD), we examine recordings of chemical explosions to better understand the generation of shear waves by explosive sources. The well is near-vertical at 1.5km and gradually transitions to a dip of 38 degrees at the deepest recording location. The chemical shots are high velocity chemical shots buried between 10-30 m and fired electrically, of size approximately 36 kg. The shotpoints are offset from the wellhead by 1 to 2 km. Previous analysis of zero-offset recordings (Pollitz et al., 2012) gave a velocity structure ranging from 1500 m/s (meters per second) in the upper 50 meters to 5000 m/s at the bottom of the well, as well as attenuation structure. The larger-offset recordings analyzed here have a strong, impulsive P arrival polarized as a longitudinal wave, and S waves composed of dominantly converted P to SV at the internal discontinuities and, to a lesser extent, the upward P to downgoing S converted wave pS. 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 a hybrid 1D structure consisting of the local Bleibinhaus et al. (2007) structure determined from an active-source experiment combined with the nearwell structure determined from the zero-offset shots. Using forward modeling on this velocity structure, both observed P and S wave energy are identified with the traveltimes expected for direct and/or reflected phases as well as the moveout associated with the local velocity structure around the receiver array. For larger-offset shots, S-wave energy is polarized primarily along the radial and propagation direction, consistent with converted P to SV energy.