Interpretation of S waves generated by near-surface chemical explosions at SAFOD

Abstract

A series of near-surface chemical explosions conducted at the San Andreas Fault Observatory at Depth (SAFOD) were recorded by high-frequency downhole receiver arrays in separate experiments in November 2003 and May 2005. The 2003 experiment involved ∼100  kg shots detonated along a 46-km-long line (Hole–Ryberg line) centered on SAFOD and recorded by 32 three-component geophones in the pilot hole between 0.8 and 2.0 km depth. The 2005 experiment involved ∼36  kg shots detonated at Parkfield Area Seismic Observatory (PASO) stations (at ∼1–8  km offset) recorded by 80 three-component geophones in the main hole between the surface and 2.4 km depth. These data sample the downgoing seismic wavefield and constrain the shallow velocity and attenuation structure, as well as the first-order characteristics of the source. Using forward modeling on a velocity structure designed for the near field, both observed P - and S -wave energy for the PASO shots are identified with the travel times expected for direct and/or reflected phases. Larger-offset recordings from shots along the Hole–Ryberg line reveal substantial SV and SH energy, especially southwest of SAFOD from the source as indicated by P -to- S amplitude ratios. The generated SV energy is interpreted to arise chiefly from P -to- S conversions at subhorizontal discontinuities. This provides a simple mechanism for often-observed low P -to- S amplitude ratios from nuclear explosions in the far field, as originating from strong near-field wave conversions.