Analysis of P- and S-wave VSP Data from the Salton Sea Scientific Drilling Project

Abstract

As part of the Salton Sea Scientific Drilling Project at California State Well 2-14, vertical seismic profile (VSP) data were collected from P an S wave sources at two distances from the well. Use of a three component geophone, along with rotation of the recorded data traces into a wave front based coordinate system, allows analysis of many aspects of seismic wave propagation properties around the well. Standard VSP analysis techniques were used to measure interval P and S wave velocities and to identify reflecting horizons both within and below the survey interval (from 455 to 1735 m). A fraction from below the survey interval, seen with both P and S sources, seems to e associated with a fractured reservoir near 2100m. Indications of fracturing were observed, including vertical scattering of P waves from a zone near 915 m. Orthogonally polarized shear waves were generated at each offset to study anisotropy by travel time measurement and particle motion analysis of the shear waver arrivals. Three component particle motion analysis of shear waver arrivals was found to be effective for characterizing the subtleties in the S wave splitting throughout the various zones in the well. The SHt source (horizontal and transverse to eth well) produced complicated elliptical particle motion while the SV source (inline with the well) produced linear particle motion. The difference in linearity of particle motion from orthogonally polarized shear wave sources was unexpected, and may be related to regional tectonics. Anomalous zones may be related to transition depths in the Salton Sea Geothermal Field. Travel time different between SV and SH waves, while clearly observable indicates only about 1% average anisotropy.