Anomalous Velocities in Salton Sea Area

Jack Cameron

As part of the Salton Sea Scientific Drilling Project in California, several vertical seismic profiles (VSP) were run in State Well 2-14. Data were recorded downhole using a three-component receiver with vertical and horizontal surface sources offset 300 ft and 2,300 ft.

The compressional and shear-wave velocities determined from these VSPs show a near linear increase from the surface to a depth of 4,000 ft. The Vp/Vs ratio over this interval ranges from 10 at the surface to 1.5 at 4,000 ft. In terms of Poisson's ratio, the change is remarkably linear, decreasing with depth.

The Imperial Valley is one of the most seismically active areas in California. The liquidlike characteristics of the near surface manifest themselves during earthquakes by surface liquefaction and sand boils.

The rapid change in Vp/Vs with depth results in very different ray paths for the compressional waves and shear waves, which is apparent on the offset VSPs.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.