Seismic Reflection Studies in Long Valley Caldera, Califomia

Abstract

Seismic reflection studies in Long Valley caldera, California, indicate that seismic methods may be successfully employed to image certain types of features in young silicic caldera environments. However, near-surface geological conditions within these environments severely test the seismic reflection method. Data quality are degraded by static, reverberation, and band-limiting problems due to these near-surface conditions. In Long Valley, seismic reflection and refraction methods were used to image both the shallow and deep geothermal aquifers within the area. The deep geothermal aquifer, the welded Bishop Tuff, was imaged as a fairly continuous reflector across the western moat of the caldera. Near-surface refraction information indicates that there may be a buried paleochannel system or horst and graben system that could control the shallow geothermal flow pattern. High-amplitude events observed in a wide-angle survey were originally interpreted as reflections from a contemporary magma body. However, a migration of the events utilizing the new generalized cellular migration algorithm indicates that these events are probably reflections from the faults of the caldera ring fracture system. The reflections may be caused by the high acoustic impedance contrast associated with the juxtaposition of relatively low-velocity, low-density, caldera fill against the granite plutons and metasediments of the Sierran basement along this fault system.