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Body and Surface-Wave Ambient-Noise Seismic Interferometry Across the Salton Sea Geothermal Field, California

L. E. Sabey1, J. A. Hole1, L. Han1, J. M. Stock2, and G. S. Fuis3
1Dept. of Geosciences, Virginia Tech, Blacksburg, VA 24061
2Caltech, Pasadena, CA, 94025
3U. S. Geological Survey, Menlo Park, CA, 91125

Seismic reflection and refraction data were acquired in southern California as a part of the 2011 Salton Seismic Imaging Project. Alongside traditional explosive source recording, a dense linear array of 486 seismometers across the Salton Sea Geothermal Field and Brawley Seismic Zone recorded 135 hours of ambient noise. The line crosses the Imperial Valley along the southern end of the Salton Sea. Volcanism associated with rifting processes provides a heat source that makes the Salton Sea Geothermal Field one of the largest and hottest geothermal fields in North America. There are abundant surface and subsurface ambient seismic noise sources near the array, including tectonic and induced earthquakes, geothermal pumping operations, a railroad, two highways, farming operations, and Salton Sea wave action.

Seismic interferometry uses continuous recordings of ambient noise to create a “virtual source” by cross-correlation of data from a reference receiver with other station data, stacking over a long time. This method has been successful for surface waves and a few previous studies have shown evidence of body waves and reflections. As anticipated, the abundant natural and induced noise sources within our study area produced both surface and body waves in interferometric virtual source gathers. Strong surface waves are observed at 1-6 Hz, despite the use of 4.5 Hz geophones. Body waves are observed up to 30 Hz and 30 km distance. The highest quality virtual source gathers are produced near anthropogenic noise sources. In particular, one large geothermal plant acted as a strong long-duration point source, producing both virtual source signal and artifacts similar to inclusion of a large explosive source. Inclusion of the >100 recorded earthquakes with normalized amplitudes improved virtual-source data quality. The virtual shot gathers compare well to the twelve explosive shots along the line, but provide additional, smaller virtual shots between the sparser explosions. Future studies will include surface wave dispersion analysis and attempt body wave reflection imaging to illuminate subsurface structure.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013