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Pacific Coastal & Marine Science Center

Coastal and Marine Earthquake Studies


Western Coastal and Marine Earthquake Studies.

 

Central California/San Francisco Bay Earthquake Hazards Project, 1998

R/V David Johnston

High-Resolution Seismic-Reflection Studies of Submerged Fault Zones throughout the San Francisco Bay Area

Childs, J. R., Hart, P. E., Sliter, R., McCarthy, J., and Marlow, M. S. (all at U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025)

In July, 1993, tests of a high-resolution multichannel seismic (MCS) air gun profiling system were conducted from the R/V David Johnston, a 40-foot research vessel, in the south San Francisco Bay between the San Francisco and Oakland airports. Based on the success of these tests, MCS air gun and concurrent very-high-resolution (VHR) single-channel profiles were collected in May, 1994 and June, 1995 in four areas: the vicinity of San Bruno Shoal in South San Francisco Bay, the offshore extension of the San Andreas fault system west of the Golden Gate, the Hayward Fault to Rodgers Creek Fault step-over in San Pablo Bay, and the Kirby Hills Fault where it crosses the western Sacramento Delta. These surveys were designed to investigate the sedimentary section in the uppermost kilometer.

The high-resolution multichannel system consisted of a 24-channel (6.25-meter group interval; 150 meters active length) solid-core streamer with "thin-film" hydrophones This streamer, manufactured by Innovative Transducers, Inc. utilizes cylindrical hydrophones constructed from poly-vinylidene fluoride (PVDF) plastic, which are characterized by high sensitivity and broad band (5-5000 Hz) frequency response. The seismic data were recorded with a Geometrics Strataview recorder, a compact, PC-based floating point instrument.

The air gun source consisted of one or two Bolt 600-series air guns with 20 cubic-inch chambers, fired at either 6.25-meter or 12.5-meter intervals. Factors limiting the firing interval were air compressor capacity and the time required by the recording system to process each shot. At speeds below 3.5 knots the 6.25-meter fire interval could be maintained; higher speeds (generally tide-controlled) necessitated the longer fire interval. Resulting common-mid-point (CMP) data were either 12-fold or 6-fold, with a 3.125-meter CMP interval.

VHR (> 1 kHz) profiles were collected with a variety of systems. These systems typically consisted of a 100 to 400 joule sled-mounted boomer source (1-3 kHz bandwidth) and receiver, capable of resolving stratigraphic units as thin as 25 cm with penetration to 30 meters. Data were recorded digitally with a USGS-developed PC-based two-channel seismic recording system, using 16 kHz sampling with 16-bit resolution. Positioning was accomplished with real-time differential GPS.

Post-collection digital processing was used to minimize, if not completely eliminate, acoustic interference between the two systems, obviating the need to gate the VHR system, which would have resulted in irregular trace spacing and data loss. Air gun interference in the VHR data was removed with a custom noise-burst filtering algorithm, while VHR interference in the air gun data was suppressed through CMP stacking.

Conclusions

Newly-available technologies enable collection of digital high-resolution multichannel seismic-reflection profiles and concurrent very-high resolution single-channel profiles in waters as shallow as 4-feet.

Concurrent high-resolution and very-high-resolution profiling can provide decimeter-scale resolution in the upper 20 to 30 meters of the sedimentary section, and meter-scale resolution to 500 meters.

The application of common-midpoint methods to high-resolution profiling results not only in improvement of the reflection image thru standard MCS processing methods, but has the potential to provide valuable information on physical properties of the sediments thru analysis of the offset data, making these methods useful to a broad range of geohazards, geotechnical and environmental problems.

 

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