Field Activity Details for field activity G195SF

AKA: none

Other ID: G-1-95-SF

Status: Completed

Organization(s):

Funding Program(s): Central California Earthquake Hazards (CENCAL) (MU041)

Principal Investigator(s): Patrick Hart, Jill McCarthy

Affiliate Investigator(s):

Information Specialist(s):

Data Type(s): Location-Elevation: Navigation, Seismics: Sub Bottom Profiler

Scientific Purpose/Goals: The purpose of this study (co-funded by the Marine and Coastal Surveys Program and the National Earthquake Hazards Reduction Program) was to test a new method for acquiring deep crustal seismic reflection data in San Francisco Bay. In contrast to the earlier 1991 BASIX experiment, which deployed single hydrophones at a spacing of 100 m, the Bay Cable experiment deployed a continuous, 2400-m-long cable with several hundred hydrophones directly onto the seafloor. Our goal was to determine whether an increased number of hydrophones and a decrease in the level of water-born noise would combine to provide improved images of the deep crust.

Vehicle(s):

Start Port/Location:

End Port/Location:

Start Date: 1995-04-17

End Date: 1995-04-28

Equipment Used: GPS, seismic

Information to be Derived:

Summary of Activity and Data Gathered: Deep seismic reflection data were acquired at three separate localities in San Francisco Bay. Data were acquired in a two-step process. First the 48-channel, 2400-m streamer was deployed onto the sea floor and anchored in a fixed position; the recording system was stationed with the streamer, separate from the sound source. Once the streamer was in place, the Robert Gray towed the 12-element, 5858 cubic inch airgun array past the receiver array, achieving shot-receiver offsets of 10-25 km. Shots were fired approximately every 90 seconds, and 14 seconds of two-way traveltime data were recorded. The three streamer deployments were located in central and south San Francisco bays and provide approximately 50 km of deep crustal seismic reflection profiling along the north-south axis of the bay. The streamer was deployed three times; the most northerly of these deployments was positioned over an area where the 1991 BASIX survey had experienced particularly good success in imaging a deep crustal reflector at 6 seconds two-way traveltime. The goal of the 1995 Bay Cable study was to reoccupy this site and see if an even better image of the 6-s arrival could be provided using the new methodology. The second and third deployments were designed to provide further testing of this technique and to help constrain the north-south extent of the 6-s reflector along the length of the San Francisco Bay Block.

Staff: Michael Boyle, Dennis Mann, Mike Marlow, Ray Sliter, Hal Williams, Jonathan Childs

Affiliate Staff:

Notes: "BAY CABLE" CRUISE NEWS: The bay cable flotilla has been successfully working through problems asthey arise (or drift away). Mike, Jon, Pat, and Co. are seeing great first break arrivals on the recorders that seem to be stronger than BASIX 1991. They believe this means the very deep reflections will be that much more resolvable. Working with the variable winds and rough chop have presented difficulties, but nothing unexpected as of yet. The streamer was deployed on Wed.; shooting began on Thurs.; and retrieval begins today. Jill reports that the radio trigger is experiencing interference as they approach the San Francisco waterfront causing some missed shots. Curious sailboats are understandably keeping their distance. Kevins MARFAC team and Curtis Gray team are doing an outstanding job keeping everything "safe and sane".- Ray Sliter For more information about G195SF -- check out the BASIX homepage.

Staff information imported from InfoBank
Jill McCarthy (USGS Western Region) - Chief Scientist
Pat Hart (USGS Western Region) - Chief Scientist
Dennis Mann (USGS Western Region) - Geophysicist
Jon Childs (USGS Western Region) - Geophysicist
Mike Marlow (USGS Western Region) - Geologist
Ray Sliter (USGS Western Region) - Geophysicist
Mike Boyle (USGS Western Region) - Electronic Tech
Hal Williams (USGS Western Region) - Mechanical Tech

Location:

San Francisco Bay

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Publications

Portals/Viewers

Data Acquired