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Metadata
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ID
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G-1-95-SF
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Abstract
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Chief Scientists: Jill McCarthy, Pat Hart. Deep-Crustal
Seismic data (seismic) of field activity G-1-95-SF in San
Francisco Bay from 04/17/1995 to 04/28/1995
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Project/Theme
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Central California Earthquake Hazards (CENCAL)
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National Plan
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Hazards
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Chief Scientist
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Jill McCarthy
Pat Hart
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Activity Type
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Deep-Crustal Seismic
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Platform
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Robert Gray
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Area of Operation
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San Francisco Bay
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Location map
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Bounding Coordinates
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38.01217
-122.45324 -122.11396
37.49946
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Dates
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04/17/1995 (JD 107) to 04/28/1995 (JD 118)
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Analog Materials
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No analog holdings.
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Index map
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Crew
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Jill McCarthy
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Chief Scientist, USGS Western Region
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Pat Hart
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Chief Scientist, USGS Western Region
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Dennis Mann
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Geophysicist, USGS Western Region
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Jon Childs
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Geophysicist, USGS Western Region
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Mike Marlow
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Geologist, USGS Western Region
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Ray Sliter
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Geophysicist, USGS Western Region
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Mike Boyle
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Electronic Tech, USGS Western Region
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Hal Williams
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Mechanical Tech, USGS Western Region
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Equipment Used
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Purpose
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The purpose of this study (co-funded by the Marine
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and Coastal Surveys Program and the National Earthquake Hazards Reduction
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Program) was to test a new method for acquiring deep crustal seismic
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reflection data in San Francisco Bay. In contrast to the earlier 1991
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BASIX experiment, which deployed single hydrophones at a spacing of 100 m,
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the Bay Cable experiment deployed a continuous, 2400-m-long cable with
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several hundred hydrophones directly onto the seafloor. Our goal was to
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determine whether an increased number of hydrophones and a decrease in the
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level of water-born noise would combine to provide improved images of the
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deep crust.
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Summary
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Deep seismic reflection data were acquired at three separate localities
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in San Francisco Bay. Data were acquired in a two-step process. First the
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48-channel, 2400-m streamer was deployed onto the sea floor and anchored in a
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fixed position; the recording system was stationed with the streamer, separate
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from the sound source. Once the streamer was in place, the Robert Gray towed the
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12-element, 5858 cubic inch airgun array past the receiver array, achieving
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shot-receiver offsets of 10-25 km. Shots were fired approximately every 90
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seconds, and 14 seconds of two-way traveltime data were recorded. The three
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streamer deployments were located in central and south San Francisco bays and
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provide approximately 50 km of deep crustal seismic reflection profiling along
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the north-south axis of the bay. The streamer was deployed three times; the most
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northerly of these deployments was positioned over an area where the 1991 BASIX
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survey had experienced particularly good success in imaging a deep crustal
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reflector at 6 seconds two-way traveltime. The goal of the 1995 Bay Cable study
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was to reoccupy this site and see if an even better image of the 6-s arrival
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could be provided using the new methodology. The second and third deployments
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were designed to provide further testing of this technique and to help constrain
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the north-south extent of the 6-s reflector along the length of the San
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Francisco Bay Block.
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Notes
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"BAY CABLE" CRUISE NEWS: The bay cable flotilla has been
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successfully working through problems asthey arise (or drift away).
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Mike, Jon, Pat, and Co. are seeing great first break arrivals on the
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recorders that seem to be stronger than BASIX 1991. They believe
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this means the very deep reflections will be that much more
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resolvable. Working with the variable winds and rough chop have
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presented difficulties, but nothing unexpected as of yet. The
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streamer was deployed on Wed.; shooting began on Thurs.; and
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retrieval begins today. Jill reports that the radio trigger is
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experiencing interference as they approach the San Francisco
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waterfront causing some missed shots. Curious sailboats are
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understandably keeping their distance. Kevin's MARFAC team and
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Curtis' Gray team are doing an outstanding job keeping everything
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"safe and sane".- Ray Sliter
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For more information about G195SF -- check out the
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BASIX homepage.
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Publications
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McCarthy, J., Hart, P., and Childs, J.R., 1995, Deep reflective character of the San
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Francisco Bay block; results from the 1995 BASIX Bay cable experiment: Geological Society
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of America Abstracts with Programs, v. 27, no. 6, p. 287.
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Brocher, Thomas M., McCarthy, Jill, and Holbrook, W. Steven, 1996, Imaging the transform
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plate boundary in the San Francisco Bay area, California: Geological Society of America
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Abstracts with Programs, v. 28, no. 7, p. 216.
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Got Help?
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For G-1-95-SF, we would appreciate any information on -- analog materials, contract, days at sea, dive count, funding, information specialist, information to be derived, kms of navigation, NGDC Info, organization, owner, ports, project number, scanned materials, seismic description, station count, station description, submersible, tabulated info.
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