The primary purpose of this cruise is to deply and retrieve ocean
bottom seismometers (OBS) and hydrophones (OBH). Instrumentation
includes 10 OBHs operated by
Woods Hole Oceanographic Inst.
(with 3-4 staff on board), and 8 OBSs operated by the US Geological Survey. Each
instrument will be deployed twice, for a total of 36 deployments and
pickups. For each deployment, the instruments are lifted off of the stern
or side, lowered to water level, then released to free-fall to the
bottom. Instruments are retrieved after they float to the surface after
being released from their anchor using an acoustic release mechanism.
The exact locations of the 36 sites has not yet been determined, but will
fall within the following geographic boundaries: 39.2N - 41.5N; 124W -
125.5W. While on the seafloor, the OBSs and OBHJs will record shots from
the R/V EWING. These shots will also be recorded by some 200 seismometers
located onshore. The data will permit us to construct an
image of the crustal structure in the region of the Mendoceno triple
junction, which will lead to a better understanding of the geological
history of western Califonria (and the future geology of Oregon in a few
million years). These data will also be useful for better understanding the
seismic hazard associated with the Cascadia subduction zone and with
the San Anbdresa fault system. Deploying and picking up OBSs and OBHs
requires 3.5 and 12 kHz echosounders to measure water depth and seafloor
character prior to deployment, and GPS navigation.
A piggyback experiment on this cruise is to collect single channel
seismic reflection data for an ODP site survey (Mitch Lyle and Alan
Mix). Several planned drilling sites for a program to understand the
paleo-oceanographic effects of the California current fall within our operating
area. While OBSs are on the seafloor, we will conduct single
channel seismic surveys over as many of the planned drilling sites as we
have time for (estimate 4-6 sites). For the SCS, we are using a digital
acquisition system and water gun operated by the USGS. Operation of the
water gun requires use of the OSU air compressor that Pete Kalk recently
fixed, and towing a streamer and gun in a grid over the planned drill
site.
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