We propose to carry out geophysical surveys at the H2O observatory site between California and Hawaii to ensure that the already funded H2O observatory will be located in a region where it will be possible to drill a deep crustal hole in the future.

The Hawaii 2 Submarine Cable system is a retired AT&T telephone cable system between San Luis Obispo, California and Makaha, on Oahu, Hawaii. A program is underway to terminate the Hawaii-2 cable approximately half-way between Hawaii and California and to install a junction box to which oceanographic instrumentation can be attached. This will provide the capability to acquire continuous data in real time from a site on the deep ocean floor. There are many types of measurements that can utilize such a facility. One particular application is the installation of a broadband seismic station for teleseismic studies and regional studies of earthquake activity in the eastern North Pacific and California. In addition, the H2O site is located on crust which formed at a "very high" spreading rate (70mm/yr half-rate) and geophysical surveys at the site (for example, basement morphology) will constrain models of crustal formation and evolution.

A proposal has been submitted to JOIDES to drill a hole into basement at the H2O site for the installation of a high quality broadband borehole seismic system. However at a recent workshop it was recognized that the site also had many of the attributes of a deep crustal penetration hole in fast spreading crust. Both the marine broadband seismology and the ocean lithosphere communities would be well served by drilling at the H2O site. A site survey is necessary at H2O to ensure that the site will be suitable for future drilling. If a site survey is not carried out there is the possibility that the cable termination will occur over an area where deep drilling will be impossible (for example, an area with very thin sediment cover) or will be difficult and expensive (for example over a rubble zone or a region of highly weathered and unconsolidated basalt). Ideally, to accomplish objective 1) requires locating the cable to within a few tens of meters in about a 10x20km area and obtaining basement morphology at two levels of resolution, 10x20km area and 10X2km area, using surface reflection and deep tow sub-bottom profiling, respectively. However because of funding constraints we are proposing a minimal program consisting of SEA BEAM bathymetry, single channel surface reflection profiling and dredging of outcrops.

We propose acquiring data on an already scheduled transit of the R/V Revelle between San Diego and Honolulu in summer or fall of 1997 or the winter of 1998. Since the transit would already be scheduled we are merely requesting the ship time and operation costs for two additional days.

The cruise schedule is roughly: Underway geophysics along the cable track - San Francisco to Honolulu - 8days for a normal 12 knot transit, plus allow 1 day to slow down to 8knots for water gun reflection profilingbetween 140-143degW plus one day for dredging and grid profiling at a to-be-determined site.

a) Join the cable track at 140degreesW (about 28.8degN) - 3.5days (It would be a good idea to acquire SEA BEAM along the whole cable track while in transit. This can be acquired at 12 knots and the cable track is along the transect from San Francisco to Honolulu. Seabeam of the whole cable track can be acquired at no additional ship time. However since the data outside of 140degW-143degW are not required by the objectives of this proposal we are not requesting funds for the additional coverage.)
b) SEA BEAM and water gun profile at 8knots to about 143degW (about 27.3degN) - about 1day
c) Somewhere between 140-143degW dredge up to three selected outcrops (0.5-1.0day)
d) Somewhere between 140-143degW carry out a detailed geophysical survey at one or two prospective sites along the cable. In a 5x10nm (10x20km) box acquire a reflection profiling survey at 1mile spacing (about 6 - 10nm lines at 8 knots plus rigging and turns - about 1.0-1.5days)
e) Leave cable track at 143degW for Honolulu - 3days

(Note: This cruise will not locate the cable. Cable location is based on 1965 navigation of repeater boxes - about 1nm resolution. If bathymetry and sediment thickness vary slowly over length scales of a few kilometers this will not be a problem for locating suitable drill sites near the cable.)

Underway Geophysical Surveys:
- magnetometer
- 3.5KHz profiling
- SEA BEAM (14km swath width in about 4500m water depth)
- single channel seismics

On Station Gear:
- Dredge and Trawl Wire

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