UNDERWAY AND SITE GEOPHYSICS

During Leg 197, seismic data were collected to refine drill site locations and resolution of the acoustic characteristics and structure of basement rock and overlying sediment. A primary goal was to attempt to acoustically distinguish and map the lateral distribution of eruptive sequences. Underway geophysics also continued to maintain traditional ODP practice by acquiring navigational GPS, magnetometer, and 3.5- and 12-kHz echo sounder data during transits. Navigational and magnetometer data were recorded in WINFROG (version 2.62_21) on a dedicated PC-compatible computer in the underway geophysics laboratory.

Navigation

Precise navigation during Leg 197 was achieved by GPS. An Ashtech GG24 receiver was used as the primary navigation device throughout the leg, and the output was provided to the underway geophysics laboratory. GPS fixes were continuously updated at 1-s intervals and were recorded in WINFROG every 60 s while in transit and on site, except during seismic surveys, when navigational data were recorded at every firing of the water gun, typically every 4-6 s. The Generic Mapping Tools software package (Wessel and Smith, 1995) was used to process and display the navigation data using UNIX workstations.

Seismic Reflection Data

Site survey data (analog records only) from previous ODP legs (Dalrymple et al., 1980; Greene et al., 1980) and a regional marine geological study (Lonsdale et al., 1993) guided initial selection of drilling locations for Leg 197 sites. However, to refine site location and acquire digital recordings, short legs of single-channel seismic data were acquired in the vicinity of the site locations. These data were digitized, processed, and interpreted to characterize the upper 150-200 m of basement and to compare the acoustic fabric with the flow morphology and stratigraphic sequence documented by recovered cores.

The acoustic source consisted of a single 80-in3 water gun developed by Seismic System Inc. (SSI) (Hutchinson and Detrick, 1984). To maximize horizontal resolution, the SSI water gun was fired at a rate faster than typical of ODP cruises. A 60-phone, 100-m-long Teledyne oil-filled streamer recorded the water gun shots. The streamer was towed ~15 m deep, while the water gun was ~4 m deep. The streamer's midpoint was ~225 m behind the water gun. The midpoint between the water gun and the active streamer's midpoint was 188 m astern from the ship's recorded GPS position. Streamer output was split to present as real-time analog displays on EPC and LSR facsimile records and using UNIX Sun Sparc5 workstations for digitizing and recording of SEG-Y files on 4-mm DAT tape and 8-mm EXABYTE tape.

Digital data were subsequently processed using SIOSEIS software for interpretation and subsequent integration with downhole logging data. Integration was planned to include the shore-based production of synthetic seismograms for each site. Generation of the seismograms will use the velocity and density data collected in the core laboratory by discrete physical properties measurements. Synthetic seismic sections will be band-pass filtered to have a power spectra that approximates the power spectra of the acquired data.

Magnetic Data

Because of equipment maintenance, measurement and recording of the total intensity of the Earth's magnetic field did not begin until the third day at sea at 4006.4900N, 14924.6980E. However, this unsurveyed region (off central and northern Japan) has been extensively covered by previous magnetic surveys. Measurements were taken with a Geometrics model G-886 proton precession magnetometer towed ~500 m astern. WINFROG navigation software recorded magnetic data at 60-s intervals in the ship's navigation files.

Bathymetry

High-resolution bathymetric measurements did not begin until the third day at sea at 4006.4900N, 14924.6980E. Two types of bathymetric data were recorded: one with a 3.5-kHz shallow seismic system and the other a 12-kHz echo sounder. Both systems used a Raytheon CESP III correlator echo sounder processor (CESP). The 3.5-kHz CESP was driven by an EDO transceiver with a single-element 10-kW transducer. The 12-kHz CESP was driven by a Raytheon PTR 105B transceiver with an EDO 323B 2-kW transducer. Uncorrected 3.5- and 12-kHz depths were recorded on EPC 8082 analog line-scan recorders. Uncorrected depths were read visually from the recorders every 5 min and entered into a Microsoft Excel spreadsheet by the underway technician. The line-scan recorders automatically annotated the data with ship speed and heading every 5 min and ship position every 30 min. These annotations were also logged by WINFROG. The vessel's sonar dome is located 45.5 m toward the bow from the ship's recorded GPS position.

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