Underway geophysical data were collected during all transits. To reconfirm the position of a proposed drill site, a short single-channel seismic (SCS) survey was conducted on the approach to most sites during Leg 187. Onboard instrumentation used included a precision echo sounder (3.5 kHz), gyrocompass (Lehmkul LR40), seismic survey system, and Global Positioning Systems (GPSs).
Throughout Leg 187, three GPS units were available, with output provided to the Underway Geophysics Laboratory. The Ashtech GG24 system was the primary navigation device. Older Omnistar and Magnavox systems were not used.
GPS fixes were available continuously (1-s updates) and recorded at 60-s intervals. Event data were recorded at 60-s intervals on site and in transit. Navigation data were logged by the WinFrog software system, which was installed on a dedicated personal computer in the Underway Geophysics Laboratory. Subsequent processing and display of navigation data were performed using the Generic Mapping Tools (GMT) software package (Wessel and Smith, 1995) on shipboard UNIX workstations.
Both 3.5- and 12-kHz echo sounders (precision depth recorders [PDRs]) were used to acquire bathymetric data as well as high-resolution reflection records of the uppermost sediment layers. Although the resolution of 12-kHz data is higher than that of 3.5-kHz data, the 12-kHz PDR has little penetration ability in these water depths. Data were recorded on two EPC 8082 analog line-scanning recorders at 3.5 and 12 kHz, respectively. The 3.5-kHz system used a Raytheon CESP III correlator echo sounder processor driven by a Raytheon PTR105B transceiver with a 2-kW sonar transmitter. This system included a single EDO-Western type 323c transducer mounted in a sonar dome on the hull 40 m forward of the center of the moonpool. This location was chosen to reduce ship-generated noise and signal attenuation from aeration beneath the hull. The recorder annotated automatically at fixed intervals; ship speed and heading were marked every 5 min and position every 30 min. Depth readings were taken manually at 5-min intervals and entered into an Excel spreadsheet.
The SCS data were received using a single-channel Teledyne model 178 hydrophone streamer with a 100-m-long active section containing 60 hydrophones, a 25-m-long "stretch" section, and a 150-m lead section. The gun and streamer were both towed at 12-18 m depth. Ship speed was kept at ~5.5 kt during all seismic surveys. One 80-in3 water-gun source operated at 2000 psi and was triggered from the WinFrog navigation system at a shot interval of 12 s, equivalent to ~33 m at 5.5 kt. The gun was towed ~20 m astern. Analog data were recorded on one Raytheon model 1807M recorder, displaying scan intervals of 4-12 s and band-pass filtered from 30 to 100 Hz throughout the seismic survey. A Krohn-Hite model 3550 analog filter was used to filter signals to the recorder.
The seismic data from each shot were sampled every 1 ms from 0 to 8 s and were digitally recorded on a Sun Sparcstation 10 in SEGY format, using the "a2d" acquisition package after applying an anti-aliasing filter with a corner frequency of 250 Hz. Seismic data were copied to both 4- and 8-mm digital audiotapes during the site survey and then processed using the SIOSEIS software package (Paul Henkart, Scripps Institute of Oceanography) and displayed on a HP 650C Design Jet plotter. Processing of SCS water-gun data acquired during Leg 187 included water-bottom mute, band-pass filter (25-120 Hz), automatic gain control using a 500-ms window, and removal of every other trace.