Underway geophysical data were collected during all transits. Onboard instrumentation used included a precision echo-sounder (3.5 kHz), magnetometer, gyrocompass (Lehmkul LR40), and GPS navigation systems.


The GPS was used throughout Leg 183. Three GPS systems were available for operation, with output provided to the underway geophysics laboratory. A GPS from Ashtech GG24 was used as the primary navigation device throughout the leg. Older Omnistar and Magnavox GPS systems were available, but not used.

GPS fixes were available continuously (1-s updates) and were 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, mounted on a dedicated PC 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.


One 3.5-kHz echo-sounder (precision depth recorder) was used to acquire bathymetric data as well as high-resolution reflection records of the uppermost sediment layers. Data from the system were recorded on an EPC 8082 analog line-scanning recorder. 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 and included a single EDO-type 323c transducer. The transducers are 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 was annotated automatically at fixed intervals; ship speed and heading were marked every 5 min, and position, every 30 min. Depth readings were taken manually every 5 min and entered into an Excel spreadsheet.


Total intensity measurements of the Earth's magnetic field were obtained with a Geometrics Model G-886 proton precession magnetometer towed ~500 m astern. Magnetic data were recorded during transits at 1-min intervals on navigation files produced by WINFROG navigation software. The data were reduced to magnetic anomalies at ODP/TAMU (Texas A&M University) after Leg 183.