Drilling during Leg 191 occurred at four sites, the last three characterized only by underway profile data collected by the JOIDES Resolution owing to extraordinary circumstances. The International Ocean Network seismometer hole at Site 1179 (proposed Site WP-2A) was sited on data collected in 1996 by the Hakuho-Maru (Ocean Research Institute, University of Tokyo) during cruise KH96-3 (Leg 1). Hammer-drill tests, originally scheduled for a site on Shatsky Rise, were moved to two seamounts near Guam after problems with the drilling equipment dictated picking up spare parts in Guam. No geophysical data were on board for these hammer-drill test sites, but because the goal was to find a basalt outcrop near a seamount summit, it was sufficient to survey for possible sites with the Resolution's echo sounders. No single-channel seismic (SCS) data were collected during Leg 191 because it was not deemed necessary. The main site (1179) was characterized by multichannel seismic lines navigated by Global Positioning System (GPS), so it was possible to precisely locate the desired drilling location without resurveying. Furthermore, the hammer-drill test sites are on the tops of young seamounts where SCS data would have provided little useful information.
Underway geophysical data collected during all Leg 191 transits and surveys were 3.5-kHz and 12-kHz echo-sounder profiles and total field magnetic data collected with a proton precession magnetometer.
GPS positioning was used throughout Leg 191. Three GPS systems were available for operation, with output provided to the underway geophysics laboratory. An Ashtech GG24 receiver 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 software package (Wessel and Smith, 1995) on shipboard UNIX workstations.
Two echo sounders (precision depth recorders) were used during transits: 3.5-kHz and 12-kHz frequency units. The former was used to acquire bathymetric data as well as high-resolution reflection records of the uppermost sediment layers. Data from both systems were recorded on EPC 8082 analog line-scanning recorders. 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 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.