The location of Site 1149 is based on 1976 vintage multichannel and sonobuoy seismic data and a short, single-channel seismic survey conducted on approach to the site during Leg 185. Hole 801C is a fully cased reentry site installed during Leg 129 and therefore did not require any seismic site survey data for location. Underway geophysical data were collected during transits to both Sites 801 and 1149. On board instrumentation included a precision echo sounder (3.5 kHz), magnetometer, gyrocompass (Lehmkul LR40), and global positioning satellite navigation systems (GPS).
The GPS was used throughout Leg 185. Three GPS systems were available for operation, with output provided to the underway geophysics laboratory. GPS from Ashtech GG24 was used as the primary navigation device throughout the leg. 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.
One 3.5-kHz echo sounder (the PDR) was used to acquire bathymetric data as well as high-resolution reflection records of the uppermost sediment layers. Data from this 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 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.
Site selection for the second drill site of Leg 185 (proposed site BON 10, ODP Site 1149) was based on a multichannel channel seismic (MCS) and sonobuoy survey conducted during the Robert D. Conrad Cruise 2005 in 1976 and included two MCS lines intersecting 50 km west of the Site 1149 location (Fig. F2). Three potential sites were identified from these data, but a gap in the MCS record existed to the west of the preferred drill site (BON 10). An ~7-hr single-channel seismic (SCS) survey was conducted on approach to Site 1149 (JOIDES Resolution [JR] SCS Lines 1, 2, and 3) to partially fill this data gap and to ensure the correct site location by comparison of the GPS navigated SCS data with the 1976 MCS images (Fig. F2). Site 1149 is located at the intersection of JR SCS Lines 1 and 3, just 3.5 km northwest of proposed site BON 10. Hole collapse in the sedimentary section (chert and clay) and drill-string sticking problems limited basement penetration to <30 m in Holes 1149B and 1149C and prompted us to place Hole 1149D at a location where the sediment section appeared to be significantly thinned. A dome-shaped rise in basement, accompanied by a thinning of the sediment section from 0.38 to 0.28 s thick is apparent along C2005 MCS Line 39, ~5 km southeast of Holes 1149A, 1149B, and 1149C (Fig. F2). This relatively shallow basement location lies beneath a distinctive seafloor slope, and a short 3.5-kHz survey was conducted to match the distinctive bathymetry apparent in the MCS and 3.5 kHz of C2005 Line 39. We intended to drill at the apex of the basement high; however, it is likely that we were located slightly downdip. Hole 1149D reached basement at 307 mbsf, ~100 m shallower than at Holes 1149B and 1149C, and was successful in yielding a significant basement section (~130 m thick).
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 guns and streamer were both towed at 12-18 m depth. The ship speed averaged 5.4 kt during the initial approach to the site on a 337° course and averaged 5.57 kt during a second site crossing on a 70° course (see Fig. F6 in the "Site 1149" chapter). The two synchronized 80-in3 water-gun sources operated at 1900 psi and were triggered from the WINFROG navigation system at a shot interval of 13 s, equivalent to ~36 m at 5.4 kt. The guns were towed ~24 m astern and 14 m apart. Analog data were recorded on two Raytheon Model 1807M recorders ("Analog-1" and "Analog-2"), displaying scan intervals of 4-12 s and band-pass filtered from 30 to 100 Hz throughout the seismic survey. Krohn-Hite Model 3550 analog filters were used to filter signals to both recorders.
The seismic data from each shot were sampled every 1 ms from 0 to 11 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 at 250 Hz. Seismic data were copied to both 4- and 8-mm digital audio tapes during the site survey, processed using the SIOSEIS software package (Paul Henkart, Scripps Institution of Oceanography), and displayed on a HP 650C DesignJet plotter. Processing of SCS water-gun data acquired during Leg 185 included water-bottom mute, band-pass filter (30-100 Hz), weighted three-trace running mix, and automatic gain control (AGC) using a 500-ms window, and removing every other trace (see "Site Geophysics" in the "Site 1149" chapter).
At its November 1988 meeting in Miami, the ODP Planning Committee established the scientific drilling plan for Leg 130 to obtain geochemical reference sections at sites designated BON 8 and MAR 4 on the Pacific plate just east of the Izu-Bonin and Mariana Trenches (JOIDES proposal, Langmuir and Natland, 1986). In 1989, we (Abrams, Diebold, and Larson) reprocessed and reinterpreted MCS data collected in 1976 during a Conrad cruise across these two proposed sites in the western Pacific, which were scheduled for ODP drilling in January 1990. Although this geochemical reference leg was canceled, the reprocessed MCS data were used to locate Site 1149 (BON 10) seaward of the Izu-Bonin arc nearly 10 yr later.
In October and November 1976, 24-channel seismic data were acquired during Conrad Cruises C2005 and C2006. The motivation for C2005 was the imaging of Moho via two-ship constant offset profiling, but several single-ship profiles, along with sonobuoy data, were also recorded in the area of the Izu-Bonin Trench, including MCS Lines 39 and 46. Lines 39 and 46 intersect 50 km west of Site 1149; the final site location is on JR SCS Line 1 ~3.5 km northwest of its intersection with MCS Line 39 (see "Site Geophysics" in the "Site 1149" chapter). C2005 was a regional survey, and both Lines 39 and 46 extend across the trench axis to the west and more than 100 km east and southeast of the site. The seismic source used during acquisition of Lines 39 and 46 consisted of four identical 466-in3 air guns, with a nominal shot spacing of 50 m. The arrivals were received by a 24-channel analog streamer with 100-m group spacing and were digitized and recorded using a DFS IV system. After acquisition, these data received "standard" processing. Since 1976, techniques and equipment enabling a higher level of interaction in choosing processing parameters, such as deconvolution operators and stacking velocities, as well as time and depth migration, have also been introduced. We, therefore, reprocessed ~220 km each of Lines 39 and 46 beginning with demultiplexed and common depth point gathered data on nine-track tapes in the Lamont-Doherty Earth Observatory (LDEO) archives. Reprocessing included repicking of stacking velocities, prestack predictive deconvolution and noise filtering, spherical divergence correction, restacking, poststack deconvolution and time-varying filtering, and, finally, migration. The resulting displayed sedimentary sections of the migrated and unmigrated data are far superior to the original data primarily because of the significant reduction in the bubble pulse from four identical air guns.
Sonobuoy 171 was acquired along Line 46 ~50 km west of Site 1149. The analog record of sonobuoy 171 was reinterpreted and indicates the onset of high velocities (>4.1 km/s) at 528 mbsf, marking the top of igneous basement (J. Diebold, pers. comm., 1989, and JOI/USSSP report JSC10-89, 1989).