Site 1157 was located based on 1997 SCS site survey data and confirmed by a short 3.5-kHz PDR and SCS presite survey from the JOIDES Resolution (JR). Onboard instrumentation included a precision echo sounder, gyrocompass, seismic system, and GPS receivers.
Site selection for Site 1157 was based on a SCS survey conducted during R/V Melville cruise Sojourner 5 in 1997. A 1-hr SCS and 3.5-kHz PDR survey was conducted on the approach to Site 1157 (JR SCS line S6; Fig. F35) to ensure the correct site location. The ship's average speed was 5.5 kt during the survey. The water gun was triggered at a shot interval of 12 s, equivalent to ~34 m at 5.5 kt. Data acquisition and processing parameters are described in "Underway Geophysics" in the "Explanatory Notes"chapter. Survey line S6 was conducted on a heading of 115°and roughly perpendicular to the site survey line. The final position of Site 1157 was chosen ~1.85 km east of the prospectus site AAD-28a (Fig. F35), where sediment cover appears to be thinner. We marked the position of Hole 1157A near seismic shotpoint 157 of line S6, where the water depth is 5080.4 m (see Fig. F36). Although there are significant out-of-plane reflectors, the sediment cover (Fig. F36) extends from 6.83 s to at least 6.98 s in two-way traveltime, equivalent to a minimum of 150 m of sediment. Hole 1157A penetrated 200 m of sediment before basement was reached, whereas Hole 1157B, about 200 m west of Hole 1157A, encountered only ~136 m of sediment, indicating significant basement roughness.