Site 1160 was located based on 1996 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 1160 was based on a SCS survey conducted during the R/V Melville cruise Boomerang 5 in 1996. A 1.5-hr SCS and 3.5-kHz PDR survey was conducted on approach to Site 1160 (JR SCS line S8; Fig. F20) to ensure the correct site location by comparison of the GPS-navigated SCS data with the 1996 SCS image and to verify sediment thickness. Survey line S8 crossed the prospectus site AAD-14c from south to north. The ship's average speed was 4.7 kt. The water gun was triggered at a shot interval of 12 s, equivalent to ~29 m at 4.7 kt. Data acquisition and processing parameters are described in "Underway Geophysics" in the "Explanatory Notes" chapter. We marked the position of Hole 1160A in 4625 m water depth near seismic shotpoint 206 of line S8, ~1.3 km north of prospectus site AAD-14c (Fig. F21). Hole 1160B is 200 m north of Hole 1160A.
The sediment cover (Fig. F21) appears to extend from 6.24 to 6.48 s in two-way traveltime, equivalent to at least 240 m of sediment, and there are several distinct reflectors within this interval. However, both Holes 1160A and 1160B were drilled through only 166 and 160.1 m of sediment, respectively, before basement was reached, obviously thinner than we estimated from the SCS profile. One possible explanation is that the sediment is much more unconsolidated than that encountered so far, delaying the two-way traveltime of the basement signal. Alternatively, and perhaps more likely, the basin may be partially filled by horizontal flows with interflow rubble and/or pillow zones. This explanation would be consistent with the actual recovery in 46 m of penetration of three massive flows, ranging between 2 and 3 m thick, separated by broken pillow material.