A short geophysical survey was performed upon approach to Site 1165 to verify prior multichannel seismic reflection (MCS) data used for site selection. The MCS survey had been conducted with transit satellite navigation, resulting in possible navigational errors. The Site 1165 presite survey consisted of one track line across the site (Fig. F3) using 3.5-kHz bathymetry and water gun seismic systems. A description of these systems is provided below.
The seismic line was run from southeast to northwest across the site at an average speed of ~6.5 kt. Good quality data were obtained (Fig. F4) following initial problems with the performance of the water gun in the cold water conditions. The presite survey confirmed that the proposed site was in the proper stratigraphic position, and drilling was completed as planned.
Navigation data were collected in the underway geophysics laboratory using Ashtech GG24 conventional GPS navigation and were recorded by the WINFROG software system on a dedicated PC. Ship position was recorded every 60 s during transit and every 10 s during presite seismic surveys.
Bathymetric data were obtained using a 3.5-kHz echo sounder (PDR) with a Raytheon CESP III Correlator driven by a Raytheon PTR105B transceiver. The 2-kW transmitter included a single EDO-type 323C transducer mounted in a sonar dome attached to the hull 40 m forward of the center of the moonpool. The signal was recorded on an EPC 8082 analog line-scanning recorder at a sweep speed of 1 s (750-m scale). The recorder was automatically annotated at fixed intervals with navigation system information. Depth readings were taken manually and entered into an Excel spreadsheet.
The seismic reflection profile was obtained using one 80-in3 water gun as the seismic source. The gun was operated at ~1900 psi, towed ~20 m behind the ship, and fired every 10 s. The shot interval was equivalent to ~33 m at 6.5 kt. Reflections were recorded by a 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-long lead section. The seismic source and streamer were towed at estimated depths of 12-18 m. Depth-stabilizing "birds" were not available for use on the streamer.
Analog seismic data were displayed in real time on two graphic recorders, Raytheon model 1807M and EPC 8082, at sweep rates of 2 and 4 s with band-pass filter settings of 30-150 Hz. Data from each 10-s shot were sampled every 4 ms over a 4-s window. They were digitally recorded on a Sun Sparcstation 10 in SEG-Y format, using the "a2d" acquisition package, after application of a 20- to 250-Hz filter. Seismic data were copied to both 4- and 8-mm digital audiotapes during the site survey, then transferred to the Geoquest IESX system in the downhole measurements laboratory for evaluation and calculation of synthetic seismic traces. A large-scale seismic section was plotted in the underway geophysics laboratory for use during Site 1165 drilling.