Site survey data (single-channel analog records only) from a regional marine geological study (Lonsdale et al., 1993) guided the initial selection of proposed Sites HE-3B and HE-3A, which became Site 1203. A single-channel seismic survey, 3.5-kHz PDR, and magnetometer survey were conducted in the vicinity of these proposed sites to ensure proper hole location and suitability for basement drilling. Figure F90 shows the track line of Leg 197, Survey 1, superimposed on ETOPO5 seafloor bathymetry with a contour interval of 100 m. Tick marks along Survey 1 mark half-hour intervals. Analog lines from Lonsdale et al. (1993) are labeled Thomas Washington, 88, and are annotated with corresponding Julian day and Universal Time Coordinated time markers.
Survey Lines 1, 3, and 6 pass within 130 m of Hole 1203A. Approximately 5-km-long sections from Lines 1, 3, and 6 that cross the site position are shown in Figures F91, F92, and F93, respectively. The midpoint of the streamer was ~450 m astern of the water gun source, which was ~4.5-6 m deep. The water gun was fired every 6 s. Each shot record is 5 s in length, beginning 100 ms before the water gun was triggered. This 100-ms delay, created by the trigger control panel, was removed in SIOSEIS record processing. Ship speed averaged 6.36 kt (3.27 m/s) during Line 1, 5.64 kt (2.90 m/s) during Line 3, and 6.10 kt (3.14 m/s) during Line 6.
All three lines were processed with SIOSEIS seismic processing software (version 2001.3; http://sioseis.ucsd.edu), using multiwindow predictive deconvolution, bandpass filtering from 40 to 100 Hz, and finite-difference migration applying a simple velocity model. Table T18 shows the SIOSEIS script processing parameters for each line. The top of basement volcanic units is imaged at 4.0 s in two-way traveltime (TWT) on the southwestern ends of the lines and gradually deepens to the northeast to ~4.06 s TWT at Hole 1203A. Drilling documents that the overlying sedimentary section is ~462 m thick. The depth to the seafloor is 3.52 s TWT. Thus, the corresponding average sediment velocity is 1711 m/s (462 m/0.54 s TWT/2). The volcanic basement is essentially flat-lying and unfaulted near Hole 1203A, but seismic data document structure elsewhere (e.g., faulted basement north of proposed Site HE-3B [Line 1] and basement exhibiting a dipping internal acoustic fabric [Line 4]). Lines 1 and 6 document that a slope-mantling drape of sediment ~0.1 s TWT thick unconformably lies over more flat-lying beds. This angular unconformity likely records a pattern of erosion and deposition affected by a strong bottom current, presumably that which shaped the Meiji sediment drift that thickly blankets the summit and flanks of Detroit Seamount.