Seismic Reflection Profiling

Initial selection of proposed Site HE-6A, the target location for Site 1206, was based on data gathered at DSDP Site 308. Previous drilling at this site penetrated ~70 m of clay and volcaniclastic sandstone but did not reach basement (Larson, Moberly, et al., 1975). We conducted a single-channel seismic survey, 3.5-kHz PDR, and magnetometer survey in the vicinity of Site 308 to ensure proper hole location and suitability for basement drilling. Analog seismic data (S.P. Lee, 8-76-NP, 9 October 1976, 0700 hr universal time coordinated) were available in the vicinity of Site 308. Figure F65 shows the track line of the S.P. Lee cruise and the Leg 197 survey superimposed on ETOPO5 seafloor bathymetry with a contour interval of 250 m. Tick marks along our survey lines mark half-hour intervals. Line 1 is approximately in the same location and azimuth as the SP Lee 8-76 line.

Survey Lines 3 and 6 pass directly over Site 1206, which is ~6.2 km south of DSDP Site 308. Approximately 2.5-km-long sections from Lines 3 and 6 are shown in Figures F66 and F67, respectively. The midpoint of the active part of the streamer was ~218 m astern of the water gun source, which was ~4.5-6 m deep. The midpoint between the active section of the streamer and the water gun source was ~191 m astern of the ship's recorded GPS position. Firing of the water gun was set at 4 s. Each shot record is 3 s in record length, beginning 100 ms before the water gun was triggered. This 100-ms delay, created by the trigger control panel, was removed during SIOSEIS record processing. Ship speed averaged 6.02 kt (3.25 m/s) during Line 1 and 6.05 kt (3.27 m/s) during Line 6. Both lines were processed with SIOSEIS seismic processing software (version 2001.3) (http://sioseis.ucsd.edu), using predictive deconvolution, bandpass filtering from 55 to 150 Hz (Line 3) and 45 to 120 Hz (Line 6), and finite-difference migration using a simple velocity model. Table T18 shows the SIOSEIS script processing parameters for each line.

At Site 1206, a thin sequence of internally reflective sediment, ~57 m thick, overlies a reverberant volcanic basement. A series of laterally coherent reflections characterizes this section, which has a reflection time of 0.04 s two-way traveltime (TWT). Acoustically defined bedding generally conforms to basement topography. A sharp decrease in lateral continuity of reflection events delineates the top of the volcanic basement. From ~2.1 s TWT at the northeast end of Line 3 (Fig. F66), the top of the basement slopes gently upward to ~2.075 s TWT near shotpoint 3080. Relatively large-amplitude, laterally coherent basement reflections are attributed to the presence of volcaniclastic units and shallow-water sediment interbedded with lava flows. Truncation of slightly southwest-dipping basement reflections between 2.125 and 2.2 s TWT and shotpoints 3083 and 3163 on Line 3, indicated by arrows in Figure F66, might record a boundary separating two sequences of lava flows. Line 6 (Fig. F67) shows a sharp apparent offset, nearly 0.1 s TWT down to the southeast, in the sediment sequence at shotpoint 4947. This apparent offset is probably a small fault scarp, although the apparent offset might be due to the inability of SIOSEIS's finite-difference migration algorithm to properly migrate the steep dip of the sediment reflections. Finally, because elsewhere the sediment layer appears to conform to basement topography, another possible explanation is that the sediment sequence along Line 3 may have been largely deposited as a drape over a preexisting step in the basement.