Initial selection of proposed Site HE-3, which became Site 1204, was based on data gathered at ODP Site 883, where basement was reached at ~820 mbsf (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). A single-channel seismic survey, 3.5-kHz PDR, and magnetometer survey were conducted in the vicinity of Site 883 to ensure proper hole location and suitability for basement drilling. A digital seismic line acquired by the Faranella in the vicinity of Site 883/1204 supplemented data acquired during our survey. Figure F61 shows the track line of the Leg 197 survey superimposed on ETOPO5 seafloor bathymetry with a contour interval of 100 m. Tick marks along our survey lines mark half-hour intervals. The Faranella seismic line is labeled F2-87-AA (Faranella) and is annotated with its corresponding Julian day and Universal Time Coordinated (UTC) time markers.
Survey Lines 1 and 3 pass directly over Site 883. Approximately 4-km-long sections from Lines 1 and 3 are shown in Figures F62 and F63, 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. 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 5.91 kt (3.04 m/s) during Line 1 and 5.35 kt (2.75 m/s) during Line 3. All three lines were processed with SIOSEIS seismic processing software (version 2001.3) (http://sioseis.ucsd.edu), using predictive deconvolution, bandpass filtering from 40 to 100 Hz, and finite-difference migration applying a simple velocity model. Table T17 shows the SIOSEIS script processing parameters for each line. Holes 1204A and 1204B are ~575 m along Line 3 southeast of Hole 883F.
Sites 883 and 1204 are located in the middle of a broad depression in the basement. This swale is ~1-2 km across and slightly >0.1 s two-way traveltime (TWT) deep. The ~1 s TWT to the basement at both sites corresponds to a sediment cover ~820 m thick, thus, to an average velocity of ~1640 m/s. A strong, nonreverberant, and low-frequency reflection, which is flat where Holes 1204A and 1204B are situated, marks the volcanic basement contact at ~4.2 s TWT in the deepest part of the basin. The basement contact can be identified elsewhere on the sections by an abrupt decrease in reflection coherency. The lack of laterally coherent internal reflectivity in the basement is consistent with the massive nature of the basalt cores recovered from Site 1204. This contrasts with Site 1203, where the presence of volcaniclastic sediment interbedded with massive lava flows caused reverberations that lasted >0.2 s TWT (see "Underway Geophysics" in the "Site 1203" chapter).
The sediment cap also exhibits some notable structure. The far left side of Figure F63 between 3.8 and 3.9 s TWT shows successive southeast-dipping reflectors propagating to the right and downlapping to the southeast. Another large clinoform was imaged just above the basement on the far left of Figure F63 at ~4.1 s TWT. The southeast-downlapping, sigmoidal nature of the clinoforms indicates a low-energy depositional environment with a sediment source somewhere to the northwest. Unlike Site 1203, the upper part of the sediment cover at Site 1204 lacks a low angular unconformity that would indicate a shifting pattern of erosion and deposition by the bottom current that shaped the formation of the Meiji sediment drift.