Site selection for Site 1158 was based on a single-channel seismic survey conducted during the R/V Melville cruise Sojourner 5 in 1997 (Fig. F9). The prospectus site is near shotpoint 680 of the south-north seismic profile, toward the northern side of a significant basin structure. This seismic profile shows clear sediment cover at Site 1158 from 6.85- to 7.08-s two-way traveltime, giving an estimated sediment thickness of ~230 m. Hole 1158A was drilled through 199 m of sediment before basement was reached, whereas Holes 1158B and 1158C, ~273 and 444 m north of Hole 1158A, encountered only 126 and 108 m of sediment, respectively.