A good local map of the sedimentary sequence and basaltic basement topography is of special interest at ocean observatory sites. The deployments of some of the instrument suites at these sites need to take into account the attributes of sediments and basement in the immediate area. The imaging described here of the acoustic interfaces in the immediate vicinity of a borehole provides an estimate of the small-scale topography of the interfaces immediately around the borehole, thereby complementing the recovered samples and logging results.

Echo sounding profiles made with sound sources operating in the lower kilohertz range commonly show reflections from interfaces beneath the seafloor in sedimented areas. An early example was reported by Smith (1958). At these frequencies, the wavelength of the sound in water is ~0.37 m, and in unconsolidated sediments it ranges from 0.4 to 0.5 m, depending on their compressional wave velocity. Echo sounding surveys of drill sites with hull-mounted sounders (3.5 kHz) have commonly been performed throughout the Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP). Jacobi et al. (1985) is a good example from Leg 86.

At Site 1224, a suite of holes was drilled 1.5 km northeast of the Hawaii 2 Observatory (H2O) junction box (Fig. F1; from Stephen, Kasahara, Acton, et al., 2003). The shipboard 3.5-kHz echo sounding profile (Fig. F2; from Stephen, Kasahara, Acton, et al., 2003) across the drill site shows that the seafloor dips smoothly ~6 m from the H2O junction box to the drill site. One subbottom horizon at ~9 m is fairly uniform throughout the area. Based on drilling (incomplete recovery), this is a midsediment section reflector. A second reflector at ~30 m below the junction box is basaltic basement; it appears only occasionally in the recording. The objective of lowering the 4-kHz source to the seafloor was to improve the resolution of the sediment and basement reflections. Note that with using the 3.5-kHz hull-mounted source (Fig. F2), the basement reflector could be identified at the H2O junction box but not at the drill site.