Estimation of the seabed acoustic impedance structure from normal‐incidence reflections: Somali Basin

Abstract

Geological interpretation of marine seismic reflection profiles is usefully supplemented by systematically measuring the energy (and its variations) of the reflection events typically observed in low‐frequency, normal‐incidence profiles. Analyses of the low frequency signals received via a passband of at least an octave (e.g., 30–60 Hz) indicate that much of the variations in reflection energy can for the purposes of the experimentalist be ascribed to vertical and lateral changes in the acoustic impedance of the seabed. Using a simple multilayer reflection model, impedances can be estimated from the energetics measurements. Techniques and rationale are given for making these observations of seabed reflectance and deriving impedance estimates. Reflectance and estimated impedance characteristics of the seabed are compared at three stations in the Somali Basin northwest of Chain Ridge and three southeast of the southwesterly extension of the ridge, which bounds the basin to the southeast. One station to the southeast is near Deep Sea Drilling Project site 235, where the impedance structure down through the sediments into basement was estimated from core measurements. The impedance structure inferred from the reflectance at the station is in good agreement with that measured at the site. Having established this agreement, the impedance structures estimated from the reflectance measurements at the stations within the basin northwest of the ridge are compared with the structures at the stations to the southeast. Although recent sedimentation is similar throughout the region, as evidenced by surficial cores, seafloor reflectivity measurements, and a water depth of about 5100 m at each station, reflectance measurements and impedance estimates show the sedimentation within the basin evolved differently from that southeast of the ridge. Two clearly different impedance structures are evident. Northwest of Chain Ridge estimated impedance increases nearly linearly from about 2 × 106 rayls (ρv, kg/m2‐sec) down through the sediment to about 16–18 × 106 rayls in basement. South of the basin proper a nearly uniform low impedance of about 4 × 106 rayls is evident throughout the sediment column with sharp increases at, or near basement. Reflectance profiles show that the lateral continuity of material properties within the seabed in the southern area is more uniform than in the northern area. Northwest of the ridge small scale variations over distances of about 2 km are observed in seafloor reflectivity and in seabed reflectance.