Approaches to Acoustic Backscattering Measurements From the Deep Seafloor

Because the average ocean depth is four kilometers, seafloor investigations are mostly remote sensing operations. The primary means to determine the morphology, the structure, and the texture of the seafloor are acoustic. This paper considers the current seafloor remote sensing approaches involving acoustic backscattering. The physical constraints imposed by the ocean as a propagation medium, by the seafloor as a backscattering boundary, and by the measuring instruments are briefly reviewed. The sonar systems currently used by the oceanographic community for deep seafloor acoustic backscattering measurements deal with these constraints differently, depending on their specific application and on whether they are towed behind a ship or mounted on her hull. Towed sidescan systems such as Gloria II (U.K.), the Sea Mapping and Remote Characterization (Sea MARC) I and II, the Deep Tow system of the Marine Physical Laboratory (MPL), and hull-mounted systems, such as Swathmap all give a qualitative measure of backscattering by converting echo amplitudes to gray levels to produce a sidescan image of the seafloor. A new approach is presented which uses a Sea Beam multibeam echo-sounder to produce similar acoustic images. Quantitative measurements of backscattering have been attempted in recent experiments using the Deep Tow system and Sea Beam. Such measurements provide some insight into the geological processes responsible for the acoustic backscatter, with useful applications for geologists as well as designers and operators of bottom-interacting sonars.