Effects of the top transition layer in marine sediments on seabed roughness and volume scattering

Abstract

Most models of high-frequency seabed scattering normally assume that the sediment is a statistically homogeneous half space and there is a discontinuity of acoustical parameters at the water–sediment interface. It is known, however, that in many cases properties of the very top layer of real marine sediments actually represent almost continuous transition within a few centimeters from those in water to those essentially constant in sediments at greater depths. At the Key West site, e.g., a definite transition layer in silty sand was found to be of 6-cm thickness. The discontinuity at the water–sediment interface, if it exists, is actually much smaller than assumed in conventional models of high-frequency scattering, where roughness scatter component is usually predicted to be strong and dominating for most types of sediments except very fine ones, such as mud and fine silt. In this paper, effects of the transition layer on both roughness and volume-scattering components are considered. These effects are shown to reduce substantially the role of roughness scattering at high frequencies. In particular, the roughness contribution is shown to be strongly dependent on real discontinuity of acoustical parameters at the water–sediment interface, which can be significantly reduced even for some sands due to existence of the transition layer. The possibly dominating role of volume scattering caused by continuous and discrete heterogeneity of the sediment is discussed. The necessity of more detailed study of physical properties of the transition sediment layer is emphasized. Related issues of interpretation of recent results on seabed scattering obtained in the Key West and SAX99 experiments are discussed as well. [Work supported by ONR-US and CNRS-France.]