High-Frequency Acoustic Scattering from Sediment Interface Roughness and Volume Inhomogeneities.

High-frequency acoustic and geoacoustic data from five experiment sites with different sediment types are compared with predictions from the composite roughness model in order to ascertain the relative contribution of interface roughness and sediment volume scattering. Model fits to backscattering data from silty sediments indicate that volume scattering predominates, but measured bottom roughness was sufficient to explain the backscattering measured from a rippled, sandy sediment. Fluctuations in sediment porosity and sound velocity probably causes volume scattering which is described by a free parameter in the composite roughness model comparisons. High -resolution vertical profiles of sediment porosity and compressional wave velocity collected from 14 diverse sites on continental shelves are used to calculate vertical spatial autocorrelation functions, variance of the fluctuations, and the dependence of sediment sound velocity and density on sediment porosity for the purpose of parameterizing sediment volume inhomogeneity. Correlation lengths calculated from autocorrelation functions show maximum variability in poorly sorted sediments. The variance of porosity and velocity fluctuations, which determines the strength of volume scattering, exhibits wide variation with sediment type and depends on the processes that mix and transport sediments. Comparison of data from a large number of locations on continental shelves suggests that fluctuations in sediment porosity are due to biological and sedimentological processes and fluctuations in sediment velocity are due to hydrodynamic processes.