The evolution of rippled seafloor topography with acoustic implications

Abstract

Rippled seafloors are often responsible for anisotropic patterns of acoustic backscattering and allow penetration of high-frequency energy into the seafloor below the critical angle. Both natural and manipulative experiments conducted during the Sediment Acoustic Experiment (SAX99) demonstrate the importance of understanding the temporal evolution and characterizing the spatial statistics of naturally occurring ripple fields for prediction of sonar performance and the detection of buried targets. Current and wave-induced sand ripples evolve in a more or less predictable pattern. Numerous empirical and semi-empirical predictive models, based on well-established principles of sediment transport, allow prediction of ripple wavelength, height, and shape. Degradation of sand ripple fields, especially by biological processes such as feeding, burrowing, and emergence is less known and has not been modeled. The temporal evolution of rippled topography measured with sector scanning sonar in high-energy environments is presented. These high-fidelity and nearly continuous observations coupled with measurements of bottom currents and near-bottom wave-induced orbital motion provide improved insights and new models of the evolution of rippled seafloor topography. In low-energy environments (SAX99 and the proposed SAX04) the longer times between storms allow characterization of rates of biological processes which destroy ripple structure and create isotropic roughness. [Work supported by ONR.]