Local suppression of bioturbation and its acoustic consequences

Abstract

Seafloor roughness in shallow waters can be both spatially and temporally dynamic. While wind-driven waves and bottom currents can generate bedforms such as ripples on sand sediments, bottom-feeding fish tend to limit the lifetime of these anisotropic features. This greatly reduces the time during which high frequency sonars can exploit the ripples to coherently penetrate the sediment at subcritical angles. Sonar performance prediction in these environments therefore relies heavily on sediment transport models to generate roughness predictions that accurately reflect changes on these time scales. The competition between the hydrodynamics and biology of the environment can be further complicated by the natural introduction of materials which can locally suppress the bioturbation. Two examples are discussed here: The first comes from the Sediment Acoustics Experiment in 2004 where a storm deposited a layer of mud over some areas of seafloor protecting the bedforms from fish. The second is from the recent Target and Reverberation Experiment where there is evidence that spatially-varying shell content can also reduce the effects of bioturbation. The consequences for sonar performance in these two cases are discussed. [Work supported by the U.S. Office of Naval Research and the Strategic Environmental Research and Development Program.]