Abstract
The roughness of sandy seafloors is constantly changing due to biological and hydrodynamic activity. Even when these processes are in equilibrium so that the average acoustic backscattering intensity does not change, the character of successive backscatter echoes changes on time scales from minutes to days, with strong frequency dependence. This change can be characterized by the temporal correlation of the backscattered signal between consecutive pings, expressed as a lag-dependent correlation coefficient. A simple model for roughness change has been developed based upon the diffusion equation. This model assumes that newly created roughness is uncorrelated with existing roughness. It predicts exponential decay of the correlation with e-folding time 1/(DK2B), where KB is the Bragg wave number, and D is the horizontal diffusivity. During SAX99 and SAX04, photographic and acoustic observations were made of changing natural and artificially generated roughness. The acoustic observations were made at 40 and 300 kHz, with the higher frequency exhibiting much more rapid decorrelation as predicted by the model. The diffusivities obtained by fitting the model to photographic and acoustic data ranged from roughly 10−9 to 10−10 m2s−1 with the highest diffusivities found at locations where bottom-feeding fish were active. [Work funded by ONR.]
Published Version
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