Abstract
Liquid undissolved fluid pollutants covering the sea bottom, in general, form a thin layer of a few cm, which cannot be detected acoustically by normal sound incidence due to the small difference of impedance compared to water. Such layers, however, can strongly affect the spherical reflection coefficient at grazing sound incidence. Using broadband chirp signals, the interference pattern at the receiving hydrophone is found to provide information on the physical properties of the sediment surface and its coating. Laboratory measurements in a small water tank with flat and rippled sand sediments coated by various different chemical substances revealed a systematic dependence of the measured excess attenuation function (EAF) on the acoustic impedance of the chemical, the layer thickness and the grazing angle of the incident sound. Results of numerical modeling performed with a fast field program (fflags) show good agreement with the measurement results. In order to prove the applicability of the new detection method, measurements in shallow water will be presented. [Work supported by BMBF.]
Published Version
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