Backscattering from an elastic target near a water-sediment interface at oblique incidence: first results of tank experiments
Résumé
Sound scattering by a target close to a rough interface has been studied in laboratory conditions in order to control separately all the parametres involved in the scattering process. Elastic spherical objects of different sizes were insonified with wide band transducers in order to cover the frequency range 200 kHz-1MHz and their backscattering cross section was measured in free space conditions. When the target is close to a perfectly reflecting interface, we can still measure its backscattering cross section which is system independent, but when the interface is a rough interface then the backscattered signals become system dependant and the backscattered level depends on transmitter beamwidth, frequency of ensonification, interface roughness and distances transducer/ object and object / interface. The rough surface which has been considered here is a flattened sand surface which was studied in details by the authors [A.N. Ivakin, and J.-P. Sessarego, High frequency scattering from flattened sand sediments: effects of granular structure, J. Acoust. Soc. Am., 122, (5) 2007, A. Ivakin, J-P Sessarego, High frequency scattering from water-saturated granular sediments: scaling effects, 154th Meeting of the Acoustical Society of America, 27-30 November, New Orleans]. Side scan sonar images will also be presented in order to show the difficulty in the interpretation of sonar images of objects near very rough boudaries.