Bistatic scattering in sediments: Comparison of model and scaled tank experiments at 238 kHz

Multiple-aspect scattering is increasingly used to investigate seabeds and objects, buried or not. However, high-frequency scattering processes on/in sediments need to be better understood, particularly when the structure and/or composition of these sediments is not fully homogeneous. Scaled tank experiments were conducted with a 238-kHz sonar (10° beam-width) imaging a silt seabed at 45°. Scattering angles varied between ∼16° and ∼70° (50 distinct values); bistatic angles varied 40° either side of in-plane with a 2.5° step (33 distinct values). Bottom returns were picked through two methods (automatic and manual) and converted into scattering strengths. This large dataset was compared with the APL-UW model for bistatic scattering, intended for 10-100 kHz but successfully tested at 240 and 455 kHz by other workers. Recursive fitting of model parameters to the experimental values assesses the influence of sediment variations (backed by microscope measurements) and the importance of even small tilts in the surface of the sediments. More importantly, they show the importance of the beam pattern and the sidelobes in the accurate calculation of bistatic scattering strengths. These results can be used to further validate scattering models as well as to design future surveying strategies.