Bottom loss modeling for sonar performance prediction

Characterizing sediment type can aid in assessing sonar performance in littoral areas. One method for sediment characterization is inverting bottom loss measurements, specifically normal incidence bottom loss from echo sounder data. In this study, bottom loss data at a range of grazing angles and frequencies were compared with different models to assess the validity of the physical models of the sediment and the effects of interface scattering. The data were taken at the Experimental Validation of Acoustic modeling techniques (EVA) sea test, conducted off the coast of Isola d´Elba, Italy in 2006. Interface roughness was concurrently measured to a high degree of accuracy with a laser profiler system mounted on a remotely operated vehicle. The data were compared with three models: a flat interface, elastic model based on geo-acoustic measurements; a flat interface, poro-elastic model based on the Biot/Stoll model; and a rough interface model based on the measured interface roughness power spectrum. The measurements were most consistent with the poro-elastic model. Scattering was found to be a significant mechanism for data taken at frequencies of 20 kHz and higher. Comparisons were also made with the APL-UW reflection and backscattering models. It was found that the APL-UW models accurately described the reflection loss at low frequencies through an unphysically low value of the sediment density which acted as proxy for poro-elastic effects. At higher frequencies, the model did not completely account for the roughness scattering. This may lead to incorrect sediment characterization for echo sounder data.