Applying physical acoustics to near-seafloor object echo-structure estimation

Abstract
In HF sonars (50kHz-500kHz), the assumption of a perfect rigid body for targets of interest is often admitted and the ratio "wave-length/object-length" leads to use physical acoustics theory to describe target-wave interactions. This has been done extensively for the determination of target strengths, especially for targets isolated in the water volume. However, it is experimentally well-known that echoes of near-seafloor objects are different in amplitude and shape from echoes of the same objects in free water. Despite the presence of such boundaries (seasurface or seafloor), physical acoustics theory can still be used in this case as a first approximation. In this paper, we propose a general method to rapidly evaluate strengths and structures of echoes from simple objects located near the seafloor and we present the validation of this target echo model by means of a specific experiment : a small scale experiment (1/5 scale) made around 200 kHz in a large water test tank and using the sea surface as reflecting interface. The results obtained during this experiment well square with the envisaged target-wave interaction mechanism.