Objective acoustic shadow detection with phase-measuring sidescan sonars

Abstract

Interpretation of seafloor acoustic images obtained with sidescan sonars is readily improved by applying conventional radiometric corrections that compensate for known fixed gains, for spreading and absorption losses, and for the effective instantaneous area ensonified by the transmitted pulse within the sonar beam. This area is a function of the angle of incidence on the bottom, which is obtained by ray-tracing from the angle of arrival of the echo relative to vertical estimated at receiver pairs in phase-measuring sidescan sonar systems equipped with inertial motion sensors. A further improvement in the acoustic imagery is achieved by estimating the in-situ roll-plane beam pattern of the sonar using all pings in a survey, then applying the resulting normalized beam pattern function to echoes from each ping at their respective attitudes. Thus effects of the sonar system and the environment are removed while preserving the physical characteristics of the seafloor acoustic backscattering process. Then, acoustic shadows can be objectively defined below a specific threshold in the signal-to-noise ratio computed from the sample complex cross-correlation coefficient function between receivers in each pair. A single shadow threshold value applies to all the data from an entire survey, with potential benefits to automatic target recognition algorithms.