Bottom detection for multibeam sonars with active contours

In multibeam sonar bathymetry, a receiving array is used to form a fan-shaped beam sector. Bottom echoes of near-normal incident angles are usually detected based on the amplitude of the received signal. To detect bottom echoes of oblique incident angles, it is common to use the phase information obtained by subarray interferometry. The type of the bottom echo is not known a priori. Hence, which of the two bottom detection methods to use for a particular beam is determined by ad hoc criteria. Localization errors of the bottom are often the consequence of choosing the wrong bottom detection method. In this paper, the authors propose to use a bottom image transform and an active contour model to integrate the amplitude and phase information, so the bottom echoes from all directions can be detected by a single algorithm. Promising results were obtained in the authors' evaluation using Simrad EM3000 sonar data recorded from shallow water.