I. Introduction
Shallow water represents an important yet challenging setting for many sonar systems. Sidescan sonar is often used to survey in shallow water, but sidescan systems are particularly susceptible to the type of interference that may arise in this environment. Because the surface of the water and the seafloor are simultaneously within the “view” of a sidescan sonar system in shallow water, acoustical signals may be received that have scattered or reflected off either boundary. The end result of this behavior is the presence of “multipath” signals that contaminate the 2-D sidescan sonar images by obscuring the image of the bottom and masking important bottom features such as target highlights and shadows. While some of these multipath signals arrive from the direction of the surface and are easily suppressed by fixed across-track beamforming [22], others arrive from the direction of the bottom and are difficult to suppress. The purpose of this paper is to explore the use of time-varying across-track beam processing (on receive) as a general approach to suppress multipath signals that arrive from the direction of the bottom.