Time-Varying Across-Track Beamforming for the Suppression of Bottom-Bounce Multipath Effects in Sidescan Sonar

Sidescan sonars are used to provide a high-resolution 2-D image of the seafloor, but when used in shallow water, these side-looking systems are vulnerable to multipath interference. In some cases, this interference affects image interpretation and downstream processing such as target recognition or bottom classification. However, it is possible to suppress multipath interference by using a small array featuring a vertical stack of receivers. Multipath signals that arrive from the direction of the surface are easily suppressed using across-track receive beamforming, however multipath signals that arrive from the seafloor are not so easily removed. This paper investigates the use of time-varying across-track receive beamforming as a method to suppress these bottom-bounce signals. Two sidescan images are presented that illustrate the impact that bottom-bounce multipath can have on sidescan sonar images. A theoretical model is presented that gives the relative intensity of the received signals and illustrates how their intensities are changed by altering the receive beampattern. In the first example, a bottom-surface-bottom signal arriving from nadir is suppressed by simply reducing the extent of the main lobe before the signal is received. In the second example, two multipath signals arriving near broadside are suppressed by introducing a null into the main lobe. It is concluded that an array employing the proposed beam processing is capable of rejecting bottom-bounce multipath, assuming that the angle and time of arrival of the interference and bottom signal are known.