PROJECT TITLE :

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

ABSTRACT:

Sidescan sonars are used to supply a high-resolution a pair of-D image of the seafloor, but when employed in shallow water, these side-trying systems are prone to multipath interference. In some cases, this interference affects image interpretation and downstream processing such as target recognition or bottom classification. But, it's doable to suppress multipath interference by using a little array that includes a vertical stack of receivers. Multipath signals that arrive from the direction of the surface are easily suppressed using across-track receive beamforming, but multipath signals that arrive from the seafloor don't seem to be therefore easily removed. This paper investigates the employment of time-varying across-track receive beamforming as a technique 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 offers the relative intensity of the received signals and illustrates how their intensities are modified by altering the receive beampattern. In the primary example, a bottom-surface-bottom signal arriving from nadir is suppressed by merely reducing the extent of the main lobe before the signal is received. Within the second example, two multipath signals arriving close to broadside are suppressed by introducing a null into the main lobe. It's 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.