Enabling Shallow-Water Sidescan Sonar Surveys: Using Across-Track Beamforming on Receive to Suppress Multipath Interference

Sidescan sonars are used to provide a high resolution 2D 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. This thesis investigates the extent to which conventional beamforming techniques, used on receive, can suppress multipath interference. Two novel techniques are proposed, one that suppresses surface interference and one that suppresses bottom-bounce multipath. Experimental data are presented which illustrate the impact that multipath interference has on sidescan images. To establish the effectiveness of the proposed methods, the methods are applied to theoretical models of the experimental data, and are applied to the experimental data as well. This thesis proposes the use of a fixed broad beam to suppress interference arriving from the surface. A sidescan array utilizing a vertical stack of six receive elements was constructed, and is shown to be effective at providing a clear view of the seafloor when surface interference is present. Experimental data collected with the experimental array are presented. A theoretical analysis examines the relative path strengths of the received signals for different across-track beam patterns, and examines how these signals are affected when beamforming is applied on receive. The experimental data are processed using the proposed method and are significantly improved, revealing a shipwreck that was not visible in the original image. Suppressing bottom-bounce multipath signals is more difficult, and this thesis proposes the use of time-varying conventional beamforming techniques as a method to realize this second goal. Two sidescan images, both contaminated with bottom-bounce multipath, are presented and modeled. The experimental data are processed using the proposed methods, and a clear view of the seafloor is provided. It is concluded that a sidescan sonar which employs across-track beamforming on a receive array is a valuable tool for suppressing the multipath and surface interference which arise in shallow water surveys.