Wake removal for clear side-scan images

Abstract

Multi-angle swath bathymetry (MASB) sonars are typically used for bathymetry applications but their unique characteristics make them useful for generating clear side-scan images of the bottom, free from artifacts produced by wakes, surface bounce multi-path signals and water column targets. MASB sonars use a small array of long vertically stacked array elements to estimate the angle of arrival for backscatter. Unlike interferometric sonars which can estimate only one angle of arrival, MASB sonars can estimate the arrival angle of multiple same-time targets and therefore the potential exists for separating bottom backscatter from unwanted backscatter. Side-scan sonar is used to obtain high resolution images of the bottom but because of the wide beam it also picks up backscatter from wakes, water column targets, and surface bounce multi-path. This unwanted backscatter obscures the bottom image sometimes making it necessary to resurvey the area if these artifacts are present. Wakes are particularly bothersome in high traffic water ways and busy harbors where it may be necessary to get clear images of the bottom for security or search and recovery applications. This paper shows how MASB processing techniques coupled with beam steering can be used to generate artifact free images of the bottom. The paper begins by briefly outlining MASB sonar principles and then presents the methodology for generating clear bottom images. Element and composite beam patterns are presented for an actual MASB sonar system and these patterns are discussed in the context of target discrimination and artifact removal. Examples are presented of actual side-scan images contaminated by wakes, water column targets, and surface bounce multi-path signals. The data for these images is processed using the techniques described and new side-scan images are presented free of artifacts. A byproduct of the process is that side-scan images of wakes and water column targets can be produced alone without the bottom. This type of image is useful for situations where the water column or wake targets are of primary interest. Finally, conclusions are drawn with regard to the application of these techniques for obtaining unobscured bottom images or images of water column targets or wakes alone for security, and search and survey applications in high traffic areas.