Correction of beam overlap-induced athwart distortion in multibeam sonars

Abstract

A method is proposed to estimate and correct athwart-beam distortion of multibeam sonars to advance sonar-based abundance estimation. We illustrate its application using data from a Simrad SN90 multibeam sonar aboard a tropical tuna purse-seiner, targeting mixed fish aggregations around drifting Fish Aggregating Devices (dFADs) in the Atlantic Ocean. To calculate the distortion in the horizontal swath, athwart-beam measurements were compared with the more accurate simultaneous along-beam measurements. For the vertical swath correction, we used as reference a vertically oriented single, split beam echosounder simultaneously monitoring the same aggregations. Along-to-athwart ratios of similar to 0.6 and similar to 0.3 were estimated for the horizontal and vertical swaths, respectively, at ranges involved during purse-seine operations in this fleet (from similar to 225 to similar to 325 m). Additionally, equations were developed to analytically describe the athwart distortion due to overlap between contiguous beams. Once corrected for distortion, typical school morphology measures were provided for tuna aggregations around dFADs.