A simple method to measure beam overlap in multibeam sonars using calibration spheres

Abstract

In multibeam sonar systems, beam overlap introduces distortion in across-beam size measurements, leading to overestimation of target dimensions. A previously developed method estimated beam overlap by comparing along-beam and across-beam measurements of fish schools during purse seine maneuvers, demonstrating the presence of overlap and proposing a metric for its quantification. However, this approach required substantial effort and had limited precision on vertical swaths. In this study, we present a simplified and reproducible method for estimating beam overlap using standard calibration spheres commonly employed in fisheries acoustics. The procedure involves placing a sphere of known diameter at the center of a sonar beam. Provided the range is sufficient for the beam to fully encompass the target, the degree of overlap is estimated by counting the number of beams that detect the sphere. This method yielded results similar to the previous approach, while requiring considerably less effort. It can be performed nearshore in a few hours, and allows for overlap estimation in both horizontal and vertical planes with similar accuracy. The proposed technique offers a practical and efficient alternative for beam overlap calibration, facilitating its integration into standard acoustic calibration protocols.