Modeling split-beam sonar to evaluate fish target strength accuracy

Abstract

Split-beam echosounders enable estimation of fish size by directly measuring their target strength. This measurement is achieved using compound transducer array geometries with accurate knowledge of resulting beam patterns. The multiple transducer components and required accurate calibration can present challenges in predicting the exact sonar capabilities. We present a model of the split-beam sonar system that can be used to evaluate and optimize sonar performance of a given transducer before committing to a hardware design. The model has been used to generate beam patterns to match prototype instruments and to simulate acoustic signals based on the scattering of sound from particles in a three-dimensional domain. Different split-beam sonar algorithms have been compared to measure the position and target strength of particles in the generated signal. The model's prediction capabilities were evaluated through comparisons with field trials of a prototype system. The field trials were conducted by lowering a calibration target sphere to a range of 200 m in the acoustic beam. Both model prediction and prototype system performance show accuracy of σ = ±0.2 dB at 25m range. Potential future applications of the model include exploring methods of target separation and improving accuracy when presented with complicated target structures.