Numerical modelling of broadband acoustic signatures for two Argentinian swimbladdered fish species

LAVIA, E.; PRARIO, I.; MADIROLAS, A.; GONZÁLEZ, J. D.; CASCALLARES, G.; BLANC, S.; MENNA, B.; CABREIRA, A.

Conferencia; IEEE ARGECON 2020; 2020

This work presents the first results concerning the analysis in the frequency domain of measured acoustic signatures for two swimbladdered fish species (Merluccius hubbsi and Engraulis anchoita) from the South West Atlantic Ocean. Comparison against numerically simulated responses, using a model based on the Boundary Element Method (BEM) are provided. In order to groundtruth the model outputs, experimental data collected at sea off the Argentine coast, onboard the research vessel BIP V. Angelescu in December 2018, are presented. The vessel was equipped with a previously calibrated SIMRAD EK80 broadband echosounder. Broadband signals (FM pulse compressed echoes) corresponding to single fish were logged and processed with the Myriax Echoview V9.0 software for a frequency band spanning from 36 kHz to 85 kHz. The physical situation involved in the measurements is mathematically related to the Helmholtz differential equation which can be solved using a Boundary Element Method (BEM) approach considering that the swimbladder constitutes the dominant contribution to fish acoustic scattering. The numerical evaluation of the acoustic response requires integration on the scattering surface (the swimbladder) which is carried out using meshes (discretized version of the surfaces) obtained from X-ray computerized tomography scans on specimens of the two species. The backscattering TargetStrength (TS) for each species was computed considering two types of scatterers, non-penetrable soft scatterer and penetrable fluid scatterer, or what is respectively equivalent, imposing twotypes of boundary conditions on the seawater volume scatterer interface (pressure release and physical properties continuity). For both fish species, numerical dependence of TS on frequencyand tilt angle is obtained. Reported comparisons between numerical and measured results in the frequency domain provide an encouraging first set of results and reveal a broad spectrumof further work in the future.