Modelling of the sound field radiated by multibeam echosounders for acoustical impact assessment

Multi-Beam Echo-Sounders (MBES) designed for seafloor-mapping applications are today a major tool for ocean exploration and monitoring. Concerns have been raised about their impact towards marine life and especially marine mammals, although their inherent characteristics (high frequencies, short signals and narrow transmitting lobes) actually minimize this possibility. The present paper proposes an analysis of MBES radiation characteristics (pulse design, source level and radiation directivity pattern) accounting for the various geometries met today and expressed according to the metrics used for acoustical impact assessment (maximum Sound Pressure Level, and cumulative Sound Exposure Level). A detailed radiation model is proposed, including the transmission through directivity sidelobes, and applied to three typical MBES examples. A simplified radiation model is then defined, in order to extend it to the case of the cumulative insonification by a MBES moving along a survey line. An approximated analytical model is proposed for the accumulated intensity, showing good agreement with the complete simulation of insonification; it is applied to the worst-case configuration of a low-frequency (12 kHz) multi-sector system. The computation of ranges corresponding to impact thresholds accepted today shows that impacts in terms of injury are negligible for both SPL and SEL; however behavioural response impacts cannot be excluded, and should require specific experimentation.