Modelling approaches to multibeam echosounders for sound field characterization

Concern over the impact of multibeam echosounders (MBES) on marine life has increased in recent years. A thorough impact assessment of acoustic sources requires both accurate modeling of the source and radiated sound field, and a biological assessment. The Joint Industry Program Acoustic Modelling Workshop in 2022 provided a set of verification scenarios for a deep-water MBES to compare modelling approaches and assess agreement across models. This work presents several relevant models designed to compute both the MBES beam patterns and propagated acoustic field. Key acoustic metrics used in impact assessment were calculated and compared using these models. The work confirmed that geometrical acoustics is well suited to the unique radiation patterns of MBES. Ray-tracing programs are relevant as well at short ranges and at long horizontal distances in the presence of large sound speed gradients. The estimation of cumulative sound exposure along a survey track is most often dominated by exposure to the main transmit beam of each sector. Accurate modelling of the near field was demonstrated to have a direct impact on final acoustic metrics and threshold ranges for various marine mammal hearing groups.