Effects of multiple scattering, attenuation, and dispersion on fish population density imaging with ocean-acoustic waveguide remote sensing.

Abstract

Ocean-acoustic waveguide remote sensing (OAWRS) instantaneously images and continuously monitors fish populations distributed over continental-shelf scale regions. It is shown that the areal population density of fish groups can be estimated from their incoherently averaged, broadband matched filtered scattered intensities. A numerical Monte-Carlo model is developed to determine the statistical moments of the scattered returns. It uses the parabolic equation to simulate acoustic field propagation in random range-dependent ocean waveguides. The effects of (1) multiple scattering, (2) attenuation due to scattering, and (3) modal dispersion on fish population density imaging are examined. Results of the model are applied to interpret OAWRS images of shoaling Atlantic herring populations acquired during the 2006 Gulf of Maine experiment. Multiple scattering and attenuation are found to be negligible at OAWRS frequencies employed in the experiment and for the herring densities observed. Coherent multiple scattering effects, such as resonance shifts, are significant for small highly dense fish groups on the order of the wavelength, but negligible for larger groups. Optimal sound speed for charting scattered returns in range within the random waveguide is found to roughly equal the group speed of mode 1.