The Effect of Attenuation from Fish on Long-Range Active and Passive Acoustic Sensing in the Ocean

Abstract

Attenuation from fish can reduce the intensity of acoustic signals and significantly decrease detection range for long-range active and passive sensing in the ocean. This makes it important to understand the relevant mechanisms and accurately predict attenuation from fish in underwater acoustic sensing. Formulations for predicting attenuation from fish, however, depend on the accurate characterization of population density and spatial distribution of fish groups along long-range propagation paths, which is difficult to achieve using conventional survey methods. In previous investigations of attenuation from fish, population densities were inferred from reductions in the intensity of long-range acoustic signals caused by diel or seasonal shoaling patterns of fish groups. Here, Ocean Acoustic Waveguide Remote Sensing (OAWRS) is used to instantaneously image massive Norwegian herring shoals that stretch for thousands of square kilometers and simultaneously measure attenuation from these shoals within the active OAWRS transmissions, as well as attenuation to ship-radiated tonals detected by Passive Ocean Acoustic Waveguide Remote Sensing (POAWRS). Reductions in signal intensity are predicted using a normal-mode-based analytical theory derived from first principles for acoustic propagation and scattering through inhomogeneities in an ocean waveguide. The predictions of the waveguide attenuation formulation are in agreement with measured reductions from attenuation, where the position, size, and population density of the fish groups are characterized using OAWRS imagery as well as in situ echosounder measurements of the specific shoals occluding the propagation path. Common heuristic formulations that employ free space scattering assumptions for attenuation from fish groups are not in agreement with measurements here, and waveguide scattering theory is found to be necessary for accurate predictions. It is experimentally and theoretically shown that attenuation can be significant when the sensing frequency is near the resonance frequency of the shoaling fish, where scattering losses from the fish swimbladders and damping from fish flesh is most significant. Negligible attenuation was observed in previous OAWRS and POAWRS surveys because the frequency of the acoustic signals was sufficiently far from the swimbladder resonance peak of the shoaling fish or the packing densities of the fish shoals were not sufficiently high.