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

Daniel Duane, Ph.D., 2022
Nicholas Makris, Advisor
Attenuation from fish can reduce the intensity of acoustic signals and significantly decrease detection range for long-range sensing in the ocean. Here, Ocean Acoustic Waveguide Remote Sensing (OAWRS) is used to image fish 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. Reductions in signal intensity are predicted using an analytical theory 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 fish shoals. 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 surveys because the frequency of the acoustic signals was sufficiently far from the swimbladder resonance peak of the fish or the packing densities of the fish shoals were not sufficiently high.