Abstract
Attenuation from fish can reduce the intensity of acoustic signals and significantly decrease detection range for long-range passive sensing of manmade vehicles, geophysical phenomena, and vocalizing marine life. The effect of attenuation from herring shoals on the Passive Ocean Acoustic Waveguide Remote Sensing (POAWRS) of surface vessels is investigated here, where concurrent wide-area active Ocean Acoustic Waveguide Remote Sensing (OAWRS) is used to confirm that herring shoals occluding the propagation path are responsible for measured reductions in ship radiated sound and corresponding detection losses. Reductions in the intensity of ship-radiated sound are predicted using a formulation for acoustic attenuation through inhomogeneities in an ocean waveguide that has been previously shown to be consistent with experimental measurements of attenuation from fish in active OAWRS transmissions. 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. Experimental measurements of attenuation presented here confirm previous theoretical predictions that common heuristic formulations employing free space scattering assumptions can be in significant error. Waveguide scattering and propagation theory is found to be necessary for accurate predictions.
Highlights
Acoustics is the primary means of sensing self-radiating sources in the ocean such as manmade vehicles, geophysical phenomena, and vocalizing marine mammals [1,2,3]
The effect of attenuation from fish on passive ocean acoustic waveguide remote sensing (POAWRS) of ship-radiated tonals is investigated, where wide-area Ocean Acoustic Waveguide Remote Sensing (OAWRS) imagery of herring groups in Norwegian spawning grounds is used in conjunction with in situ echosounder data to measure the size and population density of the fish groups occluding the propagation path
The predictions of the waveguide attenuation model are found to be in agreement with measured reductions in the Artus tonal due to attenuation from herring, where the size, position, and population density of the three shoals occluding the propagation path are fully characterized by OAWRS imagery as well as in situ echosounder measurements (Figure 4)
Summary
Acoustics is the primary means of sensing self-radiating sources in the ocean such as manmade vehicles, geophysical phenomena, and vocalizing marine mammals [1,2,3]. Attenuation from fish has been previously measured from fluctuations in the intensity of long-range acoustic signals caused by diel or seasonal shoaling patterns of the fish, including sardines in the Bristol Channel and groups of anchovies and sardines in the Gulf of Lion [4,5,6,7]. In these experiments fish population densities were inferred from the reductions in signal intensity, since it is difficult to measure instantaneous population distributions across long ranges with conventional survey methods. The effect of attenuation from fish on passive ocean acoustic waveguide remote sensing (POAWRS) of ship-radiated tonals is investigated, where wide-area OAWRS imagery of herring groups in Norwegian spawning grounds is used in conjunction with in situ echosounder data to measure the size and population density of the fish groups occluding the propagation path
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