The Effect of Attenuation from Fish Shoals on Long-Range, Wide-Area Acoustic Sensing in the Ocean

Abstract

Acoustics is the primary means of long-range and wide-area sensing in the ocean due to the severe attenuation of electromagnetic waves in seawater. While it is known that densely packed fish groups can attenuate acoustic signals during long-range propagation in an ocean waveguide, previous experimental demonstrations have been restricted to single line transect measurements of either transmission or backscatter and have not directly investigated wide-area sensing and communication issues. Here we experimentally show with wide-area sensing over 360 • in the horizontal and ranges spanning many tens of kilometers that a single large fish shoal can significantly occlude acoustic sensing over entire sectors spanning more than 30 • with corresponding decreases in detection ranges by roughly an order of magnitude. Such blockages can comprise significant impediments to underwater acoustic remote sensing and surveillance of underwater vehicles, marine life and geophysical phenomena as well as underwater communication. This makes it important to understand the relevant mechanisms and accurately predict attenuation from fish in long-range underwater acoustic sensing and communication. To do so, we apply an analytical theory derived from first principles for acoustic propagation and scattering through inhomogeneities in an ocean waveguide to model propagation through fish shoals. In previous experiments, either the attenuation from fish in the shoal or the scattering cross sections of fish in the shoal were measured but not both, making it impossible to directly confirm a theoretical prediction on attenuation through the shoal. Here, both measurements have been made and they experimentally confirm the waveguide theory presented. We find experimentally and theoretically that attenuation can be significant when the sensing frequency is near the resonance frequency of the shoaling fish. Negligible attenuation was observed in previous low-frequency ocean acoustic waveguide remote sensing (OAWRS) experiments because the sensing frequency was sufficiently far from the swimbladder resonance peak of the shoaling fish or the packing densities of the fish shoals were not sufficiently high. We show that common heuristic approaches that employ free space scattering assumptions for attenuation from fish groups can lead to significant errors for applications involving long-range waveguide propagation and scattering.

Comparing Performances of Five Distinct Automatic Classifiers for Fin Whale Vocalizations in Beamformed Spectrograms of Coherent Hydrophone Array

“…The underwater recordings of fin whale vocalizations analyzed here are drawn from the NorEx14, conducted by a collaborative team from the Massachusetts Institute of Technology, Northeastern University, NOAA-Northeast Fisheries Science Center, Naval Research Laboratory, Penn State University and the Woods Hole Oceanographic Institution in the United States, as well as the Institute of Marine Research-Bergen (IMR) in Norway. The NorEx14 was conducted from 18 February to 8 March 2014, in conjunction with the IMR survey of spawning populations of Atlantic herring off the Alesund coast, the Atlantic cod off the Lofoten peninsula, and the capelin off the Northern Finnmark region [33,34]. The twofold objectives of the NorEx14 were to (i) image and monitor the population distributions of these large fish shoals from diverse species instantaneously over wide areas of their spawning grounds, using the Ocean Acoustic Waveguide Remote Sensing (OAWRS) and imaging system [33,[35][36][37] from which fish group behavioral patterns can be quantified, and (ii) observe marine mammal vocalizations and infer their temporal-spatial distributions over wide areas using the POAWRS technique, [1,2,9,11] combined with visual observations for species confirmation.…”

Long-Range Automatic Detection, Acoustic Signature Characterization and Bearing-Time Estimation of Multiple Ships with Coherent Hydrophone Array

“…Acoustic recordings of ship-radiated underwater sound were received on a large-aperture densely-sampled coherent hydrophone array containing 160-elements deployed during the Nordic Seas 2014 Experiment (NorEx2014) [31][32][33][34][35][36]. The three approaches for analyzing different aspects of a ship's radiated underwater sound are applied to beamformed pressure-time series data spanning 360 • horizontal azimuths about the coherent hydrophone array.…”

The effect of attenuation from fish on long-range active and passive acoustic sensing in the ocean

“…The range-dependent decay in OAWRS transmissions is corrected with a theoretical formulation that has been previously shown to be consistent with experimental measurements of attenuation from fish in a waveguide environment [43,15]. After applying the attenuation correction, wide-area population density maps can be generated even during dense shoaling activity.…”

“…OAWRS images are corrected for attenuation from fish scattering using a theoretical formulation that has been previously shown to be consistent with experimental measurements of attenuation from fish in a waveguide [43,15]. The theoretical decay due to fish attenuation depends on the average population density of fish within the sensing region, which is determined for each OAWRS transmission by modeling scattering strength uncorrected for losses from attenuation ("fish-attenuated scattering strength") and performing a leastsquares fit with measurements (Figure 3-4).…”

“…Measured sensing range for the OAWRS system in the presence of Ålesund herring and Finnmark capelin (red dots in A and B) are shown to be in agreement with expected sensing range at the relevant 𝑊 0 − 𝑁 𝐿 0 values (red lines in A and B). Physical parameters used for modeling sensing range for Finnmark capelin and Lofoten cod are shown in Table 1 of [15]. Sensing range predictions for Lofoten cod exceed the maximum possible range that could be recorded in the 50 second recording window that was used in this region (red dot in C).…”