Abstract
Three approaches for instantaneous wide-area analysis of ship-radiated underwater sound, each focusing on a different aspect of that sound, received on a large-aperture densely-sampled coherent hydrophone array have been developed. (i) Ship’s narrowband machinery tonal sound is analyzed via temporal coherence using Mean Magnitude-Squared Coherence (MMSC) calculations. (ii) Ship’s broadband amplitude-modulated cavitation noise is examined using Cyclic Spectral Coherence (CSC) analysis that provides estimates for propeller blade pass rotation frequency, shaft rotation frequency, and hence the number of propeller blades. (iii) Mean power spectral densities (PSD) averaged across broad bandwidths are calculated in order to detect acoustically energetic ships. Each of these techniques are applied after beamforming of the received acoustic signals on a coherent hydrophone array, leading to significantly enhanced signal-to-noise ratios for simultaneous detection, bearing-time estimation and acoustic signature characterization of multiple ships over continental-shelf scale regions. The approaches are illustrated with underwater recordings of a 160-element coherent hydrophone array for six ocean vessels, that are located at a variety of bearings and ranges out to 200 km from the array, in the Norwegian Sea in February 2014. The CSC approach is shown to also be useful for automatic detection and bearing-time estimation of repetitive marine mammal vocalizations, providing estimates for inter-pulse-train and inter-pulse intervals from CSC spectra cyclic fundamental and first recurring peak frequencies respectively.
Highlights
Remote monitoring of ocean vessels over instantaneous wide areas from their sounds radiated underwater, including detection, localization, characterization and classification, is of great importance in maritime surveillance and defence [1,2,3,4,5,6,7,8,9]
We demonstrate the hybrid usage of the three methods that provides a robust and complete acoustic signature characterization of ship radiated underwater sound in terms of machinery tonal and propeller rotation signatures, as well as ship broadband energetics that can be employed for efficient ship classification
We provide acoustic signature characterization along with estimated bearing-time trajectories for each vessel that include (a) key tonal frequencies for ship machinery; (b) propeller shaft rotation frequency, blade-pass frequency and propeller blade number;and, (c) received mean power spectral densities (PSD) averaged across specific broad bandwidths to quantify ship radiated sound energetics
Summary
Remote monitoring of ocean vessels over instantaneous wide areas from their sounds radiated underwater, including detection, localization, characterization and classification, is of great importance in maritime surveillance and defence [1,2,3,4,5,6,7,8,9]. The sound generated by an ocean vessel contributes to environmental ambient noise [10] that can limit detection ranges in passive and active sonar systems for a wide range of ocean remote sensing applications [1,9,11,12,13,14,15,16], as well as in ocean acoustic communication [17,18]. The overall underwater sound emanated by a ship is comprised of colored broadband noise over a wide frequency range embedded with several prominent narrowband tonal components from shipboard machinery, with time-varying amplitude modulated sound pressure levels due to propeller rotation
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