Multiple mechanized ocean vessels, including both surface ships and submerged vehicles, can be simultaneously monitored over instantaneous continental-shelf scale regions >10,000 km ${}^2$ via passive ocean acoustic waveguide remote sensing. A large-aperture densely-sampled coherent hydrophone array system is employed in the Norwegian Sea in Spring 2014 to provide directional sensing in 360 degree horizontal azimuth and to significantly enhance the signal-to-noise ratio (SNR) of ship-radiated underwater sound, which improves ship detection ranges by roughly two orders of magnitude over that of a single hydrophone. Here, 30 mechanized ocean vessels spanning ranges from nearby to over 150 km from the coherent hydrophone array, are detected, localized and classified. The vessels are comprised of 20 identified commercial ships and 10 unidentified vehicles present in 8 h/day of Passive Ocean Acoustic Waveguide Remote Sensing (POAWRS) observation for two days. The underwater sounds from each of these ocean vessels received by the coherent hydrophone array are dominated by narrowband signals that are either constant frequency tonals or have frequencies that waver or oscillate slightly in time. The estimated bearing-time trajectory of a sequence of detections obtained from coherent beamforming are employed to determine the horizontal location of each vessel using the Moving Array Triangulation (MAT) technique. For commercial ships present in the region, the estimated horizontal positions obtained from passive acoustic sensing are verified by Global Positioning System (GPS) measurements of the ship locations found in a historical Automatic Identification System (AIS) database. We provide time-frequency characterizations of the underwater sounds radiated from the commercial ships and the unidentified vessels. The time-frequency features along with the bearing-time trajectory of the detected signals are applied to simultaneously track and distinguish these vessels.

Humpback whale breathing-related sounds were recorded on elements of a coherent hydrophone array subaperture deployed vertically at the Great South Channel on the US Northeastern continental shelf in Fall 2021, where half of the hydrophones were in-air and the rest submerged underwater. In-air hydrophones recorded breathing sounds with approximately 2.5 s duration, but smaller bandwidths compared to underwater hydrophones where signal energies extended beyond 50 kHz, and a mean underwater source level of 161 ± 4 dB re 1 μPa at 1 m, based on measurements at 22.9 m. The underwater recorded humpback whale breathing sound spectra displayed a broadband dip centered at 15.7 kHz, with approximately 400 Hz half-power bandwidth, likely caused by attenuation from propagation through pulsating air bubbles. The air bubble radius for natural frequency of oscillations at 15.7 kHz is estimated to be 0.205–0.21 mm. These bubbles are capable of removing energy from the forward propagated humpback breathing sounds via resonance absorption most pronounced at and near bubble natural oscillation frequency. Humpback whale distances from the vertically deployed hydrophones are estimated and tracked by matching the curved nonlinear travel-time wavefront of its breathing sounds, since the whale was in the near-field of the subarray.

Large-aperture coherent towed hydrophone array systems are capable of surveying wide ocean areas for detection, classification and localization of various underwater and surface objects. Current ocean acoustic monitoring equipment is costly, and the robust expansion of ocean acoustic sensing has primarily been held back by the lack of affordable multi-sensor systems even as the ability to analyze this data becomes more widespread. The ocean bio-acoustic sensing community has numerous researchers using single dipping hydrophones, buoy mounted hydrophones and small-aperture fixed hydrophone systems. While these systems are readily available, they do not offer the directional sensing, wide area surveying, array gain and the enhanced Signal to Noise Ratio (SNR) capabilities of a large-aperture coherent hydrophone array. In this thesis, a prototype eight-element oil-filled hydrophone array is developed to measure the acoustic field in littoral waters. This array is equipped with heading and depth sensors as well as a compact workstation with an independent Global Positioning System (GPS) receiver and winch system. The array is developed using rapid prototyping technology and low-cost manufacturing techniques by designing 3D printing thermoplastic urethane array internals, pressure tolerant designed amplifiers, potting connector moldings using 3D printed over molds, machining stainless tube adaptors, test and verification of transducer/sensor components that are readily available and acoustic testing of noise dampening materials. The prototype array developed here will serve as the basis for the development of a 160-element large-aperture coherent hydrophone array. The prototype array was deployed during an experiment between Jeffreys Ledge and the Stellwagen Bank region off the coast of Rockport, Massachusetts USA. During the experiment, several humpback whale vocalizations and distant ship tonals were recorded. Visual confirmation of humpback moving in bearing relative to the array verifies the directional sensing from array beamforming. Beamformed spectrograms are analyzed containing humpback whale downsweep moans and mid frequency tonal wail sounds associated with feeding on sand lance (Ammodytes dubius). The Source Level (SL) and measurement of temporal-spectral characteristics have been estimated for these calls agree with previous measurements of humpback whale feeding sounds. Analysis of performance characteristics is quantified to show levels of Turbulent Boundary Layer (TBL) noise and hydrophone self-noise as well as techniques to reduce electrical and mechanical noise levels. The performance of this prototype is compared with the Office of Naval Research (ONR) Five-Octave Research Array (FORA) and future design aspects are discussed based on these findings. The future development of this prototype array includes a fiber optic tow cable, analog to digital conversion inside array tubing and a variable gain stage to optimize dynamic range and SNR.