Large-aperture Coherent Hydrophone Array Research Articles

Directional soundscapes in the Norwegian Seas observed with a coherent hydrophone array

Directional soundscaping is an efficient approach for examining marine ecosystems since it allows the study of living organisms, their behavior, and temporo-spatial interactions with other natural and man-made objects underwater, useful for ecosystem monitoring during offshore energy activities, maritime surveillance, and defense. A large aperture coherent hydrophone array was employed for remote sensing in the Norwegian and Barents Seas in Spring 2014. Extensive analysis have been conducted in post-processing of recorded hydrophone array data for automatic detection, bearing estimation, and classification of signals by different sound producers, such as whale calls, ship radiated noise, and fish sounds. Directional soundscaping through passive ocean acoustic waveguide remote sensing (POAWRS) provides bearing-time trajectories of signal detections that can be applied for geographical mapping. The received marine mammal sounds include vocalizations from baleen whales such as humpback, fin, and minke, and toothed whales, such as beluga, pilot, and sperm. We provide an insight to the vocalizations and behaviorial patterns of these whales in the Lofoten and Northern Finmark regions. The relative contributions of distinct sound sources, including whales, fish, and ships, to the directional soundscapes are quantified. We also examine their temporo-spatial dependences via geographic mapping of acoustic signals from distinct sound producers.

Large-aperture wide-bandwidth densely populated coherent hydrophone array system for ocean acoustic monitoring

Underwater acoustic monitoring with a large-aperture coherent hydrophone array is advantageous because it enhances signal-to-noise ratio of received signals, provides estimates of signal bearing, and enhances signal detection ranges by one to two orders of magnitude over that of a single hydrophone. A large aperture coherent hydrophone array system comprising >160 elements has been developed inhouse at Northeastern University. The overall acoustic aperture length is 192 m with array elements nested into multiple uniformly spaced or log spaced subapertures. Hydrophones with integrated broadband pre-amplifiers designed with a linear frequency response from 10 Hz to 50 kHz send differential pair amplified and filtered analog signals to multiple 24-bit, 32-channel analog-to-digital converters with sampling rate that is programmable up to 100 kHz per channel. Array internals are designed using field replaceable pressure tolerant components verified by pressure chamber testing. Forward and aft modules are equipped with non-acoustic sensor elements to provide depth, heading, pitch, roll and temperature measurements. Acoustic aperture telemetry is user datagram protocol (UDP) converted to single-mode fiber for transmission along 600 m of faired tow cable to a shipboard data acquisition system. Examples of passive acoustic data from array deployment in the US Northeast coast are presented illustrating array capabilities.

Development of a large-aperture 160-element coherent hydrophone array system for instantaneous wide area ocean acoustic sensing

A large-aperture coherent hydrophone array and processing system has been developed in-house at Northeastern University (NU). The overall acoustic aperture length is 194 m with 160-elements nested into four sub-apertures each with 64 hydrophones and spacing corresponding to half-wavelength at design frequencies in Hertz of 250, 500, 1000, and 2000. Hydrophones with integrated broadband preamplifiers designed with a linear frequency response from 10 to 50 kHz sending differential pair amplified and filtered analog signals to 24-bit, 32-channel Analog to Digital Converters (ADC) with sampling rate programmable up to 100 kHz per channel. Array internals are designed using field replaceable pressure tolerant components including hydrophones, preamplifiers, power modules, telemetry units, and ADC verified by testing in an in-house assembled pressure chamber. Forward and aft modules are equipped with non-acoustic sensor elements to provide depth, heading, pitch, roll, and temperature measurements. The NU array was made without specialized facilities by utilizing modular interchangeable array interconnects allowing for conventional array populating and filling methods. The system implements real-time array data acquisition and processing including beamforming, beamformed spectrogram generation and signal detections. The NU array has been sea tested in 2021 in both shallow and deep water environments of the US Northeast coast.