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To better understand spawning vocalizations of Norwegian coastal cod (Gadus morhua), a prototype eight-element coherent hydrophone array was deployed in stationary vertical and towed horizontal modes to monitor cod sounds during an experiment in spring 2019. Depth distribution of cod aggregations was monitored concurrently with an ultrasonic echosounder. Cod vocalizations recorded on the hydrophone array are analysed to provide time–frequency characteristics, and source level distribution after correcting for one-way transmission losses from cod locations to the hydrophone array. The recorded cod vocalization frequencies range from ∼20 to 600 Hz with a peak power frequency of ∼60 Hz, average duration of 300 ms, and mean source level of 163.5 ± 7.9 dB re 1 μPa at 1 m. Spatial dependence of received cod vocalization rates is estimated using hydrophone array measurements as the array is towed horizontally from deeper surrounding waters to shallow water inlet areas of the experimental site. The bathymetric-dependent probability of detection regions for cod vocalizations are quantified and are found to be significantly reduced in shallow-water areas of the inlet. We show that the towable hydrophone array deployed from a moving vessel is invaluable because it can survey cod vocalization activity at multiple locations, providing continuous spatial coverage that is complementary to fixed sensor systems that provide continuous temporal coverage at a given location.

 

Airgun source systems generate low frequency underwater sound used in reflection and refraction seismology for mapping ocean bottom stratigraphy with important applications in ocean geosciences, such as understanding plate tectonics, ascertaining ocean geological history and climate change, and offshore hydrocarbon prospecting. Seismo-acoustic airgun signals from geophysical surveying activity were recorded at very long ranges, spanning roughly 175-195 km, on a large-aperture densely-populated linear coherent hydrophone array in the Norwegian Sea during Spring 2014. Off the coast of Alesund, airgun signals were detected with 8 s inter-pulse intervals for 3 to 24 hour time periods per day over the 4 days of hydrophone array operation in that region. Here we provide a time-frequency characterization and bearing-time estimation of the received airgun pulses. By correcting for transmission losses in the range- and depth-dependent Norwegian Sea environment, we estimate the source level distribution back projected to a distance of 1 m from the airgun source system. This back-projected source level distribution is then applied to model the Probability of Detection (PoD) region for the airgun signals with the coherent hydrophone array as the receiver in the Norwegian Sea employing the passive ocean acoustic waveguide remote sensing (POAWRS) technique. The estimates of back-projected source level distribution and PoD region provide an understanding of the horizontal spatial propagation extent of the signals from the airgun source system in the shallow and deep water regions of the Norwegian Sea. These results can also be applied to studies of the potential impact of airgun signals on marine organisms. 

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.