The underwater soundscape of the North Sea

Physics-informed and machine learning-enabled retrieval of ocean current speed from flow noisea)

Episodes of exceptionally strong near-bottom currents were encountered at depths of over 2500 m in a 52-day deployment of moored autonomous acoustic noise recorders (MANRs) over the Atlantis II Seamounts in the Northwest Atlantic. A strong correlation is found between the current speed and acoustic noise intensity, especially at infrasonic frequencies below 20 Hz. Flow noise and ambient sound, including shipping noise, made comparable contributions to the measured acoustic intensity but had distinct spectral properties. This paper explores a way to identify and quantify the differences between flow noise and ambient sound in the pressure fluctuations measured by a hydrophone and find statistical characteristics of the fluctuations which contain robust information about the flow speed. A regression tree machine learning model was developed to relate the acoustic features of flow noise to directly measured current speeds. By training the model using data from a MANR equipped with a hydrophone and current meter, the time series of current speed was obtained with 1-min resolution at another MANR, where only acoustic data were available. Accuracy of the inferred current speeds was confirmed by comparing the dependence of flow noise spectra on the current speed at the two MANRs.

Observation of exceptionally strong near-bottom flows over the Atlantis II Seamounts in the northwest Atlantic

Knowledge of near-bottom ocean current velocities and especially their extreme values is necessary to understand geomorphology of the seafloor and composition of benthic biological communities and quantify mechanical energy dissipation by bottom drag. Direct measurements of near-bottom currents in deep ocean remain scarce because of logistical challenges. Here, we report the results of flow velocity and pressure fluctuation measurements at three sites with depths of 2573-4443 m in the area where the Gulf Stream interacts with the New England Seamounts. Repeated episodes of unexpectedly strong near-bottom currents were observed, with the current speed at 4443 m of more than 0.40 m/s. At 2573 m, current speeds exceeded 0.20 m/s approximately 5% of the time throughout the entire eight-week measurement period. The maximum flow speeds of over 1.10 m/s recorded at this site significantly surpass the fastest previously reported directly measured current speeds at comparable or larger depths. A strong correlation is found between the noise intensity in the infrasonic band and the measured current speed. The noise intensity and the characteristic frequency increase with the increasing current speed. Machine-learning tools are employed to infer current speeds from flow-noise measurements at the site not equipped with a current meter.

Characterising underwater noise and changes in harbour porpoise behaviour during the decommissioning of an oil and gas platform

Many man-made marine structures (MMS) will have to be decommissioned in the coming decades. While studies on the impacts of construction of MMS on marine mammals exist, no research has been done on the effects of their decommissioning. The complete removal of an oil and gas platform in Scotland in 2021 provided an opportunity to investigate the response of harbour porpoises to decommissioning. Arrays of broadband noise recorders and echolocation detectors were used to describe noise characteristics produced by decommissioning activities and assess porpoise behaviour. During decommissioning, sound pressure spectral density levels in the frequency range 100 Hz to 48 kHz were 30-40 dB higher than baseline, with vessel presence being the main source of noise. The study detected small-scale (< 2 km) and short-term porpoise displacement during decommissioning, with porpoise occurrence increasing immediately after this. These findings can inform the consenting process for future decommissioning projects.