Low Frequency Ocean Acoustic Phenomena

Abstract

Underwater soundscapes are complex as they are a combination of several components with overlapping acoustic frequencies and different timescales, from individual durations of a few seconds or less, to seasonality and long-term trends. Sounds below 100 Hz are of particular interest to compare human impacts with natural processes and animal vocalisations because of their prevalence and their very large propagation ranges, but understanding is often limited to specific signals on their own, not in combination. This thesis presents the processing interpretation and analyses of over two years (2018 and 2019) of measurements from the Lofoten-Vesterålen cabled ocean observatory, located 15 km offshore northern Norway, at a depth of 255 m, and one year of data (2018) from the Comprehensive Nuclear Test-Ban Treaty Organisation's Ascension Island station hydrophones. The northern triplet is over 11 km offshore at an average depth of 847 m, and the southern triplet is over 100 km offshore at an average depth of 860 m. We identify the different contributions of weather, whales, shipping, seismic airguns, and earthquakes, using energy detectors and deep learning to automatically detect common 20 Hz fin whale calls. The main soundscape components are quantified based on origins, frequency ranges and seasonality, and assess their variability over time. These extensive results provide a useful baseline for further studies of other low-frequency environments around the world. The thorough use of deep learning and energy detectors, in particular for 20 Hz fin whale calls, also enables traceability of this AI approach in complex marine soundscapes.

Date of Award	11 Sept 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Alan Hunter (Supervisor), Philippe Blondel (Supervisor), Gary Heald (Supervisor), Duncan Williams (Supervisor) & Ross Heyburn (Supervisor)