Estimating sea-surface ambient noise levels at mid-frequency using a compact hydrophone array towed by an autonomous surface vehicle

Ambient noise monitoring in the ocean can provide insight into physical oceanographic processes such as wind and wave generation. Additionally, accurate environmental parametrization of the noise levels is needed for SONARsystems performance prediction. Typically, passive acoustic monitoring is conducted with stationary buoys that can record for long periods of time or large hydrophone arrays towed by manned vessels, creating a trade-off between monitoring area and mission duration. Here, an autonomous surface vehicle platform (Liquid Robotics Wave Gliders—WG) instrumented for physical oceanography measurements and also outfitted with a compact four-element tetrahedron-shaped hydrophone array is presented. Multiple WGs were deployed in the vicinity of the Atlantis II seamount to perform ambient acoustic surveys at depths ranging from 10 to 150 m. During these geo-localized surveys, the measured relationship between mid-frequency ambient noise levels and local wind speed was found to be consistent with previous experiments, demonstrating the efficacy of the Wave Glider as an ambient noise monitoring platform. Additionally, the mid-frequency beamforming capabilities of their compact array are leveraged to estimate in-situ the effective sound pressure level of the local sea-surface noise sources and its parametrization on the local wind speed which is needed for ambient soundscape modeling. [Work sponsored by ONR.]