Long-term measurements of the directionality and active intensity of the underwater noise field in the shallow Beaufort Sea

Abstract

In each of the past seven years, at least 35 Directional Autonomous Seafloor Acoustic Recorders (DASARs) have been deployed over a 280 km swath of the Beaufort Sea continental shelf (20–55 m depth) during the open-water season to monitor the westward bowhead whale migration. DASARs have one omnidirectional pressure sensor and two orthogonal particle velocity sensors that permit measurements of the azimuths of both transient and continuous sounds, including diffuse ocean noise. Here, we map the azimuthal directionality of the Beaufort ambient noise field as a function of frequency and location across all seven seasons. Dominant directionalities exist in the diffuse ambient noise field, which change with frequency, time, and location. We examine how localized storms, heavy whale calling activity, seismic exploration, and other industrial activities influence the noise directionality. We also examine how both the active and reactive intensity of the noise evolves with frequency and time, by comparing the phase relationships between pressure and particle velocity. The directionality and active intensity help identify source mechanisms and help determine whether long-term changes in the ambient noise environment are occurring. [Work sponsored by the Shell Exploration and Production Company.]