Vertical noise coherence measurements in shallow water using Lagrangian drifters

Abstract

The vertical coherence of an acoustic noise field in shallow water (at frequencies dominated by sea surface noise) can be used to infer certain ocean bottom properties such as the compressional sound speed of the upper sediment layer. As coherence is a normalized measurement, it is independent of background noise level, or wind speed/sea state, and remains a stable measurement over a wide range of environment conditions. A set of three Lagrangian ambient noise drifters (LANDs) were developed for vertical coherence measurements. The drifters (drogued at 15-m depth) follow the near-surface currents, and take hourly measurements of coherence from their two hydrophones located at 50-m depth. The hydrophone separation is 1 m, and the measurements are made over a usable frequency range of 100 Hz to 1 kHz. The drifters were deployed at Western Bank (Scotian Shelf, Canada) and drifted for up to 2.5 days. The sea conditions varied from calm to 3–4-m waves. The stability of the coherence data through the various sea states, and the impact of shipping noise on the measurements will be discussed.