Measurement of horizontal anisotropy of shallow water ambient noise due to breaking surf

Abstract

Additional measurements have been made of the horizontal anisotropy of ambient noise in Monterey Bay over a wider variety of sea conditions and range from shore since those reported at the November 1980 meeting. Directionality was measured using a steerable cardioid receiving pattern formed from signals telemetered from dipole and omnidirectional hydrophones suspended from tethered buoys. During heavy surf conditions, anisotropy, defined as the amount by which the shorewardly directed cardioid output exceeds that from the seawardly steered cardioid, as 10 dB at 300 Hz at a range of 9 km from shore and 90-m water depth and is significant from 20 to 700 Hz. Anisotropy is significant at a range of 15 km and water depth of 175 m. During heavy surf, noise levels also increase significantly in the same frequency range at which the anisotropy is evident. The anisotropy effects diminish both in magnitude and frequency range with lower wave height but are still observable during light surf. Evidence is very strong that when wind and surf are high, breaking surf can contribute significantly to ambient noise in fairly deep continental shelf waters. [Supported by the NPS Foundation Research Program and the Naval Sea Systems Command.]