Abstract:

Measurements of underwater sound at very low frequencies made with a hydrophone resting on the sea bottom may be contaminated by flow noise. Although the flow velocity may be negligible at the very bottom, the turbulent boundary layer of natural currents flowing above the bottom induces nonacoustic local pressure fluctuations on the bottom which may cause self-noise interference. Estimates of these pressure fluctuations are presented based on pressures measured on the ground in an open field in the presence of wind, using dimensional analysis to extrapolate from air to water. The estimated values are compared with underwater ambient noise levels measured with bottomed hydrophones to determine how large the current must be to cause concern with self-noise interference at very low frequencies. Methods for reducing the interference are discussed.