Near-surface ocean ambient noise measurements at very low frequencies

Abstract

Ambient noise in the frequency range 1–20 Hz was measured for 1-day periods at each of four widely separated sites in the North Atlantic Ocean with freely drifting, surface-suspended hydrophones. The acoustic data were recorded aboard an attending research vessel following transmission over a radio link. Narrow-band spectral analysis was performed at 1-h intervals to form time series of noise spectral level for each site. Statistical analyses of the power-spectral time series included the determination of cumulative probabilities, standard deviation, skew, kurtosis, decorrelation times, and the wind-speed dependence of mean spectral level. Comparisons with data from bottom-mounted hydrophones and studies of inter- and intra-array coherence indicate that the data were, with few exceptions, uncontaminated by self-noise down to 1 Hz. A significant dependence of noise spectrum level upon local wind speed was observed in the 1.5- to 3-Hz frequency band at each site. At 4 Hz, the dependence upon wind speed was much weaker, suggesting that the noise sources at this frequency are more distant. Estimates of noise directionality and tests for normality were also made on the data between 1 and 5 Hz.