Sources of low-frequency ambient seafloor noise on a continental shelf

Abstract

Two days of 1- to 30-Hz ambient seafloor noise recorded simultaneously by hydrophones and geophones in shallow water are correlated with identifiable sources to determine which noise sources dominated. Ambient noise levels (in decibels) between 1 and 2 Hz are linearly correlated to the logarithm of the wind speed (and/or wave height) data gathered over the ocean bottom seismometer array with an n ( f ) = 2.4 ± 0.4. This linear dependence and the observed decrease of ambient noise levels with increasing depth argue that, for high wind speeds, a linear wind speed (turbulent pressure fluctuation) source dominates other meteorological noise sources in shallow water to frequencies as low as 1 Hz. Relative ambient pressure fluctuations (in decibels) were, within measurement errors, comparable to the ambient particle velocity fluctuations at 1 to 2 Hz. Short periods of ocean bottom current activity, teleseisms, nearby drilling rig activity, nearby airgun profiling, biologic activity, and shipping raised ambient noise levels for frequencies above 2.2 Hz by at least 10 dB. In the case of the biologically induced noise, the observed absence of acoustic signals requires that the organisms actually touched the ocean bottom seismometers.