Low-frequency noise observations in the deep ocean

Abstract

Simultaneous measurements of ocean-bottom infrasonic and ocean-bottom and sub-bottom seismic noise in the frequency band 0.1–20 Hz are presented. The data were obtained in 5.5-km-deep water in the South Central Pacific with a triaxial borehole seismograph and six triaxial ocean-bottom seismographs having externally mounted hydrophones. The borehole sensors were emplaced 54 m within basement rock overlain by 70 m of pelagic clay. In the band 0.1–1 Hz, noise propagation as seismic modes trapped in the seafloor is supported by observed spectral coherences, cross phases, and ratios between ocean-bottom pressure and vertical ground motion, and by the relatively lower noise levels in the borehole. Noise variations in this band are clearly correlated with changes in local wind direction and speed, presumably through ocean-bottom pressure fluctuations caused by nonlinear wind wave–wind wave and wind wave–swell interaction.