Very low-frequency seismo-acoustic noise below the seafloor.

The low-frequency acoustic seismic experiment (LFASE), conducted in the Blake Bahama Basin in August and September of 1989, was the first experiment to measure VLF ambient ocean noise on a seismic array within a DSDP borehole. Four three-component geophones were clamped to the borehole wall at 10, 40, 70, and 100 m below the seafloor. The frequency response of the acquisition system was flat from 4.5 to 40 Hz but meaningful ambient noise measurements were acquired down to 0.3 Hz. Both discrete sources (interpreted as local microearthquakes) and a continuous distributed source (identified as a mid-Atlantic storm) were observed. The earthquakes are characterized as sudden arrivals of high-amplitude (at least 10 dB above background), high-frequency (above 20 Hz) signals with a coda decaying to background levels within 30 s. Within the upper 10 m of the seafloor these events have dominantly horizontal motion. Vertical motion becomes dominant at 70 m and by 100 m the events are indistinguishable from background noise. These amplitude effects with depth in the array suggest that these are Stoneley waves trapped near the seafloor. Noise levels associated with the continuous source (storm) also decay significantly below the sediment/water interface. The noise levels associated with the storm appeared below 2 Hz. Between 0.3 and 1.0 Hz there is a strong directionally coherent signal. Analysis of particle motions indicates that the source is in the direction of Hurricane Erin, which was located off the West African and European coast. Below 1.0 Hz the particle motion of the storm-related energy is clearly retrograde elliptical at the seafloor indicating Stoneley wave modes. [Work supported by ONR.]