Very Low Frequency Sound Studied Using Multi-Element Seafloor Arrays

Abstract

An array of eleven instruments was deployed on the seafloor in the central North Pacific in November, 1988 to record pressure fluctuations associated with ambient noise in the band from 0.001 to 1 Hz. The coherence varies with frequency and distance between pairs of instruments within the 95 km aperture array. A maximum likelihood technique has been used to calculate wavenumber and frequency spectra for various intervals within the data set. The pressure signals below 0.03 Hz propagate as surface gravity waves and appear to be derived from the coast. No coherence is found in the band between 0.03 and 0.05 Hz, except after major earthquakes along the Pacific rim, which generate long Rayleigh wave trains that are perfectly coherent across the entire array. The single frequency microseism peak between 0.06 and 0.1 Hz is evident as a band of high coherence. These waves propagate as fundamental mode Rayleigh waves with phase velocities near 4 km/s. Pressure fluctuations measured within the double frequency microseism peak are coherent only between the most closely spaced instruments (less than 10 km) and only up to a frequency of about 0.25 Hz. The wavenumber and frequency spectra show the energy in this band also propagates as fundamental mode Rayleigh waves, but with phase velocities near 1.5 km/s. Multiple source regions or directions of propagation are evident in both the single frequency and double frequency microseism peaks.