Sources of geoacoustic noise in the ocean

Abstract

Very low-frequency (<50 Hz) underwater sound measurements made from fixed hydrophone arrays have revealed several major sources of geoacoustic noise in the deep ocean. For example, sounds from undersea volcanic eruptions are detected at transoceanic distances in the deep ocean sound (SOFAR) channel [Northrop, J. Acoust. Soc. Am. 56, 837–841 (1974)]. Underwater explosions cause about 20 000 hydroacoustic signals in the Pacific per year [Spiess, Northrop, and Werner, J. Acoust. Soc. Am. 43, 640–641 (1968)], and acoustic waves radiated from ocean margin earthquakes account for another 10 000 per year. Sounds from microearthquakes on the midoceanic ridges, however, generally are not well coupled with the SOFAR channel, but account for considerable local seismic noise in the vicinity of the ridges. Recent measurements in the eastern Pacific using sonobuoys, modified to record these very low-frequency sounds, have shown an average of one microearthquake per hour on the Rivera fracture zone at 19°15′N, 108°40′W, and 41 per hour on the Galapagos spreading center at 1 °N, 86°W. These sources of ambient noise in the ocean should be incorporated in environmental acoustic models. [Work Supported by Office of Naval Research.]