ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

4aUW3. Features of the seismoacoustic ambient noise field in a near-shore environment.

Gerald L. D'Spain

William S. Hodgkiss

Lewis F. Berger

William A. Kuperman

Marine Physical Lab., Scripps Inst. of Oceanogr., La Jolla, CA 92093-0701

This paper presents the features of the 1- to 750-Hz ambient noise field recorded during MPL's adaptive beach monitoring experiment, a near-shore seismoacoustic experiment off the Camp Pendleton Marine Base north of San Diego, CA. Signal propagation characteristics in this environment are presented in a companion paper. The data were recorded by two nearly orthogonal, horizontal hydrophone arrays located 3.4 km offshore in 20-m water. The most predominant aspect is the sounds created by biologics, particularly night-time ``cycling'' choruses of fish. Ideas on the use of these sounds for determining the geoacoustic properties of the ocean bottom are discussed. Also present at night are strong lines at 85 Hz and its two higher harmonics, associated with water pumps on the marine base used to fill wells in the surrounding hills. A still-puzzling feature is the broadband pulses (50--200 Hz) that occur at a 7.5-s repetitition period; they possibly also are of a biological nature. Fluctuations in the infrasonic band are associated with ocean surface wave propagation, thereby providing a means of determining the ocean surface wave directional spectrum. Finally, by judicious selection of data, noise generated by breaking surf can be measured. [Work supported by ONR, Code 32.]