High-Frequency Sound Interaction in Ocean Sediments
2 July 1999

Geoacoustic Parameters Determined from Measurements
of Ambient Ocean Noise and Interface Waves

Michael Buckingham and Thomas Berger
Marine Physical Laboratory
Scripps Inst. of Oceanography
La Jolla, CA 92093-0238
(619) 534-7977 -7132 (fax), mjb@mpl.ucsd.edu
(619) 534-9853, -7641 (fax), tberger@ucsd.edu

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The aim is to measure geoacoustic parameters, specifically the compressional sound speed of the sediment, over a large footprint covering the site of the SAX99 experiment using the ambient noise of the ocean. The procedure can be extended to include an active sound source with the goal of inverting for geoacoustic parameters using local and spatially integrated measurements of interface waves.

If the ambient ocean sound is primarily the wind driven noise of breaking waves and white capping, an inversion for bottom properties is straightforward. The feature to be exploited is the directionality of the noise field. The directionality indicates the compressional sound speed in the sediment and can potentially yield information about shear and attenuation. If there is little breaking wave action, there is the possibility of employing an active sound source, such as an airgun, to excite interface waves. The difference in the arrival times of the interface and water waves yields the averaged sound speed over the traveled distance, and the directionality of the received head wave signal can give the local compressional sound speed. In both cases, the experimental setup consists of a vertical hydrophone array deployed at mid-water depth in a shallow ocean. To avoid contamination from the ships' noise it may be necessary to deploy the array from a small craft at about a one-mile distance from the ships. Two or three people will perform the experiments.

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