ASA 126th Meeting Denver 1993 October 4-8

3pUW2. Spatial structure of ambient noise in shallow water.

Michael J. Buckingham

Marine Phys. Lab., Scripps Inst. of Oceanog., La Jolla, CA 92093-0213

Apart from the nature of the sources, ambient noise in shallow water is distinguished from that in deep water through the proximity of the bottom. As with acoustic propagation, the bottom introduces a modal structure into the noise, evident through noise eigenrays with discrete grazing angles that are characteristic of the modes. There is also a continuous noise component, associated with overhead sources. Since it is largely controlled by the bottom, the modal structure is characterized by the geoacoustic properties, including the stratification, of the sediment. As the modal structure determines the directionality, or equivalently the spatial coherence in the vertical, it should be possible, through measurements with vertically aligned hydrophones, to establish the bottom properties through measurements of the noise field in the water column. For example, the sound speed of a fluid sediment close to the water--seafloor interface can be estimated from observations of the vertical noise coherence over the frequency range 500 Hz to 1 kHz (M. J. Buckingham and S. A. S. Jones, J. Acoust. Soc. Am. 81, 938--946 (1987)]. This inversion technique can be extended to the case of a shear supporting bottom and, at sufficiently low frequency, may yield information on the sub-bottom structure. [Research supported by ONR.]