Michael J. Buckingham
Grant B. Deane
Marine Phys. Lab., Scripps Inst. of Oceanography, La Jolla, CA 92093-0213
Ambient noise contains broadband information about the oceanic environment. In particular, in shallow water, because of the multiple boundary interactions, ambient noise contains information about the geoacoustic parameters of the bottom. To extract this information, it is necessary to identify stable, reliable features of the noise. (Noise intensity is neither stable nor reliable since it is influenced by numerous highly variable factors.) An example of a reliable feature of the shallow-water noise field is its vertical directionality, which is controlled by the bottom. In fact, in an isovelocity channel of uniform depth overlying a fast fluid bottom, the vertical structure of the noise can be inverted to obtain an accurate estimate of the sound speed in the bottom sediment [Buckingham and Jones, J. Acoust Soc. Am. 81, 938--946 (1987)]. Thus, in this example, a simple measurement of the vertical coherence of the noise in the water column yielded an important geoacoustic parameter of the sediment. Similarly, in shallow water overlying a sloping, penetrable bottom (i.e., a range-dependent channel supporting three-dimensional acoustic propagation) the noise shows a horizontal anisotropy that could be used to determine geometric and geoacoustic parameters of the channel. In such applications, ``ambient noise oceanography'' provides a remote sensing tool for interrogating the bottom, much as seismic reflection and refraction techniques are used at present. [Research supported by ONR.]