During the two world wars, oceanographers studied both shallow- and deep-water acoustics. But during the cold war, the research emphasis shifted abruptly to deep water, where the ballistic missile submarine threat lurked (see box 1). After the cold war, the onset of regional conflicts in coastal countries shifted the focus again to shallow water. Those waters encompass about 5% of the world’s oceans on the continental shelves, roughly the region from the beach to the shelf break, where water depths are about 200 meters.

To a significant extent, the problems of shallow-water acoustics are the same as those encountered in nondestructive testing, medical ultrasonics, multichannel communications, seismic processing, adaptive optics, and radio astronomy. In these fields, propagating waves carry information, embedded within some sort of noise, to the boundaries of a minimally accessible, poorly known, complex medium, where it is detected. 1,2 Although submarine detection has driven much of...

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