Shallow-Water Acoustics

shallowand 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 the acoustics research, other important applications have emerged, such as undersea communications, mapping of the ocean’s structure and topography, locating mines or archaeological artifacts, and the study of ocean biology. Shallow water is usually a noisy environment because shipping lanes exist along coastlines. Submarines typically radiate in the same frequency band as shipping noise, less than 1 kilohertz. The proliferation of quiet submarine technology has restimulated the development of active sonar systems, which send out pulses and examine their echoes, rather than the more stealthy, passive approach of listening and exploiting the relevant physics. However, the ultimate limits of passive acoustics in terms of signal-tonoise ratio (SNR), acoustic aperture (or antenna) size, and the ocean environment are ongoing research issues. The sound speed in water is about 1500 m/s, so wavelengths of interest are on the order of a few meters. In shallow water, with boundaries framed by the surface and bottom, the typical depth-to-wavelength ratio is about 10–100. That ratio makes the propagation of acoustic waves there analogous to electromagnetic propagation in a dielectric waveguide. But the shallow ocean is an exceedingly complicated place. The surface and the ocean’s index of refraction have a spatial and time dependence, and the presence of ocean inhomogeneities and loud ships can frequently scatter, jam, or mask the most interesting sounds. So, whether using active or passive techniques, scientists are ultimately concerned with the physics of extracting a signal that propagates in a lossy, dynamic waveguide from noise influenced by that same waveguide’s complexity. In that light, and considering our still imperfect understanding of coastal oceanography, one can begin to appreciate shallow-water acoustics as an interdisciplinary blend of physics, signal processing, physical oceanography, marine geophysics, and even marine biology.