Shallow-Water Acoustics

介绍声在浅水中的传播,并简要介绍了内波的影响。

介绍声在浅水中的传播,并简要介绍了内波的影响。

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. Ocean waveguides Because the motions of ocean waves and water masses, along with the sources and receivers of signals that pass through them, are so small compared to the speed of sound in seawater, the ocean, to first approximation, can be treated as essentially a frozen medium. One can further model the ocean and seabed as horizontally stratified, a simplification that permits the basic waveguide description of signal propagation there. The vertical variation in the layering is typically much greater than the horizontal variation, a fact that minimizes out-of-plane refraction, diffraction, and scattering (with some notable exceptions). Moreover, temperature, T (in Celsius), provides an excellent characterization of sound speed, c, because the depth plays a minor role in shallow water: c 1449 ⊕ 4.6T ⊕ (1.34 ? 0.01T)(S ? 35) ⊕ 0.016z, where the depth z is in meters, S is salinity in parts per thousand, and the last term embodies density and static pressure effects. The seabed can often be approximated as a fluid medium, with only sound speed, attenuation, and density as acoustic variables. Three simple profiles, illustrated in figure 1, of the speed of sound in seawater have proven useful to researchers modeling the shallow-water acoustics problem. During World War II, Chaim Pekeris applied waveguide theory to the task and treated the ocean as a static, constant sound-speed medium. That approach addressed the many boundary interactions in shallow water, but it underestimated the medium’s complexity. Box 2 summarizes the solution to the wave equation for the canonical shallow-water acoustic waveguide. To appreciate its application to a realistic case, consider the panels in figure 2, which show the behavior of a sound wave excited by a point source in a waveguide. The propagating normal modes have different frequency-dependent group speeds, so a finite-bandwidth pulse disperses as it propagates down the waveguide and passes an array of detectors. The lower modes become trapped toward the bottom of the waveguide because sound paths bend toward regions of lower sound speeds. The lowest mode has the most direct path down the waveguide and arrives at the detectors October 2004 Physics Today 55 © 2004 American Institute of Physics, S-0031-9228-0410-030-6 first. Higher modes follow and reCoastal front a Mixed winter profile b 3-layer summer profile c fract higher in the water column’s c thermocline. Surface z The nominal resolving capamixed layer bility of an array of detectors dec(z) c2(z) c1(z) c(z) Th

介绍声在浅水中的传播,并简要介绍了内波的影响。

介绍声在浅水中的传播,并简要介绍了内波的影响。

58October 2004 Physics Todayhttp://wendang.chazidian.com

介绍声在浅水中的传播,并简要介绍了内波的影响。

http://wendang.chazidian.com

October 2004 Physics Today

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