Wave-Induced Fluctuations in Shallow-Water Sound Propagation

Abstract

Relative fluctuations ΔI/I of acoustic intensity, caused by surface-wave patches passing over a source and receiver bottom in shallow water, are analyzed theoretically and compared with Scrimger's measurements [J. Acoustic. Soc. Am. 33, 239–247 (1961)]. A patch is treated as an acoustic dipole, driven by the primary source-image field, and reradiating to interfere with the main signal. ΔI/I depends on wave height A, acoustic wavelength λ, and the patch's position along a horizontal line directly over source and receiver. For Scrimger's longer wavelengths, the following agreements with experiment appear—A/λ is a major but not sole parameter; ΔI/I∝A (λ fixed); ΔI/I varies linearly to quadratically with 1/λ (A fixed; ΔI/I is greatest for patches practically over source or receiver, least at midrange; ΔI/I has fundamental frequency equalling that of a train of patches. Combining patches into wave trains appears necessary, but may not be sufficient, to calculate absolute magnitudes and to explain, at larger A/λ, saturation of ΔI/I and appearance of harmonics in its frequency spectrum. [Supported in part by U. S. Navy Office of Naval Research.]