Abstract
Shoaling surf focuses sound, creating high intensity caustics that impact the performance of underwater communications systems and sonars in the very near shore region. Simple expressions for the time-varying caustic locus and amplitude can be derived from the Kirchhoff–Helmholtz scattering integral. The analysis shows that a focus is formed at a range equal to one-half the crests radius of curvature when the wave is directly over a shallow water source, and moves toward the crest and decreases in intensity as the wave shoals shoreward. The analysis is compared with numerical scattering calculations based on measured surface gravity wave profiles, and also with experimental data in which 10-kHz, single cycle pulses were transmitted over a 40 m path in 6 m of water just north of Scripps Pier. The impact of the caustics on sonar performance will be discussed. [Work supported by ONR.]
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