John E. Moe
Darrell R. Jackson
Appl. Phys. Lab., Univ. of Washington, 1013 NE 40th St., Seattle, WA 98195
The acoustic intensity penetrating a rough surface is analyzed using Rayleigh--Rice perturbation theory. When the grazing angle of the incident field is below the critical angle in relation to the mean surface, only the zero-order component of the transmitted field is evanescent; the higher-order components contain downward traveling waves. For an incident field below the critical angle, first-order computations using parameters appropriate to a sandy bottom show that the field below the rough surface can be much greater than the corresponding field below a flat surface. These computations are carried out using a low-frequency cutoff for the bottom relief spectrum. With regard to the accuracy of the calculation, both the short-wavelength portion of the relief profile that is retained as well as the long-wavelength portion that is discarded are considered. It is shown that the rms height of the portion retained and the rms slope of the portion discarded are sufficiently small to lend confidence in the perturbation approach. Further work is required, however, to unequivocally establish the accuracy of the method. [Work supported by ONR.]