ASA 125th Meeting Ottawa 1993 May

5aSA1. Fresnel width of the coupling regions of generalized leaky Lamb waves and Fermat's principle.

Philip L. Marston

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

Ray methods describe guided-wave scattering contributions based on local coupling processes. To estimate the effective width of coupling regions for launching and radiation of sound by leaky guided waves of phase velocity c[sub l], Fresnel volume concepts of Y. A. Kravtsov and others [see, e.g., P. L. Marston, Physical Acoustics (Academic, New York, 1992), Vol. 21, pp. 1--234] were generalized. First, the usual coupling condition on the angle of incidence (theta)[sub l] for a sphere or cylinder, sin (theta)[sub l]=c/c[sub l], is derived by Fermat's principle. Then the angular shift (Delta)(theta)[sub l] is derived for a (pi) radian phase deviation from the Fermat ray. For a far-field source or observer, |(Delta)(theta)[sub l]|(approximately equal to)[radical 2(pi)/(ka cos (theta)[inf l])[radical is a measure of the ray width. Fermat's principle is also examined for a prompt backward wave mechanism [D. H. Hughes, Ph.D. thesis, Washington State Univ. (1992)] for the high-frequency scattering enhancement by shells. [Work supported by ONR.]