Meridional ray contributions to scattering by tilted cylindrical shells above the coincidence frequency: ray theory and computations

High-frequency scattering enhancements from a submerged infinite cylindrica l shell are studied by means of partial wave series (PWS) calculations and a ray approximation. For certain oblique angles of incidence it is possible to launch a generalized leaky Lamb wave which propagates strictly in the a xial direction on the shell. This meridional leaky wave radiates with a loc ally flat wavefront curvature and produces large far-field amplitudes over a range of tilt angles. When combined with a reflection from a discontinuit y in the shell, this process can give rise to large backscattering enhancem ents at large tilt angles for thick finite cylindrical shells [S. F. Morse ed nl., J. Acoust, Sec. Am. 103, 785-794 (1998)] and is present in sonar im ages. A convolution formulation ray theory for the far-field meridional ray amplitude is compared with the exact PWS solution in the region where enha ncements are observed for both the a(0) and s(0) Lamb waves. The ray theory correctly predicts both the peak enhancement amplitude and the general sha pe of the enhancement curve. (C) 1999 Acoustical Society of America. [S0001 -4966(99)04511-7].