Abstract
A computer program has been written for a normal-mode solution of the wave equation in a medium consisting of two layers, in each of which the reciprocal of the square of the sound speed is a linear function of depth. All possible combinations of algebraic signs are permitted for the gradients in the two layers. A flat, homogeneous, nonelastic, attenuating bottom is assumed. The solution is programmed as a sum of normal modes, the depth functions being expressed in terms of Airy functions. The eigenvalues are computed by an iteration process in the complex plane, employed by Pedersen and Gordon [J. Acoust. Soc. Amer. 37, 105–118 (1965)], the initial estimates being obtained by a WKB approximation, combined with an application of ray theory. The branch line integral, which makes a significant contribution only at relatively short ranges, is neglected. The program has the capacity of computing up to 190 modes and is used for shallow-water propagation and low-frequency convergence zone propagation.
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