An efficient and robust method for underwater acoustic normal mode computations

Abstract

An efficient, numerically robust algorithm for calculating acoustic depth-dependent normal modes for stratified fluid, multichannel ocean environments is described. An Airy-layered solution is used, and eigenvalues are found by a process which avoids the numerical instabilities associated with evanescent (nonoscillatory) depth dependence. Eigenfunction solutions are valid for multichannel environments. Computation time increases only linearly with frequency or number of modes. This algorithm (called ModeLab), is compared with KRAKEN [M. B. Porter and E. L. Reiss, J. Acoust. Soc. Am. 77, 1760–1767 (1985)] and with a model designed for fast cw calculations [K. J. McCann and F. Lee-McCann, J. Acoust. Soc. Am. 89, 2670–2676 (1991)]. ModeLab achieves the fastest computational speeds by more than an order of magnitude, and simultaneously provides eigenvalue accuracies comparable to the best available from KRAKEN. [Work supported by the Advanced Surveillance and Prediction System (ASAPS) Program of the Space and Naval Warfare Systems Command (SPAWAR, PMW 183-32).]