Robust computation of acoustic normal modes in attenuating ocean waveguides

Abstract

The normal mode representation [Jensen etal., Computational Ocean Acoustics, AIP Press, 1997] provides for the mathematical analysis and numerical computation of acoustic fields in a range-independent ocean waveguide. Existing algorithms [e.g., M. B. Porter and E. L. Reiss, J. Acoust. Soc. Am. 77, 1760] provide for efficient computation of the mode eigenvalues and eigenfunctions for narrow-band acoustic fields. However, for extensive computations involving a large set of environmental parameter values or acoustic frequencies, reliability issues, such as eigenvalues omitted in the calculation, have been noted in the case of attenuating media. In this work, a computational method is presented that computes the normal modes by approximate solution of the mode depth-dependence equation on a discrete computational grid, using a selected discrete basis. Empirical evidence of the robustness of the method is provided by comparisons with established numerical benchmarks and by examining the acoustic parameter dependence of the mode spectra over thousands of parameter values. Theoretical support for the reliability of the computation is then discussed. [Work supported by the Office of Naval Research.]