Abstract:
Improvements have been achieved in the speed, robustness, and versatility of a recently developed normal-mode algorithm for fluid ocean environments [S. J. Levinson et al., J. Acoust. Soc. Am. 97, 1576--1585 (1995)]. The possibility of missing modes has been eliminated by computing both the total phase of the oscillatory mode function versus depth and the number of zeros of the function. The root-finding algorithm computes both an error function and its derivative, which allows cubic interpolation to be used to obtain eigenvalue guesses when a root is bracketed. The efficiency of the root-finder is characterized by the fact that the error function typically must be computed less than 3-1/2 times per mode. An automated broadband capability has also been implemented. Analytic first and second derivatives are used to interpolate the eigenvalues in frequency as a fifth-order polynomial. For broadband computations where inclusion of the continuum is desired, a false bottom may be automatically inserted. Its thickness is specified in terms of acoustic wavelengths in the medium and varies with the frequency, thus saving the computation of a significant number of modes. [Work supported by ONR 321OA.]