The relative effects of sound-speed variability in the water column versus the seabed on normal-mode propagation in shallow water

Abstract

Normal-mode propagation in shallow water is influenced by variations in the acoustic properties of both the water column and the seabed. Specifically, the sound speed in the water may fluctuate due to the passage of an internal wave, while the sound speed in the bottom may change due to variable geological features. Assessing the relative effects of the water column versus the seabed on the characteristics of modal propagation is critical to the understanding of both the forward and inverse problems in shallow-water acoustics. In this paper, perturbation theory is combined with a Pekeris waveguide model to provide an analytic formalism for evaluating the delicate interplay between the water column and the seabed in shallow water propagation. In particular, the relative contributions of sound-speed fluctuations in the water and the bottom to variations in the modal eigenvalues are determined. The results are affected by both the strengths of the fluctuations and the magnitudes of the background modal eigenfunctions in the water and the seabed. The complexity of the problem is illustrated with synthetic and experimental data. [Work supported by ONR.]