Stable components of sound fields in the ocean

TL;DR

This paper introduces a method to identify stable sound field components in ocean waveguides that are minimally affected by sound speed perturbations, aiding in more reliable underwater acoustic measurements.

Contribution

The paper presents a novel approach to extract stable wave field components using field expansions, demonstrated through numerical simulations in deep water environments.

Findings

Stable components differ by only a phase factor under perturbations.

Stable components experience only a time delay in transient fields.

Numerical simulations confirm the existence and robustness of stable components.

Abstract

A method is proposed for finding the wave field components which are weakly sensitive to the sound speed perturbation in the ocean acoustic waveguides. Such a component is formed by a narrow beam of rays whose spread in vertical direction, up to the observation range, remains less than the vertical scale of perturbation. These rays pass through practically the same inhomogeneities and therefore their phases acquire close increments. If the ray amplitudes vary insignificantly, then (i) the stable component of the monochromatic field in the perturbed and unperturbed waveguide differ by only a constant phase factor, and (ii) in the case of transient wave field the perturbation causes only an additional time delay of the stable component as a whole. It is shown how the stable components can be selected from the total wave field using the field expansions in coherent states or in normal modes. The existence of stable components is demonstrated by numerical simulation of sound field in a deep water waveguide. It turns out, that even though the assumptions (i) and (ii) are not met exactly, the stable components in the perturbed and unperturbed waveguide are quite close.