Stable components of sound fields in the ocean

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 are incremented by the same amount. If the ray amplitudes vary insignificantly, then (i) the stable components 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 the coherent states and in the 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 waveguides are quite close.