  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.
