On description of mode amplitude fluctuations in the ocean

Abstract
Results of Tindle and Guthrie, Felsen and other authors on studying the relationship between ray and mode representations of a field in a range-independent waveguide are used to describe wave scattering in a waveguide with large-scaled random inhomogeneities of refractive index. The notion of Fresnel zones for modes is introduced, which is analogous to the usual Fresnel zones introduced for rays. Using the Fresnel zones for modes one can simplify the analysis of mode scattering at large-scaled inhomogeneities in waveguides. Simple formulae to calculate fluctuations of mode amplitudes are obtained. They are quite similar to the well known formulae of geometrical optics and those of the Rytov method. Unlike their ray prototypes, the formulae presented here allow one to describe field fluctuations not only at the waveguide regular points but at the caustics as well.