Equations have been developed for the mean acoustic field radiated in a sea channel perturbed by random fluctuations of the distribution of sound-speed (internal waves, short-scaled tidal oscillations...). These equations extend Tatarskii's markovization or δ-correlation approximation, modified and extended to very low-frequency signals (less than a few kHz) and bounded shallow sea-channels (bottom depth up to a few 100.m's). Some properties of this equation are investigated; full numerical codes were developed and tried for the evaluation of these "random modes". The resulting set of random modes seems to be better fitted than classical modes for representing the acoustic fields, for they incorporate specific qualitative features and quantitative non-perturbative cumulative effects of sound-speed variability. A typological analysis of the acoustic fields in perturbed channels is proposed, based on a comparison between the set of "random modes" and of associated wavenumbers one hand, and the set of classical modes and associated wavenumbers on another hand.
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