Causes and Implications of Instability of Individual Acoustic Paths in the Ocean

Abstract

The sample data of the preceding paper (H8) indicate that long-range underwater transmission may have, at least sometimes, an inherent long-term instability of coherence. The fluctuations must be explained in terms either of changing conditions at the air-water interface or of changes in the velocity structure. From the frequency spectrum of the coherence instability, it does not appear likely that the former factor can be important. Ray calculations have been made to explore the effect of differential changes in velocity structure. Results are presented showing how the geometry of individual ray paths can be altered by small changes in certain of the oceanographic variables. The interest here is to appraise the influence of fluctuations in the physical state of the ocean, rather than to seek underlying causes as to why the ocean behaves the way it does. Regardless of our ability to understand and to interpret coherence fading, there is no doubt that it may exert a major influence on underwater acoustic systems involving long paths. Some hypothetical illustrations are given of possible degradation in system performance introduced by fluctuations in coherence. [This work was supported by the Office of Naval Research.]