PEDERSEN, GORDON, AND WHITE: SURFACE DECOUPLING EFFECTS 



understood. We plan a systematic investigation of those acoustic 

 characteristics that appear to be important from the standpoint of 

 application to sonar systems. This paper presents results of our 

 investigation of one of these characteristics, which we refer to as 

 the surface decoupling effect. 



This presentation is based on a project sponsored by NAVELEX 

 Code 320. This project is to make a definitive theoretical investi- 

 gation of the surface decoupling effect, which can be of vital 

 significance in low- frequency underwater sound propagation. The 

 results of this project are contained in an NUC technical publication 

 (Pedersen, Gordon, and White, 1976). About 10 percent of the material 

 in this publication has been extracted for presentation here. 



The surface decoupling effect is also referred to as the surface- 

 image effect or pressure-release effect. The effect produces high 

 propagation losses for a source or receiver near the ocean surface. 

 This effect is greatest at low frequencies, where ray theory is most 

 suspect. It is logical then to explore the concept of surface 

 decoupling from the standpoint of the more accurate normal mode 

 theory . 



Here the surface decoupling effect is inherent in the theory and 

 enters as the boundary condition that the acoustic pressure be zero 

 at the ocean surface. Although the mode theory is more accurate than 

 ray theory, it is difficult to extract the functional dependence on 

 such parameters as frequency, depth, profile characteristics, etc. 

 Using the approach of two interfering ray paths, the surface-decoupling 

 effect can also be developed by ray theory. Here the functional 

 dependence on the parameters is much more explicit. 



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