SURFACE DECOUPLING EFFECTS 



M. A. Pedersen, D. F. Gordon, and D. White 

 Naval Undersea Center 



This paper presents recent theoretical progress on the 

 surface decoupling effect, which produced high propaga- 

 tion loss for source or receiver near the ocean surface. 

 The effect is greatest at low frequencies where ray 

 theory is most suspect. Thus, the effect is approached 

 from the more accurate normal mode standpoint. These 

 normal mode investigations have provided some signifi- 

 cant new results. They predict that the highest levels 

 for distant shipping noise should occur just below the 

 surface decoupling region. They also predict that under 

 conditions of severe near-surface gradients, ray theory 

 computations for near-surface sources and receivers are 

 in serious error in that the maximum convergence zone 

 intensities of mode theory can occur in the ray theory 

 shadow zones. Two new ray theory approxima'-- ons to 

 surface decoupling loss have been developed and are 

 compared with normal mode results. These new approxi- 

 mations represent a significant improvement over the 

 old iso-speed approximation available in the past. 



The work reported here is part of a continuing NUC effort to 

 utilize normal mode theory to investigate low- frequency propagation 

 in the deep ocean. Starting in 1954 we applied this mode theory to 

 propagation in surface channels (Pedersen and Gordon, 1966). In the 

 late 1960s this model was extended to examine propagation in the 

 deep ocean. A computer program was developed by Dave Gordon and is 

 referred to as the NUC n- layer normal mode program (Gordon, 1971A) . 

 This program has been used in a variety of applications (Gordon, 

 1971B, and Gordon, 1972) . Detailed acoustic fields as calculated 

 by this program have exhibited a number of new characteristics of 

 importance in the understanding of low-frequency propagation. Some 

 of these characteristics have ray theory analogs. Others are in 

 complete variance with ray theory and the mechanisms are not 



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