PEDERSEN, GORDON, AND WHITE: SURFACE DECOUPLING EFFECTS 



4) Comparison between these ray theory approximations and 

 mode theory 



5) Functional dependence of surface decoupling on important 

 parameters as determined by ray theory. 



Figure 1 presents iso-propagation loss contours in the range- 

 receiver depth plane. The receiver depth is shown from the surface 

 to 1,000 yard depth and the range interval from 1,000 to 1,080 kyd. 

 At this range the convergence zones overlap, although there still 

 remains a structure which is cyclical with range. The important 

 feature is the marked increase in loss near the surface, i.e., at 

 receiver depths shallower than 200 yards. All normal mode contour 

 plots show this increased loss near the surface at short as well as 

 long range. 



Figure 2 presents plots of propagation loss versus range for 

 various receiver depths. The shallowest receiver depth is 5 yards 

 and the receiver depths are progressively doubled to 320 yards. The 

 region to the left represents the direct field. The region centered 

 at about 65 kyd range is the first convergence zone. The high loss 

 region centered at about 30 kyd represents the "shadow-zone" region. 

 Since bottom bounce propagation is not included in this computation, 

 this high loss region is ensonified by various diffraction mechanisms. 



For the conditions of Figure 2, the surface decoupling region is 

 shallower than 206 yards. Consider the six solid curves of Figure 2 

 for receivers in this region. The surface decoupling effect can be 

 seen as a monotonic decrease in propagation loss with increasing depth 

 and also as a repetitive shape in the loss curve. This repetitive 

 shape can be explained by normal mode theory, but is too involved for 

 this presentation. For the near-surface receivers doubling the depth 

 decreases the propagation loss by 6 dB. This will be explained later. 



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