PEDERSEN, GORDON, AND WHITE: SURFACE DECOUPLING EFFECTS 



depth in the surface decoupling region, the minimum H (minimum average 



o 



propagation loss) occurs for a receiver depth somewhat below the sur- 

 face decoupling depth. In this example the source depth is 100 yards 



and the minimum in H occurs 53 yards below the decoupling depth. 



o 



This feature yields an interesting prediction about sea noise from 

 distant shipping. Here the source of the noise is in the surface 

 decoupling region. Hence, the noisiest receiver depth should be some- 

 what deeper than the decoupling depth. Since decoupling depth depends 

 on frequency, the noisiest receiver depth should tend to increase with 

 decreasing frequency, other factors being equal. 



The surface decoupling loss, H , is also illustrated in Figure 4. 

 Note that H is a measure of the shape of H (Z) in the surface de- 

 coupling region. As such, it is also a reasonable measure of the 

 shape of H . 



The big advantage in the use of H and Z to characterize the 



S SD 



surface decoupling effect is that these quantities can be approximated 



by ray theory. These approximations exhibit the dependence on various 



parameters and allow us to obtain a quantitative feel for the surface 



decoupling effect. We now will consider ray approximations to H and 



Z . 

 SD 



Figure 5 presents three ray theory approximations to the surface 

 decoupling effect. Equations 1 and 2 apply to all approximations. 

 In Equation 2, oj is the angular frequency and At is the travel time 

 difference between the two ray paths which interfere to cause the 

 surface decoupling effect. The three approximations are obtained 

 by the use of various expressions for At and hence for x- In all 

 approximations the surface decoupling depth, Z , is that depth for 

 which X = tt/2 . At this depth, sin X = 1 ^^"^ the surface decoupling 

 loss, H , equals zero. 



5 70 



