NORTH SEA 



10 15 20 



GRAZING ANGLE, deg 



Figure 4.22. Bottom reflection loss vs grazing angle for the four sites at frequencies 

 of 200 and 1000 Hz. 



The East of Singapore summer profile has its best propagation at frequencies 

 between 1500 and 3000 Hz (Figure 4.21). This propagation is due to a weak refractive duct 

 in the sound speed profile (see Figure 3.6) with a sound axis depth of 30 m. The source at 25 m 

 is just out of this duct and energy reaches it by diffraction. Thus, the propagation here is 

 a sensitive function of source depth. (In these calculations, receiver depth is optimized, 

 i.e., taken at the depth of maximum propagation, which in this case was near 30 m depth). 



Of these four areas, East of Singapore best illustrates the phenomenon that led to 

 the initiation of this investigation: when a surface duct exists in shallow water, it dominates 

 the propagation and leads to a completely different frequency of maximum propagation. 

 It is now apparent that high bottom loss is required to display this effect fully. In this area 

 the high bottom loss and the small water depth (50 m) combined to produce a bottom 

 reflected propagation loss that is entirely off the scale of Figure 4.2 1 . A range less than 

 50 km would be required to show the nature of the bottom reflected propagation. 



57 



