A feature in Figure 4.21 is the very low propagation loss for the North Sea summer 

 profile near 250-600 Hz. This results from an unusual bottom reflection loss curve as 

 shov/n in Figure 4.23. For grazing angles less than about 1 degrees, an interval of almost 

 perfect reflection can be seen for the frequencies of 250 and 275 Hz. Several modes occupy 

 this interval and propagate with negligible loss. The source depth will strongly determine 

 how these modes will enter the propagation. The peak in Figure 4.21 is at 275 Hz. This 

 discussion also helps to explain the sudden decrease in loss in Figures 4.2 and 4.3. 



The bottom model for this site (North Sea) has a 2 m depth of sand as the first 

 sediment layer. The sound speed in this sand layer is very high and the critical angle is 

 about 33 degrees grazing. However, either its shallow depth (2 m) or large absorption 

 causes the rapid increase in reflection loss near the 10 degrees grazing angle. At lower 

 angles nearly perfect external reflection occurs. However, at high frequencies near 1 kHz, 

 the loss becomes more in line with the other areas. 



0.0 



NORTH SEA 

 _ / /225 Hz 



1 



275 Hz, 250 Hz 



^250 Hz 

 ^ 21b Hz 



1 1 1 



D 5 10 15 20 25 3 



GRAZING ANGLE, deg 



Figure 4.23. Bottom reflection loss vs grazing angle for the North Sea site 

 and frequencies of 225, 250 and 275 Hz. 



58 



