Calculated minimum propagation losses as a function of frequency are shown in 

 Figure 4.16 (winter profile) and Figure 4.17 (summer profile) for the 165 m water depth. 

 The propagation losses in the positive gradient case (Figure 4.16) are much less (10-15 dB 

 at 100 Hz) than those in the downward refraction case (Figure 4.17), especially at the 

 ranges of 100 and 1 50 km. However, at the 50 km range and optimum frequencies of 

 propagation, 250 Hz for the winter profile and 650 Hz for the summer profile (downward 

 refraction), the difference is only 5 dB. 



Figure 4.18 (winter profile) and Figure 4.19 (summer profile) show the minimum 

 propagation loss calculated for ranges of 50, 1 00 and 1 50 km for water depth of 500 m. 

 Optimum frequency of propagation for the winter profile is about 300 Hz, nearly equal 

 to that for the 165 m depth. For the downward refraction case (summer profile) the 

 optimum frequency of propagation is about 40 Hz. The propagation losses for the summer 

 profile are much greater than those for the winter profile. At 100 Hz and the range of 100 

 km, the difference is 20 dB. At 200 Hz and 50 km, the propagation loss is 15 dB greater 

 in the downward refraction case. 



Except at low frequencies (less than 100 Hz), the propagation losses for the 165 m 

 water depths (shallow) are less than the losses for the 500 m depth. For the winter profiles 

 (Figure 4.16 and Figure 4.18) the propagation losses are 10-20 dB less for the shahow 

 case at frequencies above 500 Hz. Comparisons between Figures 4.17 and 4.19 (summer 

 profiles) show even greater differences. Propagation losses are 10-60 dB less at 50 km for 

 the 165 m depth. At frequencies below 100 Hz, the propagation loss curves for the 165 m 

 depth and for 100 and 150 km ranges show greater loss with decreasing frequency so at 

 50-70 Hz the propagation losses are nearly equal at the 100 km range and are less at the 

 1 50 km range for the 500 m water depth. In these cases the optimum receiver depth was 

 near the bottom. 



4.2 Propagation Loss Comparisons 



For purpose of discussion and illustration, comparisons between the first four sites 

 will be discussed as a group. Also, a natural division arises due to the questionable results 

 for the Korea Strait and to the two water depths and two sediment models in the Straits 

 of Sicily. 



4.2.1 Comparison Among Four Areas 



Figure 4.20 compares the propagation losses calculated for the winter (positive 

 gradient) sound speed profiles for four areas, the North Sea, Lands End, Strait of Juan de 

 Fuca and East of Singapore sites, at the range of 50 km. Figure 4.21 is a similar comparison 

 for the summer profiles. With the exceptions noted below, the propagation losses for the 

 winter profiles generally are about 10 dB less than for the summer profiles. The East of 

 Singapore site is very different from the others as the result of a much greater bottom loss. 

 The sediment here is a fine grained clay with a sound speed less than that of the bottom 

 water, which is very different from the silt and sand sediments of the other three areas. 



Figure 4.22 shows bottom reflection losses for each area at two frequencies, 200 

 and 1000 Hz. These losses were determined by the normal mode program at the grazing 

 angles corresponding to the modes. Smooth curves have been drawn through those points 

 and arbitrarily brought to zero loss at zero grazing angle. The much greater bottom losses 

 for the East of Singapore site are readily seen in this figure. 



53 



