Propagation loss was then plotted as a function of frequency for ranges of 50, 100 

 and 1 50 km. The curves show a minimum loss over a frequency range which is sometimes 

 relatively broad. This indicates the optimum frequency of propagation for the conditions 

 of the sound speed profile and the sediment properties of that site and season. For the case 

 of downward refraction, the optimum propagation occurs at a relatively low frequency, at 

 which a mode exists with small bottom reflection loss and a large loop length. The loop 

 length is the dominant factor. If a surface duct exists, propagation in the duct usually 

 overpowers the bottom reflected propagation. In this case, the optimum frequency of 

 propagation occurs when the surface duct is optimized or where the propagation is driven 

 by one or two modes. The frequency is usually much higher. 



3. SITE DATA 



This section contains the physical descriptions and a summary of the profile statis- 

 tics for each site. The representative sound speed profiles and bottom sediment charac- 

 teristics are given for six of the nine areas. Propagation loss calculations were made for 

 those areas. 



3.1 North Sea 



The local area at the North Sea site, 58.5°-59°N, 1.5°-2.5°E, is shown on the 

 bathymetric chart in Figure 3.1. The area generally south of the 100 m contour is a flat 

 plateau at water depths between 80 and 100 m. In the area above 57.5°N, the sea floor 

 slopes into a flat basin below 150 m (maximum depth about 159 m). To the north and 

 west, the sea floor is more hilly and gradually deepens to the north above 59°N. At the 

 site, the average depth is 100 m. 



Figure 3.2 shows characteristic sound speed profiles for the North Sea site. The 

 winter profile has positive gradients from surface to bottom as do all of the representative 

 winter profiles (for each site). The "intermediate" profile was of a type observed in about 

 20 percent of the spring profile samples and occasionally in the summer profiles. The 

 summer profile has small negative gradients to 20 m. Table 3.1 summarizes the North Sea 

 sound speed profiles and shows the relative numbers of profiles that were classified as 

 positive or non-positive gradient profiles. 







Positive 



Positive 



Season 



Total 



Gradient, # 



Gradient, % 



Fall 



89 



82 



92.1 



Winter 



109 



103 



94.5 



Totals 



198 



185 



93.3 



Spring 



193 



64 



33.2 



Summer 



173 



42 



24.2 



Totals 



366 



106 



28.7 



Table 3.1 . North Sea profiles. 



