BACKGROUND 



OBJECTIVE 



The objective of the program underlying this work is to determine the relative 

 percentage of positive gradient surface ducts versus downward refractive conditions for 

 shallow areas of strategic interest, determine frequency regimes of optimum propagation 

 corresponding to sound speed profile classification and correlate them with geographic 

 and seasonal distributions. 



RESULTS 



1 . Nine shallow water areas were examined for the presence of positive sound 

 speed gradients. Ninety percent of the fall-winter sound speed profiles and 3 1 percent of 

 the spring-summer profiles were of the positive gradient type. 



2. Positive gradient (winter) profiles generally resulted in at least 10 dB less loss 

 at 50 km range than did the negative gradient (summer) profiles. 



3. Three of the areas, North Sea, Strait of Juan de Fuca and Lands End, in which 

 propagation losses were calculated, had coarse-grained surface sediments resulting in small 

 bottom reflection losses and relatively good propagation by bottom reflected paths. In 

 these cases optimum propagation is in the 500 Hz frequency range. 



4. Optimum frequencies of propagation for positive gradient surface ducts 

 varied from 200 to 1000 Hz. 



5. When a surface duct exists in shallow water it can dominate the propagation 

 and lead to a completely different optimum frequency of propagation. However, high 

 bottom loss is required to demonstrate this effect. This is illustrated at the East of 

 Singapore site. 



6. A change in the magnitude of the compressional wave attenuation in the 

 sediment model produces a proportional change in the bottom reflection loss and can lead 

 to large changes in the calculated propagation loss. This was evident in the Straits of Sicily 

 calculations. 



7. Propagation by bottom reflected paths is a very sensitive function of sediment 

 types. Sediment models used here produced propagation ranging from very good to very 

 poor. Many details of this propagation in the frequency domain were unexpected. 



RECOMMENDATIONS 



1 . The prevalence of surface ducts in shallow water needs to be determined in 

 many more areas. 



2. The relative dominance of a surface duct when present in shallow water 

 propagation needs further clarification. 



3. Since bottom reflection loss is a large factor in shallow water propagation the 

 dependence of bottom loss on sediment properties, particularly attenuation, should be 

 determined more precisely. 



4. Include surface scattering in an assessment of surface duct versus bottom 

 reflected paths. 



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