ANDERSON: BOTTOM PROPERTIES FOR LONG-RANGE PROPAGATION PREDICTION 



We see the indicated interference structure in the variation of 

 bottom loss with grazing angle. Why do this? If we extend the depth 

 of the layer, we will see to what depth we must go before we begin to 

 lose the effect of the clay/sand interface. 



Figure 11 shows the results when the intermediate layer is 200 

 meters thick. The interference structure is reduced, but still 

 present. Figure 12 shows results for a layer thickness of 500 meters. 

 Figure 13 is for 1,000 meters. The interference structure is gone. 

 Thus, for a layer of this thickness and the assumed attenuation, the 

 Rayleigh reflection-coefficient model indicates that the lower interface 

 with a sand layer does not influence the bottom loss. 



Doing this for the same type of clay overlying basalt, where we 

 have a considerable impedance contrast between the clay and the basalt, 

 the following results are calculated. 



In Figure 14, the highly variable curve is for 70 meters of clay 

 overlying basalt. The smooth curve is for a 1,000-meter thick clay 

 layer over basalt. 



These results indicate that if the reflection model used here 

 were valid for bottom regions described by the parameters assumed 

 here for the clay layer, and if we knew the information about the 

 sediment column to 1,000 meters depth, we wouldn't have to know 

 anything about it from there on down. 



Also, we have seen instances where there is considerable energy 

 return from 2 to 3 kilometers. The result described above is critic- 

 ally dependent on the value of attenuation that is used for the layer. 

 Also, the model used for the calculations does not include gradients 

 in the layers. This work is being extended to include gradients in 

 the bottom. 



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