ANDERSON: BOTTOM PROPERTIES FOR LONG-RANGE PROPAGATION PREDICTION 



parameters with depth. Effects of rough interfaces have been studied 

 by Horton (1976). All of these require some type of geoacoustic model. 



GEOACOUSTIC MODEL 



A geoacoustic model may be described as a quantitative descrip- 

 tion of the pertinent sediment and water parameters, particularly the 

 former. This description includes at least the following: 



• Layering that exists and depths of these layers 



• Compressional-wave speed and attentuation 



• Shear-wave speed and attenuation 



• Density 



• Gradients, if they exist, of speed and density 



• Bottom topography. 



BOTTOM- LOSS VALUES 



Figure 2, an example of an empirical model, shows the low- 

 frequency bottom loss versus grazing angle model. There are several 

 notable features of these curves. For the lower three curves, the 

 bottom loss goes to zero between 10 and 20 degrees. This feature 

 indicates a critical angle effect which implies no attenuation in the 

 sediments. But when you put attenuation in, you don't see this zero 

 bottom loss except at zero grazing angle. For the two higher bottom 

 loss curves, we see that the loss does not go to zero even at zero 

 grazing angle and this implies considerable influence of topography 

 in these two classes of the empirical model. 



Figure 3 is a mean bottom -loss curve (Urick, 1974) . Observe 

 some differences from numbers in Figure 2. 



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