ANDERSON: BOTTOM PROPERTIES FOR LONG-RANGE PROPAGATION PREDICTION 



Shear wave attenuation measurements are few. Some measurements 

 of the complex dynamic shear modulus allow prediction of a shear wave 

 attenuation. 



What does all this have to do with bottom loss and with propaga- 

 tion loss at low frequencies? What is the sensitivity of this thing 

 we call bottom loss (which is an input to ray -theory models) to 

 variations in sediment parameters? 



The simplest reflection models, using a liquid layer without any 

 attenuation, a single layer overlain by water, can fit some of the 

 things that we see in Figure 9. Judicious selection of the sound- 

 speed ratio can make the critical angles fit, and juggling the density 

 ratio can cause the bottom-loss values at normal incidence to fit. 

 Unfortunately, when this is done, the grazing -angle segment just above 

 the critical angle does not fit these data. This seems to indicate 

 that the single bottom layer is far too simple a model. Disagreement 

 is not as bad as one might expect. The important thing is that this 

 shows realistic values of speed of propagation and of density for 

 bottom sediments. One problem, however, is that some of the bottom 

 loss obviously is going to be contributed by topographic effects which 

 are not included here. 



Figure 10 shows the results for a water layer overlying a two- 

 layer bottom. This three-layer model, with a clay overlying sand in 

 the bottom, is shown merely to indicate the type of variation that 

 is shown at 100 Hz for a value of attenuation obtained by the extrapo- 

 lation process mentioned earlier. The sound speeds are 1,501 meters 

 per second in the water, 1,531 meters per second in the clay, and 

 1,657 in the silty sand, with realistic values for density and with 

 a 100-meter thickness for the clay layer. 



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