ANDERSON: BOTTOM PROPERTIES FOR LONG-RANGE PROPAGATION PREDICTION 



some K' times frequency to the first power — and we examine data that 

 Hamilton (1974) has presented on attenuation for Pacific sediments and 

 data that Smith has presented for attenuation in Atlantic sediments, 

 then we find that we can plot the values of the coefficient K' versus 

 the mean grain size of the sediment, Figure 8. The resulting rela- 

 tionship will help us select the K' to be extrapolated as a linear 

 function of frequency. 



Other bottom parameters in the geoacoustic model include bulk 

 wet density, which is usually measured with samples, and shear-wave 

 speed which has been measured only in a very limited manner. Bucker 

 (1974) appears to be one of the few who has actually made these 

 measurements. He measured Stonely waves and interpreted them in terms 

 of velocity of propagation of shear waves. 



The answer to the question raised earlier about whether these 

 sediments behave as liquids or as solids depends on what you mean by 

 the question. If the question is "Do shear waves propagate?" the 

 answer depends on whether there is a finite value of dynamic shear 



modulus. Values of dynamic shear modulus have been measured in most 



8 9 

 ocean sediments somewhere on the order of 10 to 10 dynes per square 



centimeter. Propagation speeds of the shear waves are something on 

 the order of a tenth of the value of propagation speeds for the longi- 

 tudinal waves. 



In near-shore sediments, very high porosity sediments, harbors 

 and lagoons, we find even lower values of shear modulus. The lowest 

 values of dynamic shear modulus are exhibited by freshly mixed, pure 

 laboratory clays like Kaolinite, for which values of less than 10 

 meters per second are predicted for shear-wave speeds from measured 

 values of dynamic shear modulus. 



311 



