SECT, 3] 



PHYSICAL PROPERTIES OF MARINE SEDIMENTS 



805 



from a variety of sources. In Fig. 3 observations are distinguished according to 

 sediment type. It wiU be noted that most of the observations fall within the 

 n = 4: to n = 6 range defined by the equations of Nafe and Drake. Since these 

 equations roughly take into account the effect of grain-to-grain contacts, they 

 do not fit the distribution of observations on uncompacted samples. On the 

 other hand, they do outline the main trend of points and in particular fit well 

 to trajectories on the F-^ graph of artificially compacted sediments. It is clear 



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Fig. 3. Compressional velocity-porosity data for sediments and sedimentary rocks com- 

 pared with empirical equations of Nafe and Drake. 



from Figs. 1 and 3 that many samples can be found for which the observed 

 points lie close to the Wood equation even at low porosity. Observations suggest 

 that initial distribution on the V-(f) plot is governed by the Wood equation but 

 that subsequent compaction and cementation have the effect of displacing the 

 points upward along the direction of the curves shown in Fig. 3. 



The occurrence of measured velocities less than water velocity is now well 

 documented. Katz and Ewing (1956) found evidence of a low-velocity layer at 

 the top of the sediments on twelve seismic refraction stations in the Atlantic 



