SECT. 3] PHYSICAL PROPERTIES OF MARINE SEDIMENTS 813 



known to exist. The equations given in the figure are not significant in them- 

 selves. They are simply convenient representations of the main trends of the 

 data. Velocities observed on the Blake Plateau, east of Florida, are higher than 

 those at corresponding depths of burial elsewhere in the shallow waters of the 

 North Atlantic. This is probably a consequence of the high carbonate content 

 of the sediments of the Blake Plateau and is consistent with the observation 

 that, for a given porosity, sediments of high carbonate content have higher 

 velocities than those of low carbonate content. Fig. 8 suggests, as do the 

 velocity-porosity data, that the zero porosity limit for the velocity of commonly 

 occurring sediments is near 6 km/sec. Additional evidence for the existence of 

 this approximate velocity limit may be seen in the trend of deep-water points 

 at depths of burial greater than about 1.5 km. The trend to much higher 

 velocities below 1.5 km may be a consequence of loading of the sediments by 

 the water column when water-filled interstices are isolated from each other and 

 do not have free access to the sea. 



Nafe and Drake (1957) have suggested that in deep water the principal con- 

 tribution to the increase of velocity with depth is that of compaction alone. 

 Many other processes can occur in a shallow-water environment such as circu- 

 lation of water, deposition of solid material in the interstices between particle 

 grains, and erosional unloading. All would tend to increase the compressional 

 velocity. Thus, in a shallow-water environment, velocity might well increase at 

 a greater rate with depth than in deep water. It is unlikely that the rate of 

 increase could be less. 



5. Conclusions 



Consideration of available observations together with data on sedimentary 

 rocks suggests that a known value of one quantity may limit the range within 

 which values of other quantities are likely to be found. Estimation of such 

 limits may be useful in determining the consistency of results of measurements 

 of different kinds. For example, densities to be used in gravity interpretation 

 have been assigned in accordance with seismic measurements of- velocity. 

 However useful, estimation is not measurement, and much more detailed 

 information is needed on marine sediments. The close connection between 

 density and porosity seems well established. Very little is known about shear 

 velocity or about attenuation at low frequency. Compressional velocities are 

 easily measured for near-surface samples but direct measurements of velocity 

 as a function either of density or porosity at points well within the compacting 

 sedimentary column are needed. Artificial comjDaction experiments suggest that 

 the trend of observations will be roughly that of the /i = 4 to n = Q curves of 

 Fig. 3. 



The value of a physical quantity is in a sense a function of the use for which 

 that quantity is required. A static determination of Young's modulus, as Evison 

 (1956) has pointed out, may differ widely from the Young's modulus obtained 

 dynamically through substituting into equation (4) constants found by mea- 

 suring compressional and shear velocities. Indeed, the dynamic value of Young's 



