808 



KAFE AND DRAKE 



[CHAP. 29 



compacted sediments, the occurrence reported by Nafe and Drake (1957) of 

 refracted arrivals consistent with the supposition that they propagated over 

 part of their path a's shear waves, and the evidence of Oliver and Dorman 

 (Chapter 8) that higher mode Rayleigh- and Love-wave dispersion is affected by 

 sediments having a shear-wave velocity of 0.25 to 0.60 km/sec. Hamilton has 

 computed values of rigidity by attributing the discrepancy between observed 

 compressional velocities and those predicted by the Wood equation as arising 

 entirely from rigidity. Actually, an increment to the incompressibility may also 

 be a contributing factor so that Hamilton's values are in fact upper limits to the 

 rigidity. The available information is summarized in Fig. 5 on a graph of Pois- 

 son's ratio plotted against porosity. Poisson's ratio is related to shear velocity 



0.2 



0.4 0.6 



Porosity 



0.8 



1.0 



Fig. 5. Poisson's ratio plotted against porosity. Hamilton's values are upper limits to the 

 ratio. The solid points are those reported by Laughton (1957) for artificially compacted 

 Globigerina ooze. The region outlined by solid curves represents an estimate by Nafe 

 and Drake (1957) of the range of values for ocean sediments. 



through equation 5 (Table II). The solid curves were given by Nafe and Drake 

 (1957). They represent an estimated range based on an approximate theory of 

 the compaction process. The three solid points were derived by Laughton 

 (1957) from his measurements of compressional and shear velocities in com- 

 l)acted Globigerina ooze. Points obtained by Hamilton e^ai. (1956) are indicated on 

 the figure and are upper limits. The remaining points are measurements on 

 sedimentary rocks by Evison (1956), Hughes and Jones (1951), Birch et al. 

 (1942) and others. The range of average sedimentary shear velocities required 

 by Oliver and Dorman to fit their dispersion data is an indication that estimates 

 of (T by Nafe and Drake were too low for a given porosity. It is also likely that 

 Laughton's points are higher than would be found for sediments in situ where 

 cementation can occur. Thus the dashed curve may represent an approximate 



