474 



SEISMIC METHODS 



[Chap. 9 



D. Factors Affecting Elastic Properties of Rocks 



1. In igneous rocks, the rigidity, Young's modulus, and the velocity of 

 longitudinal waves all increase with a decrease in silica content; this be- 

 havior is in harmony with that of the silicate minerals and holds for both 

 intrusive and extrusive igneous rocks. 



Intrusive and coarse-grained rocks generally have a greater elasticity 

 than do extrusive rocks, since they contain less liquo-viscous matter. It 

 is possible that the difference is partially due to differences in porosity. 

 The variation of elasticity with degree and depth of crystallization is less pro- 

 nounced than its variation with silica content. 



2. Sedimentary rocks show marked differ- 

 ences in elasticity depending on petrologic 

 composition. Clastic sediments, such as 

 sands, sandstones, and shales, are less elastic 

 than sediments composed partly or wholly 

 of crystalline matter, such as limestones, 

 dolomites, and the like. Elastic properties 

 of sedimentary rocks depend much more on 

 texture and geologic history than on mineral 

 composition. 



The effect of porosity and decomposition 

 is to decrease the modulus of elasticity and 

 the wave velocity of a sediment. In areas 

 of great thickness of sedimentary rocks the 

 porosity decreases with depth. Therefore, 

 the modulus of elasticity increases and with 

 it the wave velocity. Related to changes of 

 porosity is the variation of Young's modu- 

 lus with pressure. For small pressures, 

 rocks appear to be more compressible, since 

 any cavities present have to be closed before the pressure can begin to act 

 on the rock matter itself. Excessive compressibilities resulting from poros- 

 ity are accompanied by high values of Poisson's ratio. It must be ex- 

 pected that the ratio of longitudinal and transverse wave speeds changes 

 considerably with depth in unconsoUdated sediments. The effect of poros- 

 ity on wave velocity is of practical importance in the near-surface layer, 

 inasmuch as the delay caused by the latter must be eliminated in reflec- 

 tion shooting ("weathered" or "aerated" surface layer). The thickness 

 of this layer is of the order of 5 to 50 feet, and velocities in it range from 

 500 to 2500 feet per second. Hence, wave speeds less than the speed of 



o /O lO' to' lO to 

 pro^crtlon a/r tt tartk h^ ymhtmt 



Fig. 9-18. Calculated velo- 

 cities of sound in air-earth mix- 

 tures of various proportions 

 (after Lester). 



" O. C. Lester, A.A.P.G. Bull., 16(12), 1230-1234 (Dec, 1932). 



