Chap. 9] 



SEISMIC METHODS 



475 



sound in air are possible in this formation. On the assumption that 

 the layer is a liquid mixture of air and earth, the sound velocity 



Vi = 



Mi/lhiZ 



y [pE2 + (1 - p)Ei][p6i + (1 - p)52]' 



(9-32) 



where El = elasticity of air (1.2 10^);E2 = elasticity of earth; p = pro- 

 portion of air to total by volume; 5i = density of air (0.0012) ; 52 = density 

 of earth; and 1 — p = proportion of earth to total by volume. With a 

 Young's modulus of 5.58 10^" and a density of 1.9, the curve shown in 

 Fig. 9-18 has been computed. 



The effect of moisture, or water content, on the velocity in sedimentary 

 beds is rather involved. In consoHdated beds (sandstones, limestones, 

 slates, schists, porous igneous rocks, and the like) moisture appears to 

 decrease the velocity; in unconsolidated beds moisture increases the ve- 

 locity appreciably. In reflection work practical use is made of this increase 

 in velocity (and improvement in the transmission characteristics) by 



Oroond water 

 left/ 



Fig. 9-19. Location of ground water by refraction shooting {T.T. = travel time). 



(After data from Rieber.) 



placing the shots in or below the ground-water table. In California 

 Rieber observed distinct breaks in near-surface travel-time curves on the 

 ground-water (see Fig. 9-19). 



Many observations of elastic wave speeds appear to indicate a direct 

 relation between geologic age and elasticity. However, the controlling 

 factor is the amount of diastrophism to which a formation has been sub- 

 jected in its 'geologic history. An increased age merely increases the 

 probability that it has undergone a greater degree of dynamometamor- 

 phism. As a consequence velocities in geologic formations change less 

 with depth of burial the greater their geologic age (see Fig. 9-20). Ce- 

 mentation of clastic sediments by mineral solutions during their geologic 

 history is likewise of considerable influence upon their modulus of elas- 

 ticity. 



It follows from the above that metamorphic rocks have an increased 

 elasticity compared with the rocks from which they were derived. Fur- 

 thermore, their elastic constants are different in the direction of texture 



^* See Lester, loc. cit. 



