Chap. 9] 



SEISMIC METHODS 



457 



1. Laboratory methods, (a) Static determinations involve measurements 

 of stress-strain relations, utilizing tension, compression, bending, or torsion. 



Tension and contraction tests: Samples are used in the form of long bars 

 or drill cores in any of the Olsen, Riehl6, or Emery machines. The strains 

 may be measured by rocking-mirror devices, interferometric gauges, or 

 condenser-microphone attachments. The contraction is obtained by 

 means of a lever device measuring the reduction in diameter with two 

 contact arms, provided with mirror magnification. From the load P, the 

 area S, the original length I, and the change in length 

 Al, Young's modulus may be calculated : 



E = 



Poisson's ratio is: 



P I 



s'm' 



Ad 

 d^ 



M' 

 I 



[9-3] 



[9-46] 



Fig. 9-12. Westing- 

 house magnetic strain 

 recorder. (0, oscillo- 

 graph; R, rectifier; P, 

 potentiometer.) 



These two constants are sufficient for isotropic media 

 to calculate all other elastic coefficients and, together 

 with the densities, the velocities of the longitudinal 

 and transverse waves. 



Compressibility tests: Although the compressibility 

 may be calculated from Young's modulus and Pois- 

 son's ratio, it is preferable to measure it separately. 

 For most compressibility tests the sample is used 

 in some liquid which is compressed in a strong steel cylinder. A correction 

 must be applied for the expansion of the steel cylinder and for the compres- 

 sion of the liquid. Bridgman used chrome- vanadium steel for the cylinder; 

 Adams used n-butyl-ether for the liquid. A simple way of determining 

 the volume change is to measure the movement of the piston compressing 

 the liquid. The distance the piston should move if the liquid alone were 

 in the cylinder may be calculated from the dimensions of the steel cylinder 

 and the compressibility of the liquid, or it may be measured directly. 

 Since the volume of the sample is also reduced, the difference of the piston 

 movement without sample and the movement with sample gives the 

 volume reduction AV of the specimen. Then the compressibility, 



K = 



AV 

 V'P' 



[9-10] 



