454 SEISMIC METHODS [Chap. 9 



Table 44 is given to aid in the conversion of these units. The units based 

 upon the acceleration of gravity are referred to its value in 45° latitude 

 (980.665 cm-sec~^) by international agreement. 



Although, in the technical system of this country, elastic moduli are 

 usually expressed in pounds per square inch, the barye, or dyn cm~^, will 

 be used in the following sections, since the parameters expressed in the 

 C.G.S. system may be more readily converted into velocities of longi- 

 tudinal and transverse waves. Wave speeds are usually given in meters 

 or kilometers per second, although in seismic prospecting feet per second 

 has been widely adopted. Several of the elastic constants are not inde- 

 pendent quantities but may be calculated from one another. If Young's 

 modulus and Poisson's ratio have been determmed, the compressibilitj'^ 

 may be calculated. On the other hand, the compressibility may also be 

 measured directl3\ If the two values differ, it indicates a deviation from 

 isotropy and homogeneity. Furthermore, a comparison of values deter- 

 mined statically and dynamically in the laboratory, with those calculated 

 from velocity determinations, is of value. 



Methods for the determination of elastic constants may be divided into 

 laboratory and field methods. The latter are dynamic methods, since they 

 involve the measurement of time, while laboratory determinations may 

 be either static or dynamic or both. In the static determinations stress- 

 str9,in relations are established, while dynamic measurements are based on 

 observations of niatural frequencies. Equipment for testing elastic prop- 

 erties of construction materials has been developed to a high degree of 

 perfection. For the measurement of deformations the following devices 

 are used: (1) extensometers, (2) d^fledometers, and (3) detrusion meters; they 

 may be (a) mechanical, (6) optical, or (c) electrical. 



Mechanical devices are generally not accurate enough for geophysical 

 application. Closest in this respect are some of the more delicate types 

 of Ames gauges, for which an accuracy of 0.000025 inch is claimed. 



The simplest optical devices for measuring extensions or deflections, 

 microscopes, and cathetometers, are generallj'^ not accurate enough for 

 rock testing. Rocking mirror arrangements, however, give satisfactory 

 results. In the Martens mirror extensometer two rocking mirrors are 

 clamped to opposite sides of a specimen. These are rotated when the 

 specimen is extended (see Fig. 9-7) and are observed by separate telescopes. 

 If the arrangement is so modified (Fig. 9-8) that the light travels from one 

 mirror to the other, more magnification is obtained. If yl is the distance 

 from scale or telescope to the nearest mirror, D their distance apart, and n 



• Comprehensive descriptions may be found in J. B. Johnson's Materials of Con- 

 struction, Chapter II, John Wiley (1930), and in C. H. Gibbons, Materials Testing 

 Machines, Instruments Publishing Co. (Pittsburgh, 1935). 



