SEISMIC METHODS 659 



y^ for materials having good elastic properties.* Hence, the velocities of the 

 elastic waves depend almost entirely upon the ratio of the Young's modulus 

 to the density of the material. Also, it is evident from Equations 9 and 9a 

 that the velocity of the longitudinal wave is greater than that of the trans- 

 verse wave.** 



The velocities of longitudinal waves in various materials found in the 

 earth's crust are summarized in Table 20.*** The table shows that the 

 value of the velocity for a given material varies over a considerable 

 range. This occurs because the composition, porosity, and water content 

 affect the wave velocity appreciably. § In addition, the experimental 

 results show that the velocity varies markedly with the depth of the 

 formation. 



The variations in velocity which are most commonly encountered may 

 be summarized as follows : ( 1 ) Within a single layer of homogeneous 

 material, the velocity generally increases slowly with depth. (2) The 

 velocity is usually greater in deeper layers, although many exceptions to 

 this rule exist. (3) In many regions, the increase of velocity with depth 

 is approximately linear. 



Fig. 408. — Sketch illustrating the propagation of 

 elastic disturbances in an elastic medium. 



Seismic Wave Paths. — For simplicity consider that the waves 

 originate at a point source located in a homogeneous isotropic medium. 

 The elastic disturbances which travel outwardly from this source are 

 spheres with the source as center. (Figure 408.) Any one of these spheres 

 is a wave front, i.e., a surface such that all points on it are vibrating in 



*A great many deposits found in strata close to the surface of the earth are 

 poorly consolidated. Hence, their elastic properties are not very good and their a value 

 may be much less than Y^. 



** For the rocks composing the earth's crust, this ratio is found to be of the order 

 of 1.6. 



*** A table giving the longitudinal and transverse wave velocities, densities, and 

 Young's moduli for laboratory specimens is given by A. Sieberg, Geologische, 

 physikalische und angewandte Erdbebenkunde (Gustav Fischer, Jena, 1923), p. 171. 

 S Gutenberg, Wood, Buwalda, loc. cit., p. 213. 



