460 



SEISMIC METHODS 



[Chap. 9 



3. Earthquake waves. In the preceding section it was stated that in an 

 isotropic elastic body only two types of waves exist, longitudinal and 

 transverse waves. While the observed waves are actually longitudinal and 

 transverse in character, there are a number of t3T)es in addition to the two 

 mentioned. There is virtually but one longitudinal wave and several 

 kinds of transverse waves. The explanation is that the above theory con- 

 siders a volume element within an unbounded elastic solid and is not 

 strictly valid for surfaces between media. It is readily seen that the trans- 

 verse waves should be the ones to be influenced by such conditions, since 

 they have an arbitrary direction of oscillation in a plane at right angles to 

 their direction of propagation. This plane should depend, in its orienta- 

 tion and other characteristics, on the orientation and elastic characteristics 

 of a geologic or physical boundary. 



Raiflti 



eai//iigo{& Love (Q) 



intidtnt 

 lonaitudinal 



Fig. 9-5. Wave types on boundaries. 



When a wave consisting of longitudinal and transveree impulses strikes 

 a boundary, theoretically no less than twelve new wave types are pro- 

 duced. As shown in Fig. 9-5, two refracted (longitudinal and transverse) 

 and two reflected waves are produced by the longitudinal wave. The 

 same is true for the transverse wave, so that in this manner alone eight 

 new waves are accounted for. Further, each wave may generate the Love 

 wave, Q, which is a special type of transverse wave with its plane of 

 oscillation in the formation boundary, and the Rayleigh wave, R, which 

 is a combined longitudinal and transverse wave with plane of oscillation 

 at right angles to the surface and parallel to the direction of the propaga- 

 tion. The last two waves are frequently observed at the earth's surface; 

 it is probable that the so-called "ground roll" observed in reflection seis- 

 mology is of the Rayleigh type. 



Instead of distinguishing between longitudinal and transverse waves it 

 has become the custom in seismology to speak of preliminary and surface 



