CONTINENTAL TERRACES AND SUBMARINE VALLEYS IO7 



floor. The interval between the instant of explosion and in- 

 stant of arrival at the geophone station is called the travel-time 

 of the wave. The length of the travel-time depends on the 

 elastic properties of the material traversed — its compressibility 

 and rigidity — and on the path followed by the wave. The path 

 may be of two kinds, that of reflection or that of refraction. 



Reflection occurs if at any deep level there is a sharp change 

 in the elastic properties of the rock. In this case the seismologist 

 can use the travel-time to measure with fair accuracy the depth 

 of the "discontinuity" or change of character of the deep-lying 

 rock. 



The alternative, refraction, method may be illustrated by 

 Figure 57, where "shot point" is the place of explosion and 



SHOT POINT RECORDING POINT 

 X 

 > >— 



FIGURE 57. PATHS OF SEISMIC WAVES FOR THE REFRACTION METHOD. 



"recording point" is the location of the geophone. The di- 

 agram represents four layers of material, these chosen accord- 

 ing to the general rule that the elasticity of rock matter 

 increases with depth below the surface of the earth. The paths 

 of four different waves are marked with arrows. One follows 

 the surface. A second plunges to the first break of material, to 

 be refracted at this discontinuity, then to run some distance in 

 the top part of the second layer, and finally to be refracted 



