SEISMIC METHODS 



671 



by utilizing the concept of images. f In Figure 413 the image of the shot- 

 point is located at 0'. The ray path OBj^Sk, where Sk is the position of 

 any seismometer, is equivalent to the path O'BkSk, which is straight and 

 in a medium of constant velocity Vi. The response at the seismometers is 

 entirely unchanged by the substitution of an image shot-point. 



V, • 6000/SECONDS 



Fig. 414. — Ray paths and travel-time curves for rays reflected from a 

 sloping surface. 



In the case of a sloping interface between the two strata (Figure 414), 

 the image shot-point is located at 0', where 00' = 2h. The movement of 

 the wave emanating from 0' is indicated by a succession of arcs of circles, 

 whose origin is at 0'. Each arc designates the position of a particular 

 phase of the wave at a particular time, and therefore each arc constitutes 

 a portion of a wave front. 



t See also Z. Specht, "Problems of Inclined Layers in Seismic Reflection Methods," A.I.M.E. 

 Geophysical Prospecting, Tech. Pub. 1177, April, 1940. 



