SECT. I] 



SEISMIC REFRACTION AND REFLECTION MEASUREMENTS 



11 



', i"^ ''}^^1> 



Fig. 6. Ray diagrams defining angles used in computing velocity and thickness for sloping 

 layers. 



can compute C2 and a; 12. Using these additional values in the next step allows 

 us to compute C3 and 0123, and so on for each layer. 



For computing the thickness of each layer we again follow a stepwise process 

 using the intercept times taken from the time-distance graph. Fig. 7 shows 

 the fictitious ray paths associated with the intercept times, i.e. the travel 

 times for zero shot-detector distance. This figure is derived from Fig. 6 by 

 simply moving point B to point A and considering that the rays originating 



Fig. 7. Diagram of fictitious ray paths associated with intercepts. 



at B move also. Obviously these paths are not real, since a finite range exists 

 at which a refracted wave can be returned to the surface ; but they have a 

 mathematical significance in these computations. When point B is moved a 

 short distance toward point A, the total path is shortened principally in the 

 portion where the ray follows an interface. In Fig. 7 these portions of the ray 

 paths are, in effect, negative distances. Thus the intercept time associated with 

 a j)articular interface amounts to the sum of the travel times along each 



