78 



EWING AND NAFE 



[chap. 5 



If a reflecting interface is shallower than the depth of penetration of the 

 critical Ta,y, the Rn-Ri ciu've approaches and joins the Rs-Ri curve from below 

 as demonstrated by the arrivals of profile B in Fig. 2. For this profile Rio = 

 7 sec and the best fit to the data is with the curve for K =1.0 sec~i. 



Fig. 4 shows the results from another profile for which i?io = 6 sec and 

 K = 0.9 sec~^ There are two reflecting interfaces, both shallower than the depth 



I 



or 



Fig. 3. R^— Ri versus D for case where 2-3 interface is deeper than depths penetrated by 

 purely refracted ray. 



of penetration of the critical ray, as evidenced by the fact that both reflection 

 curves pass below the cusp of the refraction curve for iC = 0.9 sec~i. From the 

 fact that both curves indicate approximately the same value of K, it would 

 appear that the upper reflector is a thin layer, above and below which the velo- 

 city and velocity gradient are not greatly different. This can be stated only 

 qualitatively because the precision of picking the arrivals in the region where 

 the curves come together is not high. 



0.8 -•-/^ 



0.6 

 I 0.4- 



ct 



0.2- 



R n Hr 



10 

 D 



12 



14 



16 



18 



20 



Fig. 4. /t's- /?i versus D for gradient 0.9 ser-l, i?jQ = 6 sec and two reflecting interfaces. 



In the profiles shown in Fig. 1, the gradient in Layer 2 is found by the 

 methods just described to be approximately 0.5 sec^^. There is some evidence 

 that the velocity continues to increase with depth below the 2-3 interface. 

 This comes from the relationship of the Rn curves and the G3 lines. Although 

 nearly so, the observed portions of the refraction lines are not tangent to the 



