540 



SEISMIC METHODS 



[Chap. 9 



of intersection with the Vo fronts. The second interface is tangent to 

 these two contact curves. 



Wave-front interpretation may be used not only for an analysis of 

 simple travel-time curves but also for more complicated types of struc- 

 tures, such as faulted strata, salt domes, and the like. A number of 

 examples of the application of wave-front methods to such types of struc- 

 tures may be found in Thornburgh's and Ansel's articles previously re- 

 ferred to. Fig. 9-71 illustrates travel-time analysis by wave fronts for 

 a sloping terrace. 



i^— J 



Fig. 9-72a. Curved-ray paths. 



(c) Curved-ray interpretation. While vertical-ray and wave-front inter- 

 pretations are a matter of preference over standard refraction methods, 

 curved-ray interpretation becomes a necessity where the overburden por- 

 tion of a travel-time curve is not straight. This curvature may also occur 

 on later portions of travel-time curves. However, its presence in the first 

 layer is the predominant condition. Curved rays occur in thick sections 

 of sedimentary beds over basement rocks and in surface-weathered layers 

 above more consolidated formations. In each case, the type of travel- 

 time curve obtained and the interpretation problem resulting therefrom 

 are identical. The mathematical theory has been treated by several 

 authors.'*" 



" L. B. Slichter, Physics, 3(6), 273-295 (Dec, 1932). H. M, Rutherford, Amer. 

 Geophys. Union Trans. 1933 (Seism), 289-303; Soc. Petrol. Geophys. Trans., V, 



