Chap. 9] 



SEISMIC METHODS 



539 



points 1, 2, 3, 4, and so on, on the left and corresponding points on the 

 right (see Fig. 9-70). (6) Draw contact curves on both sides. (7) Draw 

 a tangent to the two contact curves, obtaining depth and dip of stratum. 

 (8) The dip of the lower layer is given by the diagonal of the parallelogram 

 formed by intersection of the two underlayer wave fronts. (9) The ve- 

 locity and spacing of wave fronts in the lower layer is given by the length 

 of this diagonal. 



The case of three inclined layers is treated in a similar manner. As 

 before, the essential point is to lower the surface shot points and 0:23 inter- 

 cepts to the first formation boundary. A secondary shot point below Si 

 is given by the angle a, which follows from sin (a -\- (p) = Vi/vs down • 



Fig. 9-71. Wave fronts, inclined terrace (after Ansel). 



Wave fronts are drawn about this point with the spacing Asz , corre- 

 sponding to the velocity V2 previously determined. The location of the 

 secondary shot point on the other side is given by the angle of emergence 

 ;8 which follows from sin (/3 - ^) = Vi/vs up . After the shot point has 

 been projected on the lower layer, wave fronts are drawn about it with 

 the spacing As2 . After secondary shot points and secondary breaks have 

 been established on the first interface, the construction proceeds as before. 

 Wave fronts of the second contact wave are constructed by drawing 

 parallel lines from the secondary breaks down to intersect with the wave 

 fronts with the velocity V2 . The inclination of the wave front of the 

 second contact wave is given by the angles £2 — (^ — ^) and i^ + (^ — ^) 

 (see formula [9-57a]). Contact curves are constructed from the points 



