1000 



2000 



3000 



4000 



Fio. 9-48. Depth calculation of two layers from intercept distances (after O. v. 



Schmidt). 



6. Three horizontal layers. 



W The seismic ray follows the 



6 path ABCDEFGH; the ve- 



> ^ locities are Vi , V2 , Vs , V4 ; 



a the angles at the formation 

 ^1 "l^ ^ boundaries are e, 8, and 



Fig. 9-49. Wave paths, three-layer case. 7 as shown in Fig. 9-49 



in which the two- and three- 

 layer paths have been indicated for comparison. The angle at D 

 is 90°; hence, sin 7 = V3/V4 . Furthermore, sin /3 = sm 5/sin 7 = V2/V3 

 and sin 8 = V2/V4 , Also, sin i = sin e/sin 5 = Vi/va and sin e = V1/V4 . 

 The travel time 



s , 2(c?3 - di) , 2(d2 - di) . . 2di .„ .. x 



^4 = - -f -^-^ COS 7 H — ^ cos 6 -I cos e. (9-46a) 



V4 V3 V2 Vi 



The fourth part of the travel-time curve corresponds only to the velocity 

 in the fourth layer, since dU/ds = I/V4 . For the simultaneous arrival 



