Chap. 9] 



SEISMIC METHODS 



525 



(4) compute i -\- <p,i — (p, i, and «p; (5) compute Vg ; (6) compute JEiT and h; 

 check result with s and <p. 



Fig. 9-63 illustrates steps 1 to 3 in the evaluation of two travel-time 

 curves. By step 4, sin {i — <p) = ifi^ = 0.259; t — (p = 15°; sin (i -\- (p) = 

 m% = 0.574; i + ^ = 35°; i = 25°; cp = 10°. V2 = Vx/sin i = ^^ = 

 3790; H = 235 (1 - 0.259) /2. 0.985 -0.906 = 97.5m;h = 150(1 - 0.574)/ 

 1.97-0.906 = 35.9 m; s = 350 m; (i^ - h)/s = tan (p; V^tf = tan 10°. 

 Determination of dip and depth of dipping layers does not necessarily 

 • require reversal of direction of shooting. Shots may be fired at two points 

 on the same side of the receiver spread. Shooting down dip, we find that 

 the second part of the travel-time curve is the apparent velocity V2d ; 



Fig. 9-63. Dip and depth calculation from up- and down-dip refraction profiles. 



shooting up dip it is Vju as before. The intercept of hd with ti moves up 

 when shooting down dip, whereas it moves down when shooting up dip. 

 Depths h and H may be determined from extended ordinate intercept 

 times.'^ 



The relations derived in this section hold only if profiles are shot down 

 or up the total dip, that is, at right angles to the strike. If profiles are 

 not shot in this direction, only an apparent dip in the direction of the 

 profile is obtained. In Fig. 9-64a^^ the plane AFED is the same as that 

 represented in Fig. 9-61. When this plane is rotated about the shot point 

 A in the direction AD', the rays still travel in a plane at right angles to 

 the dipping bed, AFE'D', whose apparent dip is <P. If the azimuth of 



^* For formulas, see B. Gutenberg, Lehrb. Geophys., 3, p. 599. 

 »« F. Gassmann, Beitr. ahgew. Geophys., 4(3), 358-363 (1934). 



