526 



SEISMIC METHODS 



[Chap. 9 



the profile AD' is a (from the direction of maximum dip) the relation of 

 apparent and true dip is given by 



sin 4> = sin v? cos a. 



(9-56a) 



The azimuth a may be determined by shooting two profiles each, up 

 and down dip, at right angles to one another (Fig. 9-646), which give 



Fig. 9-64o. Relation of apparent dip, true dip, and azimuth (adapted from 



Gassmann). 



s/r/j(ie 



y/r/jh? 



<f/mfion 

 of dip 



Fig. 9-646. Two profiles for strike and dip 

 determination (adapted from Gassmann). 



four apparent velocities or 

 two groups of up- and down- 

 dip velocities: 



Vk = 



Vi 



sin {i -f (pk)' 



(9-566) 



where k = 1, 2, 3, 4. The 

 travel-time curves obtained in 

 this case are indicated in Fig. 

 9-64c, and the azimuth of dip 

 with respect to profile 1 is 



lit 

 V1V3 V2 — V4 

 ^7 i'~~i 



V4V2 V3 — Vi 



, = tan a. (9-56c) 



If the apparent angle of dip 

 <I»i_3is measured in profile 1-3 

 and the angle <I>4-2 in profile 4-2 by using the relations given in eq. 

 (9-54), then from (9-56a): 



sin ^^2 

 sin ^1-3 



= tan a. 



(9-56d) 



