522 



SEISMIC METHODS 



[Chap. 9 



parent velocity Wzd , and increased intercept distance xa . When the 

 stratum dips up from the origin, the ray travels a greater distance through 

 the lower high-speed medium. Travel times are reduced; the apparent 

 velocity (v2«) is greater than in the case of a horizontal bed and the inter- 

 cept distance (xu) is less. The apparent velocity depends upon both dip 

 and velocity ratio. One profile is not sufficient to determine velocities 

 and dip. Hence, shots are fired at two points on one side of the spread, or 

 else the direction of shooting is reversed and profiles are shot up dip and 

 down dip. This may be accomplished by two shot points on either end 

 of the profile, or by one shot point in the center of two receiver spreads, 

 the former bemg the preferred procedure. 



In Fig. 9-61 let <p be the dip, H the depth vertically below the shot 

 point, and h the depth below the receiving point. Depths normal to the 

 stratum are Z and z. The profile is assumed to be at right angles to the 



■Z¥^ 



Fig. 9-61. Refraction path in dip shooting. 



strike, that is, in the direction of maximum dip. The first part of the 

 travel-time curve is given by h = s/vi . For the second part the time 

 for the path ABCD = ^= (ZB_+ CD)/vi -\- BCM^ Since AB = 

 Z/cos i, DC = z/cos i, AD = s, FE = ID^ = s cos <p,_AI =^sin <p = 

 Z - z,FB = Z tan i,CE = z tan i, and BC = ID - FB - CE, 



Z -\- z • , s cos >p 

 tz = cos t + 



(9-50a) 



Vi V2 



For the up-dip case, substitute for 2: Z — s sin <p, and sin i/vi for I/V2 ; then 

 fe„ = ?^^^' + ^ sin a - ^). (9-506) 



Vi Vi 



If for the down-dip case Z = z -\- s sin cp, 



2z cos I , s . ,. , V 

 tu = — - — + - sm {i -\- <p). 



Vi Vi 



(9-50c) 



