Chap. 9] 



SEISMIC METHODS 



567 



All formulas derived above for 

 dipping beds imply that the tra- 

 verse is shot up or down in the 

 direction of maximum dip. If the 

 traverse makes the angle a with 

 the direction of dip, the true dip v? 

 follows from the ^'apparent" dip 

 4», as determined along the tra- 

 verse, from 



x-» 



sin ^ 



cos a 



— sm (p. 



[see 9-56a] 



Fig. 9-89. Location of shot-point image 

 from three travel times. 



Directions of strike and of total 

 dip may be determined in two re- 

 flection profiles at right angles to 

 each other; if the apparent dip 

 angles are <I» and 4>', sin O'/sin 

 4» = tan a. Another method of 



determining both dip and strike consists of locating the image of the shot 

 point in three profiles at right angles to one another. At three points of 

 equal distance c from the shot point (see Fig. 9-89) the travel times h , 

 U , and h are given by 



yH\ = (X - cf + y' + D' 



^'tl = x'-V{y- cf + D' 



vH\ ={x + cf + if 4- D\ 



where a;, y, and D are the coordinates of the image point. Then 



V X.2 



^ = 4-0^^^ 



t\) (identical with [9-75c]) 



2/ = ^ {t\ - 2l\ -f t\) 

 D = V?tl - y' -{x- c)\ 



(9-80) 



The coordinates of the reflecting point are x/2, y/2, and D/2; the dip is 

 given by tan (p = -s/x^ + y'^/D, and the strike by tan a = y/x. 



The dip-shooting method does not give small dips accurately. This is 

 because of the inherent failure of the travel-time curve to respond to small 



" S. J. Pirson, Oil Weekly, April 26, 1937. 



