548 



SEISMIC METHODS 



[Chap. 9 



The constants C and Co are obtained from refraction profiles shot at 

 the starting point in two directions. It remains necessary to reduce all 

 shot points to one datum, that is, to eliminate differences in their shot- 

 point constants by referring all times to the Co of a reference point. If 

 at a reception point common to two overlapping fans a difference in time 

 of hi — to2 corresponds to shots from two different points, their difference 

 in shot-point constant is 



Coi — .Co2 — ^01 ~ to2 



(9-67d) 



Adjusted times are plotted against the location of depth points; points 

 with equal time differences are connected by isochrons which, barring 

 velocity variations and steep dips, give a true picture of the depth con- 

 tours of the structure. 



(6) Method of differences {ABC system). This method has been applied 

 by the Imperial Geophysical Experimental Survey for the determination 

 of irregular bedrock surface in gold placer channels and is in widespread 

 use in reflection shooting for calculating the delay in the weathered layer. 

 It consists of shooting at A (Fig. 9-75) and receiving at B and C, then 



Sf 



\ / \ / 





A 



o*- 



300' 



d ^so'Y- c 



*o o © o o* fXf — *o 



^ , Deteeton 



l9t$M 2ii<i5hot 



Fig. 9-75. ABC system (method of differences). 



shooting at C and receiving at B. This gives the depth under B where 

 one or several receivers may be set up. One receiver is sufficient at A 

 or C. Since in placer and weathered-layer problems, the velocity contrast 



*2 A. B. Edge and T. H. Laby, Principles and Practice of Geophysical Prospecting, 

 pp. 339-341, Macmillan (1931). 



