Chap. 9] 



SEISMIC METHODS 



499 



the time scale in fan shooting in terms of normal geologic depth-velocity 

 sequence, a refraction profile is first shot. There are combinations of fan 

 and refraction shooting in which absolute instead of relative depths arc- 

 obtained; refraction profiles may be tied in with reflection traverses to 

 obtain average velocities to reflecting beds. Finally, refraction shooting 

 for weathered layer corrections is part of everyday reflection practice. 



B. Fan-Shooting Method 



1. In oil exploration the fan-shooting method was appHed extensively 

 on the Gulf coast for the location of salt domes from 1924 to 1929, which 



Fig. 9-36. Preliminary profile and fan layouts (after Barton). 



are ideally suited for this work, since their velocity differs considerably 

 from that of the surrounding formations. The area to be prospected is 

 covered by a series of overlapping fans (see Fig. 9-36). Receivers are 

 grouped, by repeated setups, on the circumference of a circle about the 

 shot point, at distances varying from four to eight miles. In a new area 

 a profile is first shot to determine the normal sequence of beds, that is, 

 the "normal" travel-time curve (see Fig. 9-37, and profile extending north 

 from fan shot point A in Fig. 9-36). Travel times observed at the fan 



