Chap. 9] 



SEISMIC METHODS 



501 



the standard travel-time curve, indicating that there are no media of 

 anomalous velocities within the range of the fan. Time anomalies are 

 plotted for each fan as "accelerations" (in fractions of seconds). Fig. 

 9-38 shows such fan accelerations for the Moss Bluff salt dome. 



An analysis of the length of the acceleration vectors makes it possible 

 to determine approximately the location of the dome. The corresponding 

 travel-time curve for a salt dome is given in Fig. 9-39. After a dome has 

 been located by fan shooting, the company doing the work usually blocks 



Sec. 



Km 



v//////////////////////////////////. 

 Salt Dome 



Fig. 9-39. Travel-time curve across salt dome (adapted from Barton). 



the prospect as quietly as possible and then returns to the area later to 

 detail the indication by refraction profiling, reflection shooting, or torsion 

 balance to determine the shape of the cap and the attitude of beds above 

 and around the dome. The fan-shooting method has also been applied 

 in the mapping of anticlines^' (see pages 547-548). 



2. In mining exploration the fan-shooting method can be used for the 

 location of gold-bearing placer gravel channels and similar problems. 

 Figure 9-40 shows a fan layout as applied in prospecting for gold-bearing 



" J. H. Jones, World Petrol. Congr. B.I., 169-173 (London, 1934). 



