824 



EXPLORATION GEOPHYSICS 



m 



'^i>>: 



?: 



r 



A B C D 



Fig. 508. — Group of four records from Montgomery County, Illinois, from the same shot-point. 

 Top record. A, shows three tracks of a variable area earth motion record. The next three, B, C, and 

 D, are recorded with different iiltering peaked respectively at approximately 30, 50, and 70 cycles 

 per second. This is an example of three recordings made at the same shot-point, using the same 

 amount of explosive, depth of charge and amount of stemming in the shot-hole. (Courtesy of Seis- 

 mograph Service Corporation.) 



