Chap. 9] SEISMIC METHODS 493 



ceivers are moved, the recorded energy varies with the so-called ground 

 factor, that is, with the dynamic response conditions of the surface strata. 

 Different propagation paths of refracted, reflected, and surface waves 

 further complicate the problem. 



In refraction work the amount of dynamite required increases with dis- 

 tance, probably in linear relation. In reflection shooting a relation be- 

 tween charge and depth is hardly recognized, since the recorded ampli- 

 tudes are predominantly dependent on transmission conditions of surface 

 beds at the shot point. The curves in Fig. 9-31 have been plotted from 

 data published by Barsch and Reich for a number of refraction traverses. 



ZOOO 3000 JOOO 



DistttKt in mtten 



Fig. 9-31. Dynamite charge required to produce legible impulse, as a function 

 of distance (compiled from data by Barsch and Reich). P, S-H, and*^D indicate 

 localities. 



They indicate that the amount of dynamite required varies approximately 

 in direct proportion to distance. 



4. Transmission of shot instant. The accurate transmission, from the 

 shot point to the receiver, of the instant of the explosion is of importance, 

 since virtually all interpretation methods are based on determination of 

 travel times. In refraction shooting the accuracy in time transmission 

 varies from xiir of a second for short distances to several hundredths of a 

 second for great distances, in reflection work it is of the order of two" of a 

 second. The instant of the explosion may be determined (a) by its direct 

 effects (sound, light, or temperature), or (6) by an indirect effect (electrical 

 current or radiation) released by the explosion. 



