860 



EXPLORATION GEOPHYSICS 



c= Mftai 



mm^Mm 



Figure 535 illustrates one of the most common types of construction 

 employed in electric blasting caps. 



Whereas electric blasting caps are commonly referred to as "instan- 

 taneous" caps, this term is inaccurate because the caps require measurable 

 intervals of time for their operation. In industrial 

 blasting this is of little or no interest to the consumer, 

 but in seismic work even the very short time intervals 

 involved become important. 



When an electric current is caused to flov^ in the 

 circuit of an electric blasting cap, heat is generated 

 by conduction losses {PR losses) in the bridge vi^ire. 

 This heat is conducted or radiated to the priming 

 charge. If a very low current is applied, the heat 

 generated may be insufficient to raise the temperature 

 of the priming charge sufficiently to initiate its decom- 

 position. A current of 0.3 amperes usually is the 

 minimum that will detonate a commercial cap. When 

 sufficient current is passed through the bridge wire, 

 it raises the temperature of the priming charge and 

 starts decomposition. The decomposition of the prim- 

 ing charge progresses from "burning" to "detonation" 

 very rapidly and detonates the base charge, which in 

 turn detonates the dynamite. 



The time elapsing between the application of an 

 electric current to the cap and the explosion depends 

 in part on the type of cap used, but chiefly upon the 

 magnitude of current. Figure 536 illustrates the firing 

 characteristics of two general types of electric blasting 

 caps and shows the relationship between current and time of detonation. 



The time elapsing between the detonation of an electric blasting cap 

 and the initiation of detonation in the charge of dynamite or gelatin is 

 extremely short, and because it is far below the limits of accuracy of 

 seismograph records, it may be neglected. For all practical purposes, the 

 time of detonation of the cap may be accepted as the time of the explo- 

 sion of the dynamite charge. In early seismic work the time-break was 

 obtained from the breakage of a wire wrapped either around the dynamite 

 charge or around a second cap connected in series with the cap used to 

 detonate the charge. These methods have been largely supplanted by the 

 single cap method, in which the time-break is determined from the in- 

 terruption of the firing current occasioned by the breaking of the bridge 

 wire circuit when the explosion occurs. f This method of determining the 



Fig. 535. — Section show- 

 ing the construction of a 

 typical electric blasting 

 cap. 



t G. H. Loving and G. H. Smith, "Explosives and Electric Blasting Caps for Geophysical 

 Prospecting," Journal S.P.G., Vol. 6, 1935, pp. 1-27. 



H. R. Prescott and F. L. Searcy, "Method of Making Geophysical Explorations, U. S. 

 Patent 2,046,843, issued July 7, 1936. 



