488 



SEISMIC METHODS 



[Chap. 9 



Bridge Mre 



losulahd wins 



SiJphor ctmeni 



Water pnU filltr 



Primer ckarqe 



Pressed 

 CMrge 



Fig. 9-25. Cross section of Hercules 

 electric blasting cap. 



the bridge wire is usually mercury ful- 

 minate; in the Du Pont "SS" caps it 

 is fulminate chlorate. Instead of hav- 

 ing a loose charge around the bridge 

 wire, the Atlas caps have a bead in 

 the form of a matchhead. Caps are 

 usually protected by a shunt clip 

 against accidental discharge due to 

 static or other sources of electricity. 

 This clip is removed immediately 

 before firing. 



The time characteristics of electric 

 blasting caps are of great importance 

 in seismic work. Connected to the 

 blasting cap is a circuit for transmit- 

 ting the instant of the explosion to 

 the seismic record. With the introduc- 

 tion of the reflection method, the re- 

 quirements of time accuracy went up 

 considerably, for the time of explosion 

 must be transferred to the record with 

 an accuracy of 1 to 2 thousandths of 



a second. Ordinary caps are 

 unsuited for this purpose. As 

 shown in Fig. 9-26, the delay 

 for low currents is great. For 

 high currents two breaks oc- 

 cur — one when the bridge wire 

 fuses and the next when the 

 cap fires. In caps especially 

 developed for seismic applica- 

 tions (Du Pont "SSS") not 

 only is the time difference 

 beween bridge break and deto- 

 nation eliminated but the fir- 

 ing delay is reduced consider- 

 ably. This type of cap can 

 be fired by any current greater 

 than 2 amperes. At 3 amperes the firing time is about 0.003 second 

 ± 0.0003. Batteries or blasting machines may be used for firing electric 

 blasting caps, the latter being preferable from the point of view of safety. 

 A 50-cap blasting machine furnishes about a 300-volt peak e.m.f. 



2^ 4 6 d W amps. 



Fig. 9-26. Firing current vs. firing time for 

 Du Pont seismograph blasting caps (after 

 Burrows). 1 is "SSS" bridge break and total 

 cap lag; 2A, "SS" bridge break; 2B, "SS" total 

 cap lag. 



