862 EXPLORATION GEOPHYSICS 



containing one gram of detonating material and the No. 8 caps containing 

 two grams. At the present time, however, the different manufacturers 

 use a wide variety of cap compositions. Nearly all of the No. 6 caps now 

 offered are materially higher in detonating efficiency than the original 

 No. 6 fulminate cap. No. 6 caps constitute a great majority of the caps 

 used for seismograph shooting and are amply strong for most work. The 

 present-day No. 8 caps are more efficient than the fulminate No. 8 caps 

 and offer an additional margin of safety where seismograph shooting is 

 carried out in deep holes. In addition to the regular No. 6 and No. 8 

 electric blasting caps in both standard and waterproof construction, there 

 are available special electric blasting caps which offer both an increased 

 waterproofing characteristic and a strength greatly exceeding No. 8 caps 

 in detonating efficiency. Such caps have been found advantageous under 

 extremely severe shooting conditions. Some of these caps embody the 

 "no-lag" feature. 



Important properties of electric blasting caps may be summarized as 

 follows : no time lag between rupture of firing circuit and detonation of 

 cap; sufficient detonating strength to initiate the explosion under the con- 

 ditions of use; and the various physical characteristics, such as water- 

 proofing, efficient protective shunting and a method of packaging such 

 that the wires can be "strung out" without excessive kinking or incon- 

 venience. The electric blasting caps offered by the various manufacturers 

 are furnished under various brand names and come in a variety of wire 

 lengths. The manufacturers' representatives should be consulted for 

 further particulars on any given brand. Handle caps with caution. Never 

 attempt to open a cap ; forcing the wires or rough handling may cause it to 

 explode. 



Primacord detonating fuse consists of an explosive core contained 

 within a waterproof textile covering. Relatively insensitive to flame, shock, 

 friction, and impact, it must be detonated by a blasting cap. It will not 

 deteriorate within reasonable time periods. Its explosive wave travels at 

 approximately 20,350 feet per second. 



Primacord is used in seismic work chiefly to eliminate high voltage 

 hazards when shooting near power lines. The blasting cap should be 

 attached to the Primacord by means of a special union made for this 

 purpose. The charge is made up by running the Primacord through the 

 explosive cartridge or cartridges and tying a knot at the bottom in order 

 that the Primacord cannot pull back through the charge, or by lacing the 

 Primacord through the charge. 



The essential factors governing the assembly of a charge with Prima- 

 cord are as follows. The end of the Primacord should be cut square; this 

 end must be dry, and must have the "business" end of the blasting cap 

 seated firmly against it. The Primacord should be in contact with the 



