SEISMIC METHODS 863 



explosive and must be securely fastened in the charge, particularly when 

 it is used to lower the charge in the shot hole. The joint between the 

 Primacord and the blasting cap should be about 20 feet down the shot hole. 



Electric Firing. — The electric firing current may be supplied 

 to the caps either by battery or by blasting machine. Unless a specially- 

 constructed "blaster" is used, a blasting machine is considered relatively 

 safer because certain definite movements are necessary to generate the 

 current required for the firing circuit. With batteries, current can be 

 conducted to the firing circuit simply by making contact between this 

 circuit and the battery terminals. However "blasters" are designed with 

 safety switches, practically eliminating the objections to the use of 

 batteries. 



The following general rules should be observed : Leads should not be 

 attached to blaster until ready for firing. Immediately after firing, leads 

 should be disconnected from blaster, shorted, and stretched out on the 

 ground. Blasters should be handled by the shooter only. 



Loading Procedure. — The charge is usually covered with water or 

 mud to a depth of from 10 to 100 feet or more in order to couple the 

 energy of the explosion to the earth in an effective manner. When shooting 

 in an air-filled hole a large percentage of the acoustic energy is wasted in 

 the high velocity gases which issue from the hole upon detonation. The 

 presence of the water tends to efifect a more favorable acoustic impedance 

 match between the explosive and the earth. f 



Few general rules can be given concerning the best depth for the shot 

 in seismic prospecting. Normally, the shot should be deep enough to avoid 

 surface cracking. Likewise, it generally should be below the low velocity 

 layer.l (Compare p. 849.) In a particular area it may be found that best 

 results are obtained when the shot is fired within the same stratum at all 

 shot-holes — which stratum this is can only be determined by experiment. 

 For the sake of accuracy in computing, it may be preferred to keep the 

 bottoms of the holes at as nearly constant an elevation above sea level as 

 field operating conditions will permit. 



Various methods are used for placing the electric blasting caps in the 

 cartridges of explosive. The cap may be inserted in a hole punched with 

 a wooden or bronze punch, either in the end or obliquely in the side of 

 the cartridge. The cap should be well seated at the bottom of the hole, 

 which should not be punched to a greater diameter than is necessary 

 to clear the cap. The explosive should be molded by hand over and around 

 the cap. This aids in making a good mechanical assembly and also retards 



t D. H. Clewell and R. F. Simon, Geophysics (to be published). 



X J. L. Bisch, "Means for Positioning Explosives in Bore Hole," U. S. Patent 2,433,543, Oct. 30, 

 1947. 



