SEISMIC METHODS 859 



30% also have been used successfully. However, the 60% straight, ammo- 

 nia, and semi-gelatin grades seem to be the most popular, and certainly 

 most of the intensive development work has been done on these grades 

 by the manufacturers to make them suitable for seismograph use. Because 

 of this, and because the 60 /J. gelatin grades represent about the maximum 

 explosives value per dollar over the entire range of gelatin strengths 

 available, they are to be recommended for seismic prospecting. 



An explosive to be suitable for seismic work should have the following 

 characteristics : high explosive power, effective detonation under pressure, 

 suitable plasticity, rigid packing, high density, and stable physical properties. 



It is held by some that the rate of detonation of an explosive is of great importance 

 in seismograph work, but this matter is probably controversial. Little is known of 

 actual rates of detonation developed by charges of gelatin under the conditions of use 

 in seismograph work. The rate of detonation may be a separate factor, but it is prob- 

 ably subordinate to other factors, such as the size, shape, and location of the charge, 

 and the ability of the charge to detonate completely. 



Various brands and sizes of seismograph explosives are offered by the different 

 explosives companies, and the representatives of the manufacturers should be consulted 

 before selection of the explosive to be used for an area.t 



Electric Blasting Caps 



Efficient operations in modern seismic technique require electrically 

 fired blasting caps. Because in most firing systems the electric blasting 

 cap becomes an integral part of the seismograph circuit, it is important 

 that the operator have some understanding of the construction and func- 

 tioning of an electric blasting cap. The caps offered by different manu- 

 facturers may be radically different in the details of their construction ; 

 however, the following general description is illustrative of the principles 

 employed in all electric blasting caps.t 



Essentially, an electric blasting cap consists of a waterproof metal 

 shell containing a charge of explosive material and a means of detonating 

 this material by an electric current. The caps are equipped with insulated 

 leg wires of various lengths. The explosive charge in the cap usually consists 

 of a "base charge" of a highly efficient detonating material and one or more 

 other explosive components referred to collectively as the "priming charge." 

 A fine resistance wire, known as a "bridge wire," is in intimate contact with 

 the priming charge and this wire is in circuit with the two insulated leg wires. 



t W. R. Farren and H. H. White, "Recent Developments in Explosives for Seismic Pros 

 pecting," Geophysics. Mar. 1937, pp. 114-119. 



t G. F. Rolland and H. H. White, "Developments of Essential Characteristics in Electric 

 Blasting Caps for Seismic Prospecting," Geophysics. March, 1937, pp. 119-126. 

 F. R. Seavey, "Explosives," U. S. Patent 2,415,045, Jan. 28, 1947. 

 L. A. Burrows, "Blasting Cap," U. S. Patent 2,427,899, Sept. 23, 1947. 

 E. K. Lefren, "Electric Blasting Cap," U. S. Patent 2,428,884, Oct. 14, 1947. 

 D. D. Huyett, "Electric Explosion Initiator," U. S. Patent 2,431,871, Dec. 2, 1947. 



