Chap. 9] SEISMIC METHODS 489 



When dynamite is primed with an electric blasting cap, the latter should 

 be so fastened with its lead wires around the charge that the cap does not 

 pull out when the charge is lowered into a hole. Before shooting, the firing 

 circuit should be tested for resistance to be sure that the current is passing 

 through the blasting cap and through all parts of the line. For this 

 purpose the powder companies furnish test instruments consisting of an 

 ohmmeter and a silver chloride battery which supplies less current than is 

 required to blow up a cap. The resistance of the caps plus the resistance 

 of the leads should be calculated and compared with the results of the test. 

 Both short circuits or high resistance breaks are equally objectionable. 



2. Placement of charges. In both refraction and reflection work it is 

 necessary to place the charges in the surface in such a manner that maxi- 

 mum energy transfer from the explosive to the ground is obtained. The 

 amount of energy released by the explosion itself is appreciable. It has 

 been estimated that the temperature of the gases liberated by it is of the 

 order of 3000° C, and that pressures of about 50,000 atmospheres (or 

 about 700,000 pounds per square inch) are produced by an explosion of 

 60 per cent dynamite. However, very little of this energy is likely to be 

 transmitted to the ground. If the charge were placed on the surface 

 without confinement, not only would its rate of detonation be low (unless 

 a high velocity powder is used) but most of the energy would probably be 

 converted into a compressional air wave. Hubert, as early as 1924, dem- 

 onstrated experimentally that the effect of a buried charge may be from 

 50 to 100 times greater than the effect of a surface charge. Even then, 

 most of the energy is probably expended in enlarging the hole, in crushing 

 the rock, in moving out water and mud, and in heat. Comparatryely 

 little is converted into elastic wave energy. 



Charges should be placed as deep as time, terrain, and cost permit, 

 although it may happen that deeper layers have poorer transmission 

 characteristics than do more shallow layers. Firm shales or water-soaked 

 beds are the most effective carriers of seismic energy. Reflection shooting 

 has made it a fairly general practice to place the charge at the depth of 

 the ground-water level if it is not too deep. In long-range refraction 

 shooting, usually a hole 4, 5, or 6 inches in diameter is drilled to a depth 

 of from 12 to 25 feet, and a cavity is blown at the bottom with 4 to 8 

 pounds of dynamite. The entire charge is then placed in the cavity and 

 the hole is tamped with dirt and water. The large cartridges 8 inches by 

 24 inches mentioned before are applied in marsh work and are forced down 

 with a wooden tamper as far as they will go. In open water the charges 

 are sacked and primed, and the bags are lashed together and tossed over- 



" H. E. Nash and J. M. Martin, Geophysics, 1(2), 239-251 (June, 1936). 



