486 SEISMIC METHODS [Chap. 9 



drill hole; therefore, it is necessary to place shots in as firm ground as 

 possible to insure good confinement and to tamp shot holes with mud or 

 water. Again, high water pressures will prevent the high velocity from 

 appearing in the regular anmionia gelatins. Therefore, both Hercules 

 and Du Pont have developed Hi- Velocity-type gelatins for seismic work 

 which give high unconfined velocities without special priming and with- 

 stand such water pressures as occur in the deepest reflection shot holes. 



It is obvious that those explosives which will give the greatest explosive 

 energy per dollar expended are the most desirable. In long-range refrac- 

 tion work the cost of dynamite amounts to half the cost of operation of a 

 seismic party; consumption of 1200 to 2500 pounds per day is not unusual. 

 In reflection work the cost of hole-drilling exceeds the cost of dynamite. 

 The price of special gelatins is around $17 per 100 pounds in carload and 

 $20 in ton lots. 



Commercial dynamites vary in consistency from rubber-like constitution* 

 (blastmg gelatin) to free flowing (Nitramon). For reflection work where 

 charges have to be forced into deep holes, stiff cartridges are required. 

 Therefore, special gelatins in stiff wrappers, or in tin cans (Nitramon) 

 are used lq seismic exploration. For special applications where very small 

 charges (1/16 pound) are suflScient, a plastic gelatin of sticky rubber-like 

 consistency, which may be molded around caps, is available. Lack of 

 consistency or excessive obstructions in shot holes can be overcome by the 

 use of tin torpedoes. 



Water resistance is one of the most important properties of explosives in 

 geophysical work. Blasting gelatins, straight gelatins, and ammonia 

 gelatins rank highest in water resistance; next follow the straight nitro- 

 glycerine dynamites; and lowest are certain types of ammonia dynamites. 

 By wax-dipping of wrappers or use of sealed cans (Nitramon), the water 

 resistance is increased considerably. Both dynamite and caps must be 

 highly resistant to water to be usable in the water-tamped holes required 

 in seismic exploration. 



Freezing resistance. Most gelatin dynamites are low-freezing and may 

 be used the year round throughout the United States. For severe cold 

 weather a special low-freezing grade is available. 



Safety is one of the prime considerations in any application of explosives. 

 The straight nitroglycerine dynamites are least perfect in this respect; 

 they are too sensitive to shock and friction. Gelatins and ammonia 

 dynamites rank very well. Low-density explosives, because of built-in 

 cushioning effects, are the safest but unsuited for geophysical applications 

 on account of their lack of consistency and low water resistance. Safety 

 of explosives is determined at the factory by impact testers or by dropping 

 an iron ball a distance of about 10 feet on a square section of powder resting 

 on a steel surface. 



