858 EXPLORATION GEOPHYSICS 



straight nitroglycerin dynamites, (2) the ammonia dynamites, and (3) the 

 gelatin dynamites, which may be subdivided as (3-a) ammonia gelatin 

 dynamites and (3-b) semi-gelatin dynamites.f 



The straight nitroglycerin dynamites form the standards with which high explo- 

 sives are compared for strength. They should contain the actual amount of commercial 

 nitroglycerin designated as their percentage strength. In addition to nitroglycerin 

 these explosives contain an "active base" of absorbent carbonaceous material and 

 an oxidizing salt, such as sodium nitrate. The straight nitroglycerin dynamites 

 are extremely sensitive to propagation and find little use except where this property 

 is specifically required, as for example, in ditch blasting by the propagated method. 



Ammonia dynamites (also known as "extra dynamites") contain less nitroglycerin, 

 grade for grade, than the straight nitroglycerin dynamites. Ammonium nitrate is 

 substituted for part of the nitroglycerin in amounts sufficient to give them the same 

 strength as the corresponding grades of the straight nitroglycerin dynamites. 



Gelatin dynamites are manufactured using a colloidal solution of nitrocellulose 

 in nitroglycerin instead of the nitroglycerin alone. Gelatin dynamites are dense, 

 plastic, and highly water-resistant, and are the principal explosives used in seismic 

 shooting. They are classified into three general types : namely, straight gelatins, in 

 which the nitroglycerin-nitrocellulose colloid is the explosive ingredient, ammonia 

 gelatins in which ammonium nitrate is substituted for part of the colloid, and the semi- 

 gelatins. At the present time ammonia gelatins and semi-gelatins are employed almost 

 universally for industrial blasting in this country, but all three types of gelatin dyna- 

 mites are used in geophysical prospecting. 



For relatively shallow seismic work, the ammonia gelatins and semi- 

 gelatins have been found satisfactory and are recommended because they 

 are usually sold at a lower price than the straight gelatins. While their 

 resistances to water are not as good as that of the straight gelatins, they 

 are still good enough for general use. In addition the ammonia gelatins 

 are capable of being detonated by commercial electric blasting caps under 

 moderately high heads of water. Special grades of 60% ammonia gelatin 

 or 60% semi-gelatin are available for seismograph use and because these 

 grades are usually furnished with features such as extra heavy wrapping, 

 special hardness and toughness, etc., to adapt them for seismic shooting, 

 they are to be recommended instead of the 60% ammonia gelatin ordinarily 

 used in industrial blasting. 



For the more severe conditions where explosives must be detonated 

 under very high heads of water or where "sleeper" charges must remain 

 in the ground for a long time, the explosives manufacturers have developed 

 special modifications of the 60% gelatin. These gelatins are especially 

 formulated by the manufacturers so that they will detonate with electric 

 blasting caps under very high hydraulic pressure, even after immersion 

 for considerable periods of time. 



In addition to the 60% gelatin grades described, other gelatin grades 

 have found some use in seismic shooting.^ Modifications of 80% straight 

 gelatin and 100% blasting gelatin have been developed for seismograph 

 work, but their use has been limited. The lower strength gelatins down to 



t N. G. Johnson and G. H. Smith, "Explosives for Seismic Prospecting," Geophysics, June, 

 1936, pp. 228-238. 



t H. E. Nash and J. M. Martin, Geophysics. June, 1936. 



