Chap. 9] SEISMIC METHODS 485 



glycerine. The Atlas ammonia gelatins, Du Font's Seismogel A and B, 

 and Du Pont Nitramon are in this group. 



For reasons stated later, straight d5Tiamites and blasting gelatin are not 

 so well suited for seismic applications as are ammonia and straight gelatins. 

 Explosives for geophysical as well as other applications are characterized 

 by the following properties: 



By strength of an explosive is meant the percentage of nitroglycerine 

 in straight dynamites. For any other explosive, regardless of composition, 

 the strength rating is obtained by comparing its effect with that of straight 

 dynamite. The absolute strength of an explosive is of minor importance. 

 While theoretically the effect of an explosion should be independent of the 

 type of explosive used, provided the energy (weight-strength times amount) 

 remains constant, it has been demonstrated in practice that, for the same 

 amount, variations in strength and type of gelatin within the range of 

 40 to 80 per cent have little effect. Strengths are generally referred to 

 unit weight or unit volume and are thus designated as weight-strength or 

 bulk-strength. High bulk-strength is advantageous for reducing trans- 

 portation costs and size of shot hole. 



Density is of importance in connection with strength. An explosive of 

 both high density and high strength, that is, high weight-strength, is 

 preferable. 



Propagation effectiveness is the ability of an explosion to propagate 

 through the explosive itself, as well as through air gaps or other non- 

 explosives, to another portion of explosive or cartridge. A typical indica- 

 tion of ineffective propagation from one cartridge to another is shown in 

 Fig. 2 of an article by N. G. Johnson and G. H. Smith.^* 



Rate of detonation is the speed with which the detonating wave travels 

 through a train of explosives. Experience shows that the percentage of 

 energy converted into ground vibration increases with the rate of detona- 

 tion and that the latter increases with the degree of confinement. This is 

 caused by the peculiarity of the gelatin dynamites of having two velocities, 

 one around 8500 feet and the other ranging from 13,000 to 20,000 feet, 

 depending upon the grade of the explosive. In ordinary gelatins the high 

 velocity will not be developed in the open but under close confinement in a 



** Geophysics, 1(2), 232 (June, 1936). 



