SEISMIC METHODS 857 



The time at which the hnpact occurs is usually obtained by placing a 

 seismometer fairly close to the impact stake. This seismometer is con- 

 nected to one of the recording galvanometers. 



A discussion of seismic experiments vitilizing falling weights will be 

 found in the literature, t Also, L. G. Howell, C. H. Kean, and R. R. 

 Thompson have described an electrodynamic ground shaker capable of 

 initiating continuous seismic waves of moderately high frequencies and 

 single frequencies over a range of 20 to 1400 cycles. X (The use of an 

 oscillator for generating seismic waves was described by Fessenden as 

 early as 1914.) 



Generation of Seismic Waves by Explosives. — Elastic earth waves 

 of sufficient amplitude for deep investigations are initiated by the det- 

 onation of a charge of explosive. The characteristics of the waves depend 

 on the properties of the earth formations in the vicinity of the explosive, 

 the degree of confinement of the explosive charge, and the weight and 

 type of explosive. Detailed theoretical studies of the desirable properties 

 of explosives for this and similar purposes have been reported, but only 

 certain factors of practical import for geophysical exploration need be 

 considered here. 



Types of Explosives 



An explosive is a chemical compound or mixture of chemical com- 

 pounds (usually in liquid or solid form, or both) which is capable of 

 changing rapidly into simpler chemical compounds, largely of a gaseous 

 nature, which occupy a much greater volume under standard conditions 

 than the original material. This change is accompanied by a great evolu- 

 tion of heat. The useful or destructive effect of explosives is dependent 

 upon the pressure developed by the sudden release of gases at high tempera- 

 ture and pressure, and the work done by these gases on the surrounding 

 media. 



Commercial blasting explosives may be grouped into two general 

 classes : namely, high explosives, including dynamite and gelatin dyna- 

 mites, and low explosives, i.e., explosives of the black powder type. 



The explosive decomposition of high explosives is believed to proceed 

 on a wave front along the explosive column at a high velocity, and is 

 known as detonation. Low explosives burn or deflagrate and the decom- 

 position proceeds inwardly from the surfaces of the individual particles. 

 The difference in the observed effects produced by black powder and dyna- 

 mite is due to the difference in rate of evolution of hot gas upon explosion, 

 this being much less rapid for low explosives than for high explosives. 

 Black powder is little used for seismic prospecting and only the high ex- 

 plosives need be considered here. 



The most important types of commercial high explosives are: (1) the 



t See, for example, V. G. Gabriel, "Some Experiments in Seismic Prospecting," Ergdnsungs- 

 Hefte fiir angewandte Geophvsik, Vol. 2, 1931, pp. 122-130. 

 t Geophysics, Vol. IV, No. 3, 1939; Vol. V. No. 1, 1940. 



