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ELECTRICAL METHODS 



I. INTRODUCTION 

 A. Fundamentals 



Electrical prospecting in the most general sense may be defined as 

 prospecting by electricity for mineral deposits and geologic structures. 

 Contrary to geophysical methods previously discussed, each of which makes 

 use essentially of one field of force — gravitational, magnetic, or elastic- 

 there is much greater variety in the type of electrical fields and in methods 

 of observation employed in electrical prospecting. First, an ore body may 

 act as a battery and furnish its own electrical field; second, the ground 

 under test may be energized by extraneous fields and the reaction of sub- 

 surface conductors to such fields may be measured. Both direct current 

 and alternating current are used ; the latter gives not only greater sensi- 

 tivity but additional physical quantities, which helps in the interpretation 

 of the results. Direct current can be introduced into the ground only by 

 galvanic contact, but alternating current can be applied by both contact 

 and inductive coupling. The resulting fields are measured by instruments 

 making contact with the ground by electrodes, or by employing inductive 

 coupling with reception frames. Thus, in reference to- observation meth- 

 ods, electrical methods may be divided into potential and electromagnetic 

 methods. 



Electrical exploration uses a wide range of frequencies. Those from 5 

 to about 100 will be referred to herein as low frequencies, from about 200 

 to 1000 cycles as intermediate, from 10 to 80 kilocycles as high, and from 

 about 100 kilocycles to several megacycles as radio frequencies. Low 

 frequencies are applied in potential methods, intermediate frequencies in 

 both potential and electromagnetic methods, high frequencies in electro- 

 magnetic methods, and radio frequencies in radio methods. Since depth 

 penetration decreases rapidly with frequency, the practical utility of high 

 and radio frequencies is limited. 



With the exception of the self-potential method, electrical prospecting 

 falls in the group of indirect geophysical procedures which involve an 



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