694 



ELECTRICAL METHODS 



[Chap. 10 



tacked to the ground at numerous points by steel pegs. In ore prospecting 

 their distance is 2000 feet to a half mile and their length of the same order 

 or greater. To cover large areas, the electrodes are leapfrogged, since too 

 great a distance between electrodes reduces the distinctiveness of response. 

 The normal field of line electrodes is no longer given by formula (10-21a) 

 and may be derived as follows. With reference to Fig. 10-37 the potential 

 at the point P is given by 



where E is one half the potential difference between the electrodes. Hence, 



(^1 + VTTT?)(^2 + Vrf+Tl)r^ 



Vj, = E loge 



(h + Vrl-{-n)(l2-\- Vrl-\-n)rl' 



On a Ihie of symmetry where Zi = Z2 = I, the potential is 



F, = 2E log. 



(a + x)(Z + VP+ (a-xy) 

 (a - x)il-\- Vl^ + (a + xY) 



(10-27) 



^-/T 



Fig. 10-37. Line electrodes (adapted from 

 Heine). 



The electrical (field and, therefore, 

 the current density) is greater with 

 line electrodes than it is with point 

 electrodes. 



In most equipotential, electro- 

 magnetic, and inductive surveys, 

 portable gasoline-engine driven 

 alternators of a frequency of 500 

 to 1000 cycles are used, furnish- 

 ing from 200-1000 KVA at 110 or 

 220 volts. 



The equipment for surveying 

 equipotential lines consists of two 

 search electrodes, an amplifier, and 

 headphones when out-of-phase 

 fields are negligible. It is supple- 

 mented by more elaborate bridge 

 arrangements when a complete 

 determination of A.C. potentials in 

 regard to phase and amplitude be- 

 comes necessary. In many cases, 

 even if out-of-phase fields exist, 



