Chap. 10] ELECTRICAL METHODS 693 



electrodes are laid out up to a mile or more apart so that the equipotential 

 lines over the section of ground under consideration will be approximately 

 straight and parallel to one another. In the planning of the survey, the 

 general geology of the region should be considered and electrodes should 

 be so laid out that the equipotential lines will be at right angles to the 

 strike. Modifications may be necessary to suit special conditions. For 

 instance, for determining the outline of partly accessible ore bodies, the 

 primary electrode may be connected to the ore body thus making its outline 

 an equipotential surface. In a similar technique one electrode is connected 

 to a formation in a well so that its strike and dip near the surface is re- 

 vealed by its equipotential pattern. Because of an increased conductivity 

 in the direction of the bedding planes, equipotential lines surrounding one 

 electrode in a stratified medium are elliptical, with the major axis in the 

 direction of strike. 



It is possible to number equipotential surfaces and to determine their 

 interval by the following arrangement: Near the primary electrodes two 

 auxiliary electrodes are placed in the ground in such a manner that their 

 potential difference is nearly the same as that between the primary elec- 

 trodes. These are connected to the ends of a high resistance slide wire. 

 To find the potential of a given point or line, the point or line is connected 

 through a galvanometer to the sliding contact, which is changed in posi- 

 tion until the galvanometer deflection vanishes. If the resistance on one 

 side of the tap is Ri and on the other R2 , R1/R2 = F1/F2 , which may 

 also be written Ri/{Ri + Rz) = Vi/{Vi + V2), since both the total re- 

 sistance and the total potential difference between the points A' and B' 

 are known. 



For subtracting from the measured potentials the normal potential 

 variation due to the primary electrodes, formula (10-21a) is applied. 

 Substituting, for the center line connecting the two electrodes, the value 

 h for the base length and designating by x the distance of a point P from 

 the center, the potential at that point is F = ± 8x/(6 — 4x ), the sign 

 depending on whether the point is closer to the left or right electrode. 



A.C, equipotential-line methods may be divided into two groups: 

 (1) methods applicable when elliptical polarization is negligible and (2) 

 methods for the determination of potentials in respect to amplitude and 

 phase (or by their in-phase and quadrature components). Point or line 

 electrodes may be used; their arrangement is the same in both methods. 

 Point electrodes should be laid out with their base line parallel to the 

 supposed strike. Line electrodes are laid out at right angles to the strike 

 and have the advantage that the "normal" equipotential lines are parallel 

 with the primary electrodes; therefore, distortions are more readily inter- 

 preted. Line electrodes are usually bare stranded copper wire and are 



