520 EXPLORATION GEOPHYSICS 



Method Employing Moving Current Electrodes (Figure 313f) 



Pa — 



I _ 1 +_i L 



a b L — b L — 



If Si is fixed and 6^2 moves, L, L — a, and L — b are variable quantities, 

 and a and b are constants. If ^2 is fixed and Si moves, L, a, and b are 

 variable quantities, and L — a and L — b are constants. 



Method Employing One Moving Current Electrode and One Fixed 



Current Electrode Positioned Midzuay Between the Potential 



Electrodes (Figure 313g) 



pa 



f Vp-VB \ 



~1 i 



L — 2a L 



Electrodes. — In conductive methods of electrical prospecting, con- 

 tact with the earth is made by electrodes imbedded or driven into the 

 ground. The investigation of lateral or depth variations in a given area 

 is accomphshed by means of a series of measurements made at different 

 positions or spacings of the electrodes. Usually the electrodes are moved 

 along a traverse line. The movement may be in uniform increments of 

 distance (intermittent readings) or at a continuous rate of progression 

 (continuous recording). The type and design of the electrodes depend 

 on the electrical method employed and on field conditions. 



Electrodes may be divided into two general classes: (a) non- 

 polarizing electrodes and (b) polarizing electrodes constructed of base 

 metals.* 



Non-Polarizing Electrodes 



As far as practical application to geophysical work is concerned, the 

 only type of electrode which does not change its properties appreciably 

 with the passing of small quantities of current consists of a porous cup 

 which contains a rod or sheet of metal and a saturated solution of a salt 

 of the metal. In this type of electrode the metal will go into solution or 

 will be precipitated, depending upon the direction of the current. The 

 electrochemical process is reversible, and the chemical relationships do 



* Attempts have been made to employ simple electrodes constructed of chemically 

 inactive materials, in order to avoid the inconveniences attached to employing the usual 

 type of non-polarizing electrode. For example, investigations have been made of carbon 

 and graphite electrodes, as well as electrodes constructed of base metals plated with 

 ■platinum, iridium, gold, silver, etc. Due to their relatively small electrochemical 

 activity, these materials produce smaller electrolytic potentials at the electrode. _ They 

 do not, however, have any advantages over the base metals, with regard to polarization 

 phenomena associated with the flow of current. 



