ELECTRICAL METHODS 



547 



Ground Current Compensation by Auxiliary Circuit. — As a general 

 rule, the ground currents flow over large areas and show an approxi- 

 mately regional distribution. Advantage often may be taken of this fact 

 to minimize ground potential variations in the measuring circuit, t A 

 typical arrangement is illustrated schematically in Figure 335. Two poten- 

 tiometers Pi and Po which are mounted on the same control shaft C are 

 connected to the three potential electrodes 3, 4, and 5. Electrodes 3 and 

 4 are so placed, with reference to the energizing electrode 1, that they 

 are on the same equipotential line when energizing current flows in the 

 power circuit. Electrodes 3 and 4 are spaced relatively close to 1 (usually 

 less than Yq the total separation of the energizing electrodes) so that no 

 significant shift of the equipotential line will take place when the ener- 

 gizing electrode 2 is moved. The spacing between 4 and 5 is some known 



Fig. 335.— Auxiliary circuit for neutralizing variations in ground 

 currents. 



multiple of that between 3 and 4. The potentials Eo and Ei are in the 

 same ratio as the electrode separations between 3 and 4 and between 4 

 and 5. Thus, if distance b is three times a, £2 = % ^i- The distance b 

 must be greater than a in order to minimize the efifect produced by the 

 movement of electrode 2 on the potential difference between 3 and 4. 



Continuous adjustment of Pi, to keep the galvanometer G at balance, 

 will automatically introduce a proportional neutralizing potential into 

 the circuit containing P2. Because the energizing current predominately 

 affects the potential difference between 4 and 5, the readings in that cir- 

 cuit are employed for making the resistivity calculations. If desired, 

 automatic compensation may be employed by means of a vacuum tube 

 control wherein the potential difference between 3 and 4 is applied to 



t J. J. Jakosky, "Method and Apparatus for Electrical Exploration of the Subsurface," U. S. 

 Patent 2,162,087, issued June 13, 1939. 



