512 



EXPLORATION GEOPHYSICS 



configuration because the results are dependent upon the sum of the 

 effects in the vicinity of the two energizing electrodes. If, for example, 

 a deep investigation were attempted and the energizing electrodes 

 spaced 20,000 feet apart, the two effective zones of measurement 

 probably would be from 10,000 to 15,000 feet apart. At these great dis- 

 tances, many changes could occur, especially in areas of steeply dipping 

 structures. 



J, 



5, 



^/ 



L_ 



'21 



1 Sur/ac 



\-^- 



Fig. 309. — Resistivity variations over high resistance partition, using Wenner configuration. 



The instruments employed for small scale investigations may be those 

 used for the large field studies provided they have a sufficiently large 

 scale range to enable the observer to read the very low current (usually 1 

 to 10 milliamperes) and the relatively high potentials (usually over 100 

 millivolts) encountered in the small scale investigations. 



Small scale, non-polarizing electrodes may be constructed by using 

 small diameter glass tubing. The bottom of the tube is plugged with a 

 short saturated roll of chamois skin. The tube is filled with copper sul- 

 phate solution in which a bare copper wire is partially immersed. Ordinary 

 electrodes may be made from small diameter graphite (lead-pencil car- 

 bons) or carbon rods similar to those employed for brushes in small 

 motors. Chemically active electrodes, such as iron or copper, should be 

 avoided. Finally, the electrodes should not extend into the conducting 



