500 



EXPLORATION GEOPHYSICS 



is typical for the movement of such an electrode configuration across a zone 

 of relatively high conductivity. It is the usual anomaly encountered in 

 conjunction with fault zones and other conductive zones of relatively nar- 

 row width. By like reasoning it can be seen that a zone of lower con- 

 ductivity would give an inverted type of curve having the same general 

 characteristics. 



An unsymmetrical electrode configuration which is suitable for lateral 

 investigations is illustrated in Figure 298. In this configuration, the potential 



electrodes 2 and 3 are closer to 

 power electrode 1 than to power 

 electrode 4. The distance between 

 electrodes 3 and 1 is about one- 

 third that between 3 and 4. This 

 configuration has the advantage 

 that the surface effect at one of 

 the energizing electrodes is mini- 

 mized. Thus, the predominant re- 

 sistivity variation occurs adjacent 

 the energizing electrode which is 

 near the potential electrodes. 



Movement of this configuration 

 across the zone of better conducti- 

 vity will give the type of anomaly 

 indicated in the lower part of the 

 figure. The greatest deflection oc- 

 curs when the electrode 1 crosses the zone of better conductivity. 

 A much smaller anomaly is produced when electrode 4 crosses the zone. 

 It will be shown later that if electrodes 1, 2, and 3 are maintained 

 stationary and if the depth of measurement is varied by moving only 

 electrode 4, a vertical depth exploration may be made with a minimum 

 surface effect. Hence, this type of unsymmetrical electrode configuration 

 furnishes the basis for a surface correction method useful in vertical 

 exploration. 



Figure 299 shows a regional resistivity profile plotted from data ob- 

 tained with the unsymmetrical electrode configuration shown in Figure 



TKAVeUSe DISTANCE OF LEAOINC ELECTHODE. NSI 



Fig. 298. — Unsymmetrical electrode configura- 

 tion for lateral exploration. 



Fig. 299. — Resistivity profile in Tejon Ranch Area, San Joaquin Valley, California. 



