504 



EXPLORATION GEOPHYSICS 



measurements at the small separations has a lower resistivity than the 

 deeper lying material which afifects the measurements at the large 

 separations. 



The curves also exhibit a general similarity, because the potential elec- 

 trodes occupy the same surface positions in both the shallow and the deep 

 measurements. If the current penetrates the earth uniformly, the effective 

 zone of measurement for the symmetrical shallow electrode configuration 

 will lie approximately midway between electrodes 1 and 4. For the deep 

 investigations, the effective zone of penetration lies approximately midway 

 between electrodes 1 and 5, but due to the unsymmetrical electrode arrange- 

 ment the zone of effective measurement is shifted toward the potential 

 electrodes. Hence, in comparing the shallow and deep investigations it 

 is necessary to make a shift correction. The amount of this shift will 

 depend on: (a) electrode configuration, (b) the relative conductivities 

 of the deep and shallow zones, and (c) the changes in dip or thickness of 

 section between the two zones. 



The data utilized in the interpretation are obtained by making the 

 shift correction and subtracting corresponding ordinates of the two resis- 

 tivity curves. 



Vertical Exploration 



The operating principle of the vertical exploration method will be 

 evident from Figure 302. The electrodes are placed along the traverse 

 line with the potential electrodes 2 and 3 positioned at certain fixed dis- 

 tances, usually one and four units, from the stationary energizing elec- 

 trode. The moving energizing electrode t is started a distance of about 

 four and one-half units from the stationary energizing electrode. The 

 unit length depends upon the desired depth of measurement and the rela- 

 tive conductivities of the strata in the area. The zone of investigation 

 increases in depth as the energizing electrode 4 moves out. (Figure 302A.) 



If direct current or very low fre- 

 quency alternating current is em- 

 ployed, the data recorded during 

 this movement of the electrode are 

 the values of the potential, current, 

 and electrode separation. 



Upon completion of the series 

 of measurements, the entire elec- 

 trode configuration is moved for- 

 ward along the traverse line about 

 1000 feet. Recordings are then 

 made of the potential, current, and 

 electrode spacing as the moving 

 electrode proceeds inwardly. (Fig- 



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

 Patent 2,192,404, issued March 5, 1940. 



Fig. 302.- 



-Electrode arrangement for vertical 

 exploration. 



