Chap. 2] 



METHODS OF GEOPHYSICAL EXPLORATION 



29 



"type" curves calculated for given conductivity ratios and for various 

 possible depths. 



Potential-drop-ratio methods. The essential feature of the resistivity 

 methods previously discussed is a determination of the potential difference 

 between two points at the surface and a measurement of the current in 

 the external circuit. In potential-drop-ratio methods current measure- 

 ments in the external circuit are not made and the potential drops in two 

 successive ground intervals (represented by three stakes arranged in a 



Apparent 

 Pesistlvifif 



Milhamm^ir, 



Pofenfta/Drop- 

 Qatio Curye 



■ Narma/ffafio 



Double 



Commutafor 



-#- 



impfifier^ 



i»vv^sy/VV/^\\vv!^/V\\v\vyy//'/V\\^\yy/^^^ 



'I l' l'lll'"l'l' 



-rn 



/////////////////// 



'/ / /' '/ '/ ''III mi I 'f/i I 'I - ' 



■"' '/'/' I'/'/'m'/i'i'/i'/'J • • 



"^ ','''" Mf" '"' i/i, ' ' 

 ''!'i''!''',\'m/' '/'''' * * 



Fig. 2-13. Resistivity and potential-drop-ratio mapping (schematic). 



straight line, radiating from one of the power electrodes) are compared. 

 The potential-drop-ratio method is best suited for the location of vertical 

 formation boundaries (faults, dikes, veins, and the like). The arrange- 

 ment used for the comparison of the potentials is a modified A.C. Wheat- 

 stone bridge; the two external stakes are connected to two resistors (with 

 condensers in series or parallel) whose center tap connects through the 

 indicating instrument to the center stake. When this arrangement is 

 moved across a vertical formation boundary, a potential-drop ratio greater 

 than one is obtained when proceeding from a medium of lower resistivity 

 toward a medium of greater resistivity, and vice versa (see Fig. 2-13). 



