(a + bt-c)[ 



_SU'_E_AND-£_: ELECTROMOTIVE FORCES ACROSS BOUNDARIES A, B AND C. 

 [v71 |v -c| --: POTENTIAL IN CORRESPONDING MEDIUM 



STATIC SP DIAGRAM - POTENTIAL IN MUD WHEN SP CURRENTS ARE 



PREVENTED FROM FLOWING. 

 SP LOG - POTENTIAL IN MUD WHEN SP CURRENTS ARE FLOWING 



Figure 14-6. Schematic representation of current and potential distribution. 



water, the current circulates in the direction of the arrows (from inside the 

 borehole toward the permeable bed) . 



Along its path, the SP current has to force its way through a series of 

 resistances, both in the ground and in the mud. Along a given line of flow, 

 the potential decreases continuously in the direction of the current, as indicated 

 by arrows; but across each boundary the potential increases by an amount 

 corresponding to the value of the emf at the boundary. Along a closed line of 

 current flow, the total of the ohmic-potential drops is necessarily equal to the 

 algebraic sum of the emf's encountered. 



279 



