BORE-HOLE INVESTIGATIONS 



1041 



current, as indicated by arrows, but across each boundary where an electromotive force 

 occurs, the potential increases by an amount corresponding to the value of the electro- 

 motive force. Along a closed line of current flow, the total of the potential drops is 

 necessarily equal to the sum of the electromotive forces encountered. 



Also, since the magnitude of the current flow is constant all along its path, the 

 potential drop varies according to the resistance of the section through which it flows. 

 This means that the total potential drop (which is equal to the sum of the electromotive 



^•-■-'^"'O -'— : ELECTBOMOTIVE forces ICROSS eOUXDARIES A.BtNOC. 

 H. I Vl "- 'OTEMTUL IN C0RRE3P0N0IN0 UEDIuy. 



.TIC S. P DUORAU - P0TENTI4L IK HUO WtlEN S P CURREHTS »RF HIEVtHTED FROM rLOWWt. 

 LOO - POTEHTIAL IN HUD WHEN S P CURRENTS ARE FLOWINO. 



Fig. 643. — Schematic representation of potential and current distribution in and around 

 a permeable bed. (After H. G. Doll, A.I.M.E. Tech. Pub. 2463, Sept., 1948.) 



forces) is divided between the different formations and the mud in proportion to the 

 resistance encountered by the current in each respective medium. Accordingly, the 

 potential drop in the mud of the drill hole is a measure of only part of the total E.M.F., 

 unless the electrical resistance offered by the mud is very large compared to that offered 

 by the formations. 



The Static S.P. — It is convenient to indicate the values of the electromotive forces 

 which produce the S.P. currents, and which therefore determine the S.P. log, by an 

 idealized representation based on the hypothetical case illustrated in Figure 643A. In 



