BORE-HOLE INVESTIGATIONS 



1047 



which they can return to the mud in the hole, and then through the mud back to the 

 permeable beds to complete their circuits. They cannot come back to the mud through 

 other permeable beds, because they would encounter E.M.F.'s which would oppose the 

 flow of currents in that direction. When the first conductive beds they encounter are 

 of the permeable type, they simply cross them until they reach the more conductive and 

 impervious beds. This is the case for the S.P. currents which penetrate the permeable 

 bed Po ; they have to cross the permeable bed P? in order to reach the impervious bed 

 Cb, or have to cross the permeable bed Pn in order to reach the impervious bed Cis. 



P, PERMEABLE LIME, 

 FROM CORES 



NO RECOVERY 



(a) 



:b) 



SOFT FORMATIONS (Sands ond Sholes) 



HARD FORMATIONS (Limestone) 



Fig. 646. — Comp-arisons of S.P. and resistivity logs in soft and hard formations. 

 (After H. G. Doll, he. cit.) 



All along the drill hole opposite a given hard formation, the current in the mud 

 column remains substantially the same, and so does the drop of potential per unit length 

 of hole, thus giving a constant slope as shown by the S.P. log of Figure 645A. 



At the level of each conductive bed, some S.P. current generally enters or leaves 

 the hole; therefore the slope of the S.P. log is modified. In Figure 645A, for example, 

 the S.P. log changes its slope at the level of the permeable bed P7, because part of the 

 current leaves the hole and flows into that bed. 



As a general rule, the permeable beds are characterized on the S.P. log by slope 

 changes, or curvatures, with the convexity toward the negative side of the log, and 



