1046 



EXPLORATION GEOPHYSICS 



Figure 645A represents schematically the case of four thin permeable zones Pa, P7, 

 P9 and Pii, and three thick shales Ci, Cs, and C13, separated from each other by thick, 

 compact, and highly resistive beds H2, Hi, He, Ha, Hio, and H12. In order to charac- 

 terize the problem more explicitly, it is assumed, taking the resistivity of the mud as 

 equal to unity, that the resistivity of the permeable zones, as well as that of the shales. 



C, (V @ — 



CONSTANTS- 

 STATIC S.P- 100 m¥ 

 RESISTANCE OF 10 OHMS 

 PER FOOT OF DRILL HOLE. 



(a) schematic representation of 

 formations and s.p log 



@ go9 ) APPROXIMATE VALUES 

 OF RESISTIVITY 



(B) EQUIVALENT (C) SCHEMATIC 



ELECTRICAL NETWORK DISTRIBUTION OF 

 S.P CURRENTS 



-STATIC &R DIAGRAM 



Fig. 645. — A schematic representation of S.P. phenomena in highly resistive formation. (After 

 H. G. Doll, A.I.M.E. Tech. Pub. 2463, Sept., 1948.) 



is approximately 10, whereas the hard formations have a relative resistivity of 500 or 

 more. The log shown in the illustration corresponds approximately to these figures in 

 ohm-meters. 



The E.M.F.'s involved are represented by the static S.P. diagram on which the 

 S.P. is superimposed. As can be seen, the departure of the S.P. log from the static S.P. 

 diagram is remarkable in this case, and often a log of this type is considered abnormal. 



The circulation of the S.P. currents is represented, in a schematic way, in Figure 

 645B. The S.P. currents, which are generated by the different E.M.F.'s, flow into the 

 sands. They cannot traverse the adjacent hard formations through sections located 

 close to the drill hole, because these sections are small in area and introduce large 

 resistances which practically prevent the current from flowing. On the contrary, the 

 S.P. current penetrates deeper than usual in the permeable beds and consequently enters 

 the hard formations through larger cross sections. From there on, it is easier for the 

 S.P. currents to continue their path in the hard formations without appreciable reduc- 

 tion in their cross section, as would be required if they were to converge quickly 

 toward the hole. The S.P. currents flow therefore toward conductive beds through 



