BORE-HOLE INVESTIGATIONS 



1043 



Permeable beds of different porosity, or with different dimension of grains, give 

 tlie same E.M.F.'s, if other factors are unchanged. The E.M.F.'s are also independent 

 of the permeability value, even down to fractions of one millidarcy. 



It has been observed that the salinity and the differential pressure are not always 

 constant for all permeable beds penetrated by a hole, especially at widely different 

 depths or in very different formations. Fresh-water sands or very salty sands will show 

 respectively abnormally low or large amplitude peaks. The polarity of the peak may 

 even reverse if the water in the sand is less salty than the mud. 



DRILL HOLE^ 



;.:.;■.•.•:• PERMEABLE STRATA,? 

 STATIC S.P DIAGRAM 



IMPERVIOUS STRATA, C 

 . S P LOG 



Fig. 644. — Comparison of S.P. logs for different values of-^— . (After H. G. Doll, A.I.M.E. Tech. 

 Pub. 2463, Sept., 1948.) P"" 



(b) Theoretical computations and field experience have shown that the amplitude 

 of the S.P. deflection is practically equal to the static S.P. (i.e., it gives the value of the 

 total E.M.F.'s) when the permeable beds are very thick, and when the resistivities of 

 the formation are approximately the same as that of the mud. Moreover, in this case 

 the S.P. curves mark the location of the boundaries of the bed with great accuracy. 



The amplitude of the deflection is less than the static S.P. for thin beds, and the 

 smaller the bed thickness, the smaller the peak. 



(c) On the other hand, when the resistivity of the formations pt is higher than that 

 of the mud pm, the S.P. curves are rounded off, the boundaries are marked less accu- 

 rately, and all other conditions being the same, the magnitude of the peak is less than 



when the ratio -^-is close to 1. 



Pm 



Figure 644 shows theoretical data illustrating the influence of the thickness of bed 

 and the resistivity of the formations. To facilitate the comparison, the value of the 

 static S.P. has been supposed the same for all permeable beds and equal to 100 mv. 



