1042 EXPLORATION GEOPHYSICS 



this figure it is supposed that the S.P. current is prevented from flowing by means of 

 insulating plugs, placed in the hole to intercept the electrical continuity of the mud 

 column at the boundaries between sand and clay. 



The value of the potential within each single medium, enclosed by boundaries or 

 plugs, would be constant. However, the potential would vary from medium to medium, 

 the difference of the potentials in the different media being equal to the electromotive 

 force existing at their common boundary. 



In the case illustrated it can be shown that the difference of the potentials in the 

 mud between and outside of the plugs would be equal to the total of the electromotive 

 forces a, b, and c. If the potentials in the mud could be recorded in this idealized case, 

 a log such as the dashed, cross-hatched curve on the left side of Figure 643A would be 

 recorded, and the amplitude of the deflection opposite the permeable layer would be 

 equal to the total of the electromotive forces, a -{- b -{- c. 



Such a diagram, plotted for the case in which no current is allowed to flow, i.e., 

 for the static case, is called a static S.P. diagram and is often useful in reaching a 

 better understanding of the effect of the electromotive forces. 



Factors Influencing the Shape and Amplitude of the S.P. Peaks — 

 the S.P. Log in Soft Formations'^ 



As shown in Figure 643B, the current circulates in the mud not only opposite the 

 permeable formation, but also a short distance beyond its boundaries. As a result, 

 although on the static S.P. diagram the boundaries of the permeable beds are indicated 

 by sharp breaks, the S.P. log exhibits a more progressive change in potential, extending 

 along the drill hole beyond the boundaries of that bed. 



An analysis of the circulation of the current shows that the boundaries of the layer 

 are located at the level of the inflection points on the S.P. log. This fact provides a way 

 of determining the thickness of a bed from the S.P. log. 



Moreover, since the S.P. log records only that portion of the potential drop occur- 

 ring in the mud, the amplitude of the peak of the S.P. log approaches the amplitude of 

 the static S.P. only in favorable cases, when the resistance offered to the current by the 

 sand layer and the adjacent formations is negligible in comparison with the resistance 

 of the mud in the bore-hole. 



The shape and the amplitude of the peak on the log opposite a given bed may be 

 influenced by the following factors : (a) the total electromotive forces involved (static 

 S.P.) ; (b) the thickness of the bed; (c) the resistivity of the bed, of the surrounding 

 formations, and of the mud; (d) the diameter of the drill hole; (e) the depth of pene- 

 tration of the mud filtrate in the permeable beds; (f) the presence of impervious and 

 conductive material, such as shale, inside the permeable bed. 



The S.P. log would in addition be influenced by a lack of homogeneity of the mud; 

 a change in salinity of the mud at a certain level would result in a base-line shift at 

 that level. However, it has been found in practice that such changes in salinity are very 

 seldom encountered. 



The manner in which the above-mentioned factors influence the S.P. may be 

 explained for the case of permeable beds, situated between impervious formations, the 

 resistivity of which is less than, or of the same order as, that of the permeable beds. 

 This case is typical of the so-called soft formations. The case of permeable zones in 

 hard formations (limestone) is a rather specific topic which will be discussed separately. 



(a) All other factors remaining the same, a change of the total E.M.F.'s affects 

 the amplitude but does not otherwise modify the shape of the S.P. log. 



In practice, the E.M.F.'s involved may vary in different bore holes, either because 

 the salinity of the mud (or of the formations) is quite different, or to a smaller extent 

 because the differential pressure between the mud and the formations is different. In a 

 given hole, however, and for the same type of formations and depth, there is a definite 

 tendency for the total E.M.F.'s to be the same for all beds of the same type. 



t H. G. Doll, "The S.P. Log: Theoretical Analysis and Principles of Interpretation," A.I.M.E. 

 Tech. Pub. 2463 (1948). 



