Also, since the magitude 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 is divided between the differ- 

 ent formations and the mud in proportion to the resistances encountered in 

 each respective medium. Accordingly, the potential drop in the mud of the 

 drill hole is a measure of the greater part of the total emf's because the electrical 

 resistance offered by the mud column is much greater than that offered by the 

 formations. 



Static SP 



It is convenient to indicate the value of the emf forces which produce the 

 SP currents by an idealized representation in which it is supposed that the SP 

 current is prevented from flowing. Under these conditions the potential in the 

 mud column when plotted would appear as shown as the dashed crosshatched 

 curve on the left of figure 14-6. Since this is the maximum SP that could be 

 measured, it is convenient to use this theoretical value as a reference; and it 

 is called the static SP. 



Factors Influencing Shape and Amplitude of SP Peaks 



As shown in Figure 14-6, 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 SP diagram the boundaries of the permeable 

 beds are indicated by sharp peaks, the SP 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 bed boundaries 

 are located at the inflection points on the SP log. This fact provides a means 

 of accurately determining the thickness of a bed from the SP log. 



Moreover, since the SP log records only that portion of the potential drop 

 occurring in the mud, the amplitude of the peak of the SP log approaches the 

 amplitude of the SP only when the resistance offered to the current by the bed 

 itself and the adjacent formations is negligible compared with the resistance 

 of the mud in the borehole. 



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 SP) ; (b) the thick- 

 ness 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 amount of invasion; 

 and (f) the presence of interstitial shale. All other factors remaining the same, 

 a change of the total emf's affects the amplitude but does not modify the shape 

 of the SP log. 



280 



