In practice, the emfs involved may vary from one hole to another because 

 the salinity of the mud and/or of the formation waters is quite different. The 

 amplitude of the emf also varies with the amount of shaly material inside the 

 permeable bed. In a given hole, however, and for the same type of formations 

 and for comparable depths, there is a definite tendency for the total emfs to 

 be the same, provided the beds are all essentially clean. 



Permeable beds of different porosity, or with different grain size, give 

 the same emfs. The emfs are also independent of permeability, even down 

 to fractions of one millidarcy. 



It has been observed that the salinity is not always constant for all 

 permeable beds pentrated by a hole, especially at widely different depths or 

 in very different formations. Fresh-water sands or high-salinity sands will 

 show low— or large— amplitude peaks, respectively. The polarity of the peak 

 may even reverse when the water in the sand is less salty than the mud nitrate. 



The manner in which the above-mentioned factors, besides the emfs, in- 

 fluence the SP will first be explained for the permeable beds between shales, as 

 shown on Figure 14-7. In this example, the resistivities of the permeable beds are 



100 

 H-Millivolts -*- 



100 100 



-Millivolts-*] p — Millivolts-*] 



100 



Millivolts- 



s» .. ■ — - 



CB M ■ • / • »»)»>»»*& !■ VZ7ZV 



PERMEABLE STRATA , P 

 STATIC SP DIA6RAM 



IMPERVIOUS STRATA, C 

 . S P LOG 



Figure 14.7. Comparison of SP for different values of R^IL 



281 



