1018 EXPLORATION GEOPHYSICS 



Resistance Measurements 



The Unit of Resistivity. — In electrical logging the practical unit 

 of resistivity is the ohm-meter. The resistivity of a substance in ohm-meters 

 is equal to the resistance in ohms of a cube of the substance if placed 

 between two parallel conducting plates one meter square, and separated 

 one meter. In present practice the same unit is also commonly designated 

 ohm-meters squared per meter (ohm tn~/m). In the conventional method 

 of plotting a resistivity log, an increase in resistivity causes an excursion to 

 the right. 



The Relation of Salinity and Temperature to Resistivity. — It will 

 be seen later that most problems of resistivity logging involve comparisons 

 between different resistivity measurements, such as, for instance, the 

 resistivity of the drilling mud and the resistivity of a formation or of the 

 interstitial water in the formation. Sometimes the resistivity of a formation 

 water must be deduced from a knowledge of its salt content. In making such 

 comparisons the following laws of electrolytic conduction should be borne 

 in mind : 



The conductivity of an electrolyte increases roughly in proportion to 

 the amount of chemicals in solution. Formation waters and drilling muds 

 usually contain several chemicals in solution, which differ in their ability 

 to conduct electric current. It is convenient in electrical logging calcula- 

 tions to convert the amounts of the various electrolytes present into their 

 equivalent weights of sodium chloride. The resulting equivalent concentra- 

 tion is expressed in parts per million or grains per gallon of sodium chloride. 



The resistivity of an electrolyte decreases with temperature. It is fre- 

 quently necessary to compare the resistivity of the drilling mud, which 

 has been measured at surface temperatures, to that of a formation meas- 

 ured at a much higher temperature in a deep bore hole. Obviously the two 

 values of resistivity cannot be compared without converting them to values 

 which would have been observed at a common temperature. This conversion, 

 for most drilling muds, may be made by assuming sodium chloride as the 

 electrolyte, and calculating its change in resistivity versus temperature.f 



Total Resistance Measurements. — The simplest method of measur- 

 ing the electrical resistance of the formations traversed by a drill hole 

 employs two electrodes. One electrode is lowered into the drill hole while 

 the other electrode remains fixed at the surface of the ground or at some 

 other convenient point. $ Referring to Figure 632, electrode A is arranged 



t Noyes and Coolidge, Jour. Amer. Cliem. Society, 26, 134. 

 E. B. Millard, Physical Chemistry for Colleges, McGraw-Hill. 



t C. Schlumberger, "Electrical Process and Apparatus for the Determination of the Nature 

 of the Geological Formations Traversed by a Drill Hole," U. S. Patent 1,819,923, issued 

 Aug. 18, 1931. 



