1024 EXPLORATION GEOPHYSICS 



It is advantageous to have both the shallow and the relatively deep 

 penetration curves and it is common commercial practice to furnish a log- 

 ging record which shows both curves. Interpretation is based on these two 

 resistance curves and the potential curve. 



Measurements may also be made between electrodes in two uncased 

 wells.t Broadly, this method contemplates disposing at least one electrode 

 in each of two wells and then adjusting the levels of these electrodes until 

 the resistance between them reaches a maximum or a minimum. 



Another arrangement utilizes resistance measurements along the bedding 

 planes of the strata.^ This method measures the resistance between an elec- 

 trode in the bore hole and another electrode spaced a considerable distance 

 from the hole. Another modification utilizes a distant electrode and two 

 electrodes in the hole to determine the anisotropic properties of the strata 

 by measuring vertically and along the bedding planes, and comparing the 

 two sets of measurements. § 



Electrical logging of cased holes has been proposed by Ennisff and 

 Neufeld4$ In this method a current is applied between casing and ground, 

 and the record is made from a movable electrode centered in the casing. 



The Normal Sonde.§§ — In order to remove almost entirely the 

 effects of the potential drop adjacent to the moving power electrode, resis- 

 tivity measurements are made over a short section of the total current path 

 which is sufficiently distant from the moving power electrode to be unafifected 

 by minor variations in its vicinity. 



In Figure 632 is pictured a two-electrode normal sonde,* which consists 

 of a current electrode A and a measuring electrode M, separated by a short 

 distance r, called the spacing. Current is passed between A and an electrode 

 B located at a distance from A; the potential difference created by the 

 flow of current is measured between M and an electrode A^^ which is remote 

 from both current electrodes and which is relatively at zero potential.** 

 It follows that the potential difference measured is equal to the potential E 



t p. F. Hawley, "Two-well Method of Electrical Logging and Apparatus Therefor," U. S. 

 Patent 2,183,565, issued Dec. 19, 1939. 



t .T. J. Jakosky, "Electrical Method and Apparatus for Determining Characteristics of 

 Geological Formations," U. S. Patent 2,155,133, issued April 18, 1938. 



§ J. J. Takosky. "Electrical Method and Apparatus for Determining Character of Geologic 

 Formations," U.S. Patent 2,140,798. issued Dec. 20. 1938. 



tt G. H. Ennis, "Method and Apparatus for Locating Formations in Cased Wells," U. S. Patent 

 2,414,194, Jan. 14, 1947. 



%t J. Neufeld, "Method of and Apparatus for Investigation of Cased Drill Holes," U. S. Patent 

 2,400,593, May 21, 1946. 



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

 Geological Formations Traversed by a Drill Hole," U. S. Patent 1,819,923, issued Aug. 18, 1931. 

 C. and M. Schlumberger and E. G. Leonardon, "Electrical Coring: A Method of Deter- 

 mining Bottom-hole Data by Electrical Measurements," A.I.M.E. Geophysical Prospecting, 1934, 

 p. 237. 



* In well logging practice the apparatus which is lowered into a drill hole at the end 

 of a cable, for the purpose of taking measurements of formation characteristics, is 

 often called an exploring device or sonde. 



** C. Schlumberger, "Electrical Device for the Determination of Specific Resistiv- 

 ity," U. S. Patent 1,894,328, Jan. 17, 1933. 



