BORE-HOLE INVESTIGATIONS 1021 



ured between electrode A and an electrode at a great distance, provided 

 the latter electrode is of large enough dimensions that its own mono- 

 electrode resistance is negligible. 



A modification of the method employs an electrode system wherein two 

 electrodes are lowered into the drill hole at a fixed distance apart. Varia- 

 tions in the total circuit resistance are due to changes in earth materials in 

 the vicinities of both of the electrodes. For example, if the separation of 

 the moving electrodes were such that at any depth the two electrodes were 

 opposite two strata having different resistances, the total resistance meas- 

 ured would depend on both formations. Because these two variables cannot 

 be separated where numerous anomalies occur within distances comparable 

 to the electrode separation, it is necessary : (a) to use a small separation of 

 the electrodes so they both will be afifected by the same condition or (b) to 

 make one of the electrodes sufficiently long (about 100 feet or more) so 

 that it is not affected appreciably by the formations being passed.t If the 

 electrodes are separated by too small a distance, the penetration into the 

 strata will not be great enough to give resistance data which are sufficiently 

 independent of variations in the size of the bore hole and the composition 

 of the drilling mud. (The penetration is usually less than l^ the effective 

 separation of the electrodes.) 



In a later development of the two-electrode method, t an electrode 

 system is lowered into the drill hole and measurements are made of the 

 alternating current properties of the subsurface included between the two 

 electrodes. The electrodes are maintained a fixed distance apart, and the 

 entire assembly is moved in the drill hole. Measurements are preferably 

 made with a moderate or high frequency alternating current. 



Blau and Gemmer (§) utilize a similar electrode arrangement wherein 

 an alternating current may be passed between two electrodes in the hole or 

 between a single moving electrode and a stationary surface electrode. The 

 variations in current which are caused by changes in the effective resistance 

 of the strata are measured in a suitable auxiliary circuit. A modifica- 

 tion of this method utilizes an electrical transient instead of an alter- 

 nating current. ft 



McDermott:j:$ has utilized the single moving electrode arrangement in 

 a bore-hole exploring method which does not require removal of the drill 

 pipe from the bore hole before making the measurements. In this method, 

 the drill and tubing are raised the desired distance off bottom, and an insu- 

 lated conductor, on the end of which is a weighted electrode, is lowered 

 through the inside of the drill pipe. The electrode passes out of the bit and 



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

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



% T. J. Takoskv. "Method and Apparatus for Alternating Current Investigation of Uncased 

 Drill Holes," U. S. Patent 2,038,046. issued April 21, 1936. 



§ Ludwig S. Blau and Ralph W. Gemmer, "Method and Apparatus for Logging a Well," 

 U.S. Patent 2.037.306, issued April 14. 1936. 



tt L. W. Blau and L. Statham, "Electrical Transient Well Logging," U. S. Patent 2,165,213, 

 issued Tulv 11, 1939. 



tt E. McDermott, "Method of Electrically Exploring Bore Holes," U. S. Patent 2,070,912, 

 issued Feb. 16, 1937. 



