1022 EXPLORATION GEOPHYSICS 



is employed for mapping the resistance changes in its vicinity. The method 

 is essentially one of mapping short intervals of formation, after which the 

 drilling operations are resumed. The chief intention of the invention is to 

 avoid the necessity of removing the drill pipe and bit from the bore hole 

 before making the survey. 



Karcherf has developed a modification of the single moving electrode 

 method which is designed for exploring bore holes continuously during the 

 process of drilling. This method utilizes an arrangement which causes an 

 electric current to flow through a conductor, the drill bit, and thence into 

 the earth. Both the conductor and the drill bit are insulated from the drill 

 pipe. From an operator's viewpoint, the method has the advantage of allow- 

 ing continuous exploration of the bore hole during the drilling process and 

 the disadvantage of necessitating a special type of insulating cable passing 

 through the drill pipe. In addition, it is necessary that the bit be electrically 

 insulated from the drill stem by an insulating stem or section which is 

 mechanically capable of taking the entire drilling load. An alternate 

 arrangement consists in insulating the entire drill stem, thereby leaving 

 only the bit exposed for contact with the formations. 



Another continuous logging method| has been developed wherein the 

 measurements are made between the entire drill rod assembly, including 

 the rotary equipment, and an electrode which is placed at a distance from 

 the well comparable to the depth of the hole. In this system, the electrical 

 properties of the earth included between the entire drill mechanism and the 

 distant electrode are measured. A recording is made of the variations in 

 electrical properties as the drill stem traverses the strata. Measurements 

 may be made with direct or alternating current. The electrical logs show 

 variations of an electrical property versus depth. Usually, simultaneous 

 continuous recording is made: (a) of the fluctuating potentials associated 

 with the cutting action of the bit and (b) of the change in electrical resist- 

 ance or impedance between the drill stem and the distant electrode. 



Several modifications of this method are used. In one modification the 

 drilling is done with oil or other poorly conducting fluid, such as is utilized 

 in many areas to avoid sealing of the producing formations and other dam- 

 aging effects of drilling muds.§ Contact of the drill stem with the wall of 

 the hole is prevented by use of the rubber protectors commonly employed 

 during drilling operations. A second modification employs direct current for 

 energizing the ground. The back E.M.F. due to polarization around the 

 drill pipe provides a sufficient potential drop to cause a detectable portion 

 of the current to travel down the drill stem and flow into the earth at a point 



t John C. Karcher, "Method and Apparatus for Exploring Bore Holes," U. S. Patent 1,927,664, 

 issued Sept. 19, 1933. 



% J. J. Jakosky, "Method and Apparatus for Continuous Investigation of Drill Holes," U. S. 

 Patent 2,150,169, issued March 14, 1939. Reissue 21,102, May 30, 1939. 



J. T. Jakosky, "Method and Apparatus for Continuous Exploration of Bore-holes," U. S. 

 Patent 2,153.802, issued Apr. 11, 1939. 



J. J. Jakosky, "Method and Apparatus for Continuous Exploration of Bore-holes," U. S. 

 Patent 2.181.601, issued Nov. 28, 1939. 



§ J. J. Jakosky, "Method and Apparatus for Continuous Exploration of Bore Holes," U. S. 

 Patent 2,153,802, issued April 11, 1939, and Foreign Patents. 



