1058 EXPLORATION GEOPHYSICS 



formations are highly resistant, the measurements are not too reliable 

 because of poor or intermittent contact. To overcome these difficulties 

 induction logging may be employed. 



Induction logging does not require any direct contact between the 

 exploring device and the mud or the formations. It v^^ould work just as 

 well in a hole cased with a highly resistive material such as bakelite or 

 concrete pipe. Like conventional electrical logging, however, it will not 

 work in holes cased with metallic pipe. Induction logging has been oper- 

 ated successfully in water-base drilling muds, and produces a curve very 

 similar to the conventional resistivity curve. 



In this method of logging,f the formations are energized by electro- 

 magnetic induction. This energizing field is created by an alternating 

 current flowing through a "transmitter" coil, which is one component of 

 the exploring device. The alternating magnetic field induces eddy currents 

 in the earth surrounding the coil. These eddy currents set up a secondary 

 magnetic field, and induce an electromotive force in a "receiver" coil, 

 which comprises the other component of the exploring device. If the 

 energizing current is maintained at a constant value and frequency, the 

 electromotive force thus induced in the receiver coil is proportional to the 

 mutual inductance; i.e., (1) to the spacing between the two coils, and (2) 

 to the conductivity of the earth, and consequently is inversely proportional 

 to the earth resistivity. See pages 605 to 607 for a discussion of the 

 factors affecting the mutual inductance between two parallel coils. A further 

 development of inductive logging has been made using high frequencies. | 



Figure 654A shows schematically a simple induction logging system in which an 

 energizing coil and a receiver coil are wound coaxially on a supporting insulating 

 mandrel. The distance between the coils, designated in the figure as L, is called the 

 spacing and remains constant during a run. The larger the value of L, the greater the 

 eflfective penetration of the flux into the strata. 



An alternating current of constant magnitude and frequency (in certain cases, of 

 two different frequencies§) is fed to the transmitter coil from an oscillator, which is 

 housed with the amplifier in a steel cartridge above the coil system. (See Figure 6S4B.) 



The voltage induced in the receiver coil is amplified and then rectified into direct 

 current for transmission through the cable to the recorder at the surface. As in con- 

 ventional electrical logging, the readings are recorded continuously on film as the 

 instrument traverses the bore hole. The point of measurement is taken to be at point O, 

 midway between the coils. 



Before entering the bore hole, the instrument is adjusted (in air) to a reading of 

 desired value. The receiving coil actually comprises two coils : one spaced at distance 

 L (whereby its mutual inductance with the energizing coil is affected by the resistance 

 of the material through which the magnetic flux penetrates) and another coil of very 

 small area and number of turns. This latter coil is positioned close enough to the 

 energizing coil so that its coefficient of coupling is substantially constant regardless of 

 the material surrounding the bore hole. The initial adjustment (to one side of the null 



t H. G. Doll, "Introduction to Induction Logging and Application to Logging of Wells Drilled 

 with Oil-Base Mud," A.I.M.E. meeting, San Francisco, February, 1949. 



t J. W. Millington, "Electrical Prospecting Method and Apparatus," U. S. Patent 2,376,610, 

 May 22, 1945; also U. S. Patent 2,398,800, April 23, 1946. 



C. B. Aiken, "Electrical Logging," U. S. Patent 2,390,409, Dec. 4, 1945. 



§ C. B. Bazzoni and J. W. Millington, "Electrical Prospecting Apparatus," U. S. Patent 

 2,408,029, Sept. 24, 1946. 



