394 Subsurface Geologic Methods 



the hole. The record thus produced, which is frequently called an "induc- 

 tion log" because of the way in which it is obtained, shows the variations 

 of the ground conductivity — and, consequently, of its inverse, the ground 

 resistivity — with respect to depth. It is, therefore, equivalent to the 

 resistivity log obtained by the conventional method of electric logging 

 with electrodes in water-base mud. 



The advantages of the method are more immediate, especially in view 

 of the difficulties encountered by the conventional method of electric log- 

 ging. This does not mean that the induction-logging method will not work 

 in water-base mud; on the contrary, it is believed that in that case also 

 the method will have important advantages. Experience in water-base 

 mud is, however, still very limited, not only because the available instru- 

 ments were all applied to oil-base-mud operations, where they were badly 

 needed, but also because certain improvements, which are still being 

 studied, are to be introduced for best operation in water-base mud. This is 

 why it is felt advisable not to discuss the use of induction logging in water- 

 base mud at this time and to wait for the results of field tests that will be 

 made for that case. 



In conventional logging practice, the resistivity unit is the ohm-meter. 



Conductivities are expressed in mhos per meter. It is preferred, however, 



to use units of millimhos per meter for induction logging in order to get 



a range of values that does not require an extensive use of decimal figures. 



Accordingly, 



1,000 



C mmhos/m = -^ — — 



K ohm/m 



Thereby, a bed with a resistivity of 100 ohm/m has a conductivity of 

 10 mmhos/m. 



Resistivity Measurements by Induction Logging 



The apparatus used for induction logging is shown schematically in 

 figure 176; it is in fact a mutual-impedance bridge. It comprises essen- 

 tially a transmitter coil T, fed with alternating current by an oscillator, 

 and a receiver coil R connected through an amplifier to the recording 

 galvanometer. In the absence of any conductive medium around the ap- 

 paratus, as, for example, when it is suspended in the air from a wood 

 frame high enough above ground, the coupling between the transmitter 

 and receiver coils is fully balanced, so that the measuring apparatus reads 

 "zero." When the apparatus is in a drill hole, the alternating field set up 

 by the transmitter coil produces in the surrounding medium, i.e., in the 

 ground, induced currents, generally known as "eddy currents," which 

 are proportional to the conductivity of the ground. The electromotive 

 force induced in the receiver coil by the eddy currents, referred to here- 

 after as the "signal," and designated by E, is proportional to the con- 

 ductivity of the ground. If, therefore, the apparatus is properly calibrated, 



