Principle 



In this method of logging, the formations surrounding the logging 

 apparatus are energized by electromagnetic induction. An alternating current 

 is made to flow through a transmitter coil. The alternating magnetic field due 

 to this current induces eddy currents in the earth surrounding the coil. These 

 eddy currents in turn have their own magnetic field, which induces an electro- 

 motive force in a receiver coil. 



If the transmitter current is kept constant, the magnitude of the eddy 

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

 inversely proportional to the earth resistivity. 



Figure 14-14 shows schematically a simple induction logging system in 

 which a transmitter coil and a receiver coil are wound co-axially on a supporting 

 insulating mandrel. The distance between the coils, designated in the figure 

 as L, is called spacing. 



An alternating current of constant magnitude and frequency is fed to 

 the transmitter coil from an oscillator which, with the amplifier, is actually 

 housed in an electronic cartridge above the coil system. 



The voltage induced in the receiver coil by linkage with the magnetic 

 field of the eddy current is fed into an electronic system where, after being 

 amplified, it is rectified into direct current for transmission to the surface, 

 where it is recorded. In logging, the point of measurement is taken to be a 

 point midway between the coils. 



In addition to the two coils referred to above, designated as the main 

 coils, the apparatus used in practice includes several additional coils. The 

 characteristics of the main coils and of the additional coils, their distribution, 

 and their respective positions are adjusted in order to minimize the influence 

 on the measurements of the mud column and of the media located above and 

 below the instrument. Such types of apparatus are called focusing sondes. 



The most usual types of instruments in the field at the present time are 

 the 27-inch and the 40-inch focusing sondes. The former has a lesser radius 

 of investigation but a slightly better vertical resolving power than the second. 



Scaling 



Some of the scales used for the presentation of the induction logs are 

 different from those of other logs. Inasmuch as the signal measured is propor- 

 tional to the conductivity of the formations, the induction log is scaled in terms 

 of conductivity. 



The conductivity scale is linear and is referred to a zero line located at 

 the right side of the log so that the deflections of the curves toward the left 

 correspond to increases of conductivity. 



299 



