Subsurface Logging Methods 397 



a measure of the signal constitutes a quantitative determination of the 

 conductivity of the ground. 



The signal is amplified and rectified into direct current for trans- 

 mission in the cable to the surface where it is automatically recorded. 

 A remote-controlled test signal is provided in the apparatus to check the 

 calibration. 



The oscillator and the amplifier are contained in a pressure-proof 

 housing, called the "electronic cartridge," on top of the coil assembly. The 

 subsurface instrument is represented schematically in B of figure 176. An 

 induction log recorded in oil-base mud by this equipment is given in figure 

 177, alongside the conventional electric log of the same well recorded 

 later in water-base mud for comparison. 



When the ground surrounding the coil system is homogeneous, as 

 is practically the case for a thick bed which is not appreciably invaded 

 by the mud fluid, the conductivity, as measured by the apparatus, is equal 

 to the true conductivity of the ground. When, however, the ground around 

 the coil system is not homogeneous, as, for example, in the case of a thin 

 bed surrounded by formations of appreciably difl'erent conductivity, the 

 conductivity, as measured by the apparatus, represents a combination of 

 the conductivities of the different media surrounding the coil system and 

 is referred to as the "apparent conductivity." This is similar to what 

 happens for electric logging with electrodes, where the apparatus also 

 measures an apparent resistivity. In both cases, a better approximation of 

 the true conductivity can be obtained by applying corrections deduced 

 from the departure curves ^^ or correction charts. 



An important advantage of the induction-logging system is that the 

 measured values, even without corrections, are already nearer to the true 

 values; furthermore, the corrections themselves are much easier to com- 

 pute than in the case of logging with electrodes, particularly when in- 

 fluence of bed thickness is to be taken into consideration. 



Geometry of Induction Logging 



In the logging method using electrodes for the determination of the 

 ground resistivity the flow of current is of the radial type, and it is not 

 possible to study separately the influence of the different regions of ground 

 surrounding the electrode system. The reason is that the lines of current 

 flow cross the boundaries between the different media, such as, for ex- 

 ample, the boundary between a given bed and the bed next to it or the 

 boundary between the mud and a bed. If the resistivity of any given 

 medium is changed, this affects the lines of current flow even in their path 

 through the other media. This is the reason that the mathematical com- 



_ ^' Departure curves for electric logging with electrodes have been published earlier in a booklet 

 entitled "Resistivity Departure Curves," 1947, by Schlumberger Well Surveying Corporation, Houston, 

 Texas. The application of the curves was discussed in a paper on "True Resistivity Determination from 

 the Electric Log — Its Application to Log Analysis," by H. G. Doll, J. C. Legrand, and E. F. Stratton, 

 presented at the 1947 spring meeting of the Pacific Coast District, Division of Production, American 

 Petroleum Institute, at Los Angeles, California. 



