1060 



EXPLORATION GEOPHYSICS 



In Figure 655 is shown a comparison of an induction log with the conventional 

 electrical log in the same well. The resemblance of the induction log to the resistivity 

 log is at once apparent. 



Since for induction logging the direct-current signal coming into the recorder is 

 proportional to the conductivity of the formation, the deflection of the trace will also 

 be proportional to the conductivity. Zero conductivity, i.e., infinite resistivity, corre- 

 sponds to zero signal, and for this case the recorded trace is made to print near the 



Fig. 655. — Comparison of S.P. and resistivity logs in 

 a conductive mud, with an induction log in an oil-base 

 insulating mud. 



righthand side of the film track. Increasing conductivity causes the trace to move to 

 the left in proportion. » 



The upper sensitivity scale on the induction log of Figure 655 is the conductivity 

 scale (in millimhos per meter), which is seen to be a linear scale. The underneath 

 sensitivity scale is the equivalent scale of resistivity in ohm-meters. The resistivity 

 scale is of course non-linear; variations of resistivity tend to be amplified for low 

 readings and minimized for high readings. 



The proper scale for the recording of an induction log will depend on the range of 

 conductivities (or resistivities) to be encountered. It is usual practice to keep the 

 sensitivity low enough so that the greatest conductivities recorded will not go off-scale. 

 However, as already pointed out, variations in resistivity in the more resistive sections 



