The unit of conductivity is the mho/m, reciprocal of ohm-m. In order 

 to avoid decimals in the evaluation of conductivity, the millimho/m is usually 

 used as a practical unit. For example, if formations have resistivity values of 

 1, 10, and 100 ohm-m, the corresponding conductivity figures will be 1000, 

 100, and 10 mmhos/m respectively. Resistivities (ohms) = 1000/Conductiv- 

 ities (millimhos) . 



The scale, if counted in terms of resistivities from the left to the right, 

 is hyperbolic. Therefore, the low resistivities are emphasized, and the high 

 resistivities are compressed. 



On many induction logs, an additional reciprocated curve is recorded 

 simultaneously. This curve, which gives the same measured values but with 

 a linear resistivity scale, makes possible an easier comparison of the induction 

 logs with the conventional logs. 



In order to conform to the usage of electrical logging, the factor resistivity 

 rather than conductivity will be generally considered in the following dis- 

 cussion. 



Interpretation 



The definition of different beds and the determination of their boundaries 

 are easier and more accurate with the induction log than with the conventional 

 logs. Sharp deflections occur exactly at the levels of the boundaries, and the 

 curve opposite a given bed shows practically no distortion. Good definition can 

 be obtained for thin beds down to a thickness of about 4 feet. Besides, the 

 hyperbolic resistivity scale brings the details quite conspicuously to light; the 

 lithological variations and the oil-water contacts are shown by large deflections 

 of the curve. 



Effect of Mud Column 



The signal contributed by the mud column has been eliminated in most 

 instances. The 40-inch induction is not influenced by the mud column when 

 centered in holes of less than 12 inches in diameter. When not centered, the 

 effect of the mud column is negligible in fresh muds. In low-resistivity muds 

 or in high-resistivity formations, the induction tool, if not centered, must have 

 at least a 1-inch standoff. 



Effect of Bed Thickness 



With induction logging, the measurements made at the level of a given 

 bed are much less affected by the adjacent formations than with the conventional 

 resistivity logs, all other conditions being the same. 



For a given bed thickness, the influence of the adjacent formations is 

 quite different, depending on whether they are more or less resistive than the 



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