398 Subsurface Geologic Methods 



putation of departure curves is rather complicated and can only lead to a 

 fair approximation when the beds become thinner and more homogeneous. 

 In induction logging the situation is entirely different. If the hole 

 is vertical, as will be assumed to simplify the discussion, the lines of 

 current flow are horizontal circumferences having their centers on the 

 axis of the hole. Since there is generally a symmetry of revolution of 

 the ground around the axis of the drill hole, each line of current flow 

 remains in the same medium all along its path and never crosses a bound- 

 ary between media of different conductivities. On the other hand, if the 

 frequency is not extremely high, the reaction of the different circular 

 currents on one another can be neglected. In this condition, the action of 

 the different regions of ground, which individually have a symmetry of 

 revolution around the hole, can be considered separately, and the measured 

 signal is simply the sum of the individual signals given by the different 

 regions. The consequence is that the theoretical computation of charts or 

 of typical logs corresponding to any distribution of ground conductivities 

 is always possible, if, of course, there be a symmetry of revolution, as is 

 usually the case. 



Conclusion 



The conductivity or the resistivity of formations traversed by a drill 

 hole can be determined by the induction-logging method. This new tech- 

 nique is particularly useful at present for logging dry holes and holes 

 filled with oil-base mud, in which direct contact with the formations is 

 difficult to establish. 



The method has great flexibility and is quite promising. Coil systems 

 can be designed to give a focusing effect in order to obtain directly on 

 the log a more accurate value for the conductivity of beds of finite thick- 

 ness. 



Since the different regions of ground generally have a symmetry of 

 revolution around the axis of the hole and since, therefore, the induced 

 currents have circular paths around that axis, the currents never cross the 

 boundary from one region to the other. In these conditions the contribu- 

 tions of the different regions to the measured signal can be considered in- 

 dependently. For that reason it is relatively easy to compute typical logs 

 and correction charts, which should greatly improve the possibilities of 

 quantitative interpretation. 



