BORE-HOLE INVESTIGATIONS 



1061 



tend to be minimized on the recording, so that if more feature in the resistive sections 

 is desired, it is necessary to make a recording through these sections using a greater 

 sensitivity. Ahhough a great deal of feature cannot be expected for very high resis- 

 tivities, much more feature is available at low resistivities than on the conventional 

 electric log. 



Investigation Characteristics 



The investigation characteristics of an induction logging sonde may be described 

 according to the manner in which the apparent conductivity reading is affected by : 

 (1) horizontal variations in the surrounding media; (2) vertical variations in the sur- 

 rounding media. 



Plot of geometricol foctor for cylindricol 

 shells of ground of infinite length 



RADIAL INVESTIGATION CHARACTERISTIC 

 ( Two-coil system spocing "l" ) 



Fig. 656. — Curve showing radial characteristic of two-coil 

 induction sonde. 



The first effect is described by means of the radial investigation characteristic, the 

 second by means of the vertical investigation characteristic. 



Radial Investigation Characteristic. — Consider a very thick formation in which 

 the conductivity does not vary with depth, but does vary with the radial distance from 

 the axis of the hole ; for example, let the formation next to the hole, invaded with mud 

 filtrate, have one value of conductivity, whereas the formation at a distance from the 

 hole has some different value of conductivity. The boundaries between regions of 

 differing conductivity, in this case, are circular cylinders coaxial with the hole. 



