Generator 



Meter 



Generator 



Meter 



a) Two -Electrode circuit b) Actual circuit 



Figure 14-9. Schematic normal device. 



Apparent Resistivity 



Up to this point a homogeneous medium has been assumed. It is obvious, 

 however, that in actual drill holes this condition does not exist. The existence of 

 the borehole rilled with mud, the inevitable nonhomogeneities in the formation, 

 and the existence of an invaded zone in permeable formations are all factors 

 that affect the current pattern surrounding the current electrodes A and the 

 resulting potentials measured. 



In a heterogeneous medium, then, the exploring device measures a quantity 

 called the apparent resistivity. The apparent resistivity is an average value 

 involving the resistivities of all the media surrounding the electrodes and do- 

 pending 0:1 tho arrangement end cpacings of the device. 



Radius of Investigation cf a Device 



The manner in which the various media surrounding the electrode affect 

 the value of apparent resistivity cbtaincd with a given device is sometimes indi- 

 cated by reference to its radius of investigation. This characteristic is depend- 



285 



