BORE-HOLE INVESTIGATIONS 



1019 



to traverse the drill hole and electrode B is fixed in position at the surface 

 of the earth. This is termed a monoelectrode system. 



The resistance may be measured by use of a conventional bridge circuit, 

 or voltmeter and ammeter. Intermittent 

 readings or continuous recording may be 

 employed as desired. The bridge may be of 

 the slide wire or switch type for intermit- 

 tent readings, or of the recording type for 

 continuous readings. 



The total resistance will depend on the 

 size of the electrodes, the resistance of the 

 connecting cables, the effective resistance 

 of the formations relatively near the elec- 

 trodes, and the salinity of the water or 

 mud in the drill hole.* The earth immedi- 

 ately adjacent to the electrodes contributes 

 a major part of the total resistance of the 

 current path. That is, aside from the mate- 

 rial within a few feet of the electrode sur- 

 faces, the main mass of earth included 

 between electrodes A and B has a rela- 

 tively low, or negligible, resistance due to 

 its large cross-sectional area. Hence, the 

 total resistance R oi the circuit may be 

 expressed by the relationship 



Fig. 632. — Schematic^ diagram of 

 arrangement of measuring electrical 

 resistance of formations traversed by a 

 bore hole. 



R = Ri-YR2-VRz = {Rx + Ro) + Rz 



(1) 



where J^i = resistance of the cable. This resistance is measured directly and 

 is a function of the cable length. It remains substantially 

 constant. 

 = effective resistance of the electrode B. This resistance consists 

 of the contact resistance between the electrode surface and the 

 earth and the effective resistance of the earth material immedi- 

 ately adjacent to the electrode. It remains substantially con- 

 stant. 

 = effective resistance of the electrode A. This resistance consists 

 of the contact resistance between the electrode surface and the 



Ri 



R, 



* The resistivity of the mud varies over fairly wide Hmits. Where the drill hole 

 penetrates strata containing materials which are readily dissolved or where saline 

 solutions from the strata enter the bore hole, the resistivity may be as low as 0.1 

 ohm-cm. In drill holes where the mud is relatively free from such effects or where 

 fresh water enters the drill hole, the resistivity may be many hundred ohm-cm. Average 

 drilling mud which is encountered in the usual drilling well varies in value from 

 about 1 ohm-cm. to 10 ohm-cm. 



