402 



Subsurface Geologic Methods 



Description of the Equipment 



The micrologging apparatus consists essentially of a rubber pad, 

 which is pressed against the wall of the drill hole, and in the face of 

 which are inserted a certain number of electrodes. Several distributions 

 of electrodes have been experimented with. One of the distributions is rep- 

 resented in figure 178. The electrodes are nearly flush with the rubber 

 surface, or slightly recessed, and each of them is connected by an insu- 



^TL^ 



\\\ h 



Figure 179. Microlog electrical setup. 



lated wire to one of the conductors of the cable used to lower the appara- 

 tus into the hole. 



The rubber pad is molded on one of the branches of a spring guide 

 whose design is such that the pressure applied to the pad is approximately 

 independent of the diameter of the hole, provided that this diameter re- 

 mains between certain limits which, for one of the guides presently in 

 use, are respectively 4^ inches and 16 inches. The rubber pad fits the wall 

 of the drill hole over a substantial area surrounding the electrodes be- 

 cause of its shape and the pressure exerted upon it. The pad also shields 

 the electrodes from the mud column, while the electrodes themselves are in 

 direct electrical contact either with the formation, or with the mud cake 

 between the pad and the formation. 



