The diameter of the beam increases, first very slowly and then more and 

 more rapidly, with the distance from the wall. Laboratory experiments have 

 shown that that part of the formations which is located beyond a distance of 

 about 3 inches from the wall has little influence on the measurement. 



In a porous and permeable formation, the electrode system is separated 

 from the formation by the mud cake. Inasmuch as the current crosses the mud 

 cake in a direction perpendicular to the wall, the distance it has to travel across 

 the mud cake is small in comparison with the length of the path through the 

 formation. Furthermore, the resistivity of the mud cake is usually less than 

 the resistivity of the formation. As a result of these two factors, the influence 

 of the mud cake on the measurement is much reduced and even negligible for 

 all practical purposes if the mud-cake thickness is not too great. It is recalled, 

 for the sake of comparison, that with the MicroLog the mud cake constitutes a 

 path for a part of the current toward the mud colmun and, therefore, generally 

 affects the measurements much more than with the MicroLaterolog, particularly 



nsulating Pad 



Bore Hole 



mpervious 

 Formation 

 Figure 14-20. Schematic drawing of microlaterolog. 



313 



