Except in very shallow invasion, the radius of investigation of the Micro- 

 Log does not extend beyond the flushed zone. The measured resistivities are 

 functions of the resistivity R xo of the flushed zone, of the hole diameter d, of 

 the resistivity R mc of the mud cake, and of the thickness t mc of the mud cake. 

 Interpretation charts that have been established on the basis of laboratory 

 determinations make possible the derivation of the value of K x0 and t mc from 

 the two MicroLog readings, provided the values of d and R mc are known. 

 The hole diameter is generally known from the bit size or, better, from a 

 section gauge or a microcaliper log. The value of R mc can be obtained, as 

 for Rmf, by direct measurements on mud cakes pressed from actual samples 

 of the mud or from average statistical data. 



When R^ is not too great in comparison with R„ lc , as in high porosity 

 formations, the MicroLog readings are not too much affected by the presence 

 of mud cake; and R xo can be obtained with a fair accuracy. The reverse 

 situation occurs when R x0 is very great in comparison with Rmc, low porosity. 

 Thus, there is a lower limit of porosity values under which the quantitative 

 interpretation of the MicroLog is not too reliable. With the hydraulic type of 

 pad, the limit is about 12 to 15 percent, depending on the character of the 

 mud cake. 



In those formations where it can give R^ with enough accuracy, the 

 MicroLog is also valuable for the determination of water saturation. The 

 knowledge of R^ is helpful to correct the readings of the standard devices, 

 if necessary, for the effect of invasion. 



induction Logging 



Induction logging is a method wherein the conductivity, or its reciprocal 

 the resistivity, of the formations is measured by means of induced current 

 without the aid of electrodes. 



The main advantages of induction logging over the conventional methods 

 are the following: 



a. The induction log gives a sharper delineation of the interfaces between 

 different values. 



b. It makes possible a more accurate determination of the true resistivity 

 of thin beds, especially when the resistivity of the bed is lower or only 

 slightly higher than the resistivity of the adjacent formations. 



c. Induction logging can investigate the uncontaminated zone of permeable 

 beds behind a highly resistive invaded zone, as in low-porosity forma- 

 tions drilled with comparatively fresh mud. 



Furthermore, induction logging is appropriate to resistivity measurements 

 in empty holes or in holes that are drilled with oil-base mud. 



298 



