on the one hand, between electrodes M_zM 3 and, on the other hand, between 

 electrode M 2 and a reference electrode at the surface. The combinations 

 AM^Mg and AM 3 -surface are generally referred to as microinverse and micro- 

 normal systems, respectively. The volume of ground involved in the measure- 

 ment is smaller for the microinverse than for the micronormal; in other words, 

 the microinverse has a smaller radius of investigation. 



An SP curve may be recorded simultaneously with the MicroLog. 



The most recent equipment includes a second pad, identical and diamet- 

 rically opposed to the first. The distance between the outer faces of the two 

 pads is continuously recorded, providing the so-called microcaliper log. The 

 microcaliper log is very accurate and can measure variations of hole diameter 

 as small as i/g inch. 



Delineation of Different Formations — Porous and Permeable Beds 



When the pad is applied to a porous and permeable bed, the mud cake 

 represents an important proportion of the small volume involved in the meas- 

 urement. The resistivity of the mud cake can be estimated to be usually at 

 most twice the resistivity of the mud in water-base mud and somewhat more 

 in oil-emulsion mud. The other part of the volume measured is the formation 

 which, according to laboratory experiments, extends about three inches from 

 the wall. Usually, the fluids originally present in this fraction of formation 

 have been almost complete^ flushed by mud nitrate, except when invasion 

 is very shallow; and the resistivity of this flushed zone is directly related to 

 that of the mud. The resistivity measured opposite a porous and permeable 

 bed, therefore, does not exceed a few times the resistivity of the mud. Fifteen 

 or twenty times the mud resistivity is a maximum for the resistivity measured 

 opposite a porous and permeable bed in fresh mud. 



Because of its smaller depth of investigation, the microinverse is more 

 influenced by the mud cake than is the micronormal. The values read with 

 the two different electrode arrays are therefore usually different. The difference 

 between the two readings is called separation. Usually the resistivity of the 

 flushed zone is greater than that of the mud cake, therefore the micronormal 

 gives a higher apparent resistivity than the microinverse, and the separation 

 is said to be positive. 



Accordingly, the criteria for the interpretation of porous permeable beds 

 are the following : 



1. Comparatively low resistivity readings, not more than 20 times the 

 mud resistivity (in fresh mud). 



2. Positive separation between the microinverse and the micronormal as 

 long as a substantial mud cake is deposited. 



295 



