Subsurface Logging Methods 403 



The resistivity of the small volume of ground surrounding the 

 electrodes can be measured, for example, by sending a current of known 

 intensity / through electrode A (fig. 179), and by measuring with galvan- 

 ometer Gi, the potential difference created by that current between elec- 

 trode Ml and the reference electrode A^. 



Similarly, a slightly larger volume of ground can be taken into 

 account by measuring, with galvanometer G^, the potential produced at 

 electrode M3 by the same current. Because the spacing AM2 is twice as long 

 as the spacing AMi, the corresponding investigation is also twice as deep. 

 This is an important feature of the system; it is possible not only to meas- 

 ure the average resistivity of the ground under the pad, but also to deter- 

 mine whether or not the resistivity varies with depth from the pad. This 

 determination is of significance, for a resistivity variation with depth 

 from the pad usually occurs when there is a mud cake. 



Electrode combinations other than AMi and AM2 can, of course, be 

 used. When a current is sent through electrode A, it is also possible to 

 measure the potential difference between electrodes Mj and M^. The three- 

 electrode system AM1M2 is more influenced by the mud cake than the 

 two-electrode system AMi, so that a combination of the device AM1M2 

 and the device AM2 would generally give a larger departure opposite 

 permeable beds than a combination of the two devices AMi and AMo. 



To simplify the wording, the two electrode and three electrode sys- 

 tems are called "normal" and "lateral" devices, respectively. It should be 

 pointed out, however, that micrologging devices, because they have much 

 smaller depth of investigation and because the electrodes are shielded 

 from the mud column, are different from the normal and lateral devices 

 used in conventional logging.^^ 



On most of the pads presently in use, the three electrodes are placed 

 on a vertical line, in the middle of the pad, with a spacing of one inch 

 between the successive electrodes. Three electrode combinations which 

 are thus obtained are listed below for reference. 



Measuring device 



AMx normal 



AM^ normal 



AM1M2 lateral 

 Reference 



V normal (or 1'') 



2" normal (or 2") 



l"xr lateral (or r'xl'') 

 The combination of the V x V lateral and the 2" normal is preferred 

 at the present time. 



The electric circuit used for the recording of the microresistivity 

 curves is somewhat similar to that used for conventional logging. Changes 



^ Doll, H. G., Legrand, J. C, Stratton, E. F., True Resistivity Determination from the Electric Log — 

 Its Application to Log Anaylsis: Oil and Gas Jour., vol. 46, no. 20, Sept. 20, p. 297 ff», 1947. 



