the short normal, the SP, the MicroLog, and the microcaliper gives the most 

 complete and accurate record of the formations. 



Induction logging will also continue as the proper method for the survey- 

 ing of empty holes and of holes drilled with oil-base mud, possibly in com- 

 bination with an additional curve run with scratcher electrodes and with the 

 radioactivity logs. 



Laterolog 



The Laterolog is a resistivity measuring method wherein the current is 

 forced to flow radially through the formations as a sheet of predetermined 

 thickness, by means of an appropriate electrode arrangement and automatic 

 control system. 



With the Laterolog, the measurement involves a portion of ground of limited 

 vertical extent; the measured value is practically unaffected by the mud column. 



The main advantages of the Laterolog over the conventional log are the 

 following : 



a. It shows sharper discrimination between different beds and more 

 accurate definition of their boundaries. 



b. It gives closer approximation to the true resistivity of thin beds, especial- 

 ly in formations drilled with high salinity mud. 



c. The Laterolog is useful for the logging of thin beds in formation of low 

 or moderate resistivities where the drilling mud is very saline and for 

 formations of high resistivity where any water-base mud is used. 



Principle 



There are two types of Laterologs, 7 and 3, corresponding to the number 

 and shape of electrodes. Laterolog 7 uses point electrodes as schematically 

 represented on Figure 14-17. The device comprises one current electrode, A , 

 and three pairs of electrodes, Mi and M 2 , M'^ and M%, and A* and A 2 , placed 

 symmetrically with respect to A , the pairs being respectively short-circuited. 

 A current of constant and calibrated intensity is sent through electrode A . 

 Additional current of the same polarity is fed through the auxiliary power 

 electrodes, Aj and A 2 . The intensity of these currents is automatically and 

 continuously adjusted in such a way that the difference of potential between 

 the short-circuited pairs, MjM 2 and M'jM'g, is maintained substantially equal 

 to zero. The potential prevailing at any of these four electrodes is measured. 



According to this system, the current emitted from A is prevented from 

 flowing upward past electrodes M^ and M # j or downward past electrodes 

 M and M # . Accordingly, the distance the current travels across the mud is very 

 small; and the mud column has very little influence on the measurements 

 except in deep caving. The current is obliged to flow within an approximately 



306 



