GEOPHYSICAL METHODS OF INVESTIGATING 



231 



effect between these two points by the passage of current 

 into the soil. 



To send forth a current by the electrode A the latter is 

 connected, by means of the insulated cable 1, to one of the 

 poles of a source of electricity E situated aboveground, 

 the other pole of the latter being earthed at any point B 

 close to the well. To measure the difference of potential 

 resulting between M and A^, these two electrodes are con- 



% 



'^y/^^^^,' 



%. 







J 



N 



"5 "J 



V 



Fig. 157. — Electric coring. 



nected by means of the insulated cables 2 and 3 to the two 

 terminals of a potentiometer placed aboveground. 



Knowing the distances r and r', the intensity i of the cur- 

 rent force (measured, for example, by an ammeter) and the 

 difference of potential A 7 between M and iV (measured 

 by the potentiometer), it is possible to calculate the average 

 resistance of the soil surrounding the measuring field AWN, 

 if the soil is uniform, in the following manner : 



The current i flowing from A into the soil creates, by 

 ohmic effect, a group of equipotential surfaces enveloping 

 A. These surfaces are, by reason of symmetry, practically 

 spheres centered in A , always excepting : 



