BORE-HOLE INVESTIGATIONS 



1025 



R = AttAM 



of electrode M. Since E is given by Equation 3 it is possible to write 

 (still neglecting the effect of the drill hole) : 



/ 



Since the factor AirAM is a constant for a given electrode arrangement, 

 it is called the coefficient of a normal sonde of spacing AM. Thus for a 

 constant value of / there is for a given spacing 

 a proportionality between the resistivity of 

 the surrounding medium and the voltage P I 



measured.* 



RECORDIMa 

 M£T£R 



Multi-Electrode Sonde. — It may be 



convenient for practical reasons to have in 

 the hole not only electrodes A and M but also 

 electrode B or electrode N. In Figure 634 

 is shown the electrode arrangement actually 

 used with certain electrical logging equip- 

 ments for recording normal curves. 



The Lateral Sonde. — In the lateral de- 

 vice (Figure 635a), the potential difference 

 between two electrodes M and N due to the 

 flow of current from A is measured. Since the 

 distance separating M and N is small in com- 

 parison with the distance between A and the 

 midpoint Oof MN, the quantity measured is 

 little different from the electrical field 

 strength, or potential gradient, at point O. 

 For this reason the lateral is often called a 

 gradient sonde, whereas the normal is called 

 a potential sonde. The distance AO is called 

 the spacing of the sonde. 



Recalling Equation 3 the potentials at the 



^^xks^y,<s^^x^yvx«j«r 



Fig. 634. — Schematic diagram of 

 three-electrode arrangement for meas- 

 uring electrical resistivities of for- 

 mations traversed by a bore hole. 



measuring electrodes are : 



E (sitN) 



and the potential difference between M and A'^ is : 



AiJ = £(atM)-£(a.iV)=^(^-JL.) 



AM 'AN AE 



or 



= 4^ 



MN 



I 



(5) 



* It is interesting to note that, from the similarity of Equations 3 and 4, a normal 

 sonde may be compared to a monoelectrode of radius AM. The spacing of the normal 

 sonde is not limited by the size of the hole, however, as is the case for the radius of a 

 monoelectrode. 



