1026 



EXPLORATION GEOPHYSICS 



The factor of proportionality Air 



AM' AN 



MN 



which is a constant for 



a given electrode arrangement, is called the coefficient of the lateral sonde. 

 In Figure 635b is shown an equivalent electrode arrangement actually 

 used in certain electrical logging equipment for recording lateral curves. 

 In this arrangement, the positions of the current electrodes and measuring 

 electrodes have been interchanged, resulting in what is termed the "recipro- 

 cal arrangement".* 



Generotor 



Generator 



(a) AMN Loteral (b) MAB Lateral 



Fig. 635. — Lateral sonde (equivalent circuits). 



Slichter t has proposed a method in which four electrodes are lowered 

 into the drill hole. The electrodes are spaced a fixed distance apart, and 

 the entire electrode system is moved as a unit along the length of the drill 

 hole. Means are provided for employing expanding prongs or contacts 

 whereby electrical contact may be made with the wall of the drill hole, in 

 the event the drill hole is not filled with water, (The electrodes may con- 

 sist of many fine spring wires that extend radially outward and form a 

 circular "brush" which sweeps along the wall of the drill hole.) The method 

 is applicable, therefore, to dry drill holes frequently encountered in mining 



* This is in accordance with the "principle of reciprocity," the general validity of 

 which has been demonstrated mathematically and experimentally and which may be 

 stated : 



Considering four electrodes, 1, 2, 3, 4, arranged in any way, the difiference of 

 potential measured between 1 and 2 due to a passage of current between 3 and 4 is equal 

 to the difference of potential that would be measured between 3 and 4 due to the 

 passage of the same current between 1 and 2. 



t Louis B. Slichter, "Apparatus for Exploring for Ore," U. S. Patent 1,826,961, issued 

 Oct. 13, 1931. 



