226 DEEP BOREHOLE SURVEYS AND PROBLEMS 



3. Combinations of 1 and 2. 



4. Electrical waves. 



Broadly speaking for our purpose electrical methods 

 can be grouped into two main groups, i.e., potential and 

 electromagnetic. A brief sketch is essential to a compre- 

 hension of the method of electrical borehole investigation 

 due to the brothers Schlumberger, the only electrical 

 method which has been employed for this purpose. 



Fig. 153. — Normal course of electric current lines between two electrodes in a 

 homogeneous underground. 



Potential Method. — If an electric potential of +e volts be 

 applied to a point a, while at another point b the voltage 

 is zero, then between the two field electrodes, a and b, a 

 pressure (potential) difference or electromotive force e 

 prevails, which produces a current i, that flows from a to 6 

 in imaginary current lines, the pressure decreasing all the 

 time from +e at a to nil at b. The planes or sections which 

 show equal potential difference toward the electrode a or 6 

 can be connected by planes of equal pressure called equi- 

 potential planes or level planes. From the potential 

 theory we learn that such equipotential planes stand every- 



