Chap. 2] 



METHODS OF GEOPHYSICAL EXPLORATION 



27 



As it is difficult to survey these lines of flux, lines of equal potential, that 

 is, lines along which no current flows, are mapped instead. In practice 

 power is supplied to two grounded electrodes from an alternating current 

 generator. Two types of primary electrodes may be used:- (1) point elec- 



Sfionhmeoui 

 Pottntiol 



r"""^ Pefenfiomehr 



Lines of / equal 

 Spontaneous/ Potential/ 



i 



^ Negative 

 \\ Center 



Porous 



\ 





'!n 



.■'l'"l'll I', r , 



Fig. 2-12. Self-potential mapping (schematic). 



trodes, so laid out that their base line is in the direction of strike, (2) line 

 electrodes laid out at right angles to the strike. Equipotential lines are 

 surveyed \vith one fixed and one moving probe which are connected to an 

 audio amplifier with head phones. Greater detail is obtainable by the 



