30 



METHODS OF GEOPHYSICAL EXPLORATION 



[Chap. 2 



Other potential methods. Surface potential measurements may be 

 made not only with continuous direct or alternating current but also by 

 the use of transients, such procedure being known as "Eltran." The 

 electrode arrangement is similar to the resistivity method. In the pri- 

 mary circuit, impulses are applied to the ground and the time change of 

 the potential between two stakes (usually placed outside the two current 

 stakes) is measured with an amplifier and cathode-ray oscillograph. 



Field 



eafuf- 



Phase Diff.O 



/ Compensator 



_ o , o . • . . o .^ o .■ p' 



+ + + +I+ 4- + + + 



+ + K^ + + + 



+ Ore m Sod^ + + + 



-I- -I- i- ^ + + + 



■t- + + ^ -*■ "•- 



+ + + 



Fig. 2-14. Semiabsolutc and relative electromagnetic prospecting methods 



(schematic). 



Electromagnetic-galvanic methods. Electromagnetic methods of elec- 

 trical prospecting differ from potential methods in that the electromagnetic 

 field of ground currents and not their surface potential (electrical field) is 

 measured. They fall into two major groups: (1) electromagnetic- 

 "galvanic" methods in which the primary energy is supplied by contact as 

 in the potential methods; (2) electromagnetic-"inductive" methods in 

 which the ground is energized by inductive coupling (with insulated loops). 

 To supply electrical energy to the ground by contact, line electrodes or 

 point electrodes are used. Line electrodes are laid out at right angles to 

 the strike, point electrodes parallel with the strike. 



