458 EXPLORATION GEOPHYSICS 



feet or so of the surface, these methods will locate the conducting bodies 

 with sufficient accuracy for practical purposes, t For deeper deposits, 

 electrical methods of better resolving power are necessary. 



FIELD PROCEDURE AND APPARATUS 



Direct Current Method. — The energizing electrodes are positioned 

 to include the area under investigation. Sufficient current is caused to flow 

 between the electrodes to create a measurable potential field. After this 

 field has been created, the field investigations are quite similar to those made 

 in the spontaneous potential method. In the previously described self- 

 potential method, only one datum is necessary at each point of measure- 

 ment: namely, the value of the natural earth field. In the B.C. equi- 

 potential line method, two data should be obtained: namely, (a) the 

 undisturbed natural earth field and (b) the resultant field due to the natural 

 earth current and the artificial current. The readings of (a) are employed 

 for correcting the values of (b). 



Power may be supplied to the energizing electrodes by a gasoline- 

 driven, direct current generator. The generator should have an output of 

 1500 to 2000 watts and a voltage up to 220 volts for extensive surveys. 

 The contact resistance in the energizing circuit should be minimized by 

 use of large extended electrodes.* Because of the losses by polarization 

 and electrolysis or contact potentials adjacent the electrodes, a larger 

 power supply is necessary when employing direct current than when em- 

 ploying alternating current. The power is constant throughout a series of 

 readings, or else the data are reduced to potential per unit current. 



Alternating Current Audio Method. — The methods employing A.C. 

 are more common in field practice than those employing D.C. for several 

 reasons : 



(a) increased portability of the movable circuit, consisting of metallic 

 electrodes, headphones, and an amplifier ; 



(b) absence of non-polarizing electrodes which are difficult to handle; 



(c) decreased power of generator necessary when vacuum tube am- 

 plification is used. 



The equipment used in the A.C. audio-frequency potential methods 

 comprises essentially : two power electrodes ; insulated connecting wire on 

 reels ; power source for energizing the ground ; two search electrodes or 

 "probes" ; amplifier ; headphones, or other balance indicating instruments ; 

 and flexible, rubber-insulated connecting cables. 



Electrodes. — The multipoint electrodes commonly employed in the 

 energizing circuit are described on page 522. The probe electrodes are 

 preferably made of duralumin and have a diameter of ^ inch and a length 



t Compare, also, Broughton Edge and Laby, Geophysical Prospecting, p. 43. 

 * The electrodes are described in detail on p. 522, et seq. 



