602 EXPLORATION GEOPHYSICS 



A simple modification of this method which permits its use for deeper 

 lying ore bodies employs a medium frequency alternator for supplying 

 the excitation energy. The equipotential lines are traced out with the 

 aid of two probing electrodes connected with a pair of sensitive head- 

 phones or an audio-frequency amplifier and phones. The two probes are 

 on the same equipotential Hne when no signal is heard in the headphones. 

 Short exploring lines (100 feet or so) should be used to minimize 

 inductive effects. 



This same technique is employed for location of buried pipe lines. 

 A contact is made with the pipe at a known location, and the other 

 electrode placed as far away as practical, and in the probable direction 

 of the pipe line. The concentration of current in the pipe will allow its 

 location by use of suitable direction-finding coils. 



INDUCTIVELY ENERGIZED METHODS 



Inductive methods comprise those techniques wherein the earth is 

 energized inductively by an alternating magnetic field, and measurements 

 are made of parameters which are associated with the secondary magnetic 

 field at the earth's surface produced by induced currents in the subsurface. 



When an alternating current is supplied to an insulated loop or coil 

 placed near the surface of the earth, the magnetic field set up by the 

 coil (energizing coil) will cut into the earth and induce a complexity 

 of varying voltages in the materials constituting the subsurface. These 

 induced voltages will in turn set up currents in the earth with a resulting 

 potential distribution at the surface and a redistribution of the magnetic 

 field at the surface. 



The effective electromagnetic field existing at the surface of the earth 

 is the resultant of the primary field created by the energizing coil and 

 the secondary fields due to the induced currents in the subsurface and 

 in any other conductors in the vicinity of the energizing coil. Hence, any 

 property of this electromagnetic field is diagnostic, theoretically at least, 

 with respect to the subsurface distribution of current. The parameters 

 most commonly measured or plotted are: contours of equal magnetic flux, 

 phase of the electric or magnetic field, magnitude of vertical or horizontal 

 component of the magnetic field, magnitude or direction of the resultant 

 magnetic field. 



The inductive methods are subject to an infinite variety of modifica- 

 tions as is attested by the diversity of inventions governing improvements 

 on these methods. There are many advantages in the use of the inductive 

 methods, chief of which are: (1) by use of electromagnetic means the 

 ground may be energized and/or the subsurface distribution of current 

 determined without the use of electrodes or other direct contact with 

 the ground, (2) at low frequencies the near-surface effects are minimized, 



