670 



ELECTRICAL METHODS 



[Chap. 10 



Details on construction and operation have been given by Edge and 

 Laby" (see also Fig. 10-22). 



Lines of equal potential are measured by locating points between which 

 no current flows. This is a null method and requires only a sensitive 

 galvanometer. A high-resistance milliammeter, zero center, with a range 

 of 300 to 400 milliamperes and reading to an accuracy of ^ milliampere, 

 is most suitable. Potential differences along "potential profiles" are 

 measured with a potentiometer whose accuracy should be about 1 milli- 

 volt, and which should read up to 1 volt. Exten- 

 sively used for both self-potential and resistivity 

 measurements is the Leeds and Northrup pH. potenti- 

 ometer, which is described in more detail in section v 

 on resistivity methods^^ (see page 724, Fig. 10-59a). 



In a new area, the field procedure is largely deter- 

 mined by terrain conditions. If the area is accessible, 

 fairly level, and reasonably moist to give good contact, 

 the equipotential lines can be traced directly. In 

 more difficult terrain straight-line traverses are first 

 laid out with tape and compass perpendicularly to the 

 supposed strike, so a^ to intersect such surface indica- 

 tions as may be available. Numbered wooden pegs 

 are fixed along them at from 20 to 100 foot intervals. 

 Having determined and noted the potential difference 

 between the two electrodes placed at the first two pegs 

 on the traverse, and noted the polarity of the forward 

 station, the operator moves the forward electrode and 

 potentiometer to the next peg forward while the as- 

 sistant moves the rear electrode to the position just 

 vacated by the operator. Readings are plotted in the 

 form of potential gradients (referred to unit distance) 

 or in the form of potentials calculated by progressive 

 addition of potential gradients. Potential differences 

 recorded on (normal) ground free from minerals are 

 usually small and irregular. Ore bodies produce large and steady grad- 

 ients which are readily recognized. If the reconnaissance shows a well- 

 defined gradient of the order of | of a miUivolt or more per foot, which 

 persists over distances of several tens or hundreds of feet, time may often 

 be saved by mapping equipotential lines. 



In this procedure, the^ potentiometer (or a milliammeter, which is faster 



Fig. 10-22. 

 Porous pot 

 electrodes {Hei- 

 land Research 

 Corp.). 



" A. B. Edge and T. H. Laby, Principles and Practice, etc., pp. 208-240. 

 " See also ibid., pp. 236-237. 



