DEEP ELECTRICAL PROSPECTING 65 
voltage was amplified by means of a calibrated D.C. amplifier and 
applied to a galvanometer, the motion of which was photographed. 
This gave a permanent record and permitted the determination of the 
voltage and time constant. The disposition of electrodes is designated 
by the current electrode separation, the distance between the nearest 
current and potential electrodes, and the potential electrode spacing. 
For instance, an electrode disposition represented by .5—.4—500’ means 
that the current electrode separation is one-half mile, the distance 
between the nearest current and potential electrodes is four-tenths 
mile and the potential electrode separation is 500 feet. A storage 
battery of several hundred volts employed as a source delivered 10-20 

Fic. 1.—Disposition of electrodes. 
amperes to the current electrodes which were a multiplicity of copper 
rods. The usual non-polarizing potential pots were used for the po- 
tential electrodes. 
Electrical prospecting is here regarded in the light of its ability to 
determine the presence of insulating rather than conducting media. It 
would seem that if the electrical method is to be of any considerable 
value to the petroleum geologist it must prove itself of value in this 
respect. As rocks filled with oil and gas exhibit much higher resistivi- 
ties than when filled with mineralized water in the usual case, this 
would seem the most logical and desirable use of the electrical method. 
It is well known that the resistivity of dry rock is very high. The 
comparatively low resistivities observed in electrical prospecting are 
due to the mineralized water in the rock pores acting as an electrolyte. 
It isapparent, therefore, that when the pore space is small or when this 
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