450 



EXPLORATION GEOPHYSICS 



In addition to these more or less steady components, various fluctuating 

 potentials occur. Earth currents have strong diurnal variations, magnetic 

 storm fluctuations, and other variations with a period longer than one day.f 



Figure 269 shows a comparison 

 of the variations in earth cur- 

 rents, magnetic activity, and sun- 

 spot numbers over a period of 

 several years 4 The curves are 

 necessarily "smoothed out" ; that 

 is, monthly, daily, and hourly 

 variations that would give the 

 curves a saw-toothed character 

 not shown. Hourly earth- 

 records are shown by 



are 



current 



Gish,§ 



Fig. 269. — Comparison 'of variations in earth 

 currents, magnetic activity, and sunspot numbers. 

 (Rooney, "Earth Currents," Terrestrial Mag- 

 netism and Electricity.) 



The errors due to varying 

 earth currents are minimized by 

 employing relatively short sepa- 

 rations between the measuring 



electrodes. In average areas, the effects of varying earth currents are of 



minor importance at separations less than 500 feet. 



Traverses Across Vein Conductors. — The self-potential method can 

 often be employed to advantage in studying veins and parallel vein systems 

 undergoing oxidation by running traverses across the area at right angles 

 to the vein system. Work of this type is much more rapid than the equi- 

 potential studies previously outlined. For this work, a potentiometer, reel, 

 and non-polarizing electrode system are employed. Readings are made at 

 suitable intervals along the traverse line. Plots may be made of the dif- 

 ferences in potential between the stations as ordinates and the traverse 

 distances as abscissas. Due to the change in the direction of current flow 

 above the oxidizing ore body, the potential curve has a minimum value 

 above the negative center. Non-symmetry of the curve about the negative 

 center may be caused by a dipping ore body, differences in the electrical 

 conductivity of the rock on each side of the vein system, variations in 

 ground water elevation and distribution, or the regional gradient. These 

 variables must be evaluated by proper geologic control and measurement 

 technique. 



t W. J. Rooney, "Earth Current Variations with Periods Longer than One Day," Terr. Mag. 

 42, No. 2, p. 166, June, 1937. 



% W, J, Rooney, "Earth Currents," p. 291 Terrestrial Magnetism and Electricity (Edited by 

 J. A. Fleming) (McGraw-Hill, 1939.) 



§ O. H. Gish, "Electrical Messages from the Earth, their Reception and Interpretation," Jour- 

 nal of the Washington Academy of Sciences, Vol, 26, No. 7, July 15, 1936. 



