ELECTRICAL METHODS 



537 



This constant potential method is more rapid and accurate than the 

 volt-ammeter method. However, its use requires care on the part of the 

 operator in reading the current at the time of galvanometer balance. In 

 addition, the usual precautions must be taken to minimize errors due to 

 natural ground potential variations. The rapidity with which the measure- 

 ments are made, usually two to four seconds, prevents appreciable error 

 due to ground currents except on occasional days when rapid magnetic 

 and electrical changes occur. 



Other methods have been proposed wherein the energizing current is 

 varied, t 



Apparatus for Neutralizing Natural Ground Potentials 



The very large potential spacings required when working to depths in 

 excess of 1000 feet cause considerable difficulty due to the natural ground 

 currents.* These currents pro- „„^^„ ^,,„„,^ , , 



r POWFR SUPPLY. I I 



. .-■ I i^-ri)-' PC "^ 



in.- — o 



duce irregular and varying 

 uni-directional potentials which 

 cannot be separated from the ^^^u- 

 potentials created by the flow ammeter 

 of the D.C. energizing current. 

 Various methods have been pro- 

 posed for eliminating or mini- 

 mizing the effects of these 

 ground potentials. Perhaps the 

 best known of these methods is 

 the one developed by Gish and 

 Rooney. 



The Gish-Rooney$ apparatus 

 (Figure 327) employs a double 

 commutating system so designed 

 that the potential and current 

 systems are reversed in syn- 

 chronism. The power, which is supplied by suitable "B" batteries, passes 

 through a direct-current milHammeter and into the commutating system. 

 The polarity of the current is reversed periodically by the commutator at 

 a frequency depending upon the speed of rotation — usually about 20-30 

 reversals per second. A second commutator mounted on the same rotating 



t H. M. Evjen, "Electrical Method of Geophysical Exploration," U. S. Patent No. 2,169,685, 

 Aug. 15, 1939. 



T. S. West and C. C. Beacham, "Method of Electrical Geophysical Prospecting and Apparatus 

 for Practicing Said Method," U. S. Patent No. 2,217,780, Oct. 15, 1940. 



J. J. Jakosky, "Method of Determining Underground Structures," U. S. Patent No. 2,250,024, 

 July 22, 1941. 



F. W. Lee, "Electric Impedisivity or Resistivity Measuring," U. S. Patent No. 2,277,701, 

 Mar. 31, 1942. 



* O. H. Gish, "The Natural Electric Currents in the Earth," Scientific Monthly, July 1936, 

 pp. 47-57. 



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

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



} W. J. Rooney and O. H. Gish, "Results of Earth-Resistivity Surveys near Watheroo, Western 

 Australia and at Ebro, Spain," Terr. Mag. 32, pp. 49-63 (1927). 



Fig. 327.- 



-Diagram of Gisli-Rooney double commutator 

 method. 



