754 



ELECTRICAL METHODS 



[Chap. 10 



Field procedure in P.D.R, surveys depends entirely on their purpose. 

 In prospecting for ore bodies, veins, contacts, and faults, field practice 

 differs somewhat from that used in stratigraphic investigations. A third 

 procedure is applied in corrosion surveys. In mining exploration, the 

 P.D.R. traverse is usually run along a line between the two power elec- 

 trodes and, if possible, in the vicinity of one of them. The electrode 

 separation is kept constant, an.d observed P.D.R. 's are corrected for normal 

 ratio. If ratio stations have been taken with 50 per cent overlap, the 

 potential gradient per unit distance may be obtained by assuming an 

 arbitrary potential gradient (usually 1) in the first A-B interval and by 

 calculating the gradients for all other intervals by successive multiplica- 

 tion. Potential gradient (electric field intensity) values are then plotted 

 against the center of the respective intervals. 



A continuous phase curve may be plotted by starting in the first interval 

 with an arbitrary phase angle and by calculating subsequent phase angles 



^vw\^^- 



-AAAMA/* 



e 



f 



A 



y^ 



Fig. 10-82. Schematic of Swedish-American Racom. 



by successive addition. As seen in Fig. 10-84, ore bodies are indicated 

 by zones of low potential gradient and maximum phase anomaly. For 

 the elimination of surface anomalies, ratio curves are surveyed in opposite 

 directions from two power electrodes which may be switched on alter- 

 natingly so that two ratio readings are taken at each setup. 



In determining depths to horizontal formations, measurements are 

 usually made in the vicinity of one electrode; the other is kept at a distance 

 from five to ten times the depth to be reached. Profiles are run out from 

 the close electrode at right angles to the base. Then the power electrode is 

 moved to another position and a second profile is run again normal to the 

 base. The potential ratios thus obtained must be corrected for normal 

 ratio. From a survey made with constant electrode interval and overlap, 

 potential gradients and apparent resistivities may be calculated. In cer- 

 tain problems, expanding electrode systems are preferred. Profiles radi- 

 ating out from the power electrode in different directions are useful for 

 determination of dip. An electrode arrangement in which the power elec- 



