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EXPLORATION GEOPHYSICS 



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Metal Stake Electrodes 



Stake electrodes may be made from any type of structural or bar 

 metal available. Angle-, H- or T-section iron bars, pipes, round and 

 square rods, etc., have all been used by various workers. For practical use, 

 however, it has been found that square steel rod is most satisfactory. 

 Steel, although costing slightly more, will outlast 

 ordinary cold rolled iron many times, and its longer 

 field life more than offsets its initially higher cost. 

 The initial electrode length is preferably 36 to 40 

 inches. The electrodes are made preferably from ^'' 

 square rod, which is sharpened at one end. 



The electrodes are driven into the ground to a 

 depth sufficient to contact the moist layer immediately 

 underneath the surface. This depth will vary with 

 local conditions but usually will be from two to twelve 

 inches below the surface. In areas where moist earth 

 cannot be contacted within that depth of the surface, 

 the electrode resistance will be relatively high, and 

 more than one electrode may be necessary. The num- 

 ber of electrodes is governed by the voltage drop which 

 can be tolerated in the energizing circuit. Obviously, 

 an increased contact resistance in the energizing cir- 

 cuit necessitates high voltages to create a sufficient flow 

 of current. 



As a general rule, from 85% to 95% of the total 

 potential drop in the circuit takes place in the imme- 

 diate vicinity of the grounded electrode contacts. In 

 dry earth it is often necessary to reduce the contact 

 resistance by "wetting down" or by employing more 

 than one electrode. Contact with the ground is a 

 typical point contact between the irregular earth 

 particles and the smooth electrode surface. Point con- 

 tacts of this type develop notoriously high resistances. 

 A lower effective resistance between the electrode and 

 the ground may be obtained by using (a) a larger 

 extended contact area or (b) an electrolyte which fills 

 the interstices between the electrode surface and the 

 earth's particles and thereby increases the contact area. 

 If an electrolyte is used, "wetting down" solutions 

 may be carried in ordinary canvas water bags, such 

 as are employed extensively for desert travel. About 

 one-half pint of saturated salt solution per electrode is 

 usually sufficient. 



Location of Multi-Point Electrodes. — A single 

 stake electrode should be driven into the earth at the 



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 b-Ld 



Fig. 315. — Recom- 

 mended arrangements 

 for stake electrodes, (d 

 is depth the electrodes 

 are imbedded in the 

 ground.) 



A — Single electrode 

 placed at point of 

 measurement, 

 marked x. 

 B — Two electrodes 

 placed on each side 

 of point of measure 

 ment. 

 C — Three electrodes ar 

 ranged in equilateral 

 triangle with poin 

 of measurement a 

 center. 

 D — Four electrodes ar 

 ranged in square 

 with point of meas 

 urement at center, 

 E — Five electrodes, < 

 more, arranged on 

 circle, with point of 

 measu remen t 

 center. 



