646 



ELECTRICAL METHODS 



[Chap. 10 



in which the middle term in the denominator is negligible, so that 



2RI 



P = 



n- 0.85' 



(10-12) 



which for large electrode separations is equal to p = 2Ra. This shows 

 that the resistivity between two electrodes depends largely on their radii 

 and therefore on the material in immediate contact with them. In the 

 double electrode setup as described, Koenigsberger used iron discs about 

 I inch thick and 10 inches in diameter, with a contact substance of claj' 

 or soil soaked with either FeCls and FeS04 or NaCl solution.^ He applied 

 frequencies between 100 and 400 cycles. 



5. Intermediate-frequency methods. The use of audio-frequencies from 

 500 to 1000 cycles facilitates the field technique of resistivity measure- 

 ments, since phones may be employed as indicating instruments. In the 



Rtctifier 



60- 



Poltntiomtttr 



Fig. 10-12. Determination of rock resistivity using 60 cycles (after PuUen). 



lower frequency range, efifects of inductance and capacitance are generally 

 negligible and some of the arrangements are used as D.C. bridges. Inter- 

 mediate frequency methods are applied (1) galvanically (by contact with 

 the specimen) or (2) inductively (by measuring the mutual inductance 

 between two loops). The latter are best suited for resistivity measure- 

 ments on outcrops or horizontally stratified ground, since they function 

 better on comparatively large volumes of earth. 



As an example of the galvanic application of intermediate frequency 

 methods, the soil resistivity bridge of the U. S. Department of Agriculture 

 is illustrated in Fig. 10-13.^ Current is supplied from a battery-operated 

 induction coil or buzzer. A container for the soil sample is in one arm of 

 the bridge, a multiplier resistance in the opposite arm ; two sides of a slide- 



* J. Koenigsberger, A.I.M.E. Geophys. Pros., 221 (1929). 

 » R. O. E. Davis, U.S. Dept. Agr. Circ. No. 423 (July, 1927). 



