642 ELECTRICAL METHODS [Chap. 10 



D. The Effects of Magnetic Permeability 



As was shown in the introduction to Chapter 8, magnetic permeability 

 is the ratio of magnetostatic induction to field strength. It plays a part 

 ia all electric induction phenomena. In accordance with Faraday's law, 

 the e.m.f. induced in a conductive body depends on the time rate of 

 change of the field : E = — dH./ dt. If the field change occurs in a medium 

 of the permeability v, the induced e.m.f. E = — y. dEL/ dt. Theoretically 

 this relation should be expected to find application in electrical prospecting 

 methods where current is induced in subsurface conductors by insulated 

 loops. In practice, however, it is of limited importance since with few 

 exceptions, good conductors (sulfide ores) to which inductive methods are 

 appHed chiefly are not very magnetic, while very magnetic (iron) ores, on 

 the other hand, are usually poor conductors, making potential methods 

 more suitable. 



E. Methods for the Determination of Rock Resistivity 



Methods for determining rock resistivity may be divided into two groups : 

 (1) laboratory determinations (on rock specimens) and (2) measurements 

 in situ (at the ground surface, outcrops, and the like). Laboratory 

 determination has the disadvantage that only small specimens are tested, 

 that the effects of unhomogeneities in the specimen not .characteristic of 

 the entire formation may be exaggerated, and that conditions in nature 

 may not be exactly duplicated. By making measurements on location, 

 these disadvantages are overcome, but their limitation is that unknown 

 near-surface strata differing in conductivity may affect the results; hence 

 an "apparent" instead of the true resistivity of the surface formation may 

 be determined. In regard to technique, two major groups may be dis- 

 tinguished : one that uses direct current and the other that uses alternating 

 current (commutated D.C., low-frequency A.C., intermediate-frequency 

 A.C., and high-frequency A.C.). Measurements at radio frequencies are 

 discussed in the next section in connection with determinations of the 

 dielectric constant. 



1. Preparation of specimens; electrodes. For resistivity determination 

 in the laboratory the sample should be cut to regular shape so that its 

 resistivity may be readily calculated from resistance and dimensions. 

 Rock saws with carborundum or diamond discs are suitable. The length 

 of the specimen should be at least four times its diameter. High contact 

 resistances may be overcome by the use of a frame shown in Fig. 10-7. 

 Between the metal plates and the specimen various layers of tinfoil should 

 be inserted. Contact resistance is likewise reduced with mercury elec- 

 trodes. Two arrangements are shown in Figs. 10-8 and 10-9, the latter 



