Earth Resistivity and Geological Structure * 



By R. H. CARD 



TN connection with inductive coordination problems it is frequently 

 -^ necessary to estimate low-frequency ground-return mutual im- 

 pedances between power and communication lines. The distribution 

 of currents in the earth is a major factor in the determination of these 

 impedances. This distribution is controlled by the resistivities of the 

 component parts of the earth's crust and the arrangement of these 

 parts. In impedance formulas that are customarily used the effect 

 of the earth is taken care of by the inclusion of a single parameter — the 

 earth resistivity. For a homogeneous earth this would be the actual 

 resistivity of the material composing it. But the crust is nowhere 

 homogeneous; hence, the resistivity used in such formulas is always of 

 the nature of an average of the resistivities of the several parts of the 

 crust — it is termed the effective earth resistivity. 



The effective earth resistivities for fundamental power-system fre- 

 quencies derived from mutual impedance measurements made in many 

 parts of the world range, in general, from 2 to 10,000 meter-ohms. In 

 a few instances values considerably higher than 10,000 meter-ohms 

 have been observed. (The resistivity of a particular material, ex- 

 pressed in terms of the meter-ohm, is equal to the resistance in ohms 

 between opposite faces of a one-meter cube of that material.) 



With such a range of earth resistivities to contend with it is to be 

 expected that estimates of ground-return mutual impedances for situa- 

 tions in areas where no earth resistivity data are available may be in 

 error by large factors. In an effort to improve upon the accuracy of 

 such estimates a study was begun several years ago of the relation 

 between effective earth resistivity and geology^. Consideration was 

 at that time given only to area! geology, the geology of the strata of 

 the crust lying immediately below the soil and other loose surface 

 materials. 



From this preliminary work, it appeared that the resistivities in 

 areas of very old rocks were high and that, in a general way, decreasing 

 resistivity corresponded to decreasing age of the rocks. There were, 

 however, a number of outstanding discrepancies that could not be 

 satisfactorily explained. 



* Digest of a paper published in Eleclrical Engineering, November, 1935, and 

 scheduled for presentation at the A. I. E. E. Winter Convention, New York, N. Y., 

 January 28-31, 1936. 



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