APPARENT RESISTIVITY, ANISOTROPY AND STRIKE MEASUREMENTS*

In the search for a descriptive system capable of accounting for the wide variations of earth resistance with orientation of electrode array, which can occur in field measurements, a modified measure of apparent resistivity, based on an anisotropic earth, is proposed.

This measure requires the sampling of earth resistance in, at least, three orientations. A practical system, the crossed square array, which effectively samples in four, is considered in detail. A system of reduction of field observations yields, besides the apparent resistivity, apparent measures of anisotropy and strike.

Field trials show that the anisotropic analysis not only accounts for the major part of the observed orientational variations in resistance, but also yields sounding curves of resistivity, anisotropy and strike, which vary systematically with electrode spacing.

The effects and limitations of applying such techniques in regions of pronounced two dimensional structure are examined.

Finally, to avoid unnecessary labour in deploying the crossed square sounding technique at sites where it is not required, a modified single square technique is proposed, which will normally serve but will also alert the observer when crossing is required.