THE INFLUENCE OF SURFACE FORMATIONS ON THE APPARENT RESISTIVITY VALUES IN ELECTRICAL PROSPECTING*)

In this first part, ¹) we have examined the disturbances on resistivity measurements caused by some simple examples of surface irregularities with a cylindrical structure. The study has been carried out applying a theory developed in a previous paper (Alfano 1959)

In the first place, we have considered the case of measurements carried out with the potential electrodes near the surface irregularities, and the current electrodes more distant from those irregularities. This case is related to those methods of prospecting in which it is preferred to keep the current electrodes fixed and to move the potential electrodes.

One method has been pointed out which will separate, in the measured data, the component of the field due to deep formations, from the components due to surface irregularities. Afterwards, we have considered the case with current electrodes near the dishomo‐geneities and potential electrodes more distant from them. For both cases, numerical examples are shown, both for secondary charge values and for apparent resistivity values.

Finally, asymptotic values of vertical electrical soundings have been calculated for some cases, rigorously showing that they, in the presence of surface disturbances, are not equal, generally, to the true resistivity of a possible infinite substratum.

It follows that the measurement of resistivity of an infinite substratum, by means of the resistivity asymptotic value of only one sounding, is not possible.

The numerical results given have been obtained by means of calculations based on the abovesaid paper, but whose operative details shall be described in the second part of this paper.

The second part also will deal with the cases of surface dishomogeneity with a non‐cylindrical structure.