Electrical and magnetometric fields in a layered earth containing buried electrodes

Abstract

The number of analytical magnetometric resistivity (MMR) results available for basic earth geometries is limited compared to that of electrical resistivity methods, which is unfortunate since MMR has advantages for certain classes of problems. This paper extends the list of MMR results by deriving the response for the homogeneous half‐space and the multilayered earth. Both are calculated for arbitrary source and receiver positions. We show how the result for the layered earth reduces to that of the half‐space when there is only one layer. Necessary procedures for successful numerical evaluation of results for the layered case are given. Sample borehole logs of electric and magnetic fields for an example of a two‐layer model illustrate an advantage of the magnetic‐field measurement; namely, that it is sensitive to the position of layer boundaries rather than to the position of transmitter electrodes. The algorithm is also applied to interpretation of MMR field data from two boreholes drilled near massive sulfide conductors. The magnetometric influence (or background) due to borehole geometry in an electrically layered earth may be computed and subtracted from the field data, after which it is possible to perform quantitative modeling of the residual MMR anomaly to define the location of ore‐related conductors.