Mapping and monitoring electrical resistivity with surface and subsurface electrode arrays

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Abstract
Electrical resistivity measurements using combinations of subsurface and surface electrodes are more sensitive to subsurface inhomogeneities than arrays confined to the surface. A further advantage of the subsurface configuration is that the strong influence of near‐surface inhomogeneities can be reduced by differencing the measured apparent resistivities with a reference set of values obtained with the subsurface electrode(s) at a particular depth. This process accentuates the response of features near the downhole electrode while canceling the response of the near‐surface features. An idealized two‐dimensional model of a nuclear waste repository has been used to demonstrate the effectiveness of this differencing scheme. It is shown that resistivity measurements using borehole electrodes well away from the repository and on the surface are sensitive to changes in the repository that could not be practically observed from surface measurements. This sensitivity is preserved in the presence of a conducting and inhomogeneous surface layer and, most importantly, it is preserved even if the resistivities of the near‐surface features also change.

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