Correcting the Effect of the Soil Covering Buried Electrodes for Permanent Electrical Resistivity Tomography Monitoring Systems

In long-term electrical resistivity tomography (ERT) monitoring projects, the cables and electrodes are usually placed under the surface. In such situations, the current flows both in the soil below the electrodes and through the soil covering the electrodes. This results in the potential field and the measured apparent resistivity values to be modified compared to when the electrodes are deployed on the ground surface. In this paper, we explain how to use the optical analogy to estimate this effect and correct the data. The results show that when the electrodes are below the surface, the measured apparent resistivity values are smaller than the real resistivity of the subsurface unless the geometrical factor is changed to take into account the electrode depth. This effect is especially important for measurements with shorter electrode spacings. We also present the correction graphs for two sites where we have installed permanent ERT monitoring systems along earthen embankments. These graphs, that are calculated based on the proposed methodology, are used as the first processing step to correct the measured apparent resistivity values. The corrected pseudosections can be then used for further processing and can be inverted using the algorithms that only have options for surface electrodes.