Optimization of Directionalized In-Hole Electrical Resistivity Tomography (ERT) and its Field Application

This study optimizes in-hole electrical resistivity tomography (ERT) by integrating surface electrodes to assess directional resistivity variations through numerical simulations and field applications. The proposed methodology involves rotating surface electrodes around a borehole at various azimuths while keeping the in-hole electrodes stationary. Four in-hole imaging arrays—A-BMN, A-MNB, AB-MN, and AM-NB—are evaluated for their directional performance using synthetic azimuthal apparent resistivity datasets. The arrays are tested under two scenarios: the in-panel scenario, where subsurface anomalies align with the surface electrodes, and the off-panel scenario, where surface electrodes are positioned opposite the anomaly. The analysis considers array measurement sensitivities, modeling accuracy, anomaly detection resolution in the in-panel scenario, and the impact of symmetric sensitivity effects in the off-panel scenario. The results reveal that the A-BMN and A-MNB arrays exhibit high measurement sensitivity and moderate modeling accuracy, but symmetric effects significantly constrain their directional performance. In contrast, the AB-MN array shows low sensitivity, poor model accuracy, a pronounced symmetric effect, and limited directional response. The AM-NB array, however, demonstrates high measurement sensitivity, improved model accuracy, minimal symmetric effects, and robust directional capabilities. Field validation involved rotating surface electrodes to eight predefined azimuths around monitoring wells while keeping in-hole electrodes stationary. Following chemical injection at a nearby injection well, measurements revealed substantial resistivity and conductivity variations at azimuths corresponding to the injection well’s orientation. Rose diagrams of resistivity data identified dominant flow paths and primary contaminant dispersion azimuths. This optimized, directionally sensitive in-hole ERT approach significantly improves multi-directional detection capabilities, enabling effective characterization of anisotropic subsurface conditions. By incorporating rotating surface electrodes, the study establishes in-hole ERT as a reliable method for identifying fluid flow pathways and elongated subsurface anomalies, advancing its application in hydrogeological, environmental, and geotechnical investigations.