Application of the surface-to-borehole direct current method for subgrade investigations in karst terrain

In karst subgrade exploration engineering, the application of the surface multi-electrode resistivity method faces challenges: on the one hand, the limitation of electrode array length constrains the effective exploration depth of this method; on the other hand, as the exploration depth increases, the available current distribution information from deeper layers decreases, resulting in a gradual weakening of the method's resolution capability, particularly in accurately predicting small-scale karst development areas at depth. To address this issue, the article proposes a solution: introducing wellbore electrodes on the basis of traditional surface multi-electrode resistivity observations to achieve “surface-to-wellbore” resistivity data acquisition. This strategy aims to increase and obtain deep current distribution information by adding wellbore electrodes, thereby enhancing the local effective exploration depth and improving the resolution of inversion results. To evaluate the effectiveness of the “surface-to-wellbore” resistivity observation method in obtaining deep karst information, the article first conducts an in-depth analysis using numerical simulation methods. Subsequently, the practical application value and effectiveness of this method are further demonstrated through inversion results based on measured data from karst subgrades. The research results show that the multi-electrode resistivity measurement technique with wellbore electrodes can significantly improve the resolution for identifying deep anomalies, providing an effective path to overcome the limitations of multi-electrode resistivity methods in deep karst exploration.