Borehole-Ground-Borehole Resistivity Imaging Method Applied for Low Water Level Conditions

The direct current (DC) resistivity imaging method has been widely used in geological hazard surveys. Utilizing the borehole-ground-borehole observation pattern enables a more refined positioning and imaging of adverse geology. Under low water level conditions, shallow electrodes fail to couple with the strata effectively, resulting in data loss and reduced imaging quality. We conducted borehole-ground-borehole electrical resistivity imaging research, enhancing existing methods from the aspects of data acquisition, forward modeling, and inversion imaging. Firstly, suitable electrode arrays for low water level conditions are selected based on sensitivity analysis. Secondly, an unstructured grid is used to model the detection area, with a coarse grid applied in regions of weak sensitivity to reduce parameters. Finally, smooth constraints based on unit area and spacing are designed and incorporated into the inversion equation. Additionally, by constraining resistivity values with borehole information, the ambiguity of inversion is further reduced. The effectiveness and practicality of the method were validated through numerical simulations and engineering case studies on weathered rock formations. The results indicate that the borehole-ground-borehole electrical resistivity imaging method exhibits high lateral resolution and can accurately delineate the distribution range of weathered rock formations.