Objective Resistivity methods allow for effective detection of seepage in reservoir dams. However, when used to detect the hidden hazards of dams, the surface resistivity method fails to accurately identify vulnerable areas across an entire dam due to blind spots near both dam abutments and insufficient resolutions for deep parts.

Methods This study proposed a seepage detection technique for earth-rock dams based on surface-to-borehole resistivity tomography, followed by the analysis of the distribution of surface-to-borehole resistivity under the lateral and vertical variations in a seepage zone through numerical simulations. This technique was applied to the Shibi reservoir dam. Using the surface-to-borehole resistivity observation system, this study determined electrode current in the dam, as well as the near-surface, cross-borehole, and surface-to-borehole resistivity. Furthermore, the reliability of the detection results was verified in combination with drilling data, reservoir water levels, geological sections, and borehole televiewer images.

Results and conlusions The results indicate that, as the preset seepage zone gradually approached the dam abutment, the surface resistivity method yielded progressively increased deviations in the depth and central location of the anomaly zone. In contrast, the surface-to-borehole resistivity tomography was slightly influenced by the high-resistivity zones of mountains on the bank slope, yielding detection results aligning well with the preset models. Despite being sensitive to shallow seepage anomalies, the surface resistivity method was insufficient to identify deep seepage zones. In contrast, the surface-to-borehole resistivity tomography revealed distinct targets at varying depths while also enabling effective characterization of mountain boundaries. In the seepage detection tests of the Shibi reservoir dam, the surface resistivity reflected a wide range of seepage anomalies along the dam body but failed to comprehensively capture seepage zones near the abutments. Although cross-borehole resistivity exhibited improved accuracy in the seepage detection of the dam body between boreholes, it showed limited coverage. In comparison, the inversion-derived surface-to-borehole resistivity sections provided broader exploration ranges while also effectively suppressing the low-resistivity anomalies of the dam body. This helps reduce the volumetric effect of surface resistivity, allowing for the precise delineation of low-resistivity anomalies in deep dam-abutment contact zones. Overall, the results of this study provide a technical basis for the targeted elimination of hidden hazards in reservoir dams and offer a new philosophy for the precise diagnosis and intelligent perception of earth-rock dams.