High-resolution electrical resistivity tomography for quantitative interpretation of sub-surface karst structures: A case study in Southwest China

• Improving soil-epikarst interpretation accuracy using the revised resistivity data. • Average epikarst thickness of hillslopes is larger than that of valley. • Soil-epikarst thickness exhibits synchronous zonality variations in the catchment. • Derived topographic characteristics are key to modeling epikarst distribution. Soil-epikarst thickness and near surface characteristics are important components in understanding surface and groundwater interactions in karst environments. However, the complex lithological conditions, non-transparent rock and soil structures, and strong spatial heterogeneity limit the accurate quantification of soil thickness (ST) and epikarst thickness (EkT). In this study, we investigated the soil-epikarst structures and their spatial distribution at key topographic locations including different hillslope position, ridge, saddle, and valley using Electrical Resistance Tomography (5268 sampling points) in a peak cluster-valley catchment in Southwest China. The application of revised inflexion points in 1D resistivity vertical profiles for improving ST and EkT characterization accuracy was assessed, with interpretations validated against borehole data. The results show that compared with the interpretation accuracy of using a specific resistivity threshold at interfaces, the revised inflexion point of the 1D resistivity vertical profile significantly improved the interpretation accuracy of ST and EkT. The average ST in the valley (3.23 m) is much greater than that in hillslopes (0.49 m), while the average EkT in the valley (3.77 m) is smaller than that in hillslopes (3.93 m). The ST and EkT demonstrated a synchronous zonality variation pattern at different hillslope positions and valley. Plan and profile curvature, flow length up/down, aspect, and elevation are key topographic characteristics affecting EkT’s spatial heterogeneity. The key findings of this study contribute to advancing the accurate interpretation of soil-epikarst structures under complex lithological conditions in karst areas, and support underground structural parameters for hydrological simulation at catchment scale.