Integrated Geophysical Characterization of Internal Structure and Preferential Seepage in Open-Pit Mine Waste Dump

The Mao open-pit coal mine waste dump in Hequ, Shanxi, is a loose, anthropogenic mass accumulated over the original topography. Following a recent sliding and significant settlement event, this dump became the subject of intense stability concerns. Due to the high moisture sensitivity of its interlayered soil and coal gangue structure, rainfall infiltration can reduce internal effective stress, triggering slope instability. Although conventional geological surveys have mapped surface fractures, implementing precise, targeted drainage control requires characterizing the internal geometric structure and preferred seepage directions. To address this, this study integrates electrical resistivity tomography (ERT), surface nuclear magnetic resonance (SNMR), and spontaneous potential (SP) methods. Multiple ERT profiles (270–600 m long) were deployed across several benches at varying elevations, supplemented by fixed-point SNMR sounding over typical low-resistivity anomalies and dense SP grid scanning. The integrated results successfully delineate the internal architecture and seepage characteristics of the dump. Specifically, ERT imaging resolves the primary geoelectrical interface (tentatively inferred as the potential sliding surface) separating the overlying loose mass from the stable underlying strata while mapping the spatial extent of the inferred water accumulation zone (IWAZ). SNMR sounding quantitatively reveals a two-layer water-bearing structure at the specific sounding site, with a deep primary water-bearing zone at 45–80 m depth. Furthermore, SP inversions illuminate the seepage process, demonstrating that meteoric water deflects along the geoelectrical interface to converge laterally toward the central axis at approximately 42°, before transitioning into a high-angle vertical deep infiltration zone (61.7°) within the axial region. These findings suggest a potential engineering direction for remediating surficial fractures and designing subsurface drainage along this 1040 m bench axis, which would mitigate future landslide risks by reducing internal pore water pressure.