Surface pseudo-3D time-lapse ERT reveals the spatiotemporal evolution mechanism of lagged water inrush in ultra-thick coal seams

The development of water-conducting fractures induced by fully mechanized top-coal caving mining of ultra-thick coal seams in the aeolian sand area of northern Shaanxi is characterized by significant nonlinearity and hysteresis, making water inrush disasters highly concealed and difficult to predict. To overcome the challenges of high-resistance shielding from dry surface sand and the extraction of weak deep signals, this study established a surface pseudo-3D time-lapse high-density electrical resistivity tomography (ERT) monitoring system at the 122105 working face of Caojiatan Coal Mine, utilizing the "artificial wet soil + deep-buried electrode" technique. Combined with anisotropy-constrained inversion and time-lapse ratio imaging technology, the dynamic process of mining-induced overburden failure was visualized and verified by in-situ mine hydrological data. The results indicate that: (1) The center of water inrush is not located at the mining advance line but exhibits a significant "spatiotemporal lag" characteristic, with a lag distance of approximately 110 m, which aligns closely with the limit breaking span of the high-position key stratum. (2) The lagged water inrush channel presents a "funnel-shaped" structure (large at the top and small at the bottom) in 3D space, revealing that its formation results from the fluid-solid coupling connection between the upper separation "catchment basin" and the lower shear "diversion pipe" at the moment of key stratum breaking. (3) The moment of channel connection captured by resistivity imaging (T2) is perfectly synchronized with the onset of the mine water inflow surge, and the water inflow rapidly reached its peak (310 m³/h) within the subsequent 24-48 h, verifying the significant consistency between the flow field and the geoelectric field response. (4) The water-conducting channel in the goaf possesses self-healing properties; as the lag distance exceeds 150 m, the bottom of the channel closes preferentially due to compaction of the caving zone. Based on these findings, a prevention concept of shifting the monitoring field of view to 0-150 m behind the working face is proposed, along with a stereoscopic control strategy of "high-level interception in the funnel zone and low-level drainage in the compacted zone."