Monitoring ground uplift in an abandoned mountainous coal mine converted to geothermal facilities using Synthetic Aperture Radar Interferometry (InSAR)

Mining activity is essential for providing the raw materials necessary for societal development. The life of an underground mine depends on multiple factors, such as geological characteristics (e.g., mineral quantity and quality), economic drivers (costs, market prices, technology), technical constraints (e.g., extractive efficiency), and environmental/social considerations (e.g. regulations, impact). Following closure, multiple processes can occur, such as subsidence, slope instability, uplift, and the flooding of galleries. Therefore, continuous monitoring of abandoned mines is essential to mitigate risks to human safety and infrastructures. Some abandoned mines are repurposed for uses such as underground tourism, gas storage, fungi cultivation or geothermal resources, among others. The Candín-Fondón underground coal mine is located in the Nalón River valley in Langreo (NW Spain). Coal extraction in the region began in 1840 at the La Nalona mountain mine, later transitioning to the deep-shaft mining complexes of Fondón and Candín. The Fondón shaft reached a depth of 667 m and remained operational until 1995, while the Candín complex reached depths of up to 717 m. During the exploitation of the mines, groundwater levels dropped by more than 600 m in some areas. Today, these sites have been repurposed for industrial heritage and geothermal energy production (up to 3.488 MWh) using mine groundwater. Consequently, the groundwater level in the galleries has gradually recovered. However, the recovery of groundwater levels to the design level for geothermal exploitation has led to an increase in pore pressure within the rock mass joints, causing rock mass expansion and subsequent ground surface uplift. This uplift was detected using European Ground Motion Service (EGMS) Synthetic Aperture Radar Interferometry (InSAR) datasets for the period 2015–2021. Positive uplift rates exceeding 10 mm/year and accumulated uplift of over 4 cm were measured. These displacements are concentrated mainly over the mining works of Candín and Fondón, with maximum displacements located near the Fondón mine shaft. The InSAR time series shows a clear sigmoidal evolution of positive displacements starting in 2016, increasing until 2019, and then stabilizing. EGMS datasets from 2019–2023 show stabilization with residual uplift rates below 2 mm/year. Therefore, this work presents an example of how high-resolution InSAR datasets, such as the EGMS, can be used to accurately characterize the non-linear surface response to groundwater rebound in abandoned mines repurposed for geothermal energy.