Unveiling two-stage deformation mechanisms in restored Emma open-pit coal mining areas, Spain, using InSAR and NMF decomposition

The evolution of open-pit coal mining over several decades has encompassed two distinct stages: resource exploitation and ecological restoration. Each phase has led to significant changes in surface coverage and elevation, accompanied by distinct patterns of surface deformation. Reconstructing deformation characteristics across both mining and restoration periods, analyzing the underlying mechanisms, and validating results are crucial for understanding the surface evolution. This study examines post-restoration deformation characteristics using archived Synthetic Aperture Radar (SAR) data and Intermittent Small Baseline Subset-Interferometric Synthetic Aperture Radar (ISBAS-InSAR) techniques. Non-negative Matrix Factorization (NMF) is then applied to decompose deformation signals and analyze the two-stage deformation mechanisms. Lastly, validation is performed through comparisons with differential Digital Elevation Models (DEM) and optical imagery acquired before and during the open-pit mining phase. The Emma mining area in Spain was selected as the study site. Sentinel-1 SAR imagery from 2017 to 2024 reveals vertical deformation rates of up to 21.6 cm/year. The two components derived from NMF correspond to long-term subsidence due to historical mining and short-term deformation resulting from slope restoration. The first NMF component is validated using differential DEM (2009–2020), revealing a positive correlation between deformation magnitude and backfill thickness. The second component is supported by optical imagery from 2015 to 2016, indicating that large-scale slope modification during ecological restoration contributed to short-term deformation. This study presents a quantitative framework for analyzing mining-induced deformation and assessing the effects of ecological restoration on terrain stability.