Open-pit mining activity and stability area mapping in the Pyhäsalmi Mine using a time series of Sentinel-1 images

Synthetic aperture radar (SAR) single look complex (SLC) Sentinel-1 data provide continuous all-weather monitoring of the Earth's surface and detection of texture change areas based on coherence maps. A mining intensity assessment method is proposed in this paper to identify activity and stability areas by calculating the temporal activity index (TAI) – the frequency indicator of surface changes. The TAI value was calculated from a time series of coherence-based normalized differential activity index (NDAI) robust to baseline and temporal decorrelation. The method was applied to monitor the surface conditions of the Pyhäsalmi Mine's (Finland) open pits and waste dumps from May to September 2018–2022, using a time series of Sentinel-1A images acquired in vertical transmit and vertical receive (VV) polarization with a 12-day time step. In each of the observation seasons, TAI maps were generated and areas of activity were identified, mainly on the surface of the waste dumps, at the bottom of the backfill open pit and on the slopes of the old open pit. Activity and stability areas were validated using unmanned aerial vehicle (UAV)-based very high spatial resolution visual orthomosaics and digital surface models (DSMs), which confirmed land cover changes in areas with high TAI values and no changes in stability areas.