Using multi-temporal interferometry and Sentinel-1 data to monitor ground instability hazards related to open-cast mining operations

Surface mining represents the predominant mineral and coal exploitation method in Europe and worldwide. Different mining methods can be involved, but most often: open-cast (open-pit) mining; terrace mining; contour strip mining; quarrying. Although surface mining is considered safer than underground mining, ground instability hazards represent an ever-present concern and important problem limiting the mining operations (e.g., slope instability in high wall open-cast mines and quarries). However, given the often large extent of areas affected by surface mining and life span of mines (tens of years), long-term monitoring via traditional in-situ methods is typically impractical (economically and technically). Here we focus on the use of an advanced multi-temporal interferomery (MTI) and Sentinel-1 imagery for mapping and monitoring of ground instabilities in open-cast mines and adjacent areas. Open-cast mines represent a good target for MTI, because they are i) often very large (from few to tens of km2); ii) free of or covered by sparse vegetation; iii) require long-term regular monitoring, which can now rely on free of charge Sentinel-1 data from the European Space Agency. However, a cursory review of the recent literature (international journals) suggests that in comparison to applications to underground mines, MTI has been relatively little exploited to investigate ground instabilities related to surface mining. One reason for this is that some portions of open-cast mines can lack measurable radar targets due to rapid changes of ground surface caused by intense mining operations (e.g., overburden stripping, waste material damping). We argue that this limitation can now be mitigated by the higher frequency and regularity of acquisitions provided by Sentinel-1 (nominally every 6 days since October 2016). To illustrate the potential of MTI for detecting and monitoring ground instabilities induced by surface mining, we present case study example of the Belchatow mine (Poland), one of the largest brown coal open-cast operations in Europe (about 12 km long and 3 km wide). The Belchatow open-cast mine reaches the depth of 310 m and has been affected by a number of slope failures in the last few decades. The failures disrupted the mining operations, destroyed in part the mining machinery and resulted in high economic losses. In this study we assess the recent and current stability conditions in the mine area by exploiting several tens of medium resolution (~20 m) images acquired by Sentinel-1 since October 2014. Despite the local lack of information (areas affected by intensive surface disturbance due to ongoing mining operations), the MTI results provided a valuable synoptic overview of the ground instability/stability condition within and outside the active mine. Although it is not simple to provide short-term forewarning of the impeding slope failure on the basis of the surface displacement time series alone, our experience indicate that long-term, high frequency MTI measurements offer the following benefits: i) better anticipation of risk of failure over large areas and more rational design of ground-based monitoring networks; ii) better planning for maintenance and management of open-cast mines.