# Applications of Satellite Radar Imagery for Hazard Monitoring: Insights from Australia
## 5 Natural Resource Extraction
### Below Ground Mining Operations  
Underground longwall mining is the principal approach used in Australian coal mines operating at depths of \(>\)300 m, involving the total extraction of the panels of coal in corridors that are often several kilometres long by hundreds of metres wide. As the panel is progressively extracted, the roof is supported. When the support is removed, the overburden may collapse into the void. This downward movement of material can result in ground-surface subsidence of up to 1 metre directly above the extracted panel. The monitoring and management of this large magnitude subsidence are particularly critical where mines are located close to population centres (e.g., Tahmoor Colliery, NSW, located below the towns of Tahmoor and Thrilmere: Figure 6) and engineering structures (e.g., Tower Colliery, NSW, is overlain by a freeway that crosses a bridge over a river gorge).  
Several studies have demonstrated the feasibility of using InSAR for monitoring subsidence at longwall coal mines in Australia (e.g., [4; 53; 54]). The evidence from the time-series analysis indicates that residual subsidence continues years to decades after  
Figure 6: Cumulative vertical displacements at the Tahmoor longwall coal mine, New South Wales, derived from the InSAR analysis of RADARSAT-2 data between 20150715 and 20190531. (**A–C**) display the cumulative vertical displacement at 1-year intervals (i.e., July 2016, July 2017, and July 2018, respectively) relative to the first epoch (July 2015). The magnitude and spatial extent of displacements increases over time. The location of longwall panels is overlaid in grey. The arrow shows the progression of longwall extractions, with the most southwestern longwall extracted first. (**D,E**) show vertical displacement time-series for two locations marked by ‘x’ and ‘+’ in (**A–C**). The epochs displayed in (**A–C**) are marked by vertical grey lines.  
mining activities cease (e.g., the Metropolitan Mine: [53]), emphasising the requirement for long-term ground displacement monitoring.  
Example Case-Study: Tahmoor Longwall Coal Mine (New South Wales)  
At the Tahmoor longwall coal mine, New South Wales (for location, see Figure 2A), we derive cumulative vertical displacements between July 2015 and May 2019 from InSAR analysis of ascending and descending orbit RADARSAT-2 data (Section 2). Maximum subsidence of \(-0.8\) m is observed (Figure 6). Operations progress from one longwall to the adjacent longwall in the direction shown by the arrow in Figure 6A. Time-series of individual pixels can be used to track the time history of displacements at different stages of longwall operations and occurring due to interactions between adjacent longwall extractions (Figure 6D,E). The major subsidence shown in Figure 6D occurs in late 2015 and late 2016 and is mainly affected by the two adjacent longwalls. In contrast, the time-series shown in Figure 6E is not only affected by longwall operations in the adjacent longwalls (in late 2016 and late 2017/early 2018) but also by longwall operations further away from the pixel location (in late 2018/early 2019).  
These observations are in qualitative agreement with ground-based measurements available in public reports [55]. However, unlike ground-based measurements, the InSAR analysis provides a dense spatial coverage of ground displacements for the full area capturing the spatio-temporal evolution of subsidence.