# The Role of Earth Observation, with a Focus on SAR Interferometry, for Sinkhole Hazard Assessment
## 4 The role of Earth Observation in Sinkhole Hazard Assessment
### Detecting Precursors to Sinkhole Development
LiDAR and terrestrial laser scanners are active remote sensing techniques that generate topographical information that can theoretically be used for monitoring deformation over large areas. However, deformation monitoring with LiDAR is challenging as typical rates of subsidence are within the error limits of LiDAR collections [26; 60]. Furthermore, reaching the necessary accuracy requires the instrument to be within 10 m to 100 m from the ground, limiting its wide area applicability [26]. On the other hand, the field of Synthetic Aperture Radar (SAR) image analysis in general, and differential SAR interferometry (DInSAR) techniques in particular have demonstrated the ability to measure small scale movements of the surface [19; 20; 42; 43; 60; 81; 83; 86; 87; 88; 89]. In fact, in ideal conditions, cm to mm scale deformation measurements are possible [22; 81; 90; 91; 92]. Orbiting satellites and satellite constellations enable measurements at high temporal frequency (as often as every couple of days) over large areas [22; 81]. Furthermore, in some regions, long-term historical archives of SAR data are available, enabling the retrospective analysis of deformation events and their evolution over time [22; 26]. An example of subsidence-induced deformation measurements from DInSAR is shown in Figure 7. In this figure, ongoing ground deformation associated with subsidence basin formation in South Africa as detected by RADARSAT-2 and Sentinel-1 SAR satellites for the period of September to November 2017 is shown. Due to the sensitivity to small scale movements, DInSAR techniques have been used extensively for the mapping and monitoring of surface deformation associated with a variety of natural and geological hazards [90; 93; 94; 95]. DInSAR has also been used for the identification of precursors to geological hazards such as mining collapse [16; 40; 42; 43; 81; 83; 96; 97; 98; 99; 100], landslides [16; 91; 97; 101; 102; 103] and volcanoes [16; 90; 95] as well as the mapping of deformation due to aquifer over exploitation [16; 56; 86; 87; 89; 96; 104]. The maturity of DInSAR techniques has reached a stage where they are applied in numerous operational systems [28]. The synoptic view and repeat observations provided by spaceborne SAR sensors mean that the data could prove to be an effective tool for the long-term monitoring of areas susceptible to sinkhole formation [2]. In fact, certain case studies suggest that DInSAR techniques could be used for the detection of precursory deformation [5; 6; 19; 20; 21; 26; 34; 92; 105; 106; 107; 108]. Several successful detections of sinkhole precursors have been reported in the literature. A summary of the investigations for which precursors to sinkholes were successfully detected is provided in Table 1.  
Figure 7: An example of differential interferograms generated from Sentinel-1 data showing ongoing ground deformation related to mining activity in South Africa. Each colour cycle represents ground surface deformation of \(\sim\)2.7 cm between image acquisitions.