# 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  
Although in situ approaches for the collection of information for sinkhole hazard assessment (including surveying, trenching, geophysical and geodetic monitoring approaches) are well established, the limitations of these approaches are well described. This includes the fact that they are time-consuming, labour intensive, costly and invasive [40; 41; 42; 43]. Recent advances in earth observation approaches, including satellite- and airborne imaging techniques, provide information that can assist sinkhole hazard assessments. Earth observation technologies are used for compiling sinkhole inventories but also have the potential to be more widely used for assessing sinkhole damage [44]. This includes the monitoring of active sinkholes [45] and even to identify environmental risk factors that can contribute to sinkhole development. Remote sensing is often used to assess the extent and magnitude of infrastructure damage using techniques involving high-resolution change detection on optical or SAR imagery before and after the event [44]. It has successfully been used after disasters such as earthquakes [44], volcanic eruptions [46] and landslides [47] and could therefore also be applied to sinkhole events. Remote sensing can also be used to monitor the evolution of active sinkholes, this requires very high-resolution data and airborne LiDAR has been shown to be an effective tool [45].  
In terms of sinkhole risk assessment and damage mitigation, it has been recognised that sinkholes in urban areas are likely to be caused by anthropogenic factors with leaking water infrastructure and groundwater extraction being particular risk factors [8; 12; 48; 49]. There is therefore an opportunity to investigate the role of remote sensing is assessing the presence of these risk factors and the relationship with sinkhole formation. Remote sensing is in fact already widely used as a tool in monitoring both leaking infrastructure and groundwater extraction. Pipe leaks can be detected by monitoring vegetation growth using vegetation indices on high resolution optical data [50; 51; 52] while groundwater extraction is often monitored using time-series SAR interferometry [53; 54; 55]. The potential therefore exists to investigate the connection between these factors and sinkhole events using remote sensing.  
Although the monitoring of risk factors can be performed, the value derived from sinkhole inventories through the reduction and management of sinkhole risk means that significant efforts have been devoted to refining techniques for the compilation of sinkhole inventories using earth observation techniques. Additionally, new advances in earth observation techniques have also resulted in several attempts to identify small-scale precursory deformations indicative of sinkhole development. The use of earth observation techniques for the population of sinkhole inventories and for the detection of precursors to sinkhole events have been conducted for various regions across the world (Figure 4). This includes the use of optical, SAR and terrain analysis approaches for sinkhole inventory compilation as well as SAR interferometry for monitoring precursors to sinkhole development. The successes, advantages, and limitations of the various approaches are elaborated on in Section 4.1 for sinkhole inventory compilation and Section 4.2 for precursor detection respectively.