# 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
Finally, in South Africa, between 17.2 mm and 42.2 mm of surface subsidence was observed 6 months before the development of a sinkhole with dimensions of 0.5 m \(\times\) 1 m.  
Most of the successes in detecting sinkhole-related deformation was reported for areas undergoing solution-type deformation leading to wide shallow subsidence features. Their large scale and the relatively small-scale of deformation events lends itself to the successful monitoring by conventional repeat-pass interferometry. However, in the case of cover-collapse type sinkholes, the small scale of sinkholes, usually occupying a couple of pixels compared to other geohazards, imply that they can easily be misinterpreted as noise of processing errors [76], especially when DInSAR approaches are adopted. In the case of subsidence-type sinkholes, despite the relative success in detecting surface deformation using DInSAR, certain limitations were also identified. For example, despite the successful detection of sinkhole-related deformation in Texas [4], the same approach performed the retrospective analysis of precursory deformation for a 16 month period prior to the collapse of a sinkhole [4]. The analysis failed to detect any precursory deformation prior to sinkhole development. It was suggested that, in this instance, failure may have been sudden, once the void was shallow enough to permit failure of the roof [4] and that small-scale deformation prior to collapse may not have been present. This points to surface-cover and lithological constraints leading to precursory deformation features not be present or visible at the surface [20]. The main limitation associated with the application of conventional DInSAR approaches for the monitoring of surface deformation, irrespective of the cause of the deformation, is the temporal and geometrical decorrelation of the signal, especially in areas of dense vegetation, erosion, changes in moisture conditions and large deformation rates [5; 16; 19; 20; 21; 27; 40; 43; 81; 90; 105; 115]. In addition to signal decorrelation, the phase distortion introduced by changes in the atmospheric water content between image acquisitions have been known to introduce erroneous valuations of deformation [16; 21; 105]. To overcome the limitation, advanced processing techniques that focus on the analysis of coherent targets have been used. This includes approaches such as persistent scatterer (PS) interferometry [116], SBAS [117], and SqueezeSAR [118] approaches. These approaches can be used to remove atmospheric interferences while focusing on targets that remain coherent over the time series, thereby overcoming limitations due to pixel decorrelation. The PSI approach was used to retrospectively analyse data captured two decades prior to the collapse of a shopping mall in the Netherlands [34]. A combination of ERS1/2, Envisat and Radarsat-2 scenes were used. A total of 8 cm of subsidence was measured with deformation rates between 3.3 mm/year and up to 15.6 mm/year reported. Importantly, the surface deformation was observed for up to 2 decades prior to the eventual collapse event. Although the interferogram stacking approaches can overcome the limitations associated with atmospheric ambiguities, one particular limitation associated with PSI approaches is that the correct model to describe the expected deformation needs to be adopted [119]. In the case of sinkholes, deformation is expected to be non-linear over time. This makes the removal of atmospheric phase contributions, which are also non-linear, problematic since deformation-related phase contributions may be mistakenly removed [119]. In fact, analysis of the spatial and temporal behaviour of cover-collapse sinkhole related deformation suggests a Gaussian model for sinkhole evolution [21]. Given the unpredictable nature of potential precursors, coupled with their strongly non-linear character and frequently abrupt natures, the selection of appropriate model in the application of interferogram stacking approaches for sinkhole precursor monitoring is potentially problematic.