# 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  
Due to the risks involved with invasive and non-invasive cavity detection approaches, a significant amount of research has focused on the development of techniques to detect and monitor deformation prior to sinkhole development. The ability to detect ground deformation indicative of impending failure allows for proactive remediation, increasing public safety and minimising infrastructural damage [21]. The presence of deformation preceding the collapse of rock masses have been known to exist and are used as early warning for rock mass failures [24]. This is particularly relevant in the mining industry where precursory deformation is observed in both surface and underground mines [35; 81]. In the case of sinkholes, deformation in the order of a few mm to cm are speculated to be present days, months or even years before the failure [19; 20; 26]. In general, measurable deformation have been observed before major displacement occurs, although the main limitation lies in the opportunity to observe such deformation, especially when they occur in the underground environments [24]. The monitoring of ground deformation is conventionally achieved with the use of in situ approaches including, for example, inclinometers, extensometers, geodetic measurements, laser transmitters and receptors [12; 71]. Microseismic networks have also been deployed in an effort to identify precursors to collapse events [82]. Although these approaches provide very high precision measurements, their limitations for operational monitoring of surface deformation, irrespective of the cause of the deformation, is well known. The limitations include the fact that measurements are point-based and therefore limited in spatial extent [22; 40; 41; 81; 83; 42]. Measurements are also labour intensive, time consuming and costly, making its use impractical over large areas [22; 43; 71; 81; 83; 84]. To address these limitations, recent advances in satellite and airborne imaging techniques introduced new ways of monitoring subtle surface deformation features [85]. Since large areas need to be investigated, the potential of using remote sensing for sinkhole precursor detection have been recognised [31]. In an effort to derive a geohazard map of cover collapse sinkholes in Belgium, high resolution SPOT, ERS and aerial photographs were inspected with the aim of detecting precursors to sinkhole development with little success. The lack of findings were attributed to insufficient spatial and temporal resolution in addition to land use activities that were masking the effects of a growing void at the surface [31]. The use of high resolution data for the construction of digital terrain models and their comparison over time have also been considered. The structure from motion (SfM) algorithm in particular has been recognised for its ability to derive three-dimensional terrain from images taken from different angles at low cost. Image acquisitions with high spatial and temporal resolution are possible, allowing observations in highly dynamic environments [71]. Using multi-temporal SfM datasets, very high resolution digital surface models can be used to quantify changes in surface topography based on surface model differencing [71]. It, therefore, not only has the potential to be used for mapping sinkhole dimensions, but also has the potential to map surface movement. However, differencing of surface models are known to be associated with error propagation and, given the centimetre range vertical accuracies reported in previous investigations [71], the accuracy is typically too low for sinkhole precursor detection through elevation model subtraction [71]. Several advanced in the field of active remote sensing technology has highlighted significant potential for the identification of precursors to sinkhole development. This includes LiDAR surveys as well as differential radar interferometry techniques which is based on the analysis of a time series of SAR data.