# The Role of Earth Observation, with a Focus on SAR Interferometry, for Sinkhole Hazard Assessment
## 5 Discussion and Perspectives--Towards Operational Sinkhole Early Warning  
In this section, we discuss the advances and limitations of the techniques employed for the extraction of information informing sinkhole hazard assessments. The main advantages and limitations of the various approaches as well as the recommendations in implementation are summarised in Table 2.  
Given the value that can be derived from sinkhole inventories through the reduction and management of sinkhole risk, significant efforts have been devoted to refining techniques for the compilation of sinkhole inventories as well as the mapping of regional conditions impacting sinkhole formation. It has been noted that the quality of predictive models are dependent on the completeness and representativeness of the sinkhole inventories on which they are based [37]. Since sinkholes are characterised by distinct landforms [7], the critical component in identifying the sinkholes generally rely on field surveys and geomorphological investigations together with historical records such as newspapers and interviews with local community members [12; 13; 14; 17; 18]. However, an underground cavity can be present without any indications being visible at the surface and the main problem for assessing sinkhole hazard is a lack of knowledge of the existence of a cavity [34]. In such instances, the presence of a cavity can be confirmed by geophysical techniques with the goal of identifying if buried sinkholes overlap with areas of planned development to apply a preventative planning strategy [22]. Through the detection of underground cavities, damage caused by sinkholes can be prevented by refilling of the cavity by material such as sand or gravel [14]. Although the geophysical approaches can identify the presence and geometry of underground cavities, certain limitations associated with these techniques have been described [12; 34]. In particular, anomalous features are frequently present on these datasets [22]. Therefore, geophysical surveys are often complemented by cone penetration and percussion borehole investigations, specifically targeting anomalous features identified by geophysical surveys [18]. However, direct observations within cavities are seldom possible due to the lack of knowledge of the location of cavities and their stability. Exploratory drilling campaigns are also expensive and inefficient for overall site characterisation and coverage over large areas [67]. Due to the limitations of in situ and geophysical approaches to collect information for the compilation of sinkhole inventories, remote sensing and earth observation approaches have been considered for the information that can be contributed for sinkhole hazard assessment. Satellite and airborne remote sensing datasets have been found useful for the extraction of information that can contribute to the various stages of sinkhole hazard assessment including the compilation of the sinkhole inventories and the measurements of precursors to sinkhole development. The recent publication frequency of sinkhole investigations (Figure 8) show that there has been increasing interest in the potential of earth observation techniques, and particularly SAR interferometry, for sinkhole hazard studies. These are typically complimented by historical record analysis, available for certain areas, and the various forms of geotechnical investigations. Figure 8: Frequency of publication of referenced sinkhole investigations that aim to build inventories or detect precursors, categorized by the main investigation method; compiling historical records in yellow, geotechnical and in situ investigations in grey, remotely sensed terrain models in green, multispectral image analysis (including aerial imagery) in red and InSAR in blue.