# 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
### Compilation of sinkhole Inventories  
The most important step in sinkhole hazard assessment is the construction of a comprehensive sinkhole inventory [11]. In fact, it has been noted that the quality and accuracy of predictive models are dependent on the completeness and representativeness of the sinkhole inventories on which they are based [37]. Theoretically, if a large inventory of past sinkhole events including information on the location, magnitude and chronology is available, the spatial and temporal probability of sinkholes can be derived [39]. Sinkhole inventories should, ideally, include information on the location, morphometric parameters, genetic type, chronology, deformation activity, conditioning and triggering factors of past sinkhole events [11]. The compilation of sinkhole inventories are usually achieved using direct observations such as interviews with local residents and geophysical investigations [15] (Section 3). However, in recent years, various remote sensing approaches have also proven to be effective in the identification of sinkholes at the surface, typically through analysis of historical aerial photographs, satellite imagery and topographic maps [12; 15; 17; 18; 22; 27]. The compilation of sinkhole inventories generally relies on two aspects: (1) the detection of buried cavities that may cause future collapse of the surface and (2) an inventory of sinkholes that formed in the past. For the detection and mapping of buried sinkholes and cavities, geophysical methods have been used with reasonable success [25; 26]. These methods can be either destructive or non-destructive. Destructive methods include drilling and trenching and are useful for in-depth characterisation of the structures and dating of sediments; information not obtainable through other methods [56]. They are, however, only successful if the excavation site is well chosen, typically by non-destructive methods [22; 56]. Non-destructive geophysical methods include the use of seismic reflection [30], micro-gravity surveys [4; 27], magnetic surveys [57] electromagnetic (EM) surveys [30; 56; 58] and ground penetrating radar (GPR) [25; 56]. A review of the geophysical methods for the detection of underground cavities has been completed [57]. In general, the most commonly used geophysical technique for the detection of underground cavities is GPR which operate by transmitting microwaves into the ground and analysing the returned signal [25; 26]. The technique has been used to identify and  
Figure 4: Earth observation techniques have been used worldwide for sinkhole precursor detection (stars) and sinkhole inventory compilation using optical remote sensing (blue triangles), remotely generated terrain models (green triangles) and DInSAR techniques (red triangles). localise subsurface cavities [26] and has also successfully identified buried sinkholes [25] to contribute to sinkhole inventories. An example of a radargram generated for a sinkhole-prone area in South Africa is presented in Figure 5. Anomalous features identified were associated with a zone of increased reflections due to the presence of a water-filled zone [59]. The value of GPR depends on the subsurface material characteristics as well as operator expertise and knowledge of the area and is limited by penetration depth of the selected frequency [22]. Gravity surveys have also been performed for the detection of buried cavities. In fact, the approach was demonstrated to be effective in the Dead Sea region when sinkholes occurred a few months to a few years after the survey revealed the presence of anomalies [27]. When using gravity surveys, areas prone to collapse are generally associated with negative gravity anomalies, highlighting shallow mass deficits and the potential for subsidence or collapse [4]. EM surveys, including the Transient Electromagnetic Method (TEM) and the Electrical Resistivity Tomography (ERT) methods, have been also used to provide a better understanding of the composition of underground deposits by profiling different resistivity zones [30].