# 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
In this way, different resistivities could be an indications of buried sinkholes and air- or water-filled cavities [58]. In one investigation, ERT was successfully used to characterise buried sinkholes by identifying the low resistivity of the clayey fill material which was contrasted against gravelly alluvium surrounding the cavity [56]. An example of ERT data for the detection of subsurface anomalies are provided in Figure 6. In this example from South Africa, a highly resistive zone was identified and inferred to be associated with an air-filled cavity [58; 59]. Additionally, more conductive zones associated with weathered dolomitic material filling two known sinkholes were observed [58; 59]. Although these examples illustrate that ERT has been found to be successful in mapping buried cavities, the limitation is that the technique relies on fill materials having contrasting resistivities when compared to surrounding materials. Furthermore, ERT is limited by a loss of resolution at depth [56]. In addition to the loss of resolution and the depth of the investigation, general limitations of geophysical surveys is related to the fact that the absence of a cavity in the geophysics results does not necessarily reject the possibility of future collapse [27]. Furthermore, although effective over small areas, terrestrial geophysical surveys cannot be applied to extensive areas since it is both time-consuming and expensive [27] and are considered to be hazardous in potentially unstable areas [27]. Therefore, earth observation technologies are increasingly being considered for the compilation of sinkhole inventories since the data can cover large areas rapidly, cost effectively and safely [44; 60; 61]. Various remote sensing platforms are used for sinkhole inventories, each with its own advantages and disadvantages. Ground-based systems such as ground-based radar and portable LiDAR are typically cost-effective and offer good resolution; however, they do not cover large areas, can easily be obstructed and it is challenging to keep the sensor stable over long periods [26; 45]. Optical or LiDAR sensors flow on airborne platforms are well suited for sinkhole investigations due to their high resolution [62; 63]. However, data acquisition campaigns are relatively expensive and are  
Figure 5: Radargram indicating a potential water-filled feature beneath a car-park in South Africa (with permission from [59]). typically not done frequently over long periods. In these instances, spaceborne platforms are ideal for monitoring large areas consistently over time, and are frequently used to investigate sinkholes [46; 61]. However, spatial resolution is a significant challenge for sinkhole research from spaceborne systems due to their relatively small scale [20]. Sinkholes are typically characterised by circular or oval shapes, conical collapse structures, concentric cracks along the outer edges, a sharp drop in elevation or basin-like depressions on the surface [64]. The mapping of sinkholes using remote sensing techniques generally rely on the detection and mapping of these distinct geomorphological features using one of two broad approaches namely; (1) feature extraction on optical imagery and (2) terrain models that are analysed for depression features. The simplest method of feature detection on optical imagery is visual inspection of aerial photography for signs of sinkholes [65]. This technique can be very valuable as sinkholes can often be dated accurately since many countries have aerial photograph archives dating back several decades [22; 33]. Sinkhole depressions can often be identified by ponding of water, concentric geomorphological structures such as tension cracks or vegetation clumps [66; 67] which are visible on remote sensing scenes. In general, for remote sensing investigations, high resolution, sharpness and the ability to be viewed stereoscopically is important considerations when selecting scenes for identifying sinkholes [22].