# The Role of Earth Observation, with a Focus on SAR Interferometry, for Sinkhole Hazard Assessment
## 2 Mechanisms of Sinkhole Formation  
Sinkholes are formed due to the presence of cavities in soluble bedrock below the surface. The underground cavities form when water absorbs carbon dioxide (CO\({}_{2}\)) from the atmosphere or soils to form weak carbonic acid (H\({}_{2}\)CO\({}_{3}\)). The weak acidic water can circulate through the subsurface along tension cracks, joints, and faults [3; 8; 29]. In the presence of calcium carbonate rocks, the acidic water will cause a leaching of the carbonate minerals [8]. In these cases, the process of dissolution progresses slowly [8] and cavity formation under natural conditions can take tens of thousands of years for small caverns to form and millions of years for complete development [3]. However, evaporites dissolve more rapidly [7; 22; 30], up to 100 times faster than calcium carbonate rocks [11; 22]. This is because evaporites have higher solubility and lower mechanical strength compared to calcium carbonates [7; 11; 12]. The higher solubility of evaporite deposits imply that sinkholes develop more rapidly and have a higher probability of occurrence in these areas [11]. In both scenarios, anthropogenic influences and the presence of urban infrastructure can expedite the process of underground cavity formation significantly [3; 8; 9; 22; 29]. This is due to the presence of leaking water-bearing service infrastructure and inefficient storm water management which is associated with an increase in water ingress and consequent acceleration of the development of underground cavities [8]. If subsurface cavities are present, sinkholes can be triggered due to natural or man-made vibrations, surface loading, the ingress of water, or as a result of dewatering [8; 14; 31]. Where cavities are filled with groundwater, a lowering of the water table (dewatering) can expose roof materials and concurrent removal of the buoyant support of these materials [11; 14]. These conditions can activate or accelerate subsurface erosion of the roof strata into the cavity [13]. In the case of water ingress, the concentrated ingress of water will result in the mobilisation of roof materials into the cavity. The triggering mechanism in question generally affects the size of the sinkholes with sinkholes triggered by dewatering resulting in larger sinkholes (up to hundreds of metres in diameter) while sinkholes induced by water ingress tend to be smaller [8]. The resulting instabilities are generally classified either as sinkholes or as subsidence depending on the local conditions leading to their formation. Subsidence, also referred to as solution sinkholes, form when soluble rocks are close to the surface or exposed [12]. This leads progressive dissolution and a gradual sagging of the surface [21]. A graphic representation of subsidence basin formation is provided in Figure 1. The illustration provides an example of water ingress due to leaking water bearing services as example (Figure 1A). The overburden is not competent enough to support arch formation, leading to gradual subsidence (Figure 1B). An example of a subsidence feature developed in a mining region in South Africa is provided in Figure 1D. Although subsidence events can be extremely damaging to buildings and infrastructure, the associated deformation of the surface is slow and gradual, and are therefore less likely to result in catastrophic events since there is time to issue warnings [13]. Figure 1: A graphic representation of subsidence basin formation. (**A**) The presence of a cavity near the surface and a triggering mechanism initiates the process of subsurface erosion; (**B**) The roof strata is not competent enough to support arch formation and leads to gradual sagging; (**C**) Significant damage to infrastructure can be experienced; (**D**) An example of a subsidence basin in South Africa. Sinkholes, on the other hand, are generally associated with sudden collapse [21; 22] which poses significant risks to human health and safety as well as infrastructure.