# 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
The ability of UAVSAR to resolve the subsidence at the edges of the standing water that collected in the subsidence basin, was deemed to be due to the interaction of the signal with the tree trunks. Furthermore, the longer wavelength L-band is less sensitive to temporal decorrelation and is able to detect larger magnitude deformation between acquisitions than shorter wavelengths that are more constrained by the deformation gradient limit [111]. In the case of space-borne SAR data, the increasing availability, coverage and temporal resolution of data have resulted in numerous investigations focussing on DInSAR for detecting sinkhole-related deformation. One area particularly favoured for testing techniques for sinkhole precursor detection is around the Dead Sea region [6; 27; 85; 109]. In this area, the increasing frequency of sinkhole and subsidence events, from less than 50 per year prior to 1999 to 380 per year since 2003 [6] implies that several cases of sinkholes and their potential precursors could be investigated. The mechanisms of sinkhole development in the area was associated with the dissolution of a subsurface salty layer due to flood events [109], decreasing water levels in the Dead Sea [6] potentially leading to the consolidation of an aquifer system or a combination of the two. Furthermore, in one investigation, subsidence features were deemed to be structurally controlled in the presence of faults and boundaries of salt domes that were reactivated due to an earthquake in April 1979 [109]. In these investigations, DInSAR techniques were able to identify several circular to elongated features exhibiting primarily vertical movements (mostly subsidence with one case of heave) [85]. In most instances, sinkhole-related deformation was characterised as wide, shallow subsidence basins with deformation rates ranging between 5 mm/year and ~20 mm/year although rates exceeding 60 mm/year was observed in one instance [85]. In another study, ~12 cm of precursory deformation could be detected up to seven years before a dyke collapsed [27]. In general, the dimension of the subsidence features investigated ranged between a few hundred metres to a few kilometres in size. Furthermore, DInSAR results were used to model precursory deformation behaviour and it was found that, in the region, sinkholes are likely to form at the edges of subsidence basins [114]. In other regions, several successful sinkhole precursors could be detected in areas such as the USA, Europe and South Africa. In the USA, ERS-1 and 2 as well as ALOS PALSAR datasets were acquired for the Holbrook basin in Arizona [110]. Dissolution of salt in the area has resulted in over 500 sinkholes and numerous subsidence-related geomorphic changes. Using SAR scenes captured at varying intervals between 1992 and 2013, over 25 land subsidence features have been identified covering an area of over 3 600 km\({}^{2}\). Deformation of between 1.7 cm and 26 cm of deformation could be detected for features ranging between 40 to 130 m and up to 3 km across. The presence of subsidence-induced geomorphic features including drainage reversals, closed-basin formation, compression ridges and expanded joints were identified by field investigations, validating the results obtained by InSAR [110]. In a sinkhole prone region in Texas, ALOS PALSAR data captured over a 6 month period in 2007 revealed the presence of vertical movements in the vicinity of two sinkholes that occurred in 1980 and 2002 [4]. The subsidence basins, formed due to salt dissolution, revealed between 10 cm and 15 cm of total deformation over areas between 300 m and 850 m across. Subsequent gravity surveys confirmed the presence of mass deficits, suggesting a high likelihood of future collapses.