# 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
A further limitation is related to the lack of coherent point targets in vegetated or rural areas and pose limitations for the large scale adoptability of the PSI approach for sinkhole precursor detection. An approach to increasing the density of coherent targets, called the small baseline subset (SBAS) technique, minimises geometric decorrelation by using combinations of scenes with small perpendicular baselines resulting in a higher density of coherent targets [40]. The SBAS technique has been considered in several investigations aiming to measure sinkhole-induced surface deformation [76; 97]. In the Ebro Valley, Spain, and area associated with sinkholes <2 m and up to 10 m in diameter as well as subsidence basins up to 100 m in length could be investigated. Up to 1.68 cm/year of deformation was measured between 1995 and 2002 using ERS-1 and 2 data [97]. Although sinkhole precursors could be detected successfully, a large portion of known sinkholes were not detected. The non-detection of precursors were attributed to the decorrelation of the signal in irrigated agricultural areas and the low spatial resolution of the ERS-1 and 2 data. In fact, for cover-collapse sinkholes, exhibiting diameters <2 m, the InSAR techniques were considered to be unsuitable due to their small size and the loss of coherence associated with rapidly deforming surfaces [97]. On the other hand, evidence of ground deformation associated with cover collapse sinkholes exceeding 10 m diameter and subsidence features exceeding 100 m in length were observed [97]. The decorrelation of the signal in rapidly deforming areas [105] as well as generally low coherence in vegetated areas are some of the main limitations associated with approaches based on coherent point targets. The coherent targets frequently correspond to highly reflective, stable structures as often found in densely constructed and urban environments [21; 40]. In areas that are scarcely populated, the absence of coherent targets in rural and vegetated regions makes the implementation of coherent target approaches difficult [40]. To increase the quality of measurements in non-urban areas, advance approaches, including the SqueeSAR and Stable Point Network (SPN) approach, incorporates the analysis of larger areas exhibiting coherent behaviour over time, called distributed scatterers, as opposed to focussing on single point targets alone [21; 113; 112; 113]. The SqueeSAR and SPN approaches increase the density and quality of measurements in rural areas by identifying regions with a statistically homogeneous response that also exhibit coherent temporal behaviour [21; 113]. The SPN approach was used to monitor railway infrastructure in the Ebro Valley, Spain, and resulted in a high density of measurement points due to the high reflectivity of railways [113]. Subsidence of as high as between 6.6 mm/year and 9.7 mm/year could be measured in this region [113] and was found to be correlated with sinkhole locations. However, in other investigations, it was observed that the coherent distributed targets are normally associated with statistically homogeneous areas such as those found in non-vegetated areas, suggesting limited applicability in vegetated regions [99]. In fact, the SqueeSAR approach was adopted in the investigation of surface deformation associated with sinkhole development and failing infrastructure in Virginia, USA [105; 112]. Movement velocities of between 0.03 and 0.22 mm/year could be detected for both known and previously unknown sinkholes. However, heavily vegetated areas proved to be an obstacle and was masked out of the analysis. In these cases, new refinements to interferogram stacking techniques that extract information about scatterers that gain or lose reflectivity over only a subset of the stack has been recommended [112]. These scatterers, known as temporary coherent targets, should be considered for inclusion in future research [107; 120; 121; 122].