geosciences Article Differential SAR Interferometry Using Sentinel-1 Imagery-Limitations in Monitoring Fast Moving Landslides: The Case Study of Cyprus Marios Tzouvaras 1,2,* , Chris Danezis 1,2 and Diofantos G. Hadjimitsis 1,2 1 Department of Civil Engineering and Geomatics, Cyprus University of Technology, 3036 Limassol, Cyprus; [email protected] (C.D.); [email protected] (D.G.H.) 2 ERATOSTHENES Centre of Excellence, 3036 Limassol, Cyprus * Correspondence: [email protected] Received: 18 May 2020; Accepted: 15 June 2020; Published: 18 June 2020 Abstract: Cyprus, being located on the Mediterranean fault zone, exhibits a unique geodynamic regime since its tectonic evolution is driven by the interaction of the Eurasian and African plate. Besides its seismological interest, many active landslides and slope instabilities in areas of steep topography occur in Cyprus, having substantial impact on the built environment, by posing an imminent threat for entire settlements and critical infrastructure. Moreover, extreme meteorological events occur rarely, like severe rainfall and thunderstorms, that combined with the geological properties in some areas and the seismically stressed ground, can lead to landslides, causing severe damages to critical infrastructure. In the present study, the DInSAR methodology is applied for the detection of two individual landslide events that were triggered by heavy rainfall in Limassol and Paphos Districts in February 2019. Six co-event interferometric Synthetic Aperture Radar (SAR) pairs were used to produce displacement maps in vertical and east-west directions to study the resulting slope deformations. The above are carried out using Sentinel-1 imagery that are freely provided under the Copernicus umbrella. The limitations that arise from the speed and complexity of the deformations under study and the adverse residing meteorological conditions that caused these phenomena are investigated, as found in literature. Indeed, the sparse vegetation at the slopes affected by the landslides, the residing meteorological conditions, the heavy rainfall that triggered the two landslides, and the temporal phase aliasing effect due to the speed of the ground deformation were found to be the main limitations for the application of DInSAR methodology, resulting in the underestimation of the ground deformation that occurred. Keywords: DInSAR; interferometry; landslides; slope deformation; limitations; SAR; Copernicus 1. Introduction Landslides can be caused by geological factors (soil stability, lithology, faults, etc.); geomorphological factors, such as soil erosion in the case of Cyprus, vegetation removal due to fire or draught and volcanic or tectonic uplift; physical factors, such as topography, tectonic activity, intense rainfall and snow melt; and factors associated with human activity, such as excavations, agricultural activities, mining and deforestation [1]. The area geology plays an important role on the behavior of land during a seismic event or a landslide. This tendency, especially in Cyprus, is intensified by the long history of powerful seismic activity in the region. Cyprus is in the earthquake zone of the Alps-Himalayas, where 15% of earthquakes occur worldwide, and exhibits a unique geodynamic regime since its tectonic evolution is driven by the interaction of the Eurasian and the African plate. In fact, 19 earthquakes of MS >5 occurred in Cyprus Geosciences 2020, 10, 236; doi:10.3390/geosciences10060236 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 236 2 of 25 duringGeosciences the last2020,120 10, xyears. FOR PEER The REVIEW seismicity of Cyprus 1896–2019 is presented in Figure1. The magnitude2 of 26 of the earthquakes influences the number of landslides that are triggered or reactivated during a seismicof the event, earthquakes in addition influences to the the terrain number and of geological landslides factors that are [2 ].triggered Besides itsor reactivated seismological during interest, a manyseismic active event, landslides in addition and to slope the terrain instabilities and geological in areas offactors steep [2]. topography Besides its occur seismological in Cyprus, interest, having substantialmany active impact landslides to the builtand slope environment, instabilities by in posing areas of an steep imminent topography threat occur for entire in Cyprus, settlements having and criticalsubstantial infrastructure. impact to the built environment, by posing an imminent threat for entire settlements and critical infrastructure. FigureFigure 1. 1.Seismicity Seismicity of Cyprus 1896–2019 1896–2019 [3]. [3]. Moreover,Moreover, Cyprus Cyprus has has a a semi-arid semi-arid climateclimate typetype (BSh), in in the the north-eastern north-eastern part part of of island, island, and and a a hothot Mediterranean Mediterranean climate climate type type (Csa), (Csa), inin thethe restrest ofof the island according according to to Köppen-Geiger Köppen-Geiger climate climate classification,classification, with with very very mild mild winters winters andand warmwarm toto hothot summers. Summers Summers are are generally generally dry dry with with rainrain occurring occurring mostly mostly between between November November andand MarchMarch [4,5]. [4,5]. In In fact, fact, based based on on data data obtained obtained from from the the DepartmentDepartment of of Meteorology Meteorology in in Cyprus, Cyprus, averageaverage precipitation is is approximately approximately 470 470 mm/year. mm/year. Extreme Extreme meteorologicalmeteorological events events occur occur rarely rarely in in Cyprus, Cyprus, suchsuch asas heavyheavy rainfalls and and severe severe thunderstorms. thunderstorms. TheseThese extraordinary extraordinary events events combinedcombined withwith the geological properties properties in in some some areas areas and and the the seismicallyseismically stressed stressed ground, ground, can can lead lead toto groundground deformation,deformation, in in terms terms of of land land subsidence/uplift subsidence/uplift or or landslideslandslides [6 ],[6], causing causing severe severe damages damages toto criticalcritical infrastructureinfrastructure and and urban urban areas. areas. The The most most severe severe landslideslandslides have have occurred occurred in in the the mountainous mountainous andand hillyhilly areas of of Paphos Paphos and and Limassol Limassol district, district, causing causing damages to residences; civil infrastructure networks, such as roads and road networks; utility damages to residences; civil infrastructure networks, such as roads and road networks; utility services services pipelines, etc. Considering the impact of these phenomena combined with damages due to pipelines, etc. Considering the impact of these phenomena combined with damages due to seismic seismic events, villages such as Statos, Agios Photios, Kivides, and Korfi have been abandoned and events, villages such as Statos, Agios Photios, Kivides, and Korfi have been abandoned and relocated relocated at safer areas [6]. at safer areas [6]. There are many techniques for monitoring ground deformation due to landslides. These techniquesThere are can many be classified techniques as ground-based for monitoring or satellite-based ground deformation geodetic, duegeotechnical to landslides. or physical, These techniquesphotogrammetric, can be classified and Earth as Observation ground-based (EO)-based or satellite-based techniques [7]. geodetic, For each geotechnical one of these categories, or physical, photogrammetric,different instrumentation and Earth and Observation equipment (EO)-basedneeds to be techniquesinstalled or [used7]. For for eachperiodic one monitoring of these categories, or for diffcontinuouserent instrumentation data collection and either equipment remotely needs or in-situ to be [8]. installed or used for periodic monitoring or for continuousArea datainaccessibility, collection bad either weather remotely conditions or in-situ and [8 high]. infrastructure cost can be limiting factors forArea the use inaccessibility, of ground-based bad techniques. weather conditions The spatial and and high temporal infrastructure characteristics costcan of the be active limiting satellite factors forsensors the use used of ground-based in remote sensing, techniques. with their The very spatial short and revisit temporal times characteristicsand high coverage, of the can active overcome satellite sensorsthe obstacles used in remotethat can sensing, hinder with the theirmonitoring very short of natural revisit timeshazards and via high conventional coverage, can means. overcome Earth the obstaclesObservation, that can through hinder the monitoringexploitation of of natural SAR images hazards that via conventionalcan be acquired means. day Earth and Observation,night and regardless of weather conditions [9], can show the way for a systematic monitoring of land Geosciences 2020, 10, 236 3 of 25 through the exploitation of SAR images that can be acquired day and night and regardless of weather conditions [9], can show the way for a systematic monitoring of land displacement and assessment of the impact of natural hazards on infrastructure resilience over large areas in a timely manner. Differential SAR Interferometry (DInSAR) is an active remote sensing technique based on the principle that, due to the very high stability of the satellite orbits, it is possible to exploit the informative contribution carried by the phase difference between two or more SAR images looking at the same