The Complex Karst Dynamics of the Lisan Peninsula Revealed by 25 Years of DInSAR Observations. Dead Sea, Jordan.

The Dead Sea (DS) area is one of the best examples of the significant impact that the uncontrolled exploitation of natural resources may have on the territory and the environment. In the last decades, the DS territory faced a profound change as consequence of the human activities that interfered with the delicate equilibrium of its ecosystem. In the 1960s, the potash industries started to heavily exploit the salty-rich water of the DS, pumping huge amount of water in the desalinization ponds located in the southern section of the lake. Furthermore, most of the freshwater that was coming from the Jordan River and from the other main feeders was diverted for urban and agricultural purposes in the region and as far as the Negev desert, in southern Israel. This, in combination with the unique climate of the territory, characterized by high annual evaporation rates (1500 mm) and very low average annual precipitations (60 mm) resulted in a strong negative water balance that caused the water level to drop with increasing speed. The decline rate, calculated as 17 cm/yr in the period from 1930 to 1973, has reached 100 cm/yr and exceeded 120 cm/yr nowadays. In the last 40 years, the level dropped by 36 m (as of March 2017, the DS water level is at -431 m m.s.l.) and the lake shrunk by more than one-third. The consequent change in the hydrogeological settings of the entire basin caused the seaward and downward migration of the fresh/saline groundwater interface forcing the freshwater to flow through the underlying evaporite layers constituted mainly by salt and gypsum. Dissolution-related phenomena such as subsidence and sinkholes started to appear all along the DS shoreline bringing heavy damage to the territory, the infrastructures (bridges, roads, earthen dikes of the desalinization ponds) and the buildings (houses, hotels, resorts, factories). The most active subsidence occurs in the areas surrounding the Lisan Peninsula (LP), located in the southern part of the DS in Jordanian territory. The Peninsula is also characterized by uplifting areas mainly as consequence of the upward movement of the underlying Lisan salt diapir. This work presents and analyses the results obtained from Differential Interferometric Synthetic Aperture Radar (DInSAR) techniques applied the monitoring of an area of about 18 km x 22 km that entirely covers the LP. The available SAR datasets consist of: 24 ERS-1/2 covering the period 06/1992-06/2000 and 31 ENVISAT covering 01/2003-06/2010, both acquired in C-band by the European Space Agency (ESA); 10 L-band ALOS PALSAR acquired by the Japan Aerospace Exploration Agency (JAXA) from 11/2007 to 02/2011; 20 X-band COSMO-SkyMed (CSK) for the period 12/2011-05/2014 acquired by the Italian Space Agency (ASI). Finally, we exploited 32 Sentinel-1A images acquired by ESA and covering the period 10/2014-05/2016. In addition, another Sentinel-1A dataset made of 30 images acquired in ascending geometry over the same period was processed and used to assess the predominant component of the movements in the area. The image processing has been carried out using the Small Baseline Subset (SBAS) technique. The removal of the topographic component of the phase was carried out using the Shuttle Radar Topographic Mission (SRTM) digital elevation model (DEM) with a resolution of 30 m x 30 m. All the datasets were multi-looked differently in order to obtain the same ground resolution with a pixel size of 20 m x 20 m. The areas along the shore that were exposed year by year by the DS lowering, were masked out in all the images using the -415 m m.s.l. contour line of the SRTM DEM, that refers to the water level in February 2000 at the time the DEM was produced. The deformation values calculated along the line of sight (LOS) of the satellites were projected to the vertical direction considering the incidence angle of each point measured with the different sensors. The novelty of this work comes from the integration of three different wavelenghts (X, C and L) to study the dynamics of a salt dome. Five displacement maps have been produced, carefully checked and then fused to provide a total cumulated map of the displacements in the area. The ground movements have been analysed by comparison with in-situ tectonic observations collected by various authors since the mid-1980s. The results show an increase in the displacement rates starting from the 2000. The uplift occurring in the north part of the peninsula is probably caused by the combination of different factors such as tectonic, diapirism, elastic rebound. Semi-circular depressions occur around minor uplifting areas in the southern part of the peninsula as consequence of the salt diapir upward movement. Furthermore, the study shows an episodic rising of the Lisan diapir. The Sentinel-1A satellite used in this study demonstrated its great potential as a tool for continuous monitoring activity over areas affected even by very fast displacements. The obtained results updated the knowledge of the complex karst dynamics in the Lisan Peninsula, and could be used as the starting point for further studies in the area.