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Satellite interferometry reveals spatial patterns of subsidence in the ancient Wieliczka Salt Mine (UNESCO Heritage Site, Poland)

Janusz Wasowski(1), Fabio Bovenga(2), Raffaele Nutricato(2), Domenico Conte(2), Alberto Refice(3) and Marek Graniczny(4)

(1) CNR-IRPI, Bari, via Amendola 122 I, 70126, Bari, Italy
(2) Politecnico di Bari, Via Amendola, 173, 70126, Bari, Italy
(3) CNR-ISSIA Bari, via Amendola 122 D, 70126, Bari, Italy
(4) Polish Geological Institute, Rakowiecka, 4, 00-975 Warszawa, Poland

Abstract

This work presents the first SAR interferometry data on the aereal distribution and amount of subsidence in Wieliczka, a town located 14 km SE of Cracow. The town is home to a unique salt mine, over 700 years old, one of the best known tourist attractions in Poland. Each year the mine is visited by about 1 million tourists from all over the world and in 1978 UNESCO placed it on its first International List of the World Cultural and Natural Heritage. Since the Middle Ages over 7.5 million m3 of underground passages have been excavated, extending from level I (64 m below the ground surface) to level IX (at 327 m depth). This includes over 2000 chambers, 200 km of galleries and numerous shafts. The salt deposit has been exploited under an area extending 7 km in E-W direction and about 1 km wide. There is evidence that the mining legacy has influenced the ground and building stability in the town, which is sited directly above the mine. One of the latest damaging events occurred in the early 1990’s, producing subsidence and associated landslide phenomena that interrupted the local railway. Furthermore, today many buildings in the town show clear signs of distress. To investigate the recent land instability in Wieliczka we used several tens of ERS satellite images covering the period 1992-2000. The application of the persistent/permanent scatterer (PS) SPINUA technique has led to the identification of numerous radar targets (over 100 PS/km2), suitable for ground motion monitoring. The results show the presence of continuing subsidence with average movements reaching 2.4 cm/yr. The SAR-detected length of the subsiding zone corresponds quite well to the E-W extent of the undeground mine, whereas its width exceeds that of the mining works. The maximum downward displacements are observed in the central part of the town, where ground topographic measurements documented over 1 m of subsidence in the period 1970-2000 (i.e. over 3 cm/yr). The lower rates of displacements revealed for the period 1992-2000 by the SAR results may be in part related to the decreasing mining activity, which terminated around the end of the last century. The SAR data show also some significant variations in the amount of downward movements, possibly indicative of local differential settlements. These phenomena present a potential hazard to houses and infrastructures and should be further investigated. In particular, the geotechnical parameter and geological boundary conditions need to better understood before the PS displacement data can be confidently used for hazard assessment purposes.

Acknowledgements - The ERS satellite images were provided by ESA under the ALOS ADEN 3595 project.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry