L- AND C-BAND SAR INTERFEROMETRY ANALYSIS OF THE WIELICZKA SALT MINE AREA (UNESCO HERITAGE SITE, POLAND) D. O. Nitti (1,2) , F. Bovenga (3) , A. Refice (3) , J. Wasowski (4) , D. Conte (1) , R. Nutricato (1) (1) Dipartimento Interateneo di Fisica, Politecnico di Bari, Italy (2) DEOS - Delft University of Technology, Delft, The Netherlands E-mail: [conte , nitti]@fisica.uniba.it , raffaele.nutricato@tiscali.it (3) CNR ISSIA, Bari, Italy E-mail: [bovenga , refice]@ba.issia.cnr.it (4) CNR IRPI, Bari, Italy E-mail: j.wasowski@ba.irpi.cnr.it BABSTRACT This work presents first results of interferometric proc- essing of ALOS PALSAR Single Look Complex SAR images (Stripmap Single Polarization Mode), concern- ing ground deformations in the Wieliczka Salt Mine area, a few km from Cracow, Poland. It follows a recent Persistent Scatterers SAR Interfer- ometry (PSI) analysis on this area, obtained by process- ing several tens of ERS satellite images covering the period 1992-2000. The results revealed the presence of a few kilometres long, slowly subsiding zone corre- sponding very well to the extent of the underground salt mine. The present work aims to extend the analysis by exploiting ALOS-PALSAR data especially for the neighbouring rural areas, relying on the lower sensitiv- ity to temporal decorrelation of L-band w.r.t. C-band radar data. Use of L band data is of interest because it could allow detecting faster ground movements related to sudden subsidence events that have occasionally oc- curred in the recent past. One such event, reported in the 1990’s, caused ground displacements locally exceeding 3 m. Furthermore, with L-band SAR data some useful information can be obtained for the rural areas, which neighbour the salt mine but lack PS. We present results obtained from Fine Beam, HH Single Polarization Mode (FBS), 34.3° Off-Nadir look-angle SAR images. The night acquisition time of FBS PAL- SAR images mitigates the atmospheric phase screen in each interferogram. No clear evidence of displacements is found on ALOS interferograms spanning about 1 year. The relatively high coherence shown by the inter- ferograms allows easier unwrapping of the differential interferometric phase, aiding further statistical investi- gations on the spatial properties of the atmospheric sig- nal. We also present considerations concerning some proc- essing aspects of ALOS data, as well as a preliminary comparison between the L-band and C-band differential interferometric phase behaviour. 1 INTRODUCTION Wieliczka, a town located 14 km SE of Cracow, 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 Inter- national List of the World Cultural and Natural Heri- tage. First Persistent Scatterers SAR Interferometry (PSI) analysis on this area has been recently derived [1] by processing, through the PSI SPINUA [2] technique, 39 ERS-1/2 descending acquisitions covering the period 1992-2000. Numerous radar targets (over 100 PS/km 2 ) have been identified, allowing ground motion monitor- ing in the Wieliczka area (see Fig. 2). In particular, the results showed the presence of continuous subsidence with average movements ranging from about 1 to 2 cm/yr in the period 1992-2000. The detected width of the subsiding zone corresponds very well to the extent of the underground salt mine, whereas its length (around 4.5 km) is somewhat shorter with respect to that of the mining works and of the known salt deposit (Fig. 3). This discrepancy results in part from the lack of suitable radar targets in the rural areas east and west of the town of Wieliczka. The maximum downward displacements were observed in the western part of the town (up to 2.4 cm/yr), where the PS pattern is indicative of the pres- ence of a few km long subsidence bowl. The ground to- pographic measurements in this area documented about 1 m of subsidence in the period 1970-2000 i.e. 3 cm/yr on average (The Wieliczka Salt Mine, unpublished Figure 1. Yellow frame refers to the ALOS PALSAR ac- quisition (PRS_70070102), green frame encloses the AOI, red frames refer to ERS acquisitions E1_19950805 and E2_19990919. Inset shows the location of the study area.