Il contributo dell'interferometria RADAR satellitare nelle indagini geomorfologiche in aree di pianura soggette a fenomeni di subsidenza: il caso dell'area di Portogruaro (VE)

In the last decades, thanks to the development of multi-stacking techniques, the Differential Interferometric Synthetic Aperture RADAR (DInSAR) became a fundamental and necessary tool for analysing and monitoring the spatial and temporal evolution of land subsidence in several coastal regions. This remote sensing technique allows to study the subsidence phenomena of large areas, with centimetric-millimetric precision. The benefit of DInSAR is also the availability of a great amount of data, acquired by different space agencies, which offers a wide coverage over the Earth surface. The aim of this work is to show an innovative use of DInSAR, considering the possible relationship between the geological and geomorphological architecture of an Italian coastal plain and the rate of subsidence, inferred through analysis of interferometric data of the last 20 years. In this perspective the analysis of Synthetic Aperture Radar (SAR) data through Advanced Differential SAR Interferometry (A-DInSAR) could be used as a tool for geomorphological and geological mapping. Furthermore the assessment of the possible causes of the subsidence in Portogruaro and Concordia Sagittaria, which is important for the territorial management and planning, is reported and discussed. The research is focused on a coastal sector of the venetian-friulian plain located upstream of Caorle lagoon, between Portogruaro and Concordia Sagittaria (north-eastern Italy), with an extent of about 50 km2. In the last 125 kyr this area experienced an average downlift rate of about 0.4-0.5 mm/yr, related to crustal flexuring and sediment compaction. But the on-going subsidence is strongly higher, probably related to the reclamation of the 19-20th centuries, which induced the compaction of the Holocene deposits and the oxidation of the organic component in the soil, and groundwater withdrawal. This sector of the eastern Veneto Region was formed by the interplay between the fluvial system of Tagliamento River and lagoon and marine environments. During Lateglacial and early Holocene, between about 19-8 kyr BP, Tagliamento River eroded several incised fluvial valleys, with a maximum depth of 15-25 m and a width of 500-2000 m. These features entrenched in the alluvial sediments of the Last Glacial Maximum (LGM) and since about 8 kyr BP the coastal and deltaic processes favoured their filling with lagoon and alluvial sediments. Downstream of Portogruaro the paleovalleys have been completely buried by younger deposits and their detection is possible only using subsoil investigations. On the contrary, these fluvial incisions are still partially visible through DEM and field observations upstream of Portogruaro. The cities of Concordia Sagittaria and Portogruaro, developed over the buried fluvial incisions and several ancient buildings, experienced important deformations related to the compaction of the soft sediments filling the former fluvial depressions. This is the case of the tower bell of Portogruaro and some other late-Medieval and Venetian palaces. A peculiar situation characterises the cathedral of Concordia, where in the ’90 of 20th century large volume of the subsoil below and near the church had been excavated to expose the archaeological remains of the Roman period. This intervention required special foundations for sustain the old building, but the on-going subsidence represents a major threat for the preservation of the ancient cathedral. Portogruaro and Concordia Sagittaria represent a good test site to check the relationship between surface displacements obtained from interferometric data and geological information, because in this area a lot of geological and geomorphological information are available and Holocene and Pleistocene deposits are affected by an evident differential subsidence. The study started from the analysis and comparison in GIS environment of the pre-existing data which allow to known the geological and geomorphological features of the area. In Portogruaro and Concordia Sagittaria a great amount of stratigraphic cores and geotechnical tests are available, allowing to reconstruct the stratigraphy of some sectors and to directly check in the field the position and the depth of the boundaries delimitating the ancient fluvial incisions, now filled by younger sediments. To investigate the downlift rates and reconstruct their areal pattern, I considered ERS, Envisat, COSMO Sky-Med and Sentinel-1 SAR data referred to the periods 1995-2000, 2003-2010, 2010-2016 and 2014-2016, respectively. Particularly I considered ERS and Envisat Persistent Scatterers (PS) acquired with ascending orbit, because these data show the best correspondence of the spatial distribution of velocity with the boundaries of buried incisions (Figures 4.2, 4.3, 4.4, 4.5). The velocity maps, obtained through interpolation of PS, show that the interferometric steady unit overlaps the outcropping LGM plain, whereas the subsidence unit (with rate of -1.5÷-3 mm/yr) is located upon the buried incisions (Figures 4.6, 4.7). This correspondence was also found in the velocity maps obtained from COSMO Sky-Med interferometric data (Figures 4.8, 4.9), but only partially in that map obtained from Sentinel data (Figure 4.10), probably because the short time interval covered by the available data. Then, through the available stratigraphic cores, I realised a stratigraphic section (Figure 4.14) and I compared this section and the stratigraphic section (Figure 4.15) realised by Alessandro Fontana (Fontana, 2006) with the velocity sections obtained from ERS and Envisat data (Figures 4.18, 4.19, 4.20, 4.21). I observed that a greater thickness of post-LGM deposits corresponds with a greater subsidence rate. I expect that the subsidence rate is linked with the thickness of fine deposits (organic clays and silts and peat) observed in the manual boring realized in PRT1 point (Figures 4.16 and 4.17). Another aim of this project is the assessment of the possible causes of the subsidence in Portogruaro and Concordia Sagittaria. Starting from the velocity maps derived from of ERS, Envisat, COSMO Sky-Med and Sentinel interferometric data, I identified some critical points, call velocity anomalies in this work, interested by a strong deformation rate, greater than 3 mm/yr (Figures 4.23, 4.24, 4.25). The observed critical points correspond to recent buildings, roads and infrastructures. These localized subsiding areas can be explained as the result of ground consolidation related to urbanization and recent overloads (Figures 4.26, 4.27). Finally through hydrogeological and bibliographic data I observed that the area of Portogruaro is affected by the extraction of groundwater resulting in depressurization of the deep thermal aquifer. I hypothesize that the depressurization of the deep thermal aquifer may caused subsidence in the study area. Another hypothesis is that lowering the water level of the shallow water table may led to the consolidation of the soil and consequently subsidence. In both cases the recognition of the limit of buried incisions through the rate of subsidence is linked to the different deformational behavior of the sediments filling the post-LGM incisions and the external LGM plain.