GNGTS 2014 - Atti del 33° Convegno Nazionale

SAR technique can identify building where the GPS permanent stations are located, so that a comparison between the deformation estimated with the two techniques can made. In this work , we have considered the displacements available on the website obtained analyzing the ascending and descending Envisat interferograms. The InSAR movements are successively compared with those estimated using the GPS observations and shown in Fig. 2. The combination of the GPS and PS observation seems to be straightforward, but actually requires a careful analysis because the PS targets do not necessarily coincide with the location of GPS permanent sites. In order to overcome this problem we have identified on the website the permanent scatters lying at a distance lower than 100 m from the GPS stations. The average PS displacements has been considered as representative of the movement detected by the SAR on the site where the GPS permanent station is located. After this step, the time series of the ascending and descending LOS displacement have been analyzed in order to estimate a secular trend. Successively, using the incidence angle of the SAR acquisition we have transformed the LOS displacement in the vertical motion, that can be compared with the motion estimated using GPS observation. Discussion. A comparison between the deformations estimated by two different techniques (SAR and GPS) points out some problems due to the reference system and about the observation time span. In particular, the PS displacements available on website are estimated using the interferograms acquired from November 2002 to December 2010, that it is different to GPS (2001-2014). Thus, the GPS vertical velocity that has been compared to the SAR rates cannot be that given in Fig. 2. It must be recomputed by using only the data observed in the 2002-2010 time interval. The results of this comparison can give important informations about the space and time variations of the subsidence phenomena in the study area. Important informations about the geological layers involved in a land subsidence process and on the time evolution of the phenomena can be deduced analyzing the observations acquired from vertical extensometers. The pipe or cable of these instruments are extend from the bottom of a cased borehole, through geological layers susceptible to compaction, to the ground surface. A recorder connected to the pipe measures the variation of the distance from the anchoring point to the benchmark at the ground surfaces. In 2005 ARPA Emilia Romagna and the Servizio Geologico, Sismico e dei Suoli Regione Emilia Romagna in cooperation with Provincia di Bologna installed two vertical pipe extensometers to a depth of 100 m and 200 m in the outskirts of Bologna town (Bonsignore et al., 2010). Recently these institutions have been installed other two vertical extensometers in the study area as shown in Fig. 3. Adetailed analysis performed to better understand the possible relations between the deformations observed by GPS and SAR techniques and the soil compaction registered from the vertical extensometers can give important information about the evolution of the land subsidence phenomena and the geological layers involved in the process. The results of these studies will be showed and discussed during the workshop. Acknowledgments. We are grateful to the following Institutions: ASI, ARPA Piemonte, FOGER (Fondazione dei Geometri e Geometri Laureati dell’Emilia Romagna), FREDNET (OGS), LABTOPO (University of Perugia), LEICA-Italpos, Regione Abruzzo, Regione Friuli Venezia Giulia, Regione Liguria, Regione Lombardia, Regione Piemonte, Regione Veneto, RING-INGV, Provincia Autonoma di Bolzano (STPOS), Provincia Autonoma di Trento (TPOS), STONEX, which have kindly made available GPS recordings. We are grateful to Ufficio Prevenzione Sismica - Regione Toscana and Servizio Geologico, Sismico e dei Suoli Regione Emilia Romagna for the support. The figures have been carried out by the Generic Mapping Tools (Wessel and Smith, 1998). References Arca S. and Beretta G.P.; 1985: Prima sintesi geodetica-geologica sui movimenti verticali del suolo nell’Italia Settentrionale. Boll. Geod. Sci. Aff., 44, 125-156. Altamimi Z., Métivier L., and Collilieux X.; 2012: ITRF2008 plate motion model. J. Geophys. Res., 117, B07402, doi:10.1029/2011JB008930. Baldi P., Casula G., Cenni N., Loddo F. and Pesci, A.; 2009: GPS-based monitoring of land subsidence in the Po Plain (Northern Italy). ����� ������� ������� ������� ���� �������� ������������������������������� Earth Planet. Science Letts., 288, 204-212, doi:10.1016/j.epsl.2009.09.023. Baldi, P., Casula, G., Cenni, N., Loddo, F., Pesci,A., Bacchetti, M.; 2011: Vertical ad horizontal crustal movements in Central and Northern Italy . ����������������� ������ �� �������� ��� �� �������� Boll.Soc.Geol.It. (Ital. J. Geosci.) 52, 4, 667-685. 138 GNGTS 2014 S essione 1.2