GNGTS 2014 - Atti del 33° Convegno Nazionale

of subsidence in the Po plain has been carried out by public Institutions as I.G.M.I. or A.R.P.A. - E.R. (Agenzia Regionale per la Prevenzione e l’Ambiente dell’Emilia-Romagna) and researcher groups (e.g Baldi et al., 2009, 2011; Bitelli et al., 2014 a, 2014 b; Devoti et al., 2011; Serpelloni et al., 2013; Teatini et al., 2011, 2012) by different techniques: levelling, GPS, SAR and vertical extensometer. The data set acquired by these activities provide the possibility of studying time and space evolution of vertical movements. The aim of this work is to describe the preliminary results about subsidence phenomena in the Po basin , obtained with an integrated monitoring by GPS and SAR methods. We have also compared the SAR and GPS results with the outcomes obtained from the analysis of some vertical extensometers located in the study area. GPS data analysis. This study is based on the GPS observations carried out by about 440 GPS permanent stations in the Italian peninsula and surroundings (Fig. 1). In particular, we have analyzed by the GAMIT software (version 10.5, Herring 10.5a) the daily data, acquired with a 30 s sampling rate, from January 1, 2001 to August 2, 2014. The network has been divided into 31 sub-networks (clusters), each including at least the following six common stations; BRAS, CAGL, GRAZ, MATE; WTZR and ZIMM (Fig. 1). The IGS precise ephemerides are included in the processing with tight constraints, such as the Earth Orientation Parameter (EOP). The Phase Centre Variation (PCV) absolute corrections for both ground and satellite antennas are included. Loose constraints are assigned to the daily position coordinates of stations. The daily loosely constrained solutions of the 31 clusters obtained after GAMIT processing are combined into a unique solution by the GLOBK software (Herring et al. , 2010b) and aligned into the ITRF2008 reference frame (Altamimi et al. , 2012) by a weighted six parameters transformation (three translation and three rotation), using the ITRF2008 coordinates and velocities of the following 17 high quality common IGS stations: BUCU, CAGL, GRAZ, IENG, LAMP,MARS,MATE,NOT1,ORID, PENC, SFER, SOFI, TLSE, VILL, WTZR,YEBE and ZIMM (inset in Fig.1). At the end of this procedure, the daily time series of the north, east and vertical geographical position components of each site included in the analysis are determined. The average velocity in the sites characterized by a time span greater than 2.5 years has been evaluated by the following procedure: 1) Data cleaning: time series have been preliminarily analyzed in order to detect and remove outliers, defined as the data with value or associated uncertainties greater than 3 times the weighted root mean square (wrms) of the series. The outlier epochs have been identified separately in each coordinate direction and then applied to all three components. 2) Preliminary processing: the time pattern of the daily position component y 1k (t) (k=1,2,3,for the north, east and vertical component respectively) has been modelled by the following relation: Fig. 1 – Locations of the permanent GPS stations used in this work. The inset shows the 17 IGS stations we have used to align the above regional network to the ITRF2008 reference frame (Altamimi et al. , 2012). GNGTS 2014 S essione 1.2 135