Displacement measurement for Lisbon, Portugal, using combined InSAR and GNSS data

Abstract

The city of Lisbon is the most important governmental, business and historical place in Portugal. The geological properties of its location turn it into a city prone to ground instability, where serious problems have occurred in the past, namely the destruction of a large part of the city in 1755 after an Mw 8.7 earthquake and the flooding of a tunnel during the expansion of a subway line in 2000. Besides, groundwater extraction has been proved to be a concerning problem, as more than one location in the city presents ground subsidence related to the decrease of the underground water level. This communication will present an analysis of displacements for Lisbon using a combination of SAR interferometry with GNSS data, for the time interval between March 2008 and November 2009. A dataset of Envisat ASAR images is used, together with GNSS data from national networks of permanent stations. Phase delays caused by atmosphere are estimated from Zenith Total Delay measurements, obtained from the GNSS data, for each permanent station. These values are converted to the direction of the ASAR line-of-sight, interpolated for all the study area and used to obtain the radar Atmospheric Phase Screen for the time of each ASAR image acquisition. This information is then utilized in the joint processing of InSAR and GNSS phase data in order to obtain displacement maps. The approach enables the construction of displacement maps with the spatial resolution of ASAR images and the high temporal resolution of GNSS, providing a new insight on the displacements occurred in the city of Lisbon during the study period.