Suitability of GNSS for analysis of soil subsidence in Recife in a highly urbanized coastal area

Recife Metropolitan Region, in the northeast of Brazil, is formed by a coastal plain bounded by hills and by the Atlantic Ocean. Recife Metropolitan Region has about 4 million inhabitants and is the sixth most populous Brazilian metropolitan region. In the last 50 years, there has been an increase in groundwater exploitation causing a piezometric level lowering that reached 100 meters in some points. Consequently, soil subsidence has become a major concern. Geodetic methods have been used worldwide to monitor and quantify vertical deformation of soil in places with large groundwater withdrawal. This article describes the GNSS methodology used in the monitoring of soil subsidence due to groundwater exploitation. Three experiments were carried out to evaluate the most adequate methodology for monitoring soil subsidence applying GNSS receivers. Two of them were carried out, under the same conditions, with the GNSS antennas located at ground level with different baselines (short baseline in experiment 1 and long baseline in experiment 2). In the third experiment, GNSS receivers were installed in landmarks located at the top of four buildings. From these experiments, it was verified that, using GNSS technology, it is possible to detect the occurrence of vertical displacements of the order of 1 cm, in obstacle-free conditions, and a tracking time of at least two or three hours depending on the baseline. Using a known displacement, it was observed that the GNSS system was able to detect the variation of vertical displacement in the order of magnitude researched.