Jakarta is the capital of Indonesia and is home to approximately 10 million people on the coast of the Java Sea. The subsidence due to groundwater extraction, increased development, natural consolidation of soil and tectonics in Jakarta has been known since the early part of the 20 th century. Evidence of land subsidence exists through monitoring with GPS, level surveys and preliminary InSAR investigations [1].World Bank studies conservatively estimate land subsidence in Jakarta occurring at an average rate of 5 cm per year, and in some areas, over 1 meter was already observed. Recent studies of land subsidence found that while typical subsidence rates were 7.5-10 cm a year, in localized areas of North Jakarta subsidence in the range 15-25 cm a year was occurring, which if sustained, would result in them sinking to 4 to 5 meters below sea level by 2025. Land subsidence will require major interventions, including increased pumping, dikes and most likely introducing major infrastructure investment for sea defence [1].With the increasing prevalence of Earth Observation (EO), the World Bank and the European Space Agency (ESA) have set up a partnership that aims at highlighting the potential of EO information to support the monitoring and management of World Bank projects. It in this framework that was defined the EOWorld projects [2]. Altamira Information, company specialized in ground motion monitoring, has managed one of those projects, focusing on the assessment of land subsidence in Jakarta.
Acquisitions of the Sentinel-1 satellite are processed and comprehensively analyzed to investigate the ground displacement during a three-year period above a double gas storage site (Lussagnet and Izaute) in Southwestern France. Despite quite low vertical displacements (between 4 and 8 mm) compared to the noise level, the cyclic motion reflects the seasonal variations due to charge and discharge during summer and winter periods, respectively. We can simulate the ground deformation at both storage sites by a simple mechanical model. However, ground movements of low-magnitude may be also induced by natural factors, such as the temperature or the soil moisture. Using a wavelet-based analysis, we show there is a soil expansion in the Lussagnet zone that contrasts both in phase and period with the seasonal deformation and that is linked to the surface soil moisture measured by the SMOS satellite. This other displacement is consistent with the water infiltration in the unsaturated zone followed by the swelling of a clay layer. This work reveals the combination of two different processes driving the ground displacement with the same order of magnitude (about 6 mm), namely the pressure variation of a deep gas reservoir and the swelling/shrinking of the shallow subsurface.
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