Geodetic/gravimetric evidence for mass loss in the subsidence area of Bandung, Indonesia

Land subsidence monitoring in Bandung, Indonesia, was initiated in the 2000 s. However, the monitoring has been limited to geometric observations only, which may restrict the further physical interpretation of the cause of the subsidence. In this study, we combine geometric and gravity observation methods to monitor surface subsidence in Bandung. 63 Synthetic Aperture Radar (SAR) images from Sentinel-1A covering the period of 2014-2020 were used to estimate the mean surface geometric changes. For the gravity observations, a hybrid gravity configuration that incorporates absolute (2008-2014) and relative (2011-2016) gravity observations were used to estimate the gravity changes. We estimated geometric changes of up to - 160 mm/yr, indicating rapid subsidence in the greater Bandung area. We obtained gravity changes ranging between - 56.7 and 40.1 & mu;Gal/yr. Upon subtracting the deformation-induced gravity field from the observed field, we produced a residual gravity field that was presumed to be dominated by the groundwater signal, which was then investigated further. We found that the gravity-derived groundwater signal was mainly negative, indicating subsurface mass loss. We further compared the signal with the modeled gravity effect from deep groundwater observations (1996-2008). The median difference between the observed and modeled groundwater gravity signal was estimated to be 2.8 & PLUSMN; 18.0 & mu;Gal/yr or equivalent to 0.08 & PLUSMN; 0.55 m/yr in terms of water height if we set the integration cap and groundwater depth to 1.4 km and 150 m, respectively. The discrepancy can be attributed to modeling (simple geohydrological assumption) and measurement (different observation periods and noise) factors. Nevertheless, both measurements indicate that the mass is decreasing due to groundwater depletion, demonstrating the potential of geometric-gravimetric observations to infer sub-surface mass loss.