InSAR tropospheric corrections on Merapi using global weather models and local GNSS network

Merapi is a strato-volcano rising at 2900 m a.s.l, located on the South coast of Java island, Indonesia. Only 30 km north to the city of Yogyakarta (2 millions inhabitants), it is considered one of the most dangerous dome building stratovolcanoes, as summit domes almost continuously grow and destruct. Merapi is therefore closely and routinely monitored by InSAR (Interferometric Synthetic Aperture Radar) to track ground deformation. To retrieve ground deformation from the full wave path, the delay due to the radar wave crossing the atmosphere needs to be corrected. In the case of Sentinel-1, interferograms are mostly biased by the tropospheric variations. Tropospheric variations are expected to be stronger in tropical regions and where topographic gradient is high, which is the case at Merapi. They can be estimated thanks to various methods, including global weather models (ERA-5 and GACOS), a linear model regarding topography, and GNSS networks.

In this work, we compare the performance of atmospheric corrections derived from two weather-based models, ERA-5 and GACOS, and those derived from the empirical method based on a linear phase-elevation correlation. The aim is to evaluate the efficiency of each model in correcting this tropospheric bias. To this end, we choose to study a period between 2016 and 2018 during which no deformation occurred on the Merapi, so that most of the phase delays corresponds to tropospheric signals.

We use three criteria to evaluate the performance: i) the reduction of the standard deviation, ii) the reduction of the sill of the semi-variogram, iii) the slope reduction of the phase-elevation correlation. We show that corrections with ERA and GACOS are efficient on only half of the interferograms.

Finally, we also use the local network of 5 GNSS stations to rely on an independent dataset. We show there is a linear relation between the GNSS tropospheric delays and the global weather models delays. However, the GNSS network at Merapi is too small to provide an efficient correction on the whole volcanic edifice. For this reason, a similar workflow has been carried on the Piton de la Fournaise, Réunion island, using a wider GNSS network. The final aim of this study would be to implement a strategy on which the most suitable tropospheric model is chosen routinely based on the evaluation of the performance criteria to obtain atmospheric-free interferograms during volcanic unrest.