Water Vapor Tomography of the Lower Atmosphere from Multiparametric Inversion: the Mt. Etna Volcano Test Case

Space techniques based on GPS and SAR interferometry allow measuringmillimetric ground
deformations. Achieving such accuracy means removing atmospheric anomalies that
frequently affect volcanic areas by modeling the tropospheric delays. Due to the prominent
orography and the high spatial and temporal variability of weather conditions, the active volcano
Mt. Etna (Italy) is particularly suitableto carry out research aimed at estimating and filtering
atmospheric effects on GPS and DInSAR grounddeformation measurements. The aim of this
work is to improve the accuracy of the ground deformation measurements by modeling the
tropospheric delays at Mt. Etna volcano. To this end, data from the monitoring network of 29
GPS permanent stations and MODIS multispectral satellite data series are used to reproduce
the tropospheric delays affecting interferograms. A tomography algorithm has been developed
to reproduce the wet refractivityfield over Mt. Etna in 3D, starting from the slant tropospheric
delays calculated by GPS in all the stations of the network. The developed algorithm has been
tested on a synthetic atmospheric anomaly. The test confirms the capability of the software to
faithfully reconstruct the simulated anomaly. With the aim of applying this algorithm to real
cases, we introduce the water vapor contentmeasured by the MODIS instrument on board
Terra and Aqua satellites. The use of such data,although limited by cloud cover, provides a
two-fold benefit: it improves the tomographic resolution and adds feedback for the GPS wet
delay measurements. A cross-comparison between GPS and MODIS water vapor
measurements for thefirst time shows a fair agreement between those indirect
measurements on an entire year of data (2015). The tomography algorithm was applied on
selected real cases to correct the Sentinel-1 DInSAR interferograms acquired over Mt. Etna
during 2015. Indeed, the corrected interferograms show that the differential path delay reaches
0.1 m (i.e. 3 C-band fringes) in ground deformation, demonstrating how the atmospheric
anomaly affects precision and reliability of DInSAR space-based techniques. The real cases
show that the tomography is often able to capturethe atmospheric effect at the large scale and
correct interferograms, although in limited areas. Furthermore, the introduction of MODIS data
significantly improves by ∼80% voxel resolution at the critical layer (1,000 m). Further
improvements will be suitable for monitoring active volcanoes worldwide.