Atmospheric water vapour tomography for DInSAR application and effect of volcanic plume on the microwaves

Abstract

A particular synergy among GPS and SAR techniques, to improve the precision of the current ground deformation monitoring techniques, is investigated. The study of atmo- spheric anomalies in the GPS EM waves propagation is useful to extrapolate information about the wet refractivity field. Because of its height and the quite variable weather conditions, the estimation of Mount Etna atmospheric anomalies using GPS measure- ments have noticeable importance to calibrate the SAR interferograms and to establish the “effective” ground deformation of the volcanic edifice. In this study we presented a method to obtain a 3D electromagnetic waves velocity tomography, starting from the GPS output data analysis. Thanks to the agreement between the University of Catania and the INGV-OE, the GPS data used in this work come from ”Etn@net” framework. The GPS processing has been carried out by using the GAMIT software, by adopting appropriate processing parameters. A new software was developed for deriving the tro- pospheric tomography from the GPS data. The code was validated by using synthetic tests which assume different structure of atmospheric anomalies and with random noise about twice severe than the typical errors of the GPS. The results of the tests proved that the tomography software is able to reconstruct the simulated anomalies faithfully. The code was applied to study the structure of the atmosphere in an actual case: the period of August 12, 2011 at 10.00 am. The results of the tomography indicate clearly important features of the refractivity field of the studied day. In conclusion, the syn- thetic tests and the application on actual data sets of the new software demonstrate that it is able to reveal the tropospheric anomalies and thus it is an useful tool to improve the results of the SAR interferometry. An indirect outcome of the use of the GPS for the atmospheric sounding on an active volcanic area is that concerning the detection of volcanic products in the atmosphere. Due to the Mt. Etna persistent activity occurred during the last two years, the capability of GPS to detect the volcanic plume was investigated. The Etna volcano is particularly suited for an in-depth investigation into the aptitude of GPS observations to detect volcanic plumes, owing to both the high frequency of explosive episodes and also the well-developed GPS network. Two different approaches were tested, in order to examine the capability of the GPS network to detect volcanic plumes at Etna. The first approach is applied on the signal strength of the GPS L2 carrier phase data, the second approach, instead, is statistical, and analyzes the single difference post fit residual of elaboration signals to assert the hypothesis that the plume affects the GPS data. The proposed method has been tested for the September 4–5, 2007 activity of Mt. Etna. Results from nineteen GPS permanent stations show that during this explosive activity, the GPS residuals definitely include the contribution of the volcanic plume. In the future, data derived from the GPS stations located on Etna’s flanks could be used to improve the alerting system of volcanic ash, already operating at the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo.