3-D water vapor field retrieval by GNSS tomography for InSAR observation correctionapplied to deformations of Piton de la Fournaise in Réunion Island

On one hand, current processing of GNSS signals has the ability to provide the amount of
water vapor between the satellite and the ground. This information is of major interest
because these observations have good temporal resolution and are insensitive to weather
conditions. Furthermore, when the station network is dense enough, it is possible to retrieve
the 3-D water vapor field through GNSS tomography.
Although this method has already proven effective, this presentation will first detail new
approaches development to adapt to the Reunion Island context and particularly over the
Piton de la Fournaise. Indeed, this area has a dense distribution of stations while the rest of
the island shows a weaker distribution. Hence, a classic grid mesh is therefore not suited for
this situation and we developed a Voronoï adaptive mesh scheme to better account for the
irregular network geometry. Likewise, the inversion scheme used is being upgraded to a
Single Value Decomposition (SVD) approach shown to be more effective in the literature.
On the other hand, InSAR technique consists of measuring ground deformation by difference
between two radar measurements of satellite/ground distance. However, these two
measurements are not carried out at the same time and, thus, the water vapor field which
impact such observations is different for each measurement, notably in tropical regions
where water vapor variability is very important.
Hence, a second aspect of our work will be to apply the improved 3-D water vapor retrieval
obtained with the new approaches defined above in order to propose a better correction
scheme for InSAR retrieval of slow ground deformation signals at Piton de la Fournaise, a
precursor sign of possible volcanic activity.