# Combining InSAR and GNSS to Track Magma Transport at Bastic Volcano
## 3 Geodetic Measurement Description  
### InSAR Data  
Synthetic Aperture Radar (SAR) images are systematically acquired by various satellites (Sentinel-1 in Strimap Mode, Cosmo Skymed, ALOS2, TerraSar-X) [38] within the framework of the national monitoring service, OI\({}^{2}\). For the 26 May 2016 event, images acquired by both Sentinel-1 and Cosmo-Skymed are available. Figure 1: Piton de la Fournaise (PdF) setting. (**A**) Location of Reunion Island; (**B**) Shaded topography of PdF volcano and location of the GNSS monitoring network. Black and grey diamonds represent the locations of the permanent GNSS stations. The black circled diamonds refer to the 10 GNSS stations used in this study. The GITg station (black diamond to the northwest) is the reference used here for the GNSS campaign and for the epoch-by-epoch processing. The May 2016 lava flow is shown in purple. ‘DoI’ marks the Dolomieu crater; (**C**) Zoom corresponding to the area indicated by a black box in B. Black arrows represent the cumulated horizontal displacement of the 10 GNSS stations between 25 May at 17:00 and 26 May at 04:05. Standard deviations for horizontal and vertical components are 1–2 cm and 3-4 cm, respectively. Associated colored circles represent the temporal evolution of the horizontal displacement for the GNSS stations. Grey arrows represent displacements from the GNSS campaign network between 31 August 2015 and 27 May 2016, with ellipses for the 95% confidence intervals. Coordinates are in kilometers (WGS84, UTM 40S). Modified after Reference [PERSON] et al. [16]  
All interferograms were produced using the DIAPASON software [39], removing the orbital contribution based on the precise orbit state vectors provided by Space Agencies as well as the topographic contribution using a 5 m Lidar DEM, produced by the French Geographic Institute (IGN) in 2008-2009. The interferograms were unwrapped using the Snaphu algorithm [40]. They were then detrended and shifted such that a null displacement in the Enclos Fouque caldera (see location on Figure 1) was used as a reference. Before being used as inputs for the inversion, the interferograms were subsampled using a quadtree decomposition algorithm [41; 42]. The number of points in each region was proportional to the displacement amplitude and to the displacement standard deviation in the region. Thresholds for the amplitude and the standard deviation were adjusted to keep a balanced number of points of around 500 per interferogram. Cosmo SkyMed acquisitions on 29 February and 31 May, enabled two interferograms to be created (CSKA and CSKD, Figures 2 and 3) spanning 3 months which covered the inter-eruptive period, the unrest, the eruption and 5 days after the eruption. Six Sentinel-1 acquisitions on 19-20 April, 25-26 May and 6-7 June, were used to compute 6 interferograms. Figure 2: **(Top)** Temporal evolution of baseline changes recorded between two pairs of permanent GNSS stations (SNEG and DSRG and BORG and DERG; see Figure 1 for station locations) between 29 February  
2016 (date of the first SAR acquisition used in this study) and 07/06/2016 (date of the last SAR acquisition used in this study). Distance samples are daily averages. Inset corresponds to a zoom on the pre-eruptive baseline changes spanned by Sentinel-1 data. The eruption onset is shown by a red arrow; (**Middle**) Coloured bars represent the time spanned by each of the 8 interferograms used in this study. Modified after [PERSON] et al. [16]. The vertical red area represents the eruptive crisis; (**Bottom**) Zoom. The vertical dashed line at 01:45 marks the time of the intermediate SAR acquisition by Sentinel-1. CSK = Cosmo-Skymed; S1 D1 = Sentinel-1 descending InSAR data spanning the beginning of the eruptive crisis; S1 D2 = Sentinel-1 descending InSAR data spanning the end part of the eruptive crisis; S1 A1 = Sentinel-1 ascending InSAR data spanning the pre-eruptive period; S1 A2 = Sentinel-1 ascending InSAR data spanning only the eruption. Figure 3: Wrapped interferograms used in this study.