# Combining InSAR and GNSS to Track Magma Transport at Bastic Volcano
## 1 Introduction  
The advent of Synthetic Aperture Radar Interferometry (InSAR) in the mid-1990s shed a new light on magma plumbing systems, which connect deep magma reservoirs to the surface at volcanoes [1; 2; 3]. Due to its exceptional spatial coverage, this remote sensing geodetic method can be used to produce ground surface displacements maps, which has led to a significant improvement in the imaging of magma storage zones. In particular, we now have a better characterization of the specific reservoirshapes (e.g., Reference [4]), the possibility to distinguish several active storage zones beneath active volcanoes (e.g., Reference [5]), as well as statistical studies on magma reservoir depth [2] and relative location compared to the volcanic edifice [6]. This has also led to better estimates of associated volume changes. In addition to the characterization of the storage zone geometry, another advance has been developing imaging of magma transport features. Magma can accumulate for months to years in a storage zone [7]. However, once the pressure inside this reservoir reaches a critical threshold, rupture is induced and magma starts to propagate towards the surface. It flows inside planar intrusions formed by fracturing of crustal rocks, called either dikes or sills depending on their orientation with respect to pre-existing features. This ascent is usually much quicker than the storage phase and lasts between a few tens of minutes to a few days [8; 9; 10]. Once the eruption has ended, part of the magma remains trapped and cools at depth leading to ground displacements detectable by InSAR. Co-eruptive interferograms, calculated by combining images acquired before the transport phase onset and after the eruption, contribute to our knowledge of magmatic intrusion geometries, revealing complex shapes which are probably influenced by the local stress field [11; 12]. The still relatively poor temporal resolution of Synthetic Aperture Radar (SAR) data remains a clear limitation for their use in tracking magma propagation. This phase, which is crucial to understanding the dynamics of magma emplacement, is usually recorded by continuous, ground-based sensors such as GNSS stations, tiltmeters or seismometers for example, References [9; 13; 14; 15]. It is not always possible to place ground-based captors in ideal locations with respect to the intrusion due to field conditions [16]. Moreover, seismicity does not always permit to track magma propagation, specifically when microearthquakes are too shallow and have a too small magnitude [17]. However, SAR temporal resolution has improved appreciably: the usual revisit time was reduced from one month to 6 days with the launch of the Sentinel-1 satellite constellation. There are a few cases for which SAR images have been acquired during a propagation phase and they all yield exceptional results. One early example is the sill emplacement preceding the 1999 Eyjafallajokull eruption. The SAR acquisition was favored by a long-lived propagation phase probably linked to limited magma driving pressure [10]. Similarly, a SAR image acquired during the 2009 eruption at Fernandina volcano, Galapagos [18] provided insight into the complex transition from circumferential intrusions to radial eruptive fissures. Also the May 2016 eruption at Piton de la Fournaise volcano was recorded by an intermediate Sentinel-1 acquisition, which occurred only 2 h before the onset of the eruption. With the increase in the number of radar imaging satellites, InSAR data are available with different Lines-of-Sight (LOS) and different precisions depending on the radar wavelength [19]. When crustal processes occur suddenly, it is possible to use all the interferograms that captured the event. However, for slow processes such as fault creep, viscous relaxation and shallow magma reservoir inflation [20; 21], or when several processes take place successively [22], this approach may lead to unreliable results or conceal subtle processes.