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International audience; The performance of Permanent Scatterers PS Interferometry (PSI) analysis is highly limited where the presence of large vegetated cover (agricultural terrains / forests) reduces signal coherence. A possible solution relies on the installation on ground of artificial devices (Corner Reflectors CR) to complement the existing PS network. Yet, the number of such CRs (spatial density typically ~ 1 / km2) can be limited when the deformation pattern affects a large area (tens of km2) especially for CO2 geological storage in open aquifers. In order to support the surveillance of such sites, we address here the question of how to estimate the spatio-temporal distribution of the ground displacements over the whole area using only the sparse CR + PS network. We propose to test the feasibility of the Geostatistics Output Perturbation (GOP) method, enabling to combine the results of the reservoir model calibrated on the limited number of ground displacements’ time series and the spatial correlation between such calibrated results and the observations. A test case was constructed using the signal measured during CO2 injection (from 2004–2009) in the KB501 well of the In-Salah site, Algeria at only a few tens of spatial measurement points either corresponding to: 1. a “realistic” PS network selected in a highly vegetated region in western France and 2. a series of CR selected through a space-filling criterion. The comparison with the observations over the whole area confirmed that 80% of the temporal observations were fitted by the GOP method over the region defined by the CR network with a density of ~ 1 CR / km2. Comparisons with the calibrated model results and with the direct application of a spatio-temporal kriging confirmed the better performance of GOP as well.

Persistent scatterers synthetic aperture radar interferometry (PSI) analysis is viewed as a promising monitoring technique for multiyear detection of ground surface displacements during long term CO2 injection into deep aquifers (of the order of 25 to 50 years). This technique was successfully implemented at the CO2 storage site of In-Salah (Algeria). Yet, this success may not be generalized to future onshore industrial-scale CO2 storage sites, which are planned in far more complex contexts mostly presenting natural terrains variationeither agricultural or vegetal areas. These areas arecharacterized by poor Persistent Scatterer (PS)density, which might locally drop to zero, hence hampering the deployment of the PSI technique. For illustration purpose, a sector in the Paris basin (of 3,455 km²), where the potential implementation of CO2 storage sites has been investigated, is characterized by a land use composed of less than 5 % of artificial or urban areas and more than 95 % of natural (mainly forests) and agricultural areas. In such natural terrains, the absence of bright man-made structures might lead to sparse PS network (with density ~10 PS/km² to less than 1PS/km²). This constitutes a major obstacle to the phase unwrapping stage, for which the solutions are directly dependent on the PS density and makes reliable estimation of deformation using PS techniques a challenging task. The present study aims at exploring the applicability of two approaches to enhance the performance of the PSI technique in such constraining conditions for long term monitoring of CO2 storage sites. The first approach relies on the implementation of network of corner reflectors (CR) corresponding to artificial devices installed on ground to complement the existing "natural" PS network. A strategy for optimization (number and position) the CR network is proposed based on the position of the existing PS and on hydro-mechanical large-scale simulations constrained by the observations (through history-matching). The second approach relies on the use of two advanced PSI processing techniques: 1. Diffuse scatterers analysis (based on Ferretti et al., 2011), which can improve considerably the density of usable "natural" scatterers on certain un-urbanised area; 2. MulSAR technique (Pinel-Puyssegur et al., 2012), which is a processing method, that produces denoised wrapped phase time series, i.e. which facilitates the retrieval of the deformation signal obscured by decorrelation noise. To test the efficiency of these approaches, a synthetic case was constructed as follows: 1. we use as a "reference case" the deformation field observed at In-Salah during CO2 injection into the KB-501 well between 2004 and 2009; 2. the surface characteristics of this reference case are then modified by "artificially" applying the surface conditions (urbanized areas, natural terrains, either agricultural or vegetal areas) of an on-shore site in Western France, which can be considered representative of future industrial-scale storage sites in Europe. 3. by comparing the deformation field retrieved by the PSI technique combined with the afore-described approaches with the one of the reference case, the level of improvement can be estimated. Results show that the direct application of advanced processing techniques may be limited for zones with very high density of natural terrains (>90 % of the surface area), but can still be efficient for zones with moderate-to-high density of natural terrains. On this basis, we propose to improve PSI analysis in highly vegetal / agricultural areas for long term CO2 storage monitoring based on a combination of advanced processing PSI methods and an optimized CR network. References Ferretti, A., Fumagalli, A., Novali, F., Prati, C., Rocca, F., Rucci, A., 2011. A New Algorithm for Processing Interferometric Data-Stacks: SqueeSAR . IEEE Transactions on Geoscience and Remote Sensing 49 (9), 3460-3470. Pinel-Puyssegur, B., Michel, R., Avouac, J-P., 2012. Multi-link InSAR time series: enhancement of a wrapped interferometric database. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 5(3), 784-794.

Flank-fissure eruptions involve lateral injection and propagation of magma in a volcanic edifice along pre-existing fractures in the direction of the rift zones where magma intrusion and lava flow production are concentrated over time. Thus, the identification and mapping of active fractures and faults is a fundamental aspect of studies of active volcanic systems. However, gradual dyke wedge emplacement at depth in well-fractured zones on volcano flanks and in volcanic rift zones does not necessarily trigger large amplitude deformation signals susceptible to be recorded months or even years before the actual eruption. Here we show that active and potentially eruptive areas can be detected up to 2 yrs before the arrival to the surface of the final eruptive dyke and venting of lava flows by processing satellite images applying a normalized difference vegetation index (NDVI) algorithm. A posteriori analysis of satellite images indeed reveals that the surficial effects of dyke wedge injection and ascent on plant growth were apparent for Mt. Etna from 2000 to 2002 and for Mt. Nyiragongo in 2001, thus months to years before they erupted.