DIAPHANE

The objective of the DIAPHANE project is to develop and apply density tomography using cosmic muons generated in atmospheric particle showers permanently produced by primary cosmic rays hitting the upper atmosphere. Density is determined from the screening attenuation produced by the rocks on the flux of muons, and the 4 field telescopes designed and constructed by us can scan rock thickness up to 2 km. Given our experience of muon tomography, we plan in the present project to go beyond the first step of muon radiography and to use muon tomography and gravity measurements to perform a detailed study of the structure and dynamics of the hydrothermal systems in the Soufriere of Guadeloupe and in Soufriere Hills. The Soufriere of Guadeloupe is a 500 years old lava dome subject to alteration caused by an increasing hydrothermal activity. Soufriere Hills is in a magmatic eruption stage with lava dome extrusion, explosions caused by gas pockets and collapses. The original experiments of the present project address the following questions: • Construct a high-resolution (10-20 m) 3D density model of the Soufriere of Guadeloupe to determine the volume and the degree of confinement of shallow hydrothermal reservoirs in relation with destabilization hazard and phreatic activity. • Produce a preliminary 3D density model of the unapproachable Soufriere Hills volcano in order to contribute to the comprehension of the heterogeneous build-up of lava domes and constrain destabilization models. • Perform a long-time monitoring of the density variations detected in the hydrothermal reservoirs in the Soufriere of Guadeloupe and in coincidence with the observed increasing vent activity. Muon tomography will be completed by continuous gravity measurements and monitoring of the vents. • Monitoring of density variations in Soufriere Hills to understand how hydrothermal circulation and alteration initiate. This will provide invaluable information about the genesis of the present highly heterogeneous structure of Soufriere of Guadeloupe lava dome. The project addresses methodological issues also pertaining to other domains of application of muon tomography: hydrology and aquifer monitoring, landslides and cliff tomography, monitoring underground storages of CO2, compressed air and nuclear wastes: • Distant muon imaging of a kilometre scale volume of rock. • Differential imaging and time tomography combined with data assimilation. • High-resolution tomography of thin rock layers. • Joint inversion with other geophysical data. • Study of atmospheric particle showers and elaboration of flux models. The project relies on a multidisciplinary collaboration initiated in 2008 between geophysicists (IPG Paris & Geosciences Rennes) and particle physicists (IPN Lyon). Our group can perform all steps of muon tomography: telescope building, field experiments on volcanoes (Etna, Soufriere of Guadeloupe, Mayon), modelling, data acquisition and inversion (Gibert et al., 2010; Lesparre et al., 2010, 2012a,b,c; Marteau et al., 2012, 2014; Jourde et al., 2013). The DIAPHANE project will reinforce our leading position as one of the two groups able to perform muon tomography on remote volcanoes with standalone and rugged instruments (the other group is led by Prof. Hiroyuki Tanaka in Japan).