FUNCTIONAL AND STRUCTURAL IMAGING OF VOLCANOES WITH COSMIC RAYS – DIAPHANE

The proposed methodology uses several complementary methods, some of which are very innovative. We have developed a technique to image the Soufrière using cosmic muons and evidence the less dense areas that correspond to pockets of vapour. For this purpose, we designed and manufactured telescopes that were arranged around the volcano to measure the flux of muons through it. When a pocket of vapour is formed, the flux of muons increases because there is less material to stop them like medical X-rays where the lungs appear clearer. We also installed a seismic noise measurement network to listen to the vibrations caused by steam and we measured the temperature in the most active fumaroles. All of the data collected makes it possible to detect and locate the pockets of vapour that appear inside the dome.

The implementation of our devices on the Soufrière of Guadeloupe allowed to validate the principle of our method and to highlight rapid phenomena such as the formation of pockets of vapour. For instance, in march 2017, we observed in about two days the formation of a vapour pocket that produced a seismic noise and temperature fluctuations in the fumaroles. These data combined with radiographies allowed to accurately locate
the vapour near the 56-crater at shallow depth below the summit of the volcanic dome.
The telescopes also made it possible to carry out a complete scanner of the dome which is a very heterogeneous structure comprising several very altered zones and low mechanical resistance.

The results obtained during the DIAPHANE project enabled to develop sensors and define a general methodology for the acquisition and processing of information. The most significant illustration of this work constitutes the article by Le Gonidec et al. [7]. This paper is a proof of concept and represents the perspectives of applications of our methods to volcanological monitoring. At the end of the DIAPHANE project, three muon
telescopes will be integrated into the network of the Soufrière volcanological observatory
and the sensor system in the fumarolic areas will be reinforced.

The DIAPHANE project has led to methodological progress and advances in volcanology. These results are the subject of articles for the most part freely available on the sites of the publishers. Some articles concern the methods of data analysis and modelling and several articles present the results of the experiments carried out on the Soufrière.

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).