PROCEEDI NGSof

CHEF201 3

Structural imaging and monitoring of volcanoes with cosmic muons

Dominique Gibert∗†, Kevin Jourde, Nolwenn Lesparre‡; Jean-Christophe Komorowski, Jean-Jacques Sibilla, Olivier Sirol Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, Paris, France E-mail: [email protected]; [email protected]

Jacques Marteau§, Bruno Carlus, Serge Gardien, Claude Girerd, Jean-Christophe Ianigro, Jean-Luc Montorio Institut de Physique Nucléaire de Lyon, Univ Claude Bernard, UMR 5822 CNRS, Lyon, France E-mail: [email protected]

Jean de Bremond d’Ars§, Bruno Kergosien, Yves Le Gonidec, Florence Nicollin, Pascal Rolland, Géosciences Rennes, Univ Rennes 1, UMR 6118 CNRS, Rennes, France E-mail: [email protected] This is the abstract for your CHEF 2013 written contribution. Please, remember that we must receive your manuscript (.tex and .pdf) by the deadline of the 31st of August 2013.

Calorimetry for High Energy Frontiers - CHEF 2013, April 22-25, 2013 Paris, France ∗ Speaker. † Project

PI, also with Rennes 1 University. with Institut de Radioprotection et de Sûreté Nucléaire, Fontenay-aux-Roses, France. § Co-PI of the project. ‡ Now

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1. Introduction Volcanoes are complex systems conveying energy from bottom to top and whose dynamics may abruptly change to adapt energy transfer capacity to variations of energy input coming from below, and markedly different types of hazards may be encountered on volcanoes depending on both their nature and their location. For explosive andesitic volcanoes like La Soufrière of Guadeloupe located in the Lesser Antilles volcanic arc (Fig. 1), the list of possible hazards established from past eruptive history is long and diversified: flank collapse [1], phreatic explosion [2], mud flow [3], phreato-magmatic paroxismal explosion [4]. Evaluating the magnitude of catastrophic events likely to occur is a major concern and modelling is a key issue [5]. Such models critically depend on initial and boundary conditions, and knowledge of the inner structure of the volcano is of a primary importance to provide an as precise as possible knowledge of the nature about the materials and of the fluids forming the volcanic edifice.

Figure 1: (left) Volcano lava domes (here La Soufrière of Guadeloupe) have steep slopes inherited from their formation and constitute serious landslide hazards as their mechanical integrity is progressively degraded by hydrothermal activity. (right) close view of the South crater with intensely active vents releasing acid gases. The red ellipse shows new vents that appeared after observing an increase of the muon flux across the lava dome (right of Fig. 3).

2. Density tomography with cosmic muons Various geophysical techniques are available to image the inner structure of volcanoes with seismic wave velocities and electrical resistivity. Up to now, imaging the density structure was done with gravity measurements and suffered from a lack of spatial resolution due to the ill-posedness of the inverse problem that involves the solution of a volume integral equation with a r−2 kernel. Density tomography using cosmic muons is a new technique (see [6, 7] for precursory work) that dramatically completes the range of geophysical imaging methods (see [8] for a short review). Indeed, by measuring the attenuation of the flux of cosmic muons caused by the screening effect of geological bodies and using detectors able to give the direction of the particles we derive the average density along straight rays (Fig. 2). Tomography is performed by collecting several radiographies obtained at different locations around the volcano and by solving an inverse Radon transform. The DIAPHANE project started in 2008 with target volcanoes located in the Lesser Antilles [9]. Accounting for the severe field conditions encountered on this type of volcanoes, we designed and 410

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Figure 2: (left) View of a telescope in operation (tarpaulin removed) on La Soufrière of Guadeloupe. (middle) Example of scanning performed from the Roche Fendue location (WGS84 XUTM = (20)643347 m YUTM = 1774036 m). (right) Telescopes must be rugged enough to support harsh field conditions and have a low (≈ 45 W ) power consumption to be stand-alone.

constructed telescopes (Fig. 2) with detectors composed of scintillator strips arranged perpendicularly in order to form matrices with 16 × 16 pixels of 5 × 5 cm2 [10]. Multichannel photomultipliers are used together with OPERA CAMEROP boards modified to have a nominal time resolution δt = 1 ns that allows time-of-flight analysis to efficiently remove fake tracks [11]. The telescopes are equipped with various environmental sensors, remotely controllable via long-range WiFi link, and the total power consumption is less than 45 W. They are modular and rugged enough to support harsh field conditions and transportation on remote isolated locations (Fig. 2). The acceptance T ≈ 12 cm2 .sr for an angular resolution δ θ ≈ 1 − 2◦ allows to acquire data necessary to produce exploitable radiographies of kilometer-sized volcanoes in several weeks [12].

3. Structural an functional imaging of hydrothermal systems An example of density radiography for La Soufrière lava dome is shown in Fig. 3. The acquisition configuration corresponds to the one of Fig. 2 (middle), and the resolution at the centre of the dome is ≈ 25 m [13]. Density radiographies are of a great interest to reveal the inner structure of the lava dome and evaluate its mechanical integrity. Thanks to the fan-like straight-ray arrangement of the data acquisition, the geometry of the density heterogeneities can reliably be reinstituted contrarily to what can be done with other geophysical imaging methods where ray paths – either seismic of electrical current – have to be reconstructed through highly non-linear inversions. For highly heterogeneous and high-contrast structures like volcanoes, such reconstructions remain largely speculative. The radiography of Fig. 2 shows that the volcano is formed with dense bodies (RF1, RF5) separated by low-density regions (RF2, RF4) filled with altered hydrothermalized materials and corresponding to shallow hydrothermal reservoirs. These reservoirs may contain large amount of fluids and thermal energy that can eventually be released during phreatic explosion [2] or produce lahars able to invade nearby rivers [3]. The low average densities found for both RF2 and RF4 indicate that cavities and wide fractures are present as observed in the South crater region. The RF2 reservoir is located below the active South crater (Fig. 1) and, interestingly, the low-density 411

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Figure 3: (left) Density radiography of La Soufrière lava dome obtained for the configuration shown in Fig. 2 (middle). North is on the right and low-density regions are in blue. (right) Time variation of the flux of muon across the region marked with a black ellipse in the radiography shown on the left.

reservoir RF4 partly coincides with the supposed location of the Spallanzani cave described in ancient reports and inaccessible since the occurrence of the 1843 earthquake [14]. Muon density imaging is also relevant to monitor active – eventually unapproachable – volcanoes. An example of such a monitoring is shown in the right part if Fig. 3 as a plot of the relative time-variation of the flux of muons through the part of reservoir RF2 marked with an ellipse in the radiography of Fig. 3. The increase of the flux indicates that a decrease of average density occurred in this part of the reservoir. In the present instance, this probably corresponds to a desaturation due to a phase change from liquid to steam triggered by overheating. Interestingly, new active vents appeared some weeks later at the summit of the volcano, just above this part of the reservoir (Fig. 1).

4. Conclusion By providing a new type of observable, muon tomography is a landmark in the history of geophysical imaging methods. Thanks to the dramatic improvement of detectors and electronic devices, low-power instruments can be designed to operate in remote area to probe the inner structure of hazardous volcanoes. Muon tomography also possesses the distinctive particularity to allow to remotely image the structure of unapproachable active volcanoes. Although further improvements are still in progress, muon tomography may be considered as operational for volcano studies.

Acknowledgments Field operations in Guadeloupe received the help from colleagues of the Volcano Observatory, from the crews of the helicopter station of the French Civil Security (www.helicodragon.com) and from the National Natural Park of Guadeloupe (www.guadeloupe-parcnational.fr). On-field maintenance and servicing of the telescope are ensured by Fabrice Dufour. Field operations on Mount Etna received the help of colleagues of the Volcano Observatory at Catania. Logistic organization was ensured by the ULISSE-IN2P3 department of CNRS (ulisse.cnrs.fr). 412

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We acknowledge the financial support from the UnivEarthS Labex program of Sorbonne Paris Cité (ANR -10- LABX -0023 and ANR -11- IDEX -0005-02).

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