Anatomy of a lava dome using muon radiography and electrical resistivity tomography Jean-François LÉNAT on behalf of the TOMUVOL Collaboration Laboratoire Magmas et Volcans, Laboratoire de Physique Corpusculaire, Clermont-Ferrand, France Institut de Physique Nucléaire de Lyon, France

Origin-nature of the muons Primary cosmic rays striking the atmospheric atoms and molecules  High-energy collisions  new secondary particles are produced, among them muons (unitary negative electric charge, 207 times the mass of an electron) and anti-muons. About muons/minute per square meter at the surface of the Earth Intro

From Tang et al., Phys. Rev. D74, (2006) Energy spectrum of muons obtained from a compilation of experimental data. Muons arriving in the horizontal direction have a higher intensity at energies above a few 100 GeV  Because muons become more penetrative as their energy increases, near horizontal muons can be used for radiographic scanning They can penetrate into the earth if their initial energy is sufficiently high Flux reasonably high to keep detectors to a reasonable size for geological applications Intro

At the detector location, an object (e.g. a volcano) is illuminated by muons from all the directions The opacity of the object is determined by comparing the flux crossing the object and a modeled or measured incident flux in open sky conditions. The attenuation of the flux of muons depends on the density and size of an object along the muon trajectories copyright by Tom Tam

Intro If the shape of the object is known (e.g. DEM for a volcano), the average density along the muons path can be determined (radiography). Tanaka et al., 2007 With acquisitions from different locations  3D reconstruction of density distribution (tomography).

Pioneer studies in muons radiography by : Alvarez et al.,1970, Chephren pyramid Nagamine et al. 1995, Tanaka et al. 2001, Volcanoes Borozdin et al muon scattering radiography to detect uranium/plutonium contraband in trucks Jenneson, 2004, Large vessel imaging and ongoing projects in different areas: Volcanoes Faults Tunnels Minerals deposits … Intro

TOMUVOL presentation TOMUVOL (Tomographie muonique des volcans = volcano muons tomography) project started in A collaboration between particle physicists and volcanologists from Clermont-Ferrand and Lyon (France) Detectors derived from equipments developed for high energy physics and cosmic particles studies.

TOMUVOL presentation The aim of TOMUVOL is to : Develop muon tomography of volcanoes Develop joint interpretations of muons and other geophysical data (gravity, electrical resistivity, …) Develop portable muons stations to study and monitor active volcanoes

Ongoing experiment on Puy de Dôme Puy de Dôme presentation A 11 ka old trachyte dome in a young volcanic chain (the Chaine des Puys) 10 km to the West of the city of Clermont-Ferrand (France) The dome is about 400 m high and 1.8 km wide at its base Interpreted as a composite dome by geologists Photo © Daniel Massacrier Ville de Clermont-Ferrand

Puy de Dôme presentation The Puy de Dôme has been selected as an experimental site for the Tomuvol project for: Proximity with the labs in Clermont-Ferrand Good accessibility Possibility to install the experimental equipments in existing shelters with electric power In addition to the muon data, the following geophysical data are (or will be) available : 2D electrical resistivity profiles High resolution gravity map

Muons experiment at Puy de Dôme Milestones 2011: Jan. - July: Muon detector at Grotte Taillerie May: Electrical resistivity measurement Dec: Muon detector at Col de Ceyssat Milestones 2011: Jan. - July: Muon detector at Grotte Taillerie May: Electrical resistivity measurement Dec: Muon detector at Col de Ceyssat Col de Ceyssat ( 1074 m) 107 deg Grotte Taillerie (867 m) 2 km 1.2 km Puy de Dôme ( 1465 m)

From a Lidar survey in March Accuracy better than 10 cm, 0.5 m grid size High resolution Digital Elevation Model Puy de Dôme Taillerie Col de Ceyssat

The detectors (CALICE GRPC’s) Muons experiment at Puy de Dôme Glass Resistive Plate Chamber Gas: 93% forane, 5% isobutane, 2% SF6; 1 l/h PCB :Printed Circuit Boards 9142 readout channels per m 2 HV~ 7.5kV insulation glass graphite PCB copper plates 1cm 2 Glass Resistive Plate Chamber gas muon 1 mm A charged particle ionizes the gas  high electric field amplifies this ionization by producing charge cascades, which in turn induce charge signals on the copper plates

Muons experiment at Puy de Dôme The muons telescope is composed of 3 planes of GRPC. The electronic part provides the location of the impacts (1 cm 2 pixels). This allows to reconstruct the muons trajectories.

Muons experiment at Puy de Dôme Positioning of the device by GPS and land surveying  accuracy better than 5 mm Remote detector control and data transfer through a WIFI link Alignment of the planes checked by minimizing the sum of the individual-track  2 s against variations in the alignment parameters.

Simulated event rates for a density of 1.6 g/cm 3 Path length of the atmospheric muons across the Puy de Dôme volcano in the direction of the detector  indicates that the interior of Puy de Dôme is accessible for muon imaging with an 1 m 2 large detector Muons experiment at Puy de Dôme

Experiments in different phases: Phase 1. January to April 2011 : two 1m 2 chambers + one 1/6 m 2 chamber, telescope extension: ~50cm Phase 2. three more weeks : three 1m 2 chambers, telescope extension: ~50cm Phase 3. until July 2011 : two 1m 2 chambers + one 0.16m 2 chamber, telescope extension: 1m x y z O Track reconstruction Clusterise the coincident (0.4 µs) hits in the three chambers Analytically minimise χ 2 w.r.t. 4 track parameters using the 3 cluster barycentres Position resolution : 0.4 cm; Angular resolution : better than 0.5° in both  and  during the first two phases. Angular resolution improved by about a factor 2 for the 1 m-long setups (phase 3). Muons experiment at Puy de Dôme

21 Jan – 6 April 2011 (65.9 days) (0.16 m 2, 0.5 m telescope) Tomuvol Preliminary 12 May- 1 July 2011 (46 days) (0.16 m 2, 1 m telescope) Tomuvol Preliminary 14 April- 12May 2011 (18 days) (1 m 2, 1 m telescope) Tomuvol Preliminary Datasets (muon shadows) currently analyzed to derive average density along the muons path across the dome

Resistivity experiment at Puy de Dôme 2 D Electrical Resistivity Tomography carried out with a multi- electrodes (64) equipment (ABEM) A 2205 m long south-north profile with 35 m electrode spacing More detailed south-north and west-east profiles in the summit area (electrode spacing 5 m)

Combination of Wenner a and Schlumberger soundings to ensure good signal and good data coverage Resistivity experiment at Puy de Dôme

Preliminary resistivity model along a south-north profile, showing the heterogeneous internal structure of the composite lava dome Resistivity experiment at Puy de Dôme

Preliminary geological interpretation of the resistivity model along a south-north profile. Resistivity experiment at Puy de Dôme

A complete interpretation of the internal structure of the dome will be carried out using : Two muon datasets acquired from different points of view (one acquired in 2011 and the second one starting in December 2011) Two long south-north (made in 2011) and west-east (to be carried out in 2012) resistivity profiles A high resolution gravity survey (to be carried out in 2012)  This experiment aims to : (1)Develop the study of volcanoes using muon techniques (2)To improve the study of the interior of volcanoes using joint interpretation of several types of geophysical data