Muons tomography applied to geosciences and volcanology

Imaging the inner part of large geological targets is an important issue in geosciences with various applications. Dif- ferent approaches already exist (e.g. gravimetry, electrical tomography) that give access to a wide range of informations but with identified limitations or drawbacks (e.g. intrinsic ambiguity of the inverse problem, time consuming deployment of sensors over large distances). Here we present an alternative and complementary tomography method based on the measurement of the cosmic muons flux attenuation through the geological structures. We detail the basics of this muon tomography with a special emphasis on the photo-active detectors.Comment: Invited talk at the 6th conference on New Developments In Photodetection (NDIP'11), Lyon-France, July 4-8, 2011; Nuclear Instruments and Methods in Physics Research Section A, 201

Design and operation of a field telescope for cosmic ray geophysical tomography

International audienceThe cosmic ray muon tomography gives an access to the density structure of geological targets. In the present article we describe a muon telescope adapted to harsh environmental conditions. In particular the design optimizes the total weight and power consumption to ease the deployment and increase the autonomy of the detector. The muon telescopes consist of at least two scintillator detection matrices readout by photosensors via optical fibres. Two photosensor options have been studied. The baseline option foresees one multianode photomultiplier (MAPM) per matrix. A second option using one multipixel photon counter (MPPC) per bar is under development. The readout electronics and data acquisition system developed for both options are detailed. We present a first data set acquired in open-sky conditions compared with the muon flux detected across geological objects

Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour

Dome-forming volcanoes are among the most hazardous volcanoes on Earth. Magmatic outgassing can be hindered if the permeability of a lava dome is reduced, promoting pore pressure augmentation and explosive behaviour. Laboratory data show that acid-sulphate alteration, common to volcanoes worldwide, can reduce the permeability on the sample lengthscale by up to four orders of magnitude and is the result of pore- and microfracture-filling mineral precipitation. Calculations using these data demonstrate that intense alteration can reduce the equivalent permeability of a dome by two orders of magnitude, which we show using numerical modelling to be sufficient to increase pore pressure. The fragmentation criterion shows that the predicted pore pressure increase is capable of fragmenting the majority of dome-forming materials, thus promoting explosive volcanism. It is crucial that hydrothermal alteration, which develops over months to years, is monitored at dome-forming volcanoes and is incorporated into real-time hazard assessments

Bayesian dual inversion of experimental telescope acceptance and integrated flux for geophysical muon tomography

International audienceDensity tomography of rock volumes with cosmic muons involves telescopes equipped with pixelized matrices of scintillator strips able to simultaneously measure the flux of muons in hundredths of directions. The resulting muon radiography images are a measure of the amount of matter integrated along each line of sight inside the geological target. This information constitutes the primary data at the root of muon density 3-D tomography. Before being used for either interpretation or tomography inversion, the radiographies must be corrected from artefacts due to imperfect detection capacity of the detection matrices. We present a correction method based on a Bayesian inversion to construct a probabilistic model of the distorted telescope acceptance from which undistorted radiographies may be obtained. The method also allows to simultaneously derive a stochastic model for the incident flux of muons. The resulting non-linear inverse problem is solved with the Metropolis-annealing algorithm, which allows to easily implement symmetry constraints to reduce the non-uniqueness. An inversion of real data acquired with one of our field muon telescopes is presented and discussed

3-D density imaging with muon flux measurements from underground galleries

International audienceAtmospheric muon flux measurements provide information on subsurface density distribution. In this study, muon flux was measured underground, in the Tournemire experimental platform (France). The objective was to image the medium between the galleries and the surface and evaluate the feasibility to detect the presence of discontinuities, for example, produced by secondary subvertical faults or by karstic networks. Measurements were performed from three different sites with a partial overlap of muon trajectories, offering the possibility to seek density variations at different depths. The conversion of the measured muon flux to average density values showed global variations further analysed through a 3-D nonlinear inversion procedure. Main results are the presence of a very low density region at the level of the upper aquifer, compatible with the presence of a karstic network hosting local cavities, and the absence of secondary faults. We discuss the validity of the present results and propose different strategies to improve the accuracy of such measurements and analysis

Electrical Resistivity Tomography (ERT) as a tool for the assessment of karst features in carbonate aquifers.

In the Mediterranean Basin, most of the groundwater resources are located in carbonate aquifers. In the Montpellier area, southern France, several regional faults affecting these carbonate reservoirs control the location of the main flow paths and thus the groundwater flow. These faults zones are characterized by a strong anisotropy of permeability, sometimes induced by an offset between compartments on both sides of the fault. Fluid flow generally occurs preferentially in the fault-parallel direction. Fractures located in the damaged zone of these faults, partly responsible for this anisotropy, are also at the origin of karst features development. Thus, to identify the main permeability components of such fault zones, it is essential to identify- -(i) the structure of the damaged zone+ -(ii) the fault offset and, -(iii) the presence of karst features and their geometry. A methodology based on Electrical Resistivity Tomography (ERT) was thus proposed with this purpose. It allowed -(i) to highlight en-echelon fracturing in the studied fault-zones+ -(ii) to identify the presence of karst feature and -(iii) to precise their geometry. Though the fault was a priori considered as a conduit-barrier, we demonstrated that groundwater flow across the fault probably occurs due to the en echelon offset of the fractures and that karst features preferentially located along bedding planes control most of the flow

Electrical tomography monitoring of the EDZ during the excavation of the Gallery Ga08 in the Mont Terri URL

2 p

EIDORS: an open software for EIT adapted for geophysical purposes

National audienc