Anisotropy of electrical conductivity of the Excavation Damaged Zone in the Mont Terri Underground Rock Laboratory

International audienceElectrical resistivity measurements were performed to characterize the anisotropy of electrical resistivity of the excavation damaged zone (EDZ) at the end-face of a gallery in the Opalinus clay of the Mont Terri Underground Rock Laboratory (URL). The data were acquired with a combination of square arrays in 18 zones on the gallery's face and in two series of four boreholes perpendicular to the face. Each data set is independently inverted using simulated annealing to recover the resistivity tensor. Both the stability and the non-uniqueness of the inverse problem are discussed with synthetic examples. The inversion of the data shows that the face is split in two domains separated by a tectonic fracture, with different resistivity values but with a common orientation. The direction of the maximum resistivity is found perpendicular to the bedding plane, and the direction of minimum resistivity is contained in the face's plane. These results show that the geo-electrical structure of the EDZ is controlled by a combination of effects due to tectonics, stratigraphy, and recent fracturing produced by the excavation of the gallery

Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics

International audienceCatastrophic collapses of the flanks of stratovolcanoes constitute a major hazard threatening numerous lives in many countries. Although many such collapses occurred following the ascent of magma to the surface, many are not associated with magmatic reawakening but are triggered by a combination of forcing agents such as pore-fluid pressurization and/or mechanical weakening of the volcanic edifice often located above a low-strength detachment plane. The volume of altered rock available for collapse, the dynamics of the hydrothermal fluid reservoir and the geometry of incipient collapse failure planes are key parameters for edifice stability analysis and modelling that remain essentially hidden to current volcano monitoring techniques. Here we derive a high-resolution, three-dimensional electrical conductivity model of the La Soufrière de Guadeloupe volcano from extensive electrical tomography data. We identify several highly conductive regions in the lava dome that are associated to fluid saturated host-rock and preferential flow of highly acid hot fluids within the dome. We interpret this model together with the existing wealth of geological and geochemical data on the volcano to demonstrate the influence of the hydrothermal system dynamics on the hazards associated to collapse-prone altered volcanic edifices

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

Joint inversion of P-wave velocity and density, application to La Soufrière of Guadeloupe hydrothermal system

International audienceWe present the result of a 3-D gravity and P-wave traveltime joint inversion applied to the hydrothermal system of La Soufri'ere of Guadeloupe. The joint inversion process is used here to overcome the different resolution limitations attached to the two data sets. P-wave traveltimes were obtained from three active seismic surveys that were conducted from 2001 to 2007. Gravity data collected during a microgravity campaign is described in a companion paper. We use a joint inversion process based on a Bayesian formulation and a deterministic iterative approach. The coupling between slowness and density is introduced through a supplementary constraint in the misfit function that tries to minimize the distance between parameter values and a theoretical relationship. This relationship is derived from measurements on samples representative of Mt Pel'ee of Martinique and La Soufri'ere volcanoes. We chose a grid discretization that leads to an under-determined problem that we regularize using spatial exponential covariance between the nodes parameters. Our results are compared to geophysical electromagnetic results obtained using resistivity and VLF surveys. They confirm the presence of highly contrasted dense/fast and light/slow zones in La Soufri'ere dome and crater basement. Our images suggest however that some non-conductive zones may be massive andesite bodies rather than argilized zones, and that these bodies may have deeper roots than hypothesized

Corrigendum to "Anomalies of noble gases and self-potential associated with fractures and fluid dynamics in a horizontal borehole, Mont Terri Underground Rock Laboratory" [Eng. Geol. 156 (2013) 46-57]

International audienceIn Maineult et al. (2013), the characteristics of the boreholes (their Table 1), extracted from provisional documents (Mont-Terri Project report TN 2008-5), were unfortunately not exact. Table 1 gives corrected azimuths, dip angles and lengths, estimated by linear regression from precise laser-positioning measurements acquired in July 2008 when the excavation of gallery Ga08 was paused, its front being 8.1 m behind the end-face of gallery Ga04. The associated view is given in Fig. 1, which replaces Fig. 5 of Maineult et al. (2013). This correction does not change the results and their interpretation

EIDORS: an open software for EIT adapted for geophysical purposes

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

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Geophysical monitoring of the EDZ during a gallery excavation in the Mont Terri URL: electrical measurements

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Imagerie de l'EDZ dans la galerie 04 du laboratoire souterrain du Mont Terri par endoscopie sismique

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