227 research outputs found
Joint inversion of muon tomography and gravimetry - a resolving kernel approach
Both muon tomography and gravimetry are geophysical methods that provide
information on the density structure of the Earth's subsurface. Muon tomography
measures the natural flux of cosmic muons and its attenuation produced by the
screening effect of the rock mass to image. Gravimetry generally consists in
measurements of the vertical component of the local gravity field. Both methods
are linearly linked to density, but their spatial sensitivity is very
different. Muon tomography essentially works like medical X-ray scan and
integrates density information along elongated narrow conical volumes while
gravimetry measurements are linked to density by a 3-dimensional integral
encompassing the whole studied domain. We develop the mathematical expressions
of these integration formulas -- called acquisition kernels -- to express
resolving kernels that act as spatial filters relating the true unknown density
structure to the density distribution actually recoverable from the available
data. The resolving kernels provide a tool to quantitatively describe the
resolution of the density models and to evaluate the resolution improvement
expected by adding new data in the inversion. The resolving kernels derived in
the joined muon/gravimetry case indicate that gravity data are almost useless
to constrain the density structure in regions sampled by more than two muon
tomography acquisitions. Interestingly the resolution in deeper regions not
sampled by muon tomography is significantly improved by joining the two
techniques. Examples taken from field experiments performed on La Soufri\`ere
of Guadeloupe volcano are discussed.Comment: Submitted to Geoscientific Model Developmen
Muon tomography applied to active volcanoes
Muon tomography is a generic imaging method using the differential absorption
of cosmic muons by matter. The measured contrast in the muons flux reflects the
matter density contrast as it does in conventional medical imaging. The
applications to volcanology present may advantadges induced by the features of
the target itself: limited access to dangerous zones, impossible use of
standard boreholes information, harsh environmental conditions etc. The
Diaphane project is one of the largest and leading collaboration in the field
and the present article summarizes recent results collected on the Lesser
Antilles, with a special emphasis on the Soufri\`ere of Guadeloupe.Comment: 7 pages, 7 figures, International Conference on New
Photo-detectors,PhotoDet2015, 6-9 July 2015, Moscow, Troitsk, Russia.
Submitted to Po
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
Monitoring temporal opacity fluctuations of large structures with muon tomography : a calibration experiment using a water tower tank
Usage of secondary cosmic muons to image the geological structures density
distribution significantly developed during the past ten years. Recent
applications demonstrate the method interest to monitor magma ascent and
volcanic gas movements inside volcanoes. Muon radiography could be used to
monitor density variations in aquifers and the critical zone in the near
surface. However, the time resolution achievable by muon radiography monitoring
remains poorly studied. It is biased by fluctuation sources exterior to the
target, and statistically affected by the limited number of particles detected
during the experiment. The present study documents these two issues within a
simple and well constrained experimental context: a water tower. We use the
data to discuss the influence of atmospheric variability that perturbs the
signal, and propose correction formulas to extract the muon flux variations
related to the water level changes. Statistical developments establish the
feasibility domain of muon radiography monitoring as a function of target
thickness (i.e. opacity). Objects with a thickness comprised between
50 30m water equivalent correspond to the best time resolution. Thinner
objects have a degraded time resolution that strongly depends on the zenith
angle, whereas thicker objects (like volcanoes) time resolution does not.Comment: 11 pages, 9 figures. Final version published in Scientific Reports,
Nature, 14 march 201
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
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