Journal
JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH
Volume 356, Issue -, Pages 243-263Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.jvolgeores.2018.03.017
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Funding
- University of Melbourne - Commonwealth of Australia [CR-2016-UNIV.MELBOURNE-147672-UMR5275]
- National Science Foundataion (NSF)'s Division of Earth Sciences, Instrumentation and Facilities Program [EAR-1043051]
- NSF [1226353, 1225810]
- Labex grant OSUG@2020 [ANR10 LABX56]
- CNRS-INSU program SYSTER
- CNRS-INSU project
- CNRS INSU project
- HVO Park
- Directorate For Geosciences
- Division Of Earth Sciences [1225810] Funding Source: National Science Foundation
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Electrical conductivity tomography is a well-established galvanometric method for imaging the subsurface electrical conductivity distribution. We characterize the conductivity distribution of a set of volcanic structures that are different in terms of activity and morphology. For that purpose, we developed a large-scale inversion code named ECT-3D aimed at handling complex topographical effects like those encountered in volcanic areas. In addition, ECT-3D offers the possibility of using as input data the two components of the electrical field recorded at independent stations. Without prior information, a Gauss-Newton method with roughness constraints is used to solve the inverse problem. The roughening operator used to impose constraints is computed on unstructured tetrahedral elements to map complex geometries. We first benchmark ECT-3D on two synthetic tests. A first test using the topography of Mt. St Helens volcano (Washington, USA) demonstrates that we can successfully reconstruct the electrical conductivity field of an edifice marked by a strong topography and strong variations in the resistivity distribution. A second case study is used to demonstrate the versatility of the code in using the two components of the electrical field recorded on independent stations along the ground surface. Then, we apply our code to real data sets recorded at (i) a thermally active area of Yellowstone caldera (Wyoming, USA), (ii) a monogenetic dome on Fumas volcano (the Azores, Portugal), and (iii) the upper portion of the caldera of Kilauea (Hawaii, USA). The tomographies reveal some of the major structures of these volcanoes as well as identifying alteration associated with high surface conductivities. We also review the petrophysics underlying the interpretation of the electrical conductivity of fresh and altered volcanic rocks and molten rocks to show that electrical conductivity tomography cannot be used as a stand-alone technique due to the non-uniqueness in interpreting electrical conductivity tomograms. That said, new experimental data provide evidence regarding the strong role of alteration in the vicinity of preferential fluid flow paths including magmatic conduits and hydrothermal vents. (C) 2018 Elsevier B.V. All rights reserved.
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