Description du sujet de thèse

Electrical and electromagnetic methods in geophysics are useful for studying the properties of the subsurface and identifying discontinuities, mainly thanks to the different electromagnetic responses of geological materials. They are therefore particularly well suited to imaging volcanic structures, as resistivity (conductivity) is a parameter that is highly sensitive to the presence of fluids in active fractured and permeable structures. The geometry, orientation, and depth of these structures are all parameters that control the stability of volcanic systems. Various methods, known as active or passive, exist for imaging this distribution of electrical resistivity (conductivity) at various depths and spatio-temporal scales, in 1D (vertical electrical sounding, Time Domain Electromagnetics and Magnetotellurics - MT), 2D and 3D (Electrical Resistivity Tomography). The initial objective of this thesis will be to develop methodologies related to the digital processing and inversion of MT data in conjunction with magnetic data measured from space and at terrestrial magnetic observatories in order to highlight discontinuities over a wide range of depths, from the surface to the depths of the mantle. To do this, Bayesian methods will be favored because they allow errors to be propagated on raw data and eliminate certain modeling assumptions (smoothing, linearization, etc.). Developments related to the analysis of measurements from space will contribute to the scientific exploitation of the European Swarm mission, the Chinese Macau Science Satellite-1 (MSS-1) mission, and will enable integration into the working group on the scientific preparation of the French CNES NanoMagSat mission, scheduled for launch in 2027. In a second phase, the methods will be applied to electrical, MT, and satellite databases, with the main targets being the Piton de la Fournaise volcano on Réunion Island and the Chaîne des Puys region. The results will be compared with the state of the art of our knowledge in terms of volcanic structure, mantle composition, water content and temperature, and “known” discontinuities based on information from experimental petrology and seismology. This combination of different types of data, at multiple scales, from both terrestrial and satellite sources, will enable promising developments in the field of volcanic monitoring and in our understanding of the dynamics and composition of the deep Earth.

Contexte de travail

The OPGC is a CNRS-INSU Universe Science Observatory (OSU) comprising two laboratories (the Magmas and Volcanoes Laboratory and the Physical Meteorology Laboratory) and a research support unit (UAR-833). The OPGC's mission is to contribute to the advancement of knowledge about the Earth by developing observation methods and acquiring, managing, and disseminating observation data in the fields covered by its two laboratories. The candidate will be assigned to the Magmas and Volcanoes Laboratory, which specializes in volcanology, experimental petrology, and geochemistry. Their work will involve contributing to fundamental knowledge of volcanic structures using geophysical methods and ongoing developments in new methods applicable from space. A good academic knowledge of geophysical methods, particularly electromagnetic methods, is required, as are programming skills (Matlab, Python). The candidate will divide their time between laboratory work and approximately 10-20% fieldwork and will have the opportunity to contribute to ongoing large-scale scientific projects.

Contraintes et risques

The project involves assignments in sometimes difficult terrain, in France, overseas, or abroad. Good writing skills are required, as well as a good method for producing technical documentation to track experiments and digital work. The project involves participating in site surveys in the field, installations, and the operation and maintenance of instruments. Some laboratory experiments will be conducted. These specific field and laboratory activities require the ability to apply standards and follow procedures and rules, particularly those relating to health and safety.