PhD position: "Development of geophysical imaging by joint analysis of muography and associated methods. Applications to near-surface problems" - IP2I (UMR5822) et LGL-TPE (UMR5276)

Short description

The PhD position is proposed for a 3-year period (36 months). The legal gross salary is €1768 per month (plus social benefits). An annual €2 000 package for travels and equipment will be allotted. The candidate is expected to submit a thesis manuscript to the university of Lyon for a formal presentation in front of a jury before the end of the 3-yr period.

Starting date of the contract: October the 1st, 2021

Research project

Low-energy active wet volcanoes recently became the focus of much attention for the risk they represent [1]. Indeed, most fatalities that occurred during the last two decades were caused by unexpected local explosions due to overpressure in the shallow hydrothermal system of touristic volcanoes with moderate activity [2]. This type of hazard is now recognized as a main topic by the volcano community [3]. In 1993, the explosive event of the Galeras volcano killed 9 people, including 6 volcanologists. More recently, in 2014, an unexpected explosive event killed 63 hikers on the Ontake volcano in Japan [4]. These local events are not likely to provoke regional disasters, but they however represent high-risk situations for people on site. Such is the case of many volcanoes, including La Soufrière of Guadeloupe localized in a National touristic park.

In the recent literature concerning the Ontake event, it is widely admitted that these explosive hydrothermal events may have a rapid and silent onset which can be as short as some hours or days. Presently, both the location and the date of occurrence of such events are considered impossible to predict and the physics of these phenomena remains largely undocumented. For the first time, one such event was described in details by ourselves [5] and could be studied by means of different kinds of data (vent temperature, seismic noise tomography and muon tomography). This study revealed that potentially explosive steam pockets originate from complex interactions between the fracture network that collects thermal energy in the deep hydrothermal system and the shallow network of large open fractures driving hot fluids to the atmosphere. In volcanoes like La Soufrière, thermal energy is transported by the steam phase and multiphase flows occur in the fracture network. Complicated phenomena then appear, such as density wave oscillations, bubble nucleation in developed bubble wakes, bubble detachment thresholds. Because of its high bulk density and compressibility, the liquid-gas mixture may oscillate and transmit strong seismic vibrations to the surrounding rock [5].

The main objective of the project is to document the sudden appearance and the dynamics of steam pockets in the shallow hydrothermal system of wet volcanoes. Predicting the time-origin of this type of events is of a primary importance with respect to risk mitigation. This is particularly challenging because the time constants of onset of the phenomena are particularly short. The study will largely rely on the monitoring of La Soufrière lava dome with cosmic muon tomography which allows to monitor density changes in the whole lave dome [6]. Other geophysical experiments will be done to complement the muon data: seismic noise recording to detect and localize hydrothermal sources of noise and thermodynamic monitoring of the main active vents on the summit of the volcano. Numerical models and data inversion will be done to constrain physical models of the phenomena. These models will consider multiphase flows in fracture networks subject to intense heat flows. Depending on the amount of energy to evacuate, different flow regimes may occur with, eventually, abrupt bifurcations producing dangerous surface phenomena. Stochastic approaches will be used to simulate flows in random networks and powerful computational resources.