Outstanding results: BRGM announces its 2020 awards

Geosciences for a Sustainable Earth

NATIONAL GEOLOGICAL SURVEY 2020

INNOVATION PRIZE

MétéEAU Nappes: a website with tools to monitor groundwater levels.

Hello. For an internal research project, we invented the MétéEAU Nappes website. Specialists collaborated on this project: computer developers, modellers, hydrogeologists. We have a real-time tool that displays water table status for the day in question and forecasts for the future. We have maps and graphs that are updated on each login. The work relies on the national piezometric network and the real-time measurement of groundwater levels of around 1,500 piezometers daily. MétéEAU Nappes is a tool that forecasts the evolution of aquifers. It provides data on the different components of the water cycle, the most recent, surface water and rainfall, the flow of rivers and the level of the water tables. Forecasts are provided for low-water periods and drought situations, but also for periods of rising water with risks of flooding due to rising water tables. This data results from modelling work. This is for a 6-month period and compares piezometric thresholds linked to water restrictions, in times of drought or thresholds in flood periods. The present website displays a dozen points. It will display more, as the number of public and private partnerships increases. The forecasts on the graph are associated with climate scenarios with heavy, medium or light rainfall. MétéEAU Nappes is a decision-support tool for managing water resources in sensitive areas, to anticipate droughts or floods, to assist management of conflicts of use and to anticipate climate change. Check out our website, which went live in early 2021.

Geosciences for a sustainable Earth

NATIONAL GEOLOGICAL SERVICE 2020 SCIENTIFIC PUBLICATION AWARD

Non-stationary statistical model of extreme values applied to the analysis of seismic fragility for the safety assessment of nuclear power plants

I'm Jeremy Rohmer. I work at BRGM on uncertainties. I'm Pierre Gehl. I work on problems linked to seismic risks. We will present our work, carried out as part of the European research project, NARSIS. This project is coordinated by the CEA, the Commission for Alternative Energies. At BRGM, the coordinator was Behrooz Barzagan, a risk expert in the Risk and Prevention Department. The overall objective of the project was to improve the methods and tools used for the safety analysis of nuclear power plants in the specific context of external natural hazards. That is, earthquakes, earthquake-tsunamis, floods, et cetera... We worked with colleagues from the IRSN, the French Institute of Radioprotection and Nuclear Safety, on a tool, fragility curves. Pierre will present it. Thank you. A fragility curve is a mathematical tool used to calculate the probability of something being damaged or destroyed   depending on the external level of aggression.   For example, the change resulting   from seismic waves at the base of a building.   Usually, they are built using statistical tools like mathematical regressions on given points. The data can be obtained by different methods: of empirical origin, by studying previous earthquakes or from experiments, tests on a vibrating table, or using digital methods, simulating events on a computer. The model, in the NARSIS framework... Collaborating with the IRSN enabled us to work on a structurally simplified model of a nuclear reactor. It's crossed by a steam line that carries the steam from a steam generator inside the reactor to a turbine outside. This steam line is vulnerable at elbows and anchor points. We must study the fragility of these to calculate the probability, very low, of leaks or ruptures in this pipe resulting in dramatic consequences. So we subjected this model to hundreds of seismic charges, accelerograms, and considered the variability of several of the model's mechanical parameters. For example, the thickness of the pipe which can change, for example, due to corrosion. We tried to improve this fragility analysis with our colleagues from the IRSN by addressing a specific problem, a common hypothesis, what we call the law of probability, which links the probability of damage to the intensity of the earthquake. For example, acceleration. Most fragility analysis studies hypothesize a law of so-called normal probability. The problem with this law is that it is not necessarily adapted to all cases. In particular, to what we call extreme values in statistics. Why extreme? Extreme because of their amplitude. Here, we watch movements induced by earthquakes, that can be very violent. And very extreme due to their exceptional character. They are movements that are highly unlikely. Rare events. There are statistical tools dedicated to modelling these extreme values. We applied them to the IRSN case, to create these fragility curves. We showed that it is more appropriate to use these statistical tools dedicated to extreme values to create these curves. We have highlighted the possibility of underestimating the probability of damage if this critical analysis of the law of probability is not used.

Geosciences for a sustainable Earth

NATIONAL GEOLOGICAL SERVICE 2020 THESIS PRIZE

Geothermics: Characterisation of the Piton des Neiges' hydrothermal system by geochemical and isotopic methods

Hello. I'm Bhavani Bénard and I did my thesis at the GéoSciences Laboratory Réunion in collaboration with BRGM. My subject was geothermics. "Geo", is the Earth, and "thermal", heat. And heat produces energy. Just as well, I live on a volcano, the Piton des Neiges on Reunion Island, which could be a source of heat. I'm not the first to have thought of that. Previous BRGM geologists drilled a borehole in the 1980s, which was 2 km deep. And they found heat. 182 degrees, to be exact. That is enough to produce energy. However, they were unable to. They could not extract the heat. It's not something solid that you can hold. You need a vector to transport it. Nature provides us with one: water. That's what we call a hydrothermal system. Water seeps into the rock in quantity, and heats upon contact, and this water can be pumped to make energy. Unfortunately for our predecessors, there was no water where they had drilled. Maybe they were in the wrong place. There are hot sources on this volcano. The temperature can be up to 49 degrees on the surface. Where does this water come from? Is it hotter deeper down? To find out, you need to speak the language of water. In water, there are minerals, calcium, sodium, bicarbonates, etc. Have you ever wondered how they got there? The atoms in water tell us their story. Some come from rain, others from rocks, and others from volcanic gas, or even seawater, etc. For my thesis, I took samples of the thermal sources around Piton des Neiges and analyzed them to discover their chemical composition. This is what I found. I know the water that recharges my hydrothermal system infiltrates mainly during cyclonic events, which have the particularity of presenting atoms of oxygen and hydrogen that are much lighter than those in light rain. Other chemical properties of my water reveal that these remain deeply buried for several years. Our waters also interact with magmatic gases. CO2 from magma buried 10 km below the surface rises up into our hydrothermal system. As for the hottest waters, their concentration of chloride and lithium indicates that they interact with more superficial magma. The hot waters rise up via routes created by magmatic intrusions during the last phases of activity of the Piton des Neiges. I observed two different reservoirs, not hydraulically connected, even if they may depend on the same heat source. Finally, I reconstructed the temperature of the deep hot waters and discovered a temperature of over 150 degrees. So we have a model of the hydrothermal system of the Piton des Neiges indicating that in two places there is water with a temperature compatible with geothermal exploitation. So geothermal energy soon? It's not that simple. The areas of greatest interest that I found are located in fairly inaccessible and protected areas. And we don't know in detail the architecture of the system underground. And this resource might be accessible elsewhere. The research continues.

Geosciences for a sustainable Earth

NATIONAL GEOLOGICAL SERVICE PhD AWARD 2020

Sedimentary, structural and thermal evolution of a hyperextended rift: from post-Hercynian inheritance to Alpine inversion, example of the Mauléon Basin (Pyrenees)

My PhD was carried out as part of the the Orogen research programme, a tripartite partnership between BRGM, Total and the CNRS. It was carried out at Bordeaux University in close collaboration with the BRGM teams, in particular, the Georesources Division. The subject was the evolution of the Pyrenees over the last 120 million years. More particularly, the Mauléon Basin. The Mauléon Basin is situated in the northwestern Pyrenees. To the west is the Bay of Biscay, composed of oceanic rocks, such as basalt, which constitute what we call the oceanic crust. To the east is the Pyrenees mountain range, composed of metamorphic sedimentary rocks and granite, what we call the continental crust. Earth's continental crust is distinct from the underlying mantle made up of green rocks called lherzolites. The history of the Mauléon Basin began 120 million years ago, during the Early Cretaceous period. During this period, plate tectonics caused the Iberian Plate to drift southwards. This phenomenon was responsible for distancing Spain and France. This resulted in the opening of the Bay of Biscay, an ocean, and the Mauléon Basin, considered a proto-ocean as it does not have an oceanic crust, that is, basalt. This tectonic episode led to the fracturing and thinning of the continental crust, allowing the local underlying mantle to rise. These green rocks, lherzolites, which make up Earth's mantle and were previously buried 30 km beneath the continental crust, rose to the floor of this proto-ocean and came into contact with sea water around 100 million years ago. At the time, the rising of Earth's mantle towards the surface was controlled by movements along the large faults in the crust and the mantle, called detachments. Then, 86 million years ago, the Iberian Plate, pushed by Africa, drifted towards the north, generating a compression phase between Spain and France. This tectonic episode was responsible for the closure of the Mauléon proto-ocean and the formation of the Pyrenees mountain range we know today. In the early stages of Spain and France's convergence, the geometry of the emerging mountain chain was controlled by the former faults which led to the opening of the Pyrenean proto-ocean. These areas of fragility were reactivated preferentially in inverse faults called overlaps. 65 million years ago, in the final stage of the convergence of Spain and France, the formation of the Pyrenees was controlled by the sinking of Spain beneath France. This was the continental collision stage. The lherzolites making up Earth's mantle, and which outcrop locally in the Pyrenees, were located 30 km below Earth's crust, 120 million years ago. First, these rocks rose up to the floor of the Mauléon Basin proto-ocean and came into contact with water 100 million years ago, when Spain and France were drifting apart. Fragments of these green rocks were then torn from the ocean floor and thrust several hundred metres up the Pyrenees mountain range when France and Spain collided. Based on this, it was possible to propose keys to understanding the formation of the oceans, when tectonic plates separate and their incorporation into the mountain range when the same tectonic plates move closer. Applied to other mountain ranges, these concepts take on a predictive character for the exploration and exploitation of natural resources, in particular on the increasingly popular theme of natural hydrogen.