Thanks to the five instruments of its rock mechanics platform, the LG-ENS can reproduce the Earth's pressure conditions from the surface crust to the asthenospheric mantle. Heating the samples and controlling the fluid pressure allows to get as close as possible to natural conditions.
Rock mechanics platform
The rock mechanics platform of the École normale supérieure consists of five instruments to deform rock samples at greater or lesser pressures. This pressure, known as containment, simulates the conditions of lithostatic pressure, linked to the depth, that prevails inside the Earth. In hydraulic presses, this pressure is generated by oil pressure, while in the Griggs press, with solid containment, it is salt (NaCl) and in the Paterson-type press, gas (argon) is used. This allows pressures ranging from a few MPa to several GPa, corresponding to depths from the Earth’s surface crust to the asthenospheric mantle. Some of these presses can also heat the samples during the experiments (from a hundred degrees in a hydraulic press to more than 1,000 °C in a Griggs) and/or control the fluid pressure that prevails in the sample, thus reproducing natural conditions as best as possible.
Pressure is the result of a force applied to a surface. A direct consequence of this is that to reproduce high pressure conditions, the simplest and safest solution is to reduce the size of the sample containment cell, and therefore the size of the sample. Thus, the sample size ranges from about ten cm for lower pressure experiments, to 1 cm for multi-GPa experiments performed in Griggs. What all these devices have in common is their geometry and the type of measurements associated with them. In all cases the samples have a cylindrical shape, and an axial force independent of the containment pressure is applied to the sample by a piston in order to induce its deformation. The starting samples can be whole and intact, to study their creep properties at high temperatures for example, or be previously sawn and separated into two halves to simulate the presence of a fault plane. The movement of the piston and the force applied by the piston are measured and recorded during the deformation, allowing the characterization of the mechanical behavior of the rocks.
In addition, the specificity of our platform is the use on each of these presses of a device allowing, during the experiments, to probe the acoustic properties of the rocks in the ultrasonic field (typically of the order of MHz). These devices provide two types of data. First, they allow a measure of the seismic velocities of compressive and transverse waves, which can then be directly compared to those measured by seismologists in nature. At the same time, they record micro-earthquakes related to the deformation of samples, in the form of acoustic emissions, just as a seismic station records earthquakes caused by the deformation of the Earth’s crust. Thus, these laboratory measurements are extremely valuable in understanding and modeling the mechanical behaviour of rocks, whether brittle and seismogenic, as in the Earth’s crust, or viscous as it is the case in the asthenosphere.
Recent or current projects
Characterization of the role of differential stress on phase transitions (Arefeh Moarefvand)
Relationships between roughness of fault planes and frictional properties (Jérôme Aubry)
Metamorphic reactions and deep seismicity (Marie Baïsset, Julien Gasc)
Rupture precursors and their high frequency radiation (Samson Marty)
Frequency dispersion and attenuation of elastic waves in saturated rocks: direct links between laboratory and field measurements (Ariel Gallagher, Jan Borgomano)
Relations between static and dynamic elastic properties (Jan Borgomano)
Fluid/rock interactions (poroelastic and chemical) in clayey sandstones (Hanjun Yin)
Influence of partial saturation or CO2 exsolution on the dynamic elastic properties of porous rocks (Chao Sun, Samuel Chapman, Jan Borgomano)