Responsable du projet Benoît Valley
Giona Preisig
Collaborateur Léa Perrochet
Résumé For the design and operation of most underground projects, some knowledge of the in-situ stress field is required. However, information on the in-situ stress field is and will remain sparse, because measuring it is difficult and expensive. Purely geostatistical interpolation between point measurements can be misleading since the stress field is heterogenous and discontinuous due to the large influence of mechanical properties of the lithological units and the presence of tectonic accidents. A more promising way of evaluating and mapping the in-situ stress field is to apply appropriate boundary conditions to calibrated geomechanical models, which take into account involved physical processes: geomechanics, fluid flow, thermal effects. However, the calibration of such models is a non-unique and under-determined problem. The quality and usefulness of such model will be directly dependant on the quality of the lithologic and structural model used and the stress calibration data available. Thus, rather than a single shot stress computation, it is important to have a modelling framework based on field observations that is dynamic, i.e. readily updated when new observation becomes available.
New observations can be data from new deep boreholes that can locally provide absolute stress
magnitude information. But more relevant than the absolute magnitude, an important question in reservoir geomechanics, geothermal and hydrogeological engineering involving the production and/or injection of fluid in the underground, is the criticality of fault zones. A critically stressed fault is a fault that is on the verge to failure and for which small perturbations can lead to rupture and seismic energy radiation. This is important because even relatively low magnitude seismic events, i.e. moment magnitude M ≈ 3.5, have been sufficient for stopping projects, as for example in the cases of the Basel or St-Gallen geothermal projects in Switzerland. We suggest that if the seismo-hydro-mechanical conditions of such critically stressed faults zone are carefully monitored, such object can be used as ”stressmeters” and be very useful to evaluate the in-stress conditions and calibrate stress models.
For various reasons, critically stressed faults move in an episodic fashion inducing repeated seismic
events that can be recorded by local seismic network. At first order level, the episodic rupture of faults reflect the seismic cycle were tectonic strain accumulate and relax successively. Contrast in static and dynamic friction of faults, refered to as state and rate friction, these cycles. In the detail, the fault behaviour is more complex and depends on the intrinsic properties of the fault zone, e.g. geometries, asperities, and on the presence of fluids, such as groundwater. Fluid/water pressure decreases the shear strength of geologic materials, favouring thus their failure in shear. In Switzerland, many examples exist of seismic events of low magnitude related to increasing groundwater pressure in response to seasonal snow melt coupled with important rainfall periods or in response to artificial lake impounding [7, 19, 14]. Aquifer systems in which large water head fluctuation are observed are particularly sensitive. These events are good indicators of the criticality of a fault zone and observed increase in groundwater pressure is thus an indirect measure of the criticality, that is the amount of stress necessary to trigger the shear slip event. The identification and 3D location of these seismic events can also be used to situate groundwater flow at depth and generate 3D numerical models or maps for targeting deep geothermal wells. Furthermore, back-analyses of the seismic events can provide important information about the stress regime, which is subsequently used as a basis for elaborating maps and 3D numerical models of the present day stress field.
The ”JuraHydroTectonics” project aims to explore the afore-mentioned topic by coupling hydrogeological and geophysical field investigations and data acquisition with advanced 3D modelling techniques in geology, hydrogeology and geomechanics. The selected study site is the folded Jura Mountains at the limestone edge of lake Neuchˆatel, which include several strike-slip faults that might interact with the karstic aquifer systems in the Malm rocks, such as the ”La Lance” fault system and the springs of ”La Raisse” and ”La Diaz” in the village of Concise (VD). It is well-known that the Malm aquifer system is confined at the lake level with groundwater pressure fluctuations due to seasonal effects that can span up to 60 m (0.6 MPa). The final contribution of this project will be clear and straightforward methodologies for assessing the criticality of fault zones, identifying and quantifying deep circulations of hydrothermal groundwater, as well as elaborating evolutive 3D numerical modelling framework for stress field mapping.
Mots-clés tectonics; earthquake; Jura
Type de projet Recherche fondamentale
Domaine de recherche Geothermics
Source de financement BFE, swisstopo
Etat En cours
Début de projet 1-5-2018
Fin du projet 1-6-2022
Contact Benoît Valley