How does surface deformation inform on subsurface groundwater flow ?

International audienceRelative deformation instruments like tiltmeters or extensometers have emerged as interesting tools for groundwater hydrology. Indeed, they are able to record accurately and precisely low amplitude deformations at high sampling rates. While these instruments were originally designed for geophysical studies of the Earth’s crust, recent technical implementations and field setup adaptations have transformed them into robust instruments to explore subsurface hydrologically-active structures from the surface. The main originality of deformation is its sensitivity to the rock response to fluid pressure changes, therefore complementing usual geophysical methods for heterogeneous/confined aquifer systems. Furthermore, these methods are intrinsically sensitive to mechanical parameters such as storativity. In this work, we will present the complementarity of different deformation tools to capture aquifer geometry and properties at ~100 m scales (tiltmeter, leveling methods, borehole strainmeter). We demonstrate the usefulness of short hydro-mechanical experiments to constrain key mechanical and flow properties of a faulted reservoir, illustrated by data collected at the Ploemeur hydrogeological observatory (France). Deformation was able to constrain the geometry of the main permeable fracture during a short-term pumping test. We also document the evolution of the flow structure, from 1D flow in the fracture to 2D exchanges with the surrounding matrix. Finally, we report recent results on the value to couple sinusoidal testing and deformation to track flow in fracture networks, including the estimation of deep fracture (up to 80 m) properties and connectivity to network