Constraining geometrical, hydrodynamical and mechanical properties of a fault zone at hourly time scales from ground surface tilt data
Résumé
Flow through reservoirs such as fractured media is powered by pressure gradients which also generate measurable
poroelastic deformation of the rock body. The combined analysis of ground surface deformation and sub-surface
fluid pressure provides valuable insights of a reservoir’s structure and hydromechanical properties, which are of
interest for deep-seated CO2 or nuclear waste storage for instance. Amongst all surveying tools, surface tiltmeters
offer the possibility to grasp hydraulically-induced deformation over a broad range of time scales with a remarkable
precision (1 nanoradian). Here, we investigate the information content of transient surface tilt generated by flow in
a kilometer scale sub-vertical fault zone and its surrounding fractured rock matrix. Our approach involves the combined
analysis of field data and results of a fully coupled poroelastic model, where fault and matrix are represented
as equivalent homogeneous domains. The signature of pressure changes in the fault zone due to pumping cycles is
clearly recognizable in field tilt data and we aim to explain the peculiar features that appear in: 1) tilt time series
alone from a set of 4 instruments; 2) the ratio of tilt over pressure. With the model, we evidence that the shape of
tilt measurements on both sides of a fault zone is sensitive to its diffusivity and its elastic modulus. In particular, we
show a few well placed tiltmeters (on each side of a fault) give more information on the medium’s properties than
well spatialized surface displacement maps. Furthermore, the ratio of tilt over pressure predominantly encompasses
information about the system’s dynamic behavior and extent of the fault zone, and allows separating contributions
of flow in the different compartments. Hence, tiltmeters are well suited to characterize hydromechanical processes
associated to fault zone hydrogeology at short time scales, where space-borne surveying methods fail to seize any
deformation signal.