Implementing ground surface deformation tools to characterize field-scale properties of a fractured aquifer during a short hydraulic test

International audienceIn naturally fractured reservoirs, fluid flow is governed by the structural and hydromechanical properties of fracturenetworks or conductive fault zones. In order to ensure a sustained exploitation of resources or to assess the safety ofunderground storage, it is necessary to evaluate these properties. As they generally form highly heterogeneous andanisotropic reservoirs, fractured media may be well characterized by means of several complementary experimentalmethods or sounding techniques. In this framework, the observation of ground deformation has been proved usefulto gain insight of a fractured reservoir’s geometry and hydraulic properties. Commonly, large conductive structureslike faults can be studied from surface deformation from satellite methods at monthly time scales, whereas meterscale fractures have to be examined under short-term in situ experiments using high accuracy intruments liketiltmeters or extensometers installed in boreholes or at the ground’s surface. To the best of our knowledge, thefeasability of a field scale ( 100 m) characterization of a fractured reservoir with geodetic tools in a short termexperiment has not yet been addressed. In the present study, we implement two complementary ground surfacegeodetic tools, namely tiltmetry and optical leveling, to monitor the deformation induced by a hydraulic recoverytest at the Ploemeur hydrological observatory (France). Employing a simple purely elastic modeling approach,we show that the joint use of time constraining data (tilt) and spatially constraining data (vertical displacement)makes it possible to evaluate the geometry (dip, root depth and lateral extent) and the storativity of a hydraulicallyactive fault zone, in good agreement with previous studies. Hence we demonstrate that the adequate use of twocomplementary ground surface deformation methods offer a rich insight of large conductive structure’s propertiesusing a single short term hydraulic load. Ground surface deformation tools therefore constitute an interestingaddition to traditional fractured media sounding technique