The deformation of natural systems in response to subsurface water storage and redistribution provides insights on the main water flow path within heterogeneous systems, including the role of faults or fractures (Longuevergne et al., 2009 ; Jacob et al., 2010). A long base tiltmeter was installed in the low noise underground laboratory of Rustrel (LSBB) to study the response of the Fontaine de Vaucluse (FDV) karst hydrosystem to water cycle. Data show a strong correlation with the discharge of the system observed at the Fontaine de Vaucluse spring while distant by 50 km (Fig. 1). During a few months, the instrument presented four measurement points of the medium deformation, thus allowing for the first time to study the gradient variation of the tilt. Such information provides a significant insight on the structure producing the measured deformation such as its geometry and distance to the tiltmeter. A borehole tiltmeter was also installed close to the long base tiltmeter in order to provide tilt azimuth. Models of the medium deformation are then computed using the ADELI code (Hassani et al., 1997) to estimate the most likely geometry of the structure inducing the measured tilt. The hydrological behavior of the FDV karst hydrosystem is then reconstructed from the tilt data using a lumped model fitted to discharge measurements (e.g. Fleury et al., 2007). Tilt data, jointly interpreted with internal behavior of hydrological model, supply information on the structures driving water to the saturated zone of the system.