Muon tomography is a method to investigate the in-situ rock density. It is based on the absorption of cosmic-ray muons according to the quantity of matter (thickness and density). Numerical simulations are performed in order to estimate the expected muon flux in LSBB Underground Research Laboratory (URL) (Rustrel, France). The aim of the muon measurements in the underground galleries of this laboratory is to characterize the spatial and temporal density variations caused by water transfer in the unsaturated zone of the Fontaine-de-Vaucluse karstic aquifer