Tomography using muons is a method to investigate the subsurface density. Muons are particles produced in the atmosphere, and their absorption depends on the quantity of matter they pass through, hence on the rock density and thickness. The Temporal Tomography of rock mass Density using Muons flux Measurement (T2DM2) project aims to characterise the spatial and temporal density variations of the first hundreds of metres of the subsurface. These variations can be induced by water transfer within the critical zone or other thermo-hydro-mechanical processes like fracturing. The imaging potential of muography and its application to hydrogeological processes are introduced. Numerical simulations are performed to estimate the significance of the muon flux fluctuations associated to rock density and composition variations for depths ranging from 60 to 1000 m. Particular attention is paid to muon scattering in rocks, which should be taken into account in order to achieve a good spatial and angular resolution.