Which subsurface processes can we monitor using seismic ambient noise? Can we monitor stress or temperature variations? Within research programme DeepNL (NWO) these are some of the questions I try to answer. We derived physics-based relationships between effective stress and seismic velocities, and we validated them in a hydrological context. These relationships enable us to develop a monitoring tool for 4D pore pressure variations in the shallow subsurface (0-200 m) of Groningen, purely based on seismic information. This also implies that it is feasible to passively monitor pore pressure developments in shallow hydrocarbon reservoirs. This physics-based approach can potentially be extended to monitoring of temperature and saturation variations using seismic ambient noise.