A method of determining in-situ a relation between the seismic velocity and the state of stress in an underground formation located under a surface subjected to time-changing surface loading conditions. A relation is selected between the seismic velocity and the state of stress containing at least one unknown parameter. A seismic source is arranged at surface or in a borehole penetrating the underground formation, and a seismic receiver is arranged at a distance from the seismic source at surface or in a second borehole. At two different times the seismic velocity of the formation along a path from the seismic source to the seismic receiver is determined. The difference in surface loading conditions at the two times is converted in a difference in states of stress in the underground formation. The unknown parameter(s) are calculated to obtain the relation between the seismic velocity and the state of stress in the underground formation.