1. A method for determining a rock strength of an Earth formation, comprising: receiving a signal representing a measured variation of velocities or slownesses as a function of radius from and azimuth around a borehole; in response to said signal, generating predictions from a Mechanical Earth Model (MEM) representing predictions of a variation of stress and hence velocity as a function of distance from, and azimuth around, the borehole, the MEM including a rock strength model adapted for generating a further prediction representing a predicted variation of velocities or slowness; changing the rock strength in the rock strength model of the Mechanical Earth Model until the further prediction substantially matches said measured variation in velocity around and from the borehole; and on the condition that the further prediction substantially matches the measured variation in velocity around and from the borehole, generating a set of parameters, wherein said parameters are used to populate the MEM and are used in subsequent calculations.