Method for determining the rigidity of a geological layer in terms of propagation velocity of the elastic waves within a geological system delimited by a free surface and comprising at least one first geological layer having a first rigidity and a second geological layer having a second rigidity and being arranged below the first geological layer, said first and second layer being separated from each other by an interface , the method comprising the steps of: - positioning a source within the second geological layer , whose rigidity is intended to be measured, the source being apt to generate seismic waves; - activating said source of seismic waves; - detecting at least one seismic signal by means of at least one sensor arranged on the free surface, wherein detecting comprises detecting a plurality of seismic signals by means of an array of sensors comprising a plurality of sensors (8a, ...,8j) arranged on the free surface , each sensor being apt to detect a seismic signal of the plurality of seismic signals and to detect elastic surface waves propagating along the free surface, wherein recording of surface waves is carried out according to a technique of multi-channel analysis of surface waves; - determining from at least one seismic signal a shear velocity of elastic waves in the second layer and thus indicative of the rigidity of the second layer, said step of determining a shear velocity of the elastic waves in the second layer comprises the following steps: - determining from at least one seismic signal propagation velocities of surface waves caused by the seismic waves generated by said source; - determining a map of said propagation velocities of the surface waves as a function of the frequency thereof; - identifying, in the velocities map, a propagation velocity whose frequency is correlated to a depth in the geological system, and - correlating at least one propagation velocity identified in the velocities map to a shear velocity of the elastic waves in the second layer and thus indicative of the rigidity of the second layer, - calculating the rigidity from the shear velocity of the elastic surface waves in the second layer. - positioning a source within the second geological layer , whose rigidity is intended to be measured, the source being apt to generate seismic waves; - activating said source of seismic waves; - detecting at least one seismic signal by means of at least one sensor arranged on the free surface, wherein detecting comprises detecting a plurality of seismic signals by means of an array of sensors comprising a plurality of sensors (8a, ...,8j) arranged on the free surface , each sensor being apt to detect a seismic signal of the plurality of seismic signals and to detect elastic surface waves propagating along the free surface, wherein recording of surface waves is carried out according to a technique of multi-channel analysis of surface waves; - determining from at least one seismic signal a shear velocity of elastic waves in the second layer and thus indicative of the rigidity of the second layer, said step of determining a shear velocity of the elastic waves in the second layer comprises the following steps: - determining from at least one seismic signal propagation velocities of surface waves caused by the seismic waves generated by said source; - determining a map of said propagation velocities of the surface waves as a function of the frequency thereof; - identifying, in the velocities map, a propagation velocity whose frequency is correlated to a depth in the geological system, and - correlating at least one propagation velocity identified in the velocities map to a shear velocity of the elastic waves in the second layer and thus indicative of the rigidity of the second layer, - calculating the rigidity from the shear velocity of the elastic surface waves in the second layer. <EOS>