Method for ultrasonic characterization of a medium, comprising generating a series of incident ultrasonic waves, generating an experimental reflection matrix Rui(t) defined between the emission basis (i) as input and a reception basis (u) as output, and determining a focused reflection matrix RFoc(rin, rout, δt) of the medium between an input virtual transducer (TVin) calculated based on a focusing as input to the experimental reflection matrix and an output virtual transducer (TVout) calculated based on a focusing as output from the experimental reflection matrix, the responses of the output virtual transducer (TVout) being obtained at a time instant that is shifted by an additional delay δt relative to a time instant of the responses of the input virtual transducer (TVin).