Method for ultrasonic characterization of a medium, in order to determine an integrated speed of sound in the medium, the method comprising: - a step of generating a series of incident ultrasonic waves (US in) in an area of said medium, by means of an array of transducers, said series of incident ultrasonic waves being an emission basis (i); and - a step of generating an experimental reflection matrix R ui (t) defined between the emission basis (i) as input and a reception basis ( u) as output; - a step of determining a focused reflection matrix RFoc ( r in, r out, δt) which comprises responses of the medium between an input virtual transducer (TV in) of spatial position r in and an output virtual transducer (TV out) of spatial position r out, the responses of the output virtual transducer (TV out) being obtained at a time instant shifted by an additional delay δt relative to a time instant of the responses of the input virtual transducer (TV in), - a step of determining a wavefront image for the input virtual transducer (TV in) and for an additional delay interval, said wavefront image being determined as a function of the speed of sound c 0 in the medium, and said wavefront image being determined based on: - the focused reflection matrix RFoc ( r in, r out, δt) and - a ballistic propagation relation of the type δt ( Δr out) = - sign (Δ z out) · |Δ r out |/ c 0, which makes it possible to extract values from the focused reflection matrix in order to construct the wavefront image, and where: δt is the additional delay, | Δr out | is the modulus of the vector between the input virtual transducer (TV in) and the output virtual transducer (TV out), with Δr out = r out - r in, Δz out is the component along a depth axis Z of the spatial position vector Δr out. said wavefront image comprising a focal spot, - a step of determining a center of the focal spot in the wavefront image, and determining the depthwise position in the direction of axis Z of said focal spot, said depthwise position being noted Δz ( r in)and - a step of calculating an integrated speed of sound c ( r in), based on the following formula: c 1 r in = c 0 1 + Δ z 0 r in z in where z in is the component along the depth axis Z of the spatial position vector r in of the input virtual transducer (TV in). - a step of generating a series of incident ultrasonic waves (US in) in an area of said medium, by means of an array of transducers, said series of incident ultrasonic waves being an emission basis (i); and - a step of generating an experimental reflection matrix R ui (t) defined between the emission basis (i) as input and a reception basis ( u) as output; - a step of determining a focused reflection matrix RFoc ( r in, r out, δt) which comprises responses of the medium between an input virtual transducer (TV in) of spatial position r in and an output virtual transducer (TV out) of spatial position r out, the responses of the output virtual transducer (TV out) being obtained at a time instant shifted by an additional delay δt relative to a time instant of the responses of the input virtual transducer (TV in), - a step of determining a wavefront image for the input virtual transducer (TV in) and for an additional delay interval, said wavefront image being determined as a function of the speed of sound c 0 in the medium, and said wavefront image being determined based on: - the focused reflection matrix RFoc ( r in, r out, δt) and - a ballistic propagation relation of the type δt ( Δr out) = - sign (Δ z out) · |Δ r out |/ c 0, which makes it possible to extract values from the focused reflection matrix in order to construct the wavefront image, and where: δt is the additional delay, | Δr out | is the modulus of the vector between the input virtual transducer (TV in) and the output virtual transducer (TV out), with Δr out = r out - r in, Δz out is the component along a depth axis Z of the spatial position vector Δr out. said wavefront image comprising a focal spot, - the focused reflection matrix RFoc ( r in, r out, δt) and - a ballistic propagation relation of the type δt ( Δr out) = - sign (Δ z out) · |Δ r out |/ c 0, which makes it possible to extract values from the focused reflection matrix in order to construct the wavefront image, and where: δt is the additional delay, | Δr out | is the modulus of the vector between the input virtual transducer (TV in) and the output virtual transducer (TV out), with Δr out = r out - r in, Δz out is the component along a depth axis Z of the spatial position vector Δr out. said wavefront image comprising a focal spot, δt is the additional delay, | Δr out | is the modulus of the vector between the input virtual transducer (TV in) and the output virtual transducer (TV out), with Δr out = r out - r in, Δz out is the component along a depth axis Z of the spatial position vector Δr out. said wavefront image comprising a focal spot, - a step of determining a center of the focal spot in the wavefront image, and determining the depthwise position in the direction of axis Z of said focal spot, said depthwise position being noted Δz ( r in)and - a step of calculating an integrated speed of sound c ( r in), based on the following formula: c 1 r in = c 0 1 + Δ z 0 r in z in