1. An imaging method using shear waves for observing a viscoelastic medium, comprising: several successive excitation steps j during which elastic shear waves are generated respectively at different excitation loci in the viscoelastic medium by an imaging device, said different excitation loci being separated from one another by a maximum distance Dm, an imaging step corresponding to each excitation step j, in which a set j of successive raw images Im j (t k ) of the viscoelastic medium at times t k are determined by said imaging device during propagation of the shear wave in said viscoelastic medium, said raw images having a resolution R which is at least equal to said maximum distance Dm, t k being a relative time counted from generation of the corresponding shear wave, an averaging step in which raw images Im j (t k ) corresponding to the same relative time t k are averaged to determine an average image Im′(t k ) corresponding to said relative time t k .