A method of imaging a target area of the subsoil from “walkaway” data having application to development of oil reservoirs or monitoring of geologic storage sites is disclosed. After acquisition of seismic data of walkaway type and estimation of the rate of propagation of the seismic waves in the subsoil, p illumination angles are selected. The seismic measurements are then converted to data Dp by illumination angle. The distribution of acoustic impedances best explaining data Dp is determined within the target by using a non-linear inversion which minimizes a difference between the data Dp obtained from measurements and data by illumination angle resulting from an estimation. This estimation is performed by solving a wave propagation equation from the velocity field, an acoustic impedance distribution and a pressure distribution at the level of the top of the target for each illumination angle.