A method for simultaneously inverting PP and PS seismic amplitudes in native time domains with optimized registration through travel times' estimations, the method comprising: obtaining PP and PS seismic data acquired during a seismic survey of a multi-layer structure; and jointly inverting the PP and the PS seismic data in a stratigraphic grid, using different time axes for PP and PS reflections, wherein a ratio of PP and of PS waves travel times inside a same layer cell is a function of a ratio of a P-wave propagation velocity (Vp) and of an S-wave propagation velocity (Vs) therein, wherein for the PP waves travel time inside a layer, an incident and a reflected P-wave have same P-wave propagation velocity and an incident angle substantially equal to a P-reflection angle, while, for PS waves travel time inside the same layer, the incident P-wave and a reflected S-wave have different propagation velocities, V p >V s , and an S-reflection angle different from the incident angle, and further wherein the jointly inverting includes: producing test models of geophysical properties inside the multi-layer structure by perturbing a previous model according to predetermined rules; generating synthetic data corresponding to the PP and to the PS seismic data, using each of the test models; and selecting one of the test models that minimizes a cost function depending on differences in amplitudes and travel times between the seismic data and the synthetic data. obtaining PP and PS seismic data acquired during a seismic survey of a multi-layer structure; and jointly inverting the PP and the PS seismic data in a stratigraphic grid, using different time axes for PP and PS reflections, wherein a ratio of PP and of PS waves travel times inside a same layer cell is a function of a ratio of a P-wave propagation velocity (Vp) and of an S-wave propagation velocity (Vs) therein, wherein for the PP waves travel time inside a layer, an incident and a reflected P-wave have same P-wave propagation velocity and an incident angle substantially equal to a P-reflection angle, while, for PS waves travel time inside the same layer, the incident P-wave and a reflected S-wave have different propagation velocities, V p >V s , and an S-reflection angle different from the incident angle, and further wherein the jointly inverting includes: producing test models of geophysical properties inside the multi-layer structure by perturbing a previous model according to predetermined rules; generating synthetic data corresponding to the PP and to the PS seismic data, using each of the test models; and selecting one of the test models that minimizes a cost function depending on differences in amplitudes and travel times between the seismic data and the synthetic data. producing test models of geophysical properties inside the multi-layer structure by perturbing a previous model according to predetermined rules; generating synthetic data corresponding to the PP and to the PS seismic data, using each of the test models; and selecting one of the test models that minimizes a cost function depending on differences in amplitudes and travel times between the seismic data and the synthetic data. <EOS>