Reliable quantification of carbon dioxide () properties and saturation is crucial in the monitoring of underground storage projects. We have focused on quantitative seismic characterization of at the Sleipner storage pilot site. We evaluate a methodology combining high-resolution seismic waveform tomography, with uncertainty quantification and rock physics inversion. We use full-waveform inversion (FWI) to provide high-resolution estimates of P-wave velocity and perform an evaluation of the reliability of the derived model based on posterior covariance matrix analysis. To get realistic estimates of saturation, we implement advanced rock physics models taking into account effective fluid phase theory and patchy saturation. We determine through sensitivity tests that the estimation of saturation is possible even when using only the P-wave velocity as input. After a characterization of rock frame properties based on log data prior to the injection at Sleipner, we apply our two-step methodology. The FWI result provides clear indications of the injected plume being observed as low-velocity zones corresponding to thin filled layers. Several tests, varying the rock physics model and properties, are then performed to estimate saturation. The results suggest saturations reaching 30%–35% in the thin sand layers and up to 75% when patchy mixing is considered. We have carried out a joint estimation of saturation with distribution type and, even if the inversion is not well-constrained due to limited input data, we conclude that the has an intermediate pattern between uniform and patchy mixing, which leads to saturation levels of approximately . It is worth noting that the 2D section used in this work is located 533 m east of the injection point. We also conclude that the joint estimation of properties with saturation is not crucial and consequently that knowing the pressure and temperature state of the reservoir does not prevent reliable estimation of saturation.