Full Waveform Inversion (FWI) and Controlled Source Electro-Magnetic (CSEM) inversion can provide quantitative estimates of useful subsurface properties. In the context of CO2 monitoring, FWI has the capability to characterize velocity changes induced by CO2 injection, while CSEM can be useful to determine the CO2 saturation. Quantifying the uncertainty of the reconstructed parameters is however challenging. In this study, we assess the reliability of the results obtained using CSEM and FWI when monitoring CO2 storage at the Sleipner pilot in the North Sea. The uncertainty evaluation methodology is described in a first stage. In a second stage, two CO2 monitoring examples are presented, one synthetic case using CSEM and one real data case using FWI. For the synthetic CSEM case, the inversion of the EM data has a clear effect on the uncertainty, reducing the covariance of the probability distribution for possible conductivity models. For the real, rather large-scale FWI case, computationally efficient strategies are considered. The covariance is not reduced to the same extent as for the CSEM case, but an improvement is observed. In both cases, the quantification approach provides a means to assess the quality of the derived models.


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