CO2 monitoring at Sleipner field using reflection oriented full waveform inversion: part 2-4D investigation

In this work, we apply a dedicated 4-D full waveform inversion workflow to short offset streamer data from the Sleipner CO$_2$ storage field in the North Sea. We consider a baseline data set acquired in 1994 and a monitor data set acquired in 2008. Accessing to only short offset data raises significant difficulties for full waveform inversion. In this case the penetration of diving waves, which controls the depth where quantitative updates of the velocity can be expected, do not reach the zone of interest where the CO$_2$ is injected. For this reason, we propose to combine an efficient time-lapse full waveform inversion strategy, which we call simultaneous, with a reflection oriented full waveform inversion workflow. The latter has been introduced in the literature as a way to circumvent short-offset limitation and increase the ability of full waveform inversion to update the macro-velocity model at depth by exploiting the reflection paths, using a prior step of impedance reconstruction. We first illustrate the interest of this combined strategy on a 2-D synthetic model inspired from the Sleipner area. Then we apply it to the Sleipner field data, using as baseline model the one we present in a companion paper, where our reflection oriented workflow is presented. Our combined approach yields reliable estimates of the changes due to the CO$_2$ injection, characterized by velocity reductions of up to 400 m s$<^>{-1}$ and strong impedance contrasts at depths of 800-1000 m, which consistent with previous full waveform inversion studies. Furthermore, the spatial distribution of CO$_2$ changes aligns with conventional seismic time-migration results from earlier studies, following a north-south migration trend.