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The Sleipner storage site hosts one of the most complex CO2 plumes in the world. The geological characteristics of the aquifer unit pose several challenges that complicate the understanding of vertical CO2 migration and its forecasting through reservoir modeling. In this study, we apply Full-Waveform Inversion (FWI) up to 70 Hz using an Ocean-Bottom Node (OBN) seismic survey acquired in 2023 over the Sleipner storage site. The inversion reveals the complex 3D structure of the CO2 plume, including vertical migration pathways, channels, and fairways that control both vertical and lateral CO2 migration at the site. By comparing our inversion results with high-quality FWI results derived from a towed-streamer (TS) dataset acquired in 2010, we analyze the evolution of CO2 layers and migration pathways to improve understanding of vertical CO2 migration at the site. Our results suggest that FWI can be applied, either repeatedly, for CO2 plume monitoring, or as a single 3D application to map vertical migration features that have the potential to improve future CO2 storage forecasting and conformance assurance.