Carbon capture and storage projects in deep reservoirs have
  significant monitoring obligations associated with regulatory needs  and social licence to operate. Downhole methods are becoming increasingly attractive because of their low surface footprint and ease of stakeholder engagement. When several wells are available, monitoring from VSP-based seismic methods is possible using downholeDAS fibre optics, and even lower footprint seismic methods are  possible using surface orbital vibrators combined with DAS.  Crosswell pressure tomography (PT) monitoring is also possible using test water injections in completions in the reservoir interval and downhole pressure gauges.  Monitoring confidence is enhanced when  all monitoring data can be explained with a single model or  range of models, which requires joint Bayesian inversion. However, simultaneous inversion of time-lapse seismic data and PT data is  generally a difficult problem involving coupled multi-physics  modelling. In the CCS context, highly buoyant and thin gas plumes make the problem susceptible to effective approximations that enable decoupling of the physics, and thereby a cascaded Bayesian inversion  for both seismic amplitudes and crosswell pressures. We show inversions for the Otway stage 3 CCS demonstration project using these ideas.