3D and time-lapse (4D) seismic imaging is central to the exploration for, and the monitoring of, hydrocarbon reservoirs and CO2 sequestration projects on WA’s Northwest Shelf. The main imaging kernel involves modelling seismic waves through a 3D velocity model. Generating usable seismic images requires repeating this modelling 1000s of times, necessitating the use of HPC facilities. A key challenge in 4D seismic monitoring is that the changes introduced by gas extraction/injection either can be very subtle (gas production) or extremely complex (CO2 sequestration). These require novel analysis methods capable of detecting small signals in, or handling strong complexity of, recorded seismic wavefields. We have developed a GPU-based 3D and 4D imaging framework capable of handling these situations, which has resulted in more detailed maps of subsurface change that provide extra information for better operational decisions in gas extraction or CO2 sequestration projects.