Joint full waveform inversion for time-lapse 4D seismic monitoring

Full waveform inversion (FWI) can be applied to time-lapse (4D) seismic data for subsurface reservoir monitoring. Usually, baseline and monitor velocity models are respectively built through a data fitting process, and the difference of models are used to estimate the time-lapse changes. However, large-scale (or low-wavenumber) velocity structures of deep reservoirs are mainly obtained by fitting the diving waves, which can be unavailable due to limited source-receiver offset apertures. Alternatively, joint full waveform inversion (JFWI) assumes a scale separation in the model space such that low wavenumbers can be recovered from both diving and reflected waves, in addition to high-wavenumber impedance perturbations. We extend JFWI to time-lapse data and improve the sensitivity of waveform inversion approaches to low-wavenumber velocity changes of deep targets. A central-difference strategy is also implemented (CD JFWI), which averages four JFWI velocity models (two baseline and two monitor) to attenuate false changes in the 4D velocity estimate, and which increases the stability of the approach in data fitting. The synthetic 4D SEAM case study shows that CD JFWI followed by 4D FWI leads to a more accurate estimate and better resolution of velocity changes than conventional 4D FWI methods in a reservoir zone that is not penetrated by diving waves.