Elastic Full Waveform Inversion with a Permanent Seismic Source ACROSS: Towards Hydrocarbon Reservoir Monitoring

Time-lapse analysis plays an essential role in EOR (enhanced oil recovery) or CCS (carbon capture and storage) management. However, conventional time-lapse seismic often cannot capture differential signals from the target interval because of near-surface heterogeneity or poor signal-to-noise (S/N) ratio. We discuss a potential application of a permanent seismic source “ACROSS” (Accurately Controlled and Routinely Operated Signal System) that can continuously excite controlled seismic signals over years. The superior repeatability of ACROSS makes it possible to subtract any waveforms that do not change over time, and hence the temporal changes due to the fluid replacement at the target interval should be enhanced. Also, it is possible to consider reverse time migration or full waveform inversion rather than just the first arrival or P-wave reflection traveltime inversion, since ACROSS precisely controls the source signature. Both vertical and horizontal single forces are reconstructed by clockwise and counter-clockwise rotations of an asymmetric cylindrical mass. In our synthetic study, we assume that two ACROSS sources are installed at a CO2 pilot injection project in Ketzin, Germany. Elastic FWI (full waveform inversion) is tested with the assumption that the medium parameters (i.e., P- and S- wave velocities and density) before the injection are known. To prevent crosstalk caused by interference of P- and S-waves, we apply a wave separation technique by extracting the scalar (P-wave component) and vector potential (S-wave component) of the elastic wavefields. The simulation results demonstrate that this approach clearly delineates the P-wave velocity decrease caused by the fluid injection. The high-repeatability of ACROSS enables an application of elastic FWI for residual P-wave velocity, which may bring a breakthrough toward CO2 and hydrocarbon reservoir monitoring.