Unlocking subsalt reservoir time-lapse insights with 4D elastic FWI imaging in Atlantis field

The quest for a clear subsalt 4D (time-lapse) signal in Atlantis field has been a paramount goal over the past decade. However, two well-known issues have hindered previous 4D processing efforts: velocity errors in the complex overburden salt bodies and weak, imbalanced illumination in subsalt areas. Recent advances in full-waveform inversion (FWI) algorithms combined with the Atlantis full-azimuth and long-offset OBN data have significantly improved the accuracy of salt velocity models and subsalt images. However, challenges in subsalt 4D from reverse time migration (RTM) remains due to poor illumination, and the improvements in 4D RTM from the FWI velocity uplifts have not been sufficient to fully reveal subsalt 4D signals in Atlantis. To address these challenges, more advanced imaging algorithms are needed. In recent years, FWI imaging, particularly elastic FWI imaging, has demonstrated its superiority in subsalt 3D imaging in various fields including Atlantis, resulting in significantly improved illumination, signal-to-noise ratio (S/N), and focusing compared to conventional methods. This made extending elastic FWI imaging to 4D a promising solution for solving the subsalt 4D imaging challenges. 4D elastic FWI imaging was successfully applied in Atlantis field with remarkable results, revealing for the first time in Atlantis production history, reliable subsalt 4D signals.