Sea Bed Diffraction and Impact on 4D Seismic Data – Observations from Synthetic Modelling and Field Data

The water layer represents a major source for non-repeatability in 4D seismic processing. In this study we have analyzed the effect of seabed 4D timeshifts due to changes in water velocity and 4D seismic acquisition mis-position for the case when diffractions are positioned at seabed. Real 4D seismic data from the Norne Field and synthetic data using a single point diffractor at seabed model are used in this study. We find that, in addition to tidal correction, timeshift correction should include water velocity correction and relative strength and position of the seabed diffraction observed. A main result from the analysis shows that 4D seismic acquisition mis-position adds non-negligible timeshifts close to the seabed diffraction apex point.