Uncertainties in the 4D seismic signature due to seawater velocity variations

Seawater velocity variations are known to influence the quality of seismic, especially in deepwater areas. Here, this topic is extended to evaluate the effect on reservoir monitoring. A 3D data set from the Gulf of Mexico has been corrected for seawater velocity variations and the corresponding time and amplitude perturbations are measured. From this, a 4D synthetic data set is modeled for realistic reservoir production scenarios. The 4D modeled seismic is perturbed using similar errors to those measured on the real data in order to mimic realistic effects due to seawater velocity variations. Differences between the perturbed and unperturbed 4D volumes are assessed in order to quantify the impact of seawater velocity variations on various reservoir monitoring attributes. Large uncertainties on AVO intercept and gradient are revealed (up to 25 %), highlighting possible ambiguities in the interpretation of time-lapse AVO-derived parameters such as pressure and saturation.