An Evaluation of the Effect of VSP and Surface Acquisition Geometries on 4D Velocity Estimates

Time-lapse (4D) seismic monitoring is a key element in reservoir management. In a marine context for localized targets, instrumented wells (VSP) and ocean bottom sensors (OBN/OBC) are common acquisitions for time-lapse monitoring. In this study, we analyze the impact of VSP and OBN acquisition geometries on estimated 4D velocity anomalies. For this purpose, we designed a synthetic experiment with 4D surface and VSP acquisitions (baseline and monitor) directly inspired by a real study. The RTM migrated image obtained from surface data shows well-defined and continuous reflectors, while the migrated image obtained from VSP data suffers from significant portions of blurred or missing reflectors. From these migrated images, 4D velocity anomalies were estimated using the “Lapseis” method (wave-equation generalization of 4D warping approaches). While the anomaly obtained from the surface data agrees relatively well with the true anomaly, the estimate obtained from the VSP data is less accurate (more pronounced wobbling effect and poorly resolved at locations away from the well). This confirms that poorly illuminated areas in VSP migrated images negatively affect the quality of estimated 4D anomalies. Therefore, improving and quantifying the quality of migrated images is critical to improving the quality and reliability of quantitative 4D monitoring results.