Uncertainties in depth imaging: A multitechnique approach on real data

Abstract

A Pre-stack and post-stack depth migration study was performed on one line from offshore Vietnam. The purpose of the study was to determine the position of a fault and the associated uncertainty to guide the drilling. Using the velocity field obtained with the Finite Differences (FD) pre-stack migration, the authors show by running a post-stack depth migration velocity scanning that the normal fault can be interpreted either by the markers terminations or by the seismic response of the fault itself. Then an additional criterion for an accurate migration was the coincidence of the fault plane response and the terminations of the markers. To evaluate the uncertainty associated with the lateral position of the fault they run several pre-stack trials by modifying the velocity field in the poorly resolved area in order to have a perfect fit between the fault trace and the terminations of the markers. The maximum lateral displacement of the fault plane was 100 meters. Depth estimates may also be altered in case of differences between vertical and horizontal velocities (``apparent anisotropy``). This results in an uncertainty on the lateral position of the fault of the order of 300 meters. An a posteriori control will well data showedmore » that their estimate on the apparent anisotropy sign was not correct, and they show by using other tools (Kirchhoff pre-stack depth migration and travel time inversion) that the misfit is related to the seismic data.« less