Forward Kirchhoff Modelling to Evaluate Potential Interpretation Pitfalls beneath Sub-vertical Velocity Discontinuities

Abstract

Summary Sharp velocity contrasts in the overburden may cause problems for seismic imaging of deeper targets. Pre-stack depth migration in combination with traveltime tomography to build the velocity model is generally considered as a solution to the problem. However, building the velocity model with sufficient lateral and vertical resolution near discontinuities is difficult and imperfect velocity models will result in imperfect seismic images. Forward modelling seismic data followed by PSDM is a way to investigate and quantify sensitivity to imperfections in the velocity model. Kirchhoff modelling offers an efficient alternative to full-wavefield modelling; it is based on advanced ray tracing and can model diffractions, but is still computationally quick. In this study Kirchhoff pre-stack modelling provides a means for testing the impact of velocity errors on the final seismic image: Kirchoff modelling is followed by Kirchhoff target migration with the model velocity field and with degraded velocity fields. Results reveal the scale of potential imaging errors under a shallow velocity discontinuity at the Golden Eagle field in the North Sea. Results show that the danger of creating false structure due to image distortion is low, but that vertical positioning due to inaccurate velocity models carries a higher risk.