This thesis investigates velocity model estimation and optimized imaging of conventional seismic data from the North Sea. Seismic velocity analysis is considered to be a central prerequisite to the main processing and imaging techniques used today. In order to produce a well-resolved image of subsurface structures, a high-quality velocity model is needed. Even though numerous studies have been conducted to obtain knowledge regarding the physical parameters of the North Sea reservoir rocks, there are still many unknowns to be resolved. In the Danish sector of the North Sea, the Chalk Group comprises important oil and gas reservoirs. By application of well-established conventional velocity analysis methods and high-quality diffraction imaging techniques, this study aims to increase the resolution and the image quality of the seismic data.
In order to analyze seismic wave propagation in a multilayered medium, 2D finite difference forward modeling was applied. The rock physical properties used for the numerical calculations are based on an exploration well in the Halfdan Field. The synthetic modeling results and field data were utilized to investigate the performance of seismic velocity analysis based on the conventional NMO corrections of CMP gathers. Our results reveal how the interference caused by multiples, converted waves, and influence of thin-bed can lead to mis-picking of the velocity model, if such effects are not accounted for. Picking the incorrect velocity values can introduce significant errors in the thickness and depth estimations of the layers. Standard processing techniques such as front muting and bandpass filtering cannot resolve the problem.
High-quality seismic imaging relies on valuable information from geological discontinuities such as faults, fractures, pinch-outs and salt delineations, which can be extracted from the diffractions. The potential of diffraction imaging techniques was studied for 2D seismic stacked data from the North Sea. In this approach, the applied plane-wave destruction method was successful in order to suppress the reflections from the stacked data. The optimized migration velocity values estimated from the isolated diffractions were used to image the stacked seismic data. The application of the diffraction imaging techniques provides better-resolved migration velocity models, which consequently leads to higher quality seismic images. This improved seismic imaging is demonstrated for a salt structure as well as for Overpressured Shale structures and the Top Chalk of the North Sea.