An integrated procedure for migration velocity analysis in complex structures of thrust belts

Abstract

Velocity analysis is a key part in seismic depth imaging because it has a direct effect on the quality of the depth-migrated seismic images. In complex structures, lateral velocity variations and complicated geometry of structures make the velocity analysis a challenging task. A variety of techniques using pre-stack seismic data exists for migration velocity analysis, including tomographic inversion methods which update velocity globally and are more efficient for complex situations. I present a procedure for migration velocity analysis in complex fault-folded structures using two types of depth tomography: cell-based tomography and layer-based tomography. The proposed method has 4 main stages including initial velocity model generation, large wavelength (regional) velocity update, short wavelength velocity update and final model generation. Pre-stack depth migration has been used to measure the quality of the estimated velocity at each stage. Relevant a priori information has been proposed for each stage to constrain the velocity analysis. The velocity analysis starts from regional updates and general structural framework reconstruction, then deals with complex structures in detail and finally finishes with a high-resolution velocity field that is consistent with the complex geometry as well as the rock types. Knowledge of structural style, well-derived information including logs, vertical seismic profiling data and the geologic markers together with detailed seismic interpretation are the main sources of information to control the inversion process of migration velocity analysis and ensure convergence to the correct answer. At the second and third stages of the proposed procedure, cell-based tomography is applied and at the last stage horizon-based tomography has been applied. The method has been tested on a real 3D dataset from the Iranian Zagros fold-thrust belt with strong topographic variations. The area has structural complexity including different deformation styles at different depth levels, fault-related folds, and various thicknesses of incompetent rock types such as salt and thick layers of competent rocks such as carbonates. The results of velocity analysis are completely consistent with the structural style of the area. It includes more detail about the rock types. The derived velocity model shows deformed incompetent layers in the shallow section and dependency of carbonate velocity on depositional environment at deeper levels. The final velocity model is structurally consistent and provides better focusing of the seismic images.