The Extraction Of Angle-domain Common Imaging Gathers On Complex Geologic Structure

The wave equation prestack depth migration is a powerful tool to solve complexstructural imaging. Sensitive to its image quality is dependent on the migration velocitymodel, so to get the right image, we must first do the migration velocity analysis. Angledomain common image gathers (ADCIGs) is considered to be the most reasonable gathersfor migration velocity analysis of a variety of common image gathers.Reflections from steep interfaces are important for velocity analysis. Most wavesilluminating reflectors with gentle dip travel in a direction that is less than30。from the vertical.Therefore, seismic reflection tomography is a limited-angle tomography, an ill posed problem.Since most waves illuminating near-horizontal reflectors travel almost vertically, the verticalresolution in seismic reflection tomography is limited. In contrast, most waves illuminatingsteep reflectors have a portion of wavepath that is nearly horizontal. Therefore using ADCIGsof steep reflectors improves the angle coverage of rays in tomography, thus reducing theproblem of poor conditioning and improving the stability.The conventional one-way wave equation has been routinely applied in the industry. It isdifficult, however, to obtain reliable ADCIGs of steep reflectors by conventional one-waywave-equation migration. First, steep reflectors are mainly illuminated by near-horizontal andoverturned waves, which cannot be modeled accurately by the one-way wave-equationextrapolator in vertical Cartesian coordinates. Second, we cannot obtain reliable ADCIGsfrom horizontal-offset-domain CIGs, because given an opening angle, the steeper the reflector,the larger the horizontal subsurface offset is needed to get a reliable ADCIG.s。Plane-wave can be generated by slant stack. This plane wave decomposition can beapplied to one-way wave equation migration, because the advantage of the direction forpropagation of the plane wave gathers. We can make the vertical Cartesian coordinate systemrotating a certain angle to get a new system: the titled coordinate system. By doing wave extrapolation in this new system, the propagation direction is close to the extrapolationdirection, so the waves from steep reflectors can be accurately propagated by plane-wavemigration in titled coordinates.Plane wave migration in tilted coordinates can extract the horizontal and verticaldirection angle domain common imaging gathers respectively. And by applying a dipinformation for both weighted, we can achieve an accurate angle domain common imaginggathers for steep reflectors. Numerical model and real data test have show that the angledomain common image gathers can improve the stability. This new angle domain gather canprovide more accurate datas for velocity analysis, AVO or AVA analysis.