Preserved-amplitude Seimic Migration Imaging Method Based On Wave Equation

Seismic data contain rich travel-time and amplitude information. Imaging precision of the wave-equation prestack depth migration based on advanced wave theory for complex media is so good that it is recommended widely, especially when the structures are intricate or when the velocities of media vary intensely in vertical or lateral. Wave equation preserved-amplitude prestack depth migration has a special function that can give true amplitudes as well as the correct locations. This method not only can make scattering energy be focused and migrated to the correct position to improve imaging accuracy, but also can export the amplitude information which reflects the correct subsurface reflection coefficients. So this method can provide more real seismic information for following attribute analysis, such as AVO/AVA.One-way wave equation is an important tool for studying the propagation modeling and migration imaging of seismic wave in complex media. This migration method using one-way wave equation to continue wavefield has been used widely in prestack depth migration of multidimensional seismic data. The traditional method of wave-equation prestack depth migration decomposes full acoustic wave equation into simple one-way wave equation to continue upward wavefield and downward wavefield downwards. This one-way wave equation is accurate in kinematics and can ensure the correctness of phases to satisfy tectonic imaging. But it ignores that amplitudes of seismic waves can be reconstructed by the parameters variation of media. It only has the ability to preserve amplitudes relatively and cannot describe the dynamics characteristics of seismic wave's propagation exactly. In this paper, we derive a high efficient and direct-viewing one-way wave equation. It can continue the pressure components of seismic wave propagating wavefield directly from full acoustic wave equation using one-way wave's preserved-amplitude decomposition based on more strict decoupling theory. This one-way wave equation composed of transmit term and scatter term can act as wavefield propagation operators of preserved-amplitude prestack depth migration. This one-way wave equation fits eikonal equation and transport equation corresponding to full acoustic wave equation in travel-time and first order amplitude. According to the perturbation theory often used in inverse solution, we split the velocity field into intraformational constant velocity background and variable velocity disturbance. Then we calculate time-shift quantity of migration and amplitude correction coefficients for wavefield depth continuation in total uniform formation and each formation. So we obtain a dual-domain preserved-amplitude prestack depth migration operator equation based on the method of Fourier finite-difference.In this paper, we investigate and modify ground boundary conditions and imaging conditions used in the traditional method of wave-equation prestack depth migration to take amplitude compensation into account in modified boundary conditions and imaging equations. The modified equations and the preserved amplitude one-way wave-equation can compensate for amplitudes affected by geometric spreading loss and incidence angle variations from three aspects above.Wave-equation prestack depth migration based on double-square-root (DSR) equation is another wave-equation migration method used widely. It plays an important role in seismic migration methods because of its advantages such as less aliasing, simple boundary process, needless to derive source wavelet, regardless of migration aperture and high computation efficiency. In this paper, beginning with the operator of preserved single-square-root equation, we achieve the formula transformation from common-shot domain to common-middle-point domain according to the theory of wavefield double downward continuation in prestack depth migration. Then we derive preserved-amplitude one-way wave equation defined by DSR equation.Prestack migration and AVO analysis are two important technologies in seismic exploration. Preserved-amplitude wave equation prestack depth migration based on more advanced seismic wave theory is an accurate imaging method with the clearest geological significance. It can eliminate the influence of seismic wave amplitudes caused by media propagation and export preserved-amplitude CIGs (common imaging point gathers) that have been focused and migrated correctly. So we can establish the relationship between seismic data processor and interpreter. AVO analysis technique directly predicts reservoir and estimates crust lithologic parameters using subsurface lithologic character and pore fluid character reflected by the rule of amplitude variation with offset. So it establishes the relationship for interpreters and exploration engineers. The link of them provides a theoretical basis for lithologic imaging and inversion. It can also provide powerful guarantee for oil exploratory development and Chinese energy resources stratagem.