| Pre-stack depth migration(PSDM)is the mostly accepted method for imaging the complex structure because it can accurately depict the propagation of seismic waves in complex subsurface situation.In PDSM,the strong coupling between depth and velocity makes the migration method relies too heavily on velocity.The uncertainty of the velocity and depth makes it difficult of PSDM method to have a best imaging section,which has immediate impact on the final result of each link in seismic exploration.Therefore,the estimation of the migration velocity is an essential step.The commonly used methods of MVA mainly include the Residual Curvature Analysis(RCA),Wave Equation Migration Velocity Analysis(WEMVA),travel-time tomography and Full Waveform Inversion(FWI).RCA based MVA methods and travel-time tomography require picking curvature.However,it is difficult or failed to pick for complex structures.FWI provides highest accuracy of inversion.When there is lack of low frequency information or the initial velocity model is far from the true velocity,FWI is easily trapped in local minima because the “half wavelength” condition is violated,in that case,FWI may lead to an unstable inversion resultIn consideration of the shortcomings mentioned above,the thesis update velocity Differential Semblance Optimization(DSO)as the criterion,aiming at the best quality of image gathers.Firstly,given an initial velocity and recorded data,we can obtain Common Image Gathers(CIGs)by using extended imaging condition.Then we build the objective function with CIGs and conduct i nversion by gradient method,making the divergent CIGs gradually convergent.When the image gathers reach best convergence,the inversed velocity can be considered accurate background velocity.The thesis firstly introduced the numerical modeling and migration based on wave equation in Chapter 2,then I studied the common image gather and it’s extraction method in Chapter 3.These are the basis and premises for WEMVA study.In Chapter 4 I took systematic studies on the principle and algorithm of DSO-based WEMVA.Model test is given to verify the effectiveness and reasonability of the proposed method,and further analysis and discussion were carried through at the end of Chapter 4. |
