Wave-equation Migration Velocity Analysis By Wavefield Extrapolation

Estimating an accurate velocity model is crucial for seismic imaging to obtain a good understanding of the subsurface structure.The objective of this thesis is to investigate methods of velocity analysis by optimizing seismic images.This method formulate wave-equation MVA with an operator based on linearization of wavefield extrapolation using the first-order Born approximation.It defines the optimization objective function in the space of migrated images,in contrast with wave-equation tomography with objective function defined in the space of recorded data.Since the entire images are sensitive to migration velocities,this method uses image perturbations for optimization,in contrast with traveltime tomography which employs traveltime perturbations picked at selected locations.This method constructs image perturbations with residual migration operators by measuring flatness of angle-domain common image gathers,or by measuring spatial focusing of diffracted energy.In this thesis,we gave a method of image perturbation basd on prestack residual migradion.A conventional seismic image is obtained by zero-lag crosscorrelation of wavefields extrapolated from a source wavelet and recorded data on the surface using a velocity model.The velocity model provides the kinematic information needed by the imaging algorithm to position the reflectors at correct locations and to focus the image.In complex geology,wave-equation migration is a powerful tool for accurately imaging the earth's interior;the quality of the output image,however,depends on the accuracy of the velocity model.Given such a dependency between the image and model,if give a series of velocity at one depth interval and imaging under different models,to a certain extent,model convergence direction can be get by imaging results.This is the thought of the residual migration.In this thesis,we gave another method of image perturbation basd on extended images.If the nonzero space-and time-lags information are preserved in the crosscorrelation,the output are image hypercube defined as extended images.Compared to the conventional image,the extended images provide a straightforward way to analyze the image quality and to characterize the velocity model accuracy.When the velocity model is inaccurate,the cone shifts along the time-lag axis.When the extended image cones shift,the distance and direction of their apex away from zero time lag constrain model errors.This information can be used to construct an image perturbation,from which a slowness perturbation is inverted under the framework of linearized wave-equation migration velocity analysis.Model trial results show that the inversion of the model can get good results when used to remigraion.