Study On Anisotropic Parameter Model Building And Reverse Time Migration In TTI-ORT Media

Anisotropic seismic velocity model building and imaging is significant for accurate imaging of subsurface media.Currently,as the rapid development of computing capability and wide-azimuth or full-azimuth seismic data acquisition,anisotropy should be considered into the imaging workflow.Reverse time migration(RTM)is based on the two-way wave equation with precise numerical solutions,thus,compared with other imaging methods,RTM has the advantages of no limiting in subsurface dips,correct imaging of turning wave and multiples with precise imaging velocity,and so on.Scalar wave equation is sufficient for wavefield simulating in isotropic media.However,P and SV wave in anisotropic media are coupled which means that there is no pure scalar wave in anisotropic media.Commonly,quasi-P(qP)wave which has the same kinematics with the P wave component was used for anisotropic imaging.In this paper,we proposed a set of anisotropic velocity model building method based on well log data and full-azimuth reflection angle gathers.Firstly,full-azimuth reflection angle gathers are extracted using full-azimuth common reflection angle migration method,and the accurate azimuthal anisotropic residual moveout(RMO)can be acquired based on the full-azimuth reflection angle gathers.Then,local anisotropic parameters at well positions are extracted using well log data.At last,the angle domain grid tomography technique is used for iterating inversion of anisotropic parameters,and building accurate anisotropic velocity model.The proposed method was tested on field seismic data,and showed good results in fractures detecting and structure imaging,which proved the applicability and effectiveness of the method.At the same time,qP wave equation system were derived based on the Christoffel equation and starting from phase velocity and dispersion relation,and simplified wave equation was derived by setting the shear wave velocity in the direction of anisotropic symmetry axis as 0 in the qP wave equation system.The simplified wave equation can be solved with much higher efficiency,and can produce stable wavefield in vertical transversely isotropic(VTI)media with criteria ?-? ?0.However,the wavefield produced by the simplified wave equation was contaminated with pseudo-S waves.Thus,the improved wavefield correction method based on the elliptic background perturbation field was proposed to solve the amplitude-preserving problem of traditional correction method.Numerical examples proved the effectiveness of the proposed method.Reverse Time Migration(RTM)are implemented in Hess VTI model and field data.The imaging results shows that,VTI RTM can get better images than isotropic RTM for the imaging of salt body,sub-salt structures and faults with steep dips.And for field data example,the relation of seismic and well log data of the VTI RTM images are much more reasonable than isotropic RTM,and the imaging accuracy are improved significantly.If the proposed wave equation system was generalized to TTI(Tilted Transversely Isotropic)media,numerical stabilities occurred in model with complex velocity and anisotropic parameters.To solve the stabilities problem of the qP wave equation for TTI media,new second-order quasi-P wave equation for TTI media was derived starting from the basic Hook's Law and equation of motion.The newly proposed equation can be solved using explicit finite difference scheme.By acoustic approximation,shear wave velocity in the direction of symmetry axis was set to 0,and for model with the same symmetry axis and ? ??,stable numerical solutions can be acquired.But for TTI media,as the anisotropic parameters are varied with the direction of symmetry axis,acoustic approximation will cause the instabilities of wavefield propagating and numerical solutions.This kind of instability was mainly caused by the space derivations of the varying anisotropic symmetry axis,and featured as weak instability with linear growing in time.Thus,the regularized finite-S-wave equation was proposed to solve the instabilities.In the end,the proposed equation was tested with Foothill TTI model and field data,and the results shows that traditional finite-S-wave equation comes up with simulating instabilities with increasing propagating steps.However,the newly proposed regularized wave equation attenuated the high wave number component,and made the whole wavefield calculating stable and the final images better than traditional finite-S-wave equation.We also preliminarily studied the full-azimuth anisotropic model building and imaging for Orthorhombic(ORT)media and pure quasi-P wave propagating operator for TTI media.The proposed solutions can get good images even in 3D TTI field data.For model areas with abrupt changes,the above-mentioned method still generates SV wave.For P-wave migration,those SV-waves are considered as noises.But for migration with field data,the contaminated SV-wave energy was not visible in the final RTM volume,and this could be the reason that SV-wave energy was much weak than the P-wave energy.The numerical examples of stable wavefield simulating and high-quality RTM images using the proposed wave equation in complex TTI model show the accuracy of the method and the applicability in field data.