Study On True-amplitude Elastic Reverse Time Migration For Isotropic Media

Since elastic reverse time migration(ERTM)uses two-way wave equations to reconstruct the source and receiver wavefields,it can take full advantage of the subsurface elastic information and provide multi-wave images.What’s more,true-amplitude ERTM can provide relatively amplitude-preserved multi-wave images,which are important for the following inversion and interpretation.So,based on the framework of ERTM,we make the detailed analysis and research of the amplitude preservation property of wavefield separation and imaging conditions.Meanwhile,we also make an exploration of the angle-domain true-amplitude ERTM.In this paper,we first give the brief introduction and analysis of the major steps of ERTM,including reconstruction strategies of the source and receiver wavefields,wavefield separation methods,imaging conditions and low-wavenumber noise suppression methods.For the wavefield separation issues,we analyze the influence of the conventional wavefield separation method on the phase and amplitude and provide the true-amplitude wavefield separation method in the wavenumber domain;as the particle velocity components used in the wavenumber-domain wavefield separation are not defined at the same grid point with the staggered-grid finite-difference method for elastic wavefield simulation,we propose the wavenumber-domain interpolation method to estimate the required values at the common grid point prior to the wavenumber-domain true-amplitude wavefield separation.Moreover,numerical experiments show that the wavenumber-domain interpolation method has high interpolation accuracy and the true-amplitude wavefield separation method shows good amplitude preservation,and further application of the proposed method to ERTM can obtain well amplitude-preserved images even in the case of some velocity error or weak anisotropy.For the imaging condition issues,we provide the polarity-consistent excitation amplitude imaging condition by properly combining the well amplitude-preserved and highly-efficient excitation amplitude imaging condition with a shot-domain polarity reversal correction method.By utilizing the relatively stable and well amplitude-preserved source-normalized crosscorrelation imaging condition as a comparison,we testify to the feasibility and validity of the proposed imaging condition in the aspects of amplitude preservation property,imaging capability of complex structures,storage cost and computational efficiency.For the angle domain issues,we make an exploration of the elastic case based on the inspiration of the angle-domain true-amplitude acoustic RTM.By combing the modified wavefield reconstruction strategy,true-amplitude wavefield separation method,extended imaging condition and angle decomposition method,we present a procedure to obtain relatively amplitude-preserved angle-domain common image gathers(ADCIGs)of the PP and PS waves.Numerical examples demonstrate that the ADCIGs are not only kinematically correct but also relatively amplitude-preserved.Finally,we make a summary of the true-amplitude ERTM methods and make a prediction of the following research subjects.