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Anisotropic elastic least-squares reverse time migration with density variations in vertical transverse isotropic media

Zhong Yu, Gu Hanming, Liu Yangting, Luo Xia, Mao Qinghui, Xu Kai

Acta Geophysica

2024

Vol. 72, no. 1

67--83

Elastic least-squares reverse time migration (ELSRTM) has the potential to provide higher-quality migration images related to the lithology and fluid by imaging multi-component seismic data than conventional elastic reverse time migration (ERTM). Oil and gas are widely stored in fractures and sedimentary rocks. The sedimentary rocks and the rocks with fractures will produce anisotropy. The anisotropy effect should be corrected in migration. In order to correct the anisotropic effect to the images of ELSRTM, a new anisotropic ELSRTM scheme is developed to image the multi-component seismic data in vertical transverse isotropic (VTI) media. This new ELSRTM method can invert high-quality images and correct the anisotropic effect in VTI media. Many ELSRTM methods assume that the density is constant. However, the constant-density assumption will generate false migration results when the density of media is variation. We derive the elastic VTI de-migration operator in the media with density variations based on Born approximation. The adjoint state equations and gradient formulas with respect to medium images in VTI media with density variations are also derived by the adjoint state method. Using the new elastic de-migration operator, adjoint state equations, and gradients in VTI media with density variations, we can produce high-resolution subsurface elastic reflectivity images. Numerical examples from the graben VTI model and modified HESS VTI model demonstrate that the proposed ELSRTM can not only generate the images with high quality but also correct the anisotropic effect in VTI media with density variations.

A new elastic least-squares reverse time-migration method based on the new gradient equations

Zhong Yu, Liu Yangting, Gu Hanming, Mao Qinghui

Acta Geophysica

2022

Vol. 70, no 6

2733--2746

Compared with single-component seismic data, multicomponent seismic data contain more P- and S-wave information. Making full use of multicomponent seismic data can improve the accuracy of seismic exploration. Elastic reverse-time migration (ERTM) is the most advanced migration technology for imaging multicomponent seismic data from complex subsurface structures. However, most conventional ERTM methods often use the adjoint operator of forward operator for approximation to the inverse operator. When the multicomponent seismic data suffer from a finite recording aperture, limited bandwidth, and imperfect illumination, the image quality of conventional ERTM is greatly reduced. In this study, we propose an elastic least-squares reverse-time migration (ELSRTM) scheme to improve the image quality of ERTM through multiple iterations. We first review the ERTM method; then, we derive the Born modeling equations, adjoint wave equations, and gradient equations of P- and S-wave images of ELSRTM. The new gradient equations, which use the time derivative of stress to replace the spatial derivative of particle velocity for improving the accuracy of gradients near the boundary, are also proposed. We compare the performance of ERTM with ELSRTM via synthetic experiments in numerical examples. Synthetic examples reveal that ELSRTM can generate high-quality images with higher resolution, fewer artifacts, and more balanced amplitude than ERTM.