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Seismic migration and inversion are closely related techniques to portray subsurface images and identify hydrocarbon reservoirs. Seismic migration aims at obtaining structural images of subsurface geologic discontinuities. More specifically, seismic migration estimates the reflectivity function (stacked average reflectivity or pre-stack angle-dependent reflectivity) from seismic reflection data. On the other hand, seismic inversion quantitatively estimates the intrinsic rock properties of subsurface formulations. Such seismic inversion methods are applicable to detect hydrocarbon reservoirs that may exhibit lateral variations in the inverted parameters. Although there exist many differences, pre-stack seismic migration is similar with the first iteration of the general linearized seismic inversion. Usually, seismic migration and inversion techniques assume an acoustic or isotropic elastic medium. Unconventional reservoirs such as shale and tight sand formation have notable anisotropic property. We present a linearized waveform inversion (LWI) scheme for weakly anisotropic elastic media with vertical transversely isotropic (VTI) symmetry. It is based on two-way anisotropic elastic wave equation and simultaneously inverts for the localized perturbations (ΔVp0 =Vp0;ΔVs0 =Vs0;Δe;Δδ) from the longwavelength reference model. Our proposed VTI-elastic LWI is an iterative method that requires a forward and an adjoint operator acting on vectors in each iteration. We derive the forward Born approximation operator by perturbation theory and adjoint operator via adjoint-state method. The inversion has improved the quality of the images and reduces the multi-parameter crosstalk comparing with the adjoint-based images. We have observed that the multi-parameter crosstalk problem is more prominent in the inversion images for Thomsen anisotropy parameters. Especially, the Thomsen parameter d is the most difficult to resolve. We also analyze the multi-parameter crosstalk using scattering radiation patterns. The linearized waveform inversion for VTI-elastic media presented in this article provides quantitative information of the rock properties that has the potential to help identify hydrocarbon reservoirs. [ABSTRACT FROM AUTHOR]

Common-image gathers are extensively used in amplitude versus angle (AVA) and migration velocity analysis (MVA). The current state of methods for anisotropic angle gathers extraction use slant-stack, local Fourier transform or low-rank approximation, which requires much computation. Based on an anisotropic-Helmholtz P/S wave-mode decomposition method, we propose a novel and efficient approach to produce angle-domain common-image gathers (ADCIGs) in the elastic reverse time migration (ERTM) of VTI media. To start with, we derive an anisotropic-Helmholtz decomposition operator from the Christoffel equation in VTI media, and use this operator to derive the decomposed formulations for anisotropic P/S waves. Second, we employ the first-order Taylor expansion to calculate the normalized term of decomposed formulations and obtain the anisotropic-Helmholtz decomposition method, which generates the separated P/S wavefields with correct amplitudes and phases. Third, we develop a novel way that uses the anisotropic-Helmholtz decomposition operator to define the polarization angles for anisotropic P/S waves and substitute these angles to decomposing formulations. The polarization angles are then calculated directly from the separated vector P- and S-wavefields and converted to the phase angles. The ADCIGs are thusly produced by applying the phase angles to VTI ERTM. In addition, we develop a concise approximate expression of residual moveout (RMO) for PPreflections of flat reflectors in VTI media, which avoids the complex transformations between the group angles and the phase angles. The approximate RMO curves show a good agreement with the exact solution and can be used as a tool to assess the migration velocity errors. As demonstrated by two selected examples, our ADCIGs not only produce the correct kinematic responses with regards to different velocity pertubatation, but also generate the reliable amplitude responses versus different angle. The final stacking images of ADCIGs data exhibit the identical imaging effect as that of VTI ERTM. [ABSTRACT FROM AUTHOR]