Vector Decoupling-Based Elastic Reverse Time Migration for OBN Data in VTI Media

The accurate imaging of converted S-waves is one of the key technical challenges in the processing of multicomponent ocean-bottom seismic data. The widespread presence of anisotropy in the seafloor environment, characterized by fluid-solid coupled media, leads to strong coupling between P- and S-waves. This coupling introduces significant crosstalk noise, which severely degrades the resolution of seismic imaging. To address this issue, we propose a vector wavefield decoupling method tailored for fluid-solid coupled media, aiming to achieve more accurate elastic vector wave imaging for ocean-bottom node (OBN) data. Specifically, we simplify the existing acoustic-elastic coupled equations (AECEs) for vertically transverse isotropic (VTI) media. Building upon the decoupling theory developed for purely elastic media-which is based on the normalized zero-order pseudo-Helmholtz operator-we derive explicit relationships between the first-order time derivative of the pseudo-stress components of quasi-P and quasi-S waves in the decoupled system and the first-order time derivative of the synthetic pressure and deviatoric stress components in the acoustoelastic coupling system. From these relationships, we obtain first-order time derivatives of the particle vibration velocity fields for quasi-P and quasi-S waves and express them explicitly in terms of the synthetic pressure and deviatoric stress, thus constructing the first-order velocity-stress equations for vector quasi-P and quasi-S waves. Wavefield decoupling tests on both homogeneous and heterogeneous media models demonstrate that the proposed method effectively separates vector quasi-P and quasi-S waves. We further apply the decoupling scheme to elastic reverse time migration (ERTM). Numerical experiments on simple and complex models show that our method significantly suppresses P- and S- waves crosstalk and mitigates the adverse effects of wave-mode coupling, thereby enabling high-quality imaging of multicomponent seismic data acquired at the seafloor.