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Vertically transverse isotropic (VTI) acoustic wave equations are widely used to simulate wave propagation in VTI media. A commonly used acoustic VTI wave equation can be derived by setting the vertical shear‐wave velocity to zero in the elastic VTI wave equation. However, the resulting acoustic VTI wave equation has a non‐symmetric propagation operator, which leads to the operator of the adjoint equation in full‐waveform inversion (FWI) being different from that of the forward equation. Consequently, two separate sets of code are required for simulating the forward and adjoint wavefields. To simplify code implementation, we propose a symmetric‐form acoustic VTI equation. This new formulation allows both the forward and adjoint equations in FWI to share the same operator, enabling a unified code for both the forward and adjoint wavefield simulation and streamlined implementation. In addition, although both the symmetric and non‐symmetric formulations yield the same gradient, the adjoint wavefield from the non‐symmetric equation shows weaker amplitudes in deeper regions compared to that from the symmetric equation. As a result, FWI using the non‐symmetric formulation may suffer from insufficient compensation when employing a spatial preconditioner based on an approximated diagonal pseudo‐Hessian, leading to slower convergence. Numerical examples using the Marmousi2 and BP anisotropic models, as well as an ocean‐bottom cable (OBC) field data set, demonstrate that the proposed symmetric‐form acoustic VTI FWI achieves better inversion results and faster convergence than its non‐symmetric counterpart.
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