Prestack wave-equation time migration

In conventional time migration, Green's function is commonly calculated using ray tracing, which may produce inaccurate images for complex structures with strongly lateral velocity variations. To mitigate this issue, we develop an accurate prestack wave-equation time migration method, borrowing from reverse time migration (RTM). First, a two-way acoustic wave equation is derived in the image-ray coordinate system. Then, we directly solve this equation using the finite-difference method to extrapolate source- and receiver-side wavefields, which does not require any approximation as in conventional ray-based time migration. Finally, a zero-lag crosscorrelation imaging condition is applied to generate the time migration image. In addition, we derive the dispersion relation and stability condition for the finite-difference method in the image-ray coordinate system, which provides guidance for temporal and spatial grid discretization. The perfectly matched layer boundary condition is used in the image-ray coordinate system to attenuate boundary reflections. Numerical examples of synthetic and field data verify the feasibility and adaptability of our approach for low signal-to-noise land data. In addition, after mapping the images from the time domain to the depth domain, the image results are even better than RTM results.