Topography-dependent first-arrival traveltime and slope tomography with redatuming for improved velocity imaging: applied to ocean-bottom seismometer data

First-arrival traveltime and slope tomography has been routinely applied to velocity imaging of marine seismic data, particularly for ocean-bottom seismometer (OBS) surveys. However, strong seafloor undulations and the presence of a water layer introduce significant challenges for accurate traveltime modeling and reliable imaging of shallow subsurface structures. In particular, wave propagation through the water layer can substantially degrade seismic illumination and, consequently, reduce the resolution of sedimentary layers beneath the seafloor. In this study, we develop a topography-dependent first-arrival traveltime and slope tomography method based on a body-fitted curvilinear grid that explicitly accounts for complex seafloor topography. The subsurface velocity structure is inverted using first-arrival waves of OBS data, enabling robust imaging of the shallow crust beneath irregular bathymetry. To mitigate the adverse effects of the water layer, we further incorporate a redatuming strategy, in which observed data are downward continued to a virtual receiver surface located at the seafloor. This approach effectively suppresses water-layer interference and enhances subsurface illumination. Synthetic checkerboard tests demonstrate that redatuming significantly improves the recovery of velocity anomalies, particularly in shallow sedimentary layers where conventional slope tomography suffers from limited resolution. We further apply the proposed method to the GO_3D_OBS benchmark model. The results show a clear enhancement in the resolution and accuracy of shallow velocity structures after redatuming, confirming the effectiveness of the proposed workflow for OBS seismic imaging in the presence of complex bathymetry.