An extended 3D first-arrival slope tomography model-building method and its application

First-arrival slope tomography (FAST) is a velocity model-building method originally introduced for two-dimensional (2D) applications using ocean-bottom seismometer datasets. To enable more effective three-dimensional (3D) applications, we reformulate 2D FAST and propose an extended 3D first-arrival slope tomography (EFAST) method. Compared with its 2D counterpart, EFAST extends the data space by incorporating xline directional derivatives of the first-arrival traveltimes at both source and receiver positions, namely, slopes information in the xline direction. First-arrival traveltimes and their directional slopes in both the inline and xline directions are jointly incorporated to better constrain ray propagation in 3D media, thereby improving velocity model-building accuracy. For receiver- or source-dominant seismic acquisition geometries, the reciprocity principle is leveraged to define first-arrival observables, enabling efficient computation of the data variables and the objective-function gradient using a limited number of the forward and inverse problem equations. Consequently, EFAST maintains acceptable computational efficiency. Combined with a multi-stage smoothing strategy, EFAST progressively improves model-building accuracy and yields more reliable near-surface velocity models for critical exploration areas, such as foothill belt zones. Theoretical overthrust model experiment and field data application in a foothill belt zone further demonstrate the superior capability of EFAST in reconstructing multiscale subsurface geological structures. Therefore, EFAST provides an effective complement to contemporary first-arrival-based methods and should be promoted and applied in structurally complex environments.