When Tomography and Semblance Disagree: Constructing a Practical Starting Velocity Model for Shallow PSDM in Active, Fault-Bounded Intramontane Basins

Pre-stack depth migration can improve imaging shallow structures in fault zones, but its use is limited by the difficulty of defining a suitable starting velocity model. This problem is relevant in small intramontane basins, where strong lateral heterogeneity, limited aperture, complex wavefields, and the absence of direct velocity measurements make first-arrival tomography and conventional semblance analysis difficult to use as standalone constraints. More demanding approaches, such as full-waveform inversion or machine-learning-based velocity modelling, are difficult to apply because these surveys commonly lack low frequencies, long offsets, and calibration data. We examine this problem on a high-resolution seismic profile from the Pantano di San Gregorio Magno basin, Southern Apennines. We compare three starting models: a first-arrival tomographic model, a semblance-derived interval-velocity model, and a horizon-guided model that combines robust elements of both. Tomography constrains the main refractors but includes short-wavelength and locally high-velocity features that are unsuitable for Kirchhoff depth migration. The semblance-derived model better matches the reflected wavefield in the basin fill but is poorly constrained where coherent reflections are weak or absent. The horizon-guided macromodel provides the most appropriate starting point for residual-moveout refinement and offers a practical strategy for shallow depth imaging where direct velocity control is unavailable.