Seismic Waveform Inversion for Near-Surface Velocity Modelling in the Hinterland Niger Delta, South South Nigeria

Near-surface velocity anomalies in the hinterland Niger Delta create statics problems that degrade the quality of deep seismic reflection data used for exploration and development planning, and conventional refraction statics methods are insufficient to resolve the complex velocity variations introduced by shallow gas sands, palaeochannels, and laterally discontinuous lithological boundaries. This study applies full waveform inversion methodology to derive a high-resolution near-surface velocity model from first-arrival waveform data extracted from a 2D seismic survey in Imo State. The FWI workflow was implemented in the frequency domain using a multi-scale inversion strategy, starting from low frequencies to avoid cycle-skipping and progressively incorporating higher frequencies to improve spatial resolution. A starting model was derived from conventional first-break tomography applied to the same dataset. After 40 frequency iterations, the FWI-derived model resolves velocity structures at lateral scales as fine as 25 metres and depth scales of 5 metres in the uppermost 150 metres. Statics corrections derived from the FWI model and applied to the full seismic stack produced improvements in reflection continuity of up to 1.8 times the pre-correction coherence values at the target horizon level. A comparison with borehole-derived velocity logs at one well confirms that the FWI model reproduces the shallow velocity stratigraphy with sub-10 percent RMS error. The study establishes FWI as a practical near-surface modelling tool for Nigerian hinterland delta seismic surveys. Keywords: full waveform inversion, near-surface velocity, seismic statics, Niger Delta, reflection seismic