Early-Arrival 3D Acoustic Full Waveform Inversion at the Asse II Repository: A Case Study

Reliable geological characterization is critical for the safety assessment and planned waste retrieval at the Asse II nuclear repository in Germany. Conventional seismic imaging provides structural information but lacks the resolution needed to resolve detailed variations in velocity. In this study, we apply 3D acoustic full waveform inversion (FWI) to a high-fold, wide-azimuth land seismic dataset acquired over the Asse II site. To reduce nonlinearity and cycle skipping, we focus on an early-arrival FWI strategy targeting refracted and diving waves, combined with careful data conditioning and a multiscale frequency inversion workflow. The inversion uses a First-arrival time tomography (FATT) P-wave velocity model as the initial model and inverts frequencies from 8 to 20 Hz. Gradient preconditioning, source wavelet estimation, offset muting and windowing are applied to stabilize convergence. The resulting 3D velocity model provides improved resolution and is consistent with the initial the established geological horizons of the area. These results provide a promising velocity model for our future investigations such as elastic FWI and support mine stability analysis, and long-term safety assessment of the Asse II repository.