Attenuating Internal Multiples in Seismic Reprocessing for Safe Nuclear Waste Repository Development in Nördlich Lägern, Switzerland

Summary The reprocessing workflow applied on the 3D seismic data acquired in Switzerland’s Nördlich Lägern region is presented. The national cooperative for the disposal of radioactive waste (Nagra) selected this location as the preferred site for a deep geological nuclear waste repository within the Opalinus Clay formation. The seismic dataset required specialized processing to address challenging imaging conditions, particularly the presence of internal multiples at target generated by shallow carbonate layers. The enhanced workflow incorporated the extended interbed multiple prediction (XIMP) algorithm after regularization, which effectively modeled internal multiples from three principal generating horizons identified via VSP analysis. Post-migration multiples adaptive subtraction preserved reflector integrity while improving the coherency and transparency of the Opalinus Clay. Achieving high-confidence subsurface imaging is critical to support construction planning for a nuclear waste repository. The seismic imaging obtained through the reprocessing workflow and the implementation of XIMP demonstrates improved definition of key target intervals and enhanced interpretability, thereby supporting the next phase of repository planning and project development.