Processing 3D Seismic Surveys Jura Ost, Nördlich Lägern and Zürich Nordost
Nagra (2024): Processing 3D Seismic Surveys Jura Ost, Nördlich Lägern and Zürich Nordost. Nagra Arbeitsbericht NAB 23-16.
pdf NAB 23-16 Processing 3D Seismic Surveys Jura Ost, Nördlich Lägern and Zürich Nordost(67.04 MB)
The sectoral plan for deep geological repositories for nuclear waste requires supplementary site-specific investigations in the proposed siting regions Jura Ost, Nördlich Lägern, and Zürich Nordost. Earth scientific investigations such as seismic surveying contribute to the site compari-son and selection. In combination with information from deep boreholes, the seismic data allows to estimate the depth trends, thickness, and spatial continuity of the Opalinus Clay host rock formation and of the confining geological units, as well as the identification of major deformation structures.
For that purpose, mixed-source 3D land seismic surveys were acquired in the three proposed siting regions in the years 2015 – 2016. In the Zürich Nordost siting region, the existing 3D survey of 1997 was merged in for larger areal coverage and processed together with the 2016 survey.
The processing of the seismic field data was optimized to support the comparison of the three siting regions for the purpose of the site selection. The work was essentially done in two stages: Quick Look Processing and Refinement Processing. The Quick Look Processing delivered seismic field records merged with geometry, picked first-break travel times as well as conditioned common midpoint seismic gathers. The conditioned gathers served as a basis for Quick Look time-domain imaged products and also as the input for the Refinement Processing. The Quick Look Processing took place in the year 2016 – 2017 and was reported in detail in Hölker & Birkhäuser (2018).
The purpose of the report at hand is primarily to document the Refinement Processing which took place in 2019 – 2021. The purpose of the Refinement Processing was to obtain higher quality and more detailed seismic images, as well as velocity information for depth imaging and depth conversion purposes. The construction of near surface and full volume velocity models in 3D was an essential component of this stage. All shot and signal processing of the field data was done in the Quick Look Processing stage and the resulting data was reused in the Refinement Processing.
Driven by laterally highly variable acoustic near surface conditions, three separate near surface velocity models were derived for all three siting regions through seismic refraction tomography. Differences in geology and data acquisition mandated region-specific procedures in near surface model building and statics evaluation.
These near surface models were used as the starting point for building full-volume velocity models by seismic reflection tomography for each of the areas. In these models the strong vertical and lateral velocity variations in the subsurface were represented. Using these three individually optimized velocity models, imaging was performed by Pre-stack depth migration, a technology which honours the lateral velocity variations and produces a result in the depth domain. The Pre-stack depth migration provided detailed results with locally improved fault imaging, which are however limited in comparability to each other due to the different modelling and processing parameter choices used per siting region.
To constrain interpretation uncertainties and to maximise the comparability of the three siting regions, a uniform Pre-stack time migration processing scheme was also carried out with -as much as possible- equal parameters and regularised geometry for all three sites. This so-called Reference Pre-stack time migration was used as the primary product for subsequent comparative interpretation as an input for the site selection.
A confident interpretation of geological horizons and identification of faults was possible for the areas of primary interest in all three siting regions by using these seismic volumes and combinations thereof (see Nagra 2024a, 2024b, 2024c). Hence, the Refinement Processing results have contributed to the selection of the Nördlich Lägern siting region as the preferred location for constructing the repository.