Estimating relative uncertainty of geological 3D models with low density of input data in geologically complex regions

Abstract This paper addresses uncertainty quantification in manually created 3D geological models of complex regions with sparse data, crucial for e.g. safety and exploration planning. Current models lack such assessments, which is particularly problematic for non-expert stakeholders. The Czech Geological Survey (CGS) developed a workflow to evaluate and visualize uncertainty in such models. Unlike models of geologically simpler areas with rich input data, these models based on sparse input data are created manually and rely on specific model-based interpretations that assume certain tectonic regime or deformation style known from a broader region and from field studies of rock outcrops. The classic uncertainty calculation based on the comparison of a range of (semi-) automatically created models cannot be employed for manually-created stand-alone geological models, and no suitable methodology has yet been determined for such cases. The methodology presented by the CGS considers three sources of uncertainty: the local lithological complexity, the distance to faults, and the so-called general uncertainty - a coefficient that increases with depth. The resulting uncertainty, normalized to the interval < 0;1>, is then reduced in the vicinity of available input data sources (boreholes, geological/geophysical profiles, etc.). The result comprises a 3D grid of the relative uncertainty of the given geological model, which can then be imported into the 3D software tools available to the users of the respective geological model. Demonstrated on the Čertovka site, a potential deep geological repository of high-level radioactive waste, the resulting uncertainty 3D grid guides users in understanding model limitations and can guide subsequent geological explorations.