Automated hierarchical 3D modelling of Quaternary aquifers: an example from the Upper Aare Valley, Switzerland

When modelling groundwater systems in Quaternary formation, one of the first steps is to construct a geological and petrophysical model. This is often repetitive because it relies on multiple manual steps which include geophysical interpretation, construction of a structural model, identification of geostatistical model parameters, facies and property simulations. Those steps are often carried out in different softwares, which makes the automation intractable or very difficult. A non automated approach requires a lot of time and is critical to update the model for integrating new available data or when some geological interpretations are modified, and to conduct a cross-validation procedure to assess the overall quality of the models. Moreover, it renders the quantification of the joint structural and parametric uncertainty tedious. To address these issues, we propose a new approach and a Python module to automatically generate realistics geological and parameter models. One of its main features is that the modelling operates in a hierarchical manner. The input data consists of a set of borehole data, surface geology, and a stratigraphic pile. The stratigraphic pile describes formally and in a compact manner how the model should be constructed. It contains the list of the different stratigraphic units and their order in the pile, their conformability (eroded or onlap), the surface interpolation method (e.g. kriging, SGS, MPS, etc.) or also the filling method (e.g. MPS, SIS, etc.). Then, the procedure is automatic. In a first step the stratigraphic unit boundaries are simulated. Secondly, they are filled with facies and finally the petrophysical property models are simulated inside the facies. All these steps are straightforward and automated once the stratigraphical pile and its related parameters have been defined. Hence, this approach is extremely flexible. It is illustrated using data from an alpine quaternary aquifer in the Upper Aare plain (south-east of Bern, Switzerland) and combined with inverse methods to estimate geophysical and hydrogeological parameters.