The subsurface is a valuable and multifunctional resource for cities, which must be carefully managed in harmony with its potential and its societal value. For example, the subsurface offers many opportunities for answering sustainable energy questions and questions related to groundwater, climate adaptation, land use and reuse of materials.

To be able to exploit and locate this potential, a detailed picture of the near subsurface is needed. Hence, the Flemish government, Department of Environment, Flemish Planning Bureau (VPO) asked VITO to create a near subsurface geological model of the urban region of Antwerp and part of its harbour as a test case for geological detailed modelling at urban level.

Stijn Janssen

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First urban subsurface model in Flanders

This near subsurface model shows the variation in the composition of the subsurface from ground level to a depth of about 50 metres. The model can serve as a reference for geological, geotechnical and groundwater applications or research and can be used for prospective purposes. It is the first urban detailed subsurface model published on the Databank Ondergrond Vlaanderen.

The model is a test case for future modelling of urban subsurface. The main objectives are to build a new and consistent 3D geological layer model with the highest possible level of detail of the subsurface layers. The model is available free of charge to every user in Flanders.

Geological layers mapped in detail

The 3D model is built up from geological model units that are modelled in depth. It shows the expected depth and thickness variation of the Boom Clay, the Neogene glauconitic sands, the Pleistocene and Holocene and an indication of the Anthropogenic under the city of Antwerp, part of its harbour and the nearby polders of the Waasland. This can be used to deduce where peat layers, palaeochannels of the Scheldt or old dike breaches are located, or where high glauconite levels are to be reckoned with, or where major human disturbance of the original geological layers is expected.

Within the geological layer model, the internal variation of the expected soil type and the glauconite content (clay mineral) was modelled. The geological model units are divided into so-called 'voxels' (volumetric pixels of 25x25x0.5m). The model consists of more than 21 million voxels to which shares of peat, clay, silt, fine sand, medium sand, coarse sand and gravel have been assigned on the basis of drilling descriptions. Together, these parameters describe a prediction of the regional variation in soil composition of the subsurface.

In addition to soil composition, the expected glauconite content was also documented for each model unit. Hence, the voxels can be used to find regions where, for example, a lot of soft sediments such as young peat and clay layers occur, or where a lot of glauconite needs to be taken into account during drilling or digging. The voxel model was created within the VLAKO reference task carried out by VITO for VPO and can be explored via the website of Databank Ondergrond Vlaanderen.