3D MODELING OF THE SHALLOW SUBSURFACE OF THE NETHERLANDS; INTEGRATING GEOLOGY AND MODELING

Increasing pressure on the scarce space in densely populated countries - caused by economic activities and environmental regulations - makes the need for detailed knowledge of the subsurface more and more pressing. Knowledge of the spatial distribution and properties of subsurface rock units is a prerequisite for proper spatial planning.

The subsurface of the Western and Northern parts of The Netherlands consists of highly unconsolidated fluvial and tidal deposits of Holocene age. Here clay, sand and peat show complex distribution patterns with a high degree of spatial variability. The remaining part of the country mainly consists of irregular distributed fluvial deposits that have been influence by a number of glacial events. This complex geological setting forms the background of the new modelling project that aims to develop a voxel-based 3D model of the upper 30-50 meter below the surface. Results are going to be used in studies regarding groundwaterflow, subsidence calculations and geotechnical studies (e.g. constructions of tunnels).

The 3D modelling at TNO starts with recognition of lithostratigraphical defined formations, members, and beds in borehole data, cone penetration tests and geophysical measurements. The main lithostratigraphic subdivision is determined by the process of origin, i.e. fluvial, aeolian, marine or glacial. Detailed identification of faciës and faciës distribution patterns form the basis for further modelling, in which faciës proportion curves are used to show the lithologic distribution versus depth.

Advanced geostatistical methods (collocated indicator co-kriging / simulations, plurigaussian simulations) are used in combination with knowledge of the spatial arrangement of specific geological features. The latter may be derived from detailed mapping surveys (e.g. channel belt pattern, tidal inlets, glacial basins) or from object-based techniques. Multiple realizations of the geometry and composition within the lithostratigraphical framework result in an ensemble of equal probable facies distributions. The next step is then to apply to each facies the appropriate parameters (hydraulic conductivity, compaction etc) in order to model groundwaterflow etc. and to assess the impact of human-induced or naturally changing environmental conditions.