DETAILED 3D GEOLOGICAL MODELLING IN A GLACIAL TERRAIN WITH FOCUS ON GROUNDWATER INTERESTS

As the Danish drinking water supply is based solely on groundwater there is a high demand for geological models intended as input for numerical groundwater models. The highly complex geological settings of Quaternary and Tertiary sediments in Denmark impose great challenges for geological modelling. Typically, several cross cutting buried valleys, glaciotectonic deformations, faults and erosional unconformities, adds a complexity to the geological picture that requires a thorough understanding of the geological framework.

Because traditional layer-cake models cannot fulfill the needs for complexity and detail a concept of combined voxel/layer modelling in a full 3D environment (GeoScene 3D, I-GIS 2010) has been developed. The model concept ensures a thorough interpretation of data as well as exploitation of the full potential that lies within a detailed understanding of the geological structure and history.

The concept has been used for building a model of about 150 km² in a hilly, glaciated terrain. The primary data consists of a dense grid of SkyTEM (Sørensen & Auken 2004) soundings, PACES (Sørensen 1996) and borehole data from the national well-database. Based on the geophysical data, a 3D resistivity grid was produced, initially supplying the voxel grid with resistivity/lithology values. Following a systematic modelling approach, the voxel grid was manually edited and modelled in accordance with stratigraphical layer boundaries in boreholes, defined by digitized point swarms and visualized by surfaces. The model concept thus encompasses a stratigraphical 3D model, in which the geological history of the model area is in focus, and an entirely voxel based lithological model covering the same volume.

The resulting model has improved the basis for the subsequent groundwater modelling and thereby secured a higher success rate for calculations of groundwater resources and implementation of groundwater protection zones.

References

Sørensen, K.I. & Auken, E. (2004): SkyTEM – A new high-resolution helicopter transient electromagnetic

Sørensen, K. (1996): Pulled array continuous electrical sounding. – First Break, 14: 85–90.