Airborne electromagnetic (AEM) data have proven successful for the purpose of near-surface geological mapping and are increasingly being collected worldwide. However, conversion of data from measured resistivity to lithology is not a straightforward task. Therefore, it is still challenging to make full use of these data. Many limitations must be considered before a successful geological interpretation can be performed and a reasonable 3D geological model constructed. In this paper, we propose a method for 3D geological modelling of AEM data in which the limitations are jointly considered together with a cognitive and knowledge-driven data interpretation. The modelling is performed iteratively by using voxel modelling techniques with tools developed for this exact purpose. Based on 3D resistivity grids, the tools allow the geologist to select voxel groups that define any desirable volumetric shape in the 3D model. Recent developments in octree modelling ensure exact modelling with a limited number of voxels.

Contamination of groundwater with pesticides and nitrate has compelled the Danish Government to launch a major hydrogeological mapping programme covering about 40% of the land area of Denmark. Numerous geophysical surveys are currently being carried out in order to acquire the necessary data. These new data are crucial for the 3-D geological models that are used in the planning of future water supply and landuse. Normally, site-specific groundwater protection zones (Thomsen et al. 2004) are based on groundwatermodelled catchment areas for each well, but proper 3-D geological models are needed in order to create a valid basis for the groundwater models. Since most of the Danish nearsurface geology is complex, a full geological understanding is required combined with in-depth interpretation of geological and geophysical data.

In Denmark water abstraction data have been collected since the late 1970s. Initially the purpose was to monitor and assess the groundwater resources available for future local water abstraction. For this reason, abstraction data were collected not only from waterworks, but also from irrigation, industry etc. Today water abstraction data are used for several purposes, for instance in water -balance calculations to estimate the available resource to wetlands, streams and lakes or to calculate the flow of chemical substances in the water environment.