3D mapping of groundwater resources in Gotland, Sweden
World-wide the groundwater resources are becoming increasingly important as a source for drinking water supply and irrigation of farm fields. If too much groundwater is extracted in areas close to the coast, salt water from the sea will penetrate and ruin the groundwater reservoirs. The airborne SkyTEM system developed at Aarhus University has been a key player for mapping the groundwater beneath the soils in Denmark.
The SkyTEM airborne electromagnetic method (AEM) is a based on the further development of ground based TEM equipment (Transient Electromagnetics). AEM is used for indirect measurement of the subsurface down to depth while at the same time providing large area coverage.
Components installed in the solution
The technology makes it possible to use an antenna suspended below a helicopter to scan the uppermost layers of the earth to a depth of 200 to 300 metres. The researchers can then view underground groundwater landscapes as 3D images with the help of specifically developed software.
Specifications of the SkyTEM system:
- Dual moment system
- Transmitter moment: 132,000A/m2
- Z and X TDEM receiver coils
- Total field magnetics
- Depth of investigation: 200-300 m
- Flight height: 30-50 m
- Flight speed: 12-25 m/s
Step 1. With the help of antenna suspended below the helicopter the system transmits an electromagnetic signal towards the ground, and translates the signal it receives back into a resistivity model.
Step 2. Furthermore, The Aarhus Workbench software allows geophysicists to turn the SkyTEM raw data into accurate, transparent and welldocumented 3D resistivity models of the subsurface, which are the geophysical basis for the groundwater mapping programme. The software is unique, as it was developed specifically for the hydrogeological application of airborne geophysical data. It incorporates tools for applying data processing and modelling protocols to ensure the results meet the quality required in groundwater mapping.
Step 3. When the geophysical data has been processed, and interpreted, specific structures in the subsurface can be identified. The task is to identify different layers and structures based on differences in electrical conductivity. Knowledge about the electrical properties of different sediments is crucial to distinguishing the various geophysical structures. This step relies heavily on information from boreholes in the area. Once the structures have been identified, geological modelling can be carried out, using the GeoScene3D tool.
Step 4. In GeoScene3D both airborne and borehole data is collated in a 3D environment. The geologist, in collaboration with the hydrogeologist and hydro chemist, can now develop a geological model for the area being studied. The result is a model that can be used in groundwater flow calculations by applying hydrological modelling software, such as MikeShe, Modflow or FeFlow.
Step 5. A groundwater model reveals how groundwater flow is determined by geological structures. All the necessary details for the groundwater model are exported directly from GeoScene3D into the hydrological modelling software. Calculations of groundwater flow path lines, to mention one example, are completely dependent on the quality of the input from the geological model especially when it comes to more complex subsurface geological structures.
In 2013 the Swedish Geological Survey, SGU, contracted surveying of the Swedish island of Gotland. Those were aimed primarily at studying the hydrogeological conditions of groundwater extraction in four areas on Gotland, which were assessed as being particularly interesting from a hydrogeological perspective.
The results have shown that the SkyTEM method is well suited for surveying the geology on Gotland. The interpretation of the data acquired has resulted in a number of areas identified as important for future investigations for groundwater extraction. For each of the survey areas a map has been drawn up, showing the depth to the level with risk of salt water encroachment.