High resolution 3D imaging of subsurface using towed transient electromagnetic system - TTEM: Case studies

Near surface geophysical methods are increasingly applied as a mapping tool in the areas of infrastructure, water supply, artificial infiltration, farming, waste deposits, construction, etc. A new towed geophysical transient electromagnetic system (tTEM) has been developed at Aarhus University (Maurya and Auken, in this conference) with a target zone of top 100 m of the subsurface. The development has been driven by the fact that geophysical methods capable of imaging this zone has limited efficiency when it comes to create full 3D images or they do not have sufficient imaging depth. In this paper, we present three case studies where the tTEM system was used for mapping the subsurface at hectare scale.In the first case, we used tTEM for mapping raw materials in the northern Jutland, Denmark. The survey was carried out to map possible sand and gravel deposits. In the tTEM models the gravel deposits show up as very high resistive bodies in the top 30 m. The deposits have significant lateral variation on a scale much less than hundred meter. In the second case study, the tTEM was used to map the thickness of a protecting clay layer above the groundwater aquifer in Vildbjerg, a town in central part of Denmark. Results shows that the top capping clay layer has sufficient thickness (>15 m) to protect the underlying aquifer from a pesticide pollution in the area. Finally, in the third case study, we used the tTEM system for mapping geology in the vicinity of a landfill. The inversion results reveal a hitherto unknown shallow buried valley-like feature within 30 m of the surface that was not identified from older, regional TEM surveys and that can be crucial for predicting the water flow. In the presentation, we will also present the comparison of the tTEM inversion result against the well-known electrical resistivity (ERT) method and borehole information.