A typical AEM system carries a transmitter loop that induces electric eddy currents in the subsurface. The ground response, picked up by one or more receiver coils (on the airborne platform), is dependent on the conductivity distribution in the Earth. Depending on the system parameters (signal strength, antenna size, flight height, noise characteristics) and the data processing, the penetration depth ranges from several tens to several hundreds of meters. A survey grid with depth sections (with a few meter spacing) along the flight lines finally provides a 3D model of the earth resistivity over the survey area.

As a rule, higher porosity and concentration of saline elements in the pore water will result in rock or sediments with higher conductivity. There is no general correlation of the lithology with resistivity, but a broad classification is possible. Moraine sediments (gravel, sand, tills) are resistive to poorly conductive (50-10000 Ωm) while clays are highly conductive (5-100 Ωm). In sedimentary areas, conductivity depends on clay content, porosity, dissolved mineral content, and water saturation.

AEM response

 
NGI's state-of-the-art expertise

  • Bedrock surface modelling by integration of AEM and geotechnical data
  • Investigation of possible quick-clay areas
  • Mapping of weakness zones in hard rock
  • Landslide studies through identification of weakness zones and sliding planes
  • Sea ice thickness surveys
  • Multi-method mapping of hydrocarbon seepage plumes

In 2013, an AEM survey was conducted along a planned motorway corridor in Norway. Through a sophisticated integration of AEM and geotechnical data, a 3D bedrock topography model was produced which was in very good agreement with existing drillings. Furthermore, areas of possible sensitive clay (quick-clay) were identified. See video of the helocopter investigation here.

A snapshot of the final AEM 3D bedrock surface model is shown in the figure below, aligned together with a normal terrain model a bedrock surface model based on drillings only. The total number of geotechnical drillholes could be significantly decreased by virtue of the AEM results. 

3D model labels EN

Existing terrain model (left), bedrock topography derived from boreholes only (middle, triangulated), and AEM bedrock surface model (right) produced from advanced integration of AEM and geotechnical data.

 
The figure below shows an AEM conductivity anomaly in a mountainous region of Norway. The anomaly was confirmed by a ground-based ERT profile and identified as a geological boundary (gneiss/phyllite) by geological fieldwork. Weakness zones in phyllite tend to contain water-saturated clay and are therefore recorded as a strong conductor in contrast to the highly resistive hard rock. Knowledge about this weakness zone was crucial for the planned tunnel corridor in the area. See video from the helicopter AEM mapping.

AEM Aurland RQ

Example of a landslide hazard assessment study where AEM mapping successfully lead to identification of weakness zones at geological boundaries and possible sliding planes.

 
Since 2011, NGI has been developing a next generation frequency domain helicopter EM system dedicated for sea ice research. This Multi-sensor Airborne Sea Ice Explorer, MAiSIE, has been in operation since 2012 and is used and financed by the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. It has been successfully used over Arctic and Antarctic sea-ice.

Research and Development

  • 3D visualization of AEM results
  • Joint inversion of AEM with ERT
  • Development of new inversion software
  • Development of new generation frequency domain AEM systems
  • Extracting rock quality data from AEM results (RockEM)

Relevant Hardware providers / collaborators and Software

AEM Related Services

  • AEM survey planning and feasibility studies
  • Selection of suitable commercial service provider
  • Interpretation of AEM data
  • Application of AEM to unconventional targets (hydrocarbon seepage, rock quality...)
  • AEM hardware development