Since the method's establishment in the 1950s for mining exploration, Airborne Electromagnetic (AEM) systems have evolved into highly accurate, quantitative mapping devices, that open up new and exciting geo-modeling applications. AEM provides 3D data in a cost efficient way, applicable even in very rough terrain with helicopter based platforms.
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 a characteristic of the conductivity distribution of earth material with depth. Depending on system parameters (signal strength, noise characteristics, auxiliary sensors for data correction, data processing ...) the penetration depth ranges from several tens to several hundreds of meters. A survey grid with depth sections every some meters along the flight lines finally provides a 3D cube of earth resistivity over the survey area.
NGI's state-of-the-art expertise
- AEM multi-method mapping of hydrocarbon seepage plumes
- Mapping of weakness zones in hard rock
- Sea ice thickness AEM
- Landslide studies
As a rule, the greater the porosity and concentration of saline elements in the pore water will result in a rock or sediment with greater 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 ohm-m) while clays are highly conductive (5-100 ohm-m). In sedimentary areas, conductivity depends on clay content, porosity, dissolved mineral content, and water saturation.
The two figures above show examples from a hydrocarbon exploration case (left), and a rock quality survey (right).
In contrast to very costly ground work especially in remote areas, AEM can provide a cost efficient image of the near surface geology. This can in turn guide the regional geological interpretation, ground work planning, seismic static correction and in selected cases potentially indicate hydrocarbon accumulation at depth. As no oil or gas reservoir is sealed off completely, hydrocarbons seeping through the lithology to the surface can cause re-mineralization in the near surface. These halos can be identified by conductivity anomalies and other properties like radioelement content or specific bacteria species.
The example to the right in the figures above, shows an AEM conductivity anomaly in Norwegian, mountainous terrain, confirmed by an ERT profile and identified as a geological boundary (gneiss/phyllite) by geological field work. Weakness zones in phyllite tend to contain clay and water and thus feature 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.
Research and Development
- 3D visualization of AEM results
- Joint inversion of AEM with ERT
- Extracting rock quality data from AEM results
- Hardware development for sea ice sensors
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