Ground conductivity meters use the same physical principle as airborne electromagnetic measurements (see AEM).  An electromagnetic field is generated from a transmitter coil. After entering the soil this electromagnetic field creates so called eddy currents resulting in a (phase-shifted) secondary field which is detected by several receiver coils. Based on these two electromagnetic fields the resistivity and susceptibility (how easily a material is magnetized) of the earth can be derived.

This service was introduced at NGI in 2013. Based on the extensive experience with electromagnetic methods at NGI (see AEM, ERT and GPR) this new tool tops off our near surface geophysics competence and can be used for.  

EM prinsipp

 Pic 1: Principle of electromagnetic measurements (Reynolds, 2011)

Applications as

  • Sediment type mapping: Sediment conductivity is largely determined by porosity, saturation and clay content. Even quick clay may be identified as quick clay possesses a high resistivity  than unleached marine clay  due to lower salt content.
  • Environmental risk assessment: Delineation of contamination plumes,  and buried Waste.
  • Agriculture: Mapping of soil horizons, soil quality and infiltration properties
  • Infrastructure: Detection of pipes, cables and subsurface installation (i.e. tanks), UXOs and permafrost features (dead-ice bodies, thawing ice).
  • Hydrogeology: Mapping of aquifers and/or aquicludes for ground-water management and drinking water prospection.
  • Waste management: Quality control of  sealing layers and potential leachates of existing landfills
  • Archeological artefacts: Buried anthropogenic structures and building remnants can be identified due to changes in soil moisture in its surroundings. Furthermore bricks and partly building stones possess a magnetic remanence which is detected by the GCM.
  • Dyke and Dam-investigation: Due to the high production rate dykes and dams can be rapidly assessed and potential seepage zones identified in an early stage.

NGI-Equipment

NGI uses a state of the art frequency domain sensor from the Canadian manufacturer DualEM.  The patented dual-geometry of these instruments simultaneously measure the conductivity and magnetic susceptibility of the ground for soil profiling and can also be used to find buried metal (see applications).  The model in use has 6 receiver coils in the range of 1m to 4m spacing. This enables the acquisition of several depths during a survey which are eventually calculated to depth sections of the underground resulting in 3D volumes of the subsurface down to a depth of 6-9m.
Due to an intensive cooperation with the Hydrogeophysics-group at Aarhus-university NGI combines state of the art equipment with the latest software-developments on the market to deliver the best technology to the end user.

Research and Development

  • Cluster analysis of geophysical, hydrologic and environmental chemistry data
  • Correlation and constraint inversion of GPR-GCM data
  • 3D-Inversion and visualization

Relevant equipment and software

 GroundEM-related services

  • Survey planning and feasibility studies
  • Data acquisition, processing and interpretation
  • Site assessment
NGI Anomaly map south

Fig 2. Example showing EM-mapped conductivity and localization of anomalies at Rissa, Norway.

References

Reynolds, J. M. (2011). An Introduction to Applied and Environmental Geophysics, Wiley.