Electromagnetics

The Method

Electromagnetic (EM) methods include some of the most commonly employed geophysical techniques used for environmental and geotechnical studies. EM methods employ active sensing technology in which a generated EM field is used to induce secondary EM response in the mediums being investigated. EM sensors can be divided into two categories, frequency domain and time domain. Frequency domain instruments measure the amplitude and phase of the induced electromagnetic field while time domain instruments measure the decay time of the induced field.

Equipment Used

Geonics EM31 DL terrain conductivity meter
Geonics EM34 DL terrain conductivity meter
Geonics EM61 and EM61-MK2 metal detector
Geonics EM63 multi-channel metal detector
Geophex GEM-3 metal detector
Abem Wadi VLF

All EM surveys provide measurements of variability in subsurface conductivity, which can be naturally occurring (differing lithologic materials), or man-made (soil/groundwater contaminants or buried metal). As compared to magnetometer-based investigations, EM instruments have the added capability of detecting non-ferrous as well as ferrous metallic objects.

Survey Design

Digital EM surveys provide a highly accurate and cost effective means of characterizing subsurface conditions at a site. A number of EM methods are available for use, each with its own advantages and limitations. With strong theoretical understanding of EM methods and years of practical experience, NAEVA Geophysics will evaluate and select the method that is most applicable to specific site conditions and project requirements.

Data Processing and Presentation

Graphical presentation of the results of an electromagnetic investigation typically include a site plan showing the area of investigation, known cultural features, and data collection points. The electromagnetic data can be presented as color or color-fill contour maps or stacked profiles, which can overlay the site plan.

For depth estimates to buried targets, electromagnetic modeling techniques can be employed. If subsurface layering information is desired, additional field measurements can be made before modeling.

Applications

  • Identify small ferrous and non-ferrous metallic objects such as unexploded ordnance (UXO)
  • Locate buried metallic objects (drums, tanks, etc)
  • Map leachate plumes
  • Map soil salinity and salt water intrusion
  • Delineate landfill and trench boundaries
  • Map soil and groundwater contaminants
  • Detect location and orientation of faults
  • Map lateral and vertical distribution of soil type
  • Locate water resources
  • Identify karst bedrock features
  • Predict areas prone to slope failure
EM31

The EM31 can measure terrain conductivity to map groundwater contaminants or potential sinkhole locations