ELECTROMAGNETICS

THE METHOD

Electromagnetic (EM) methods include some of the most commonly employed geophysical techniques used for environmental and geotechnical studies. Electromagnetic methods fall in two categories, frequency domain and time domain. Frequency domain measures the amplitude and phase of an induced electromagnetic field. Time domain measures the decay time of an electromagnetic pulse induced by a transmitter. EM surveys will measure variability in subsurface conductivity, which can be naturally occurring (differing lithologic materials), or man-made (soil/groundwater contaminants or buried metal).
 

An EM-31 survey was conducted at a military base to define covered landfill cells. The geophysical expression of the trenches is sharp, although subtle, requiring a very small contour interval to expose the features.

SURVEY DESIGN

Electromagnetic (EM) surveys provide an 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.
EQUIPMENT

NAEVA Geophysics can bring the following electromagnetic equipment to your site investigations, providing for a wide range of environmental and geotechnical applications:

  • Geonics EM-31 DL terrain conductivity meter
  • Geonics EM-34 DL
  • Geonics EM-61 and EM-61MK2
  • Geonics EM-16 and EM-16R
  • Scintrex ENVIMAG
  • Abem Wadi
DATA PROCESSING AND PRESENTATION

Graphical presentation of the results of the electromagnetic survey 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 sought, additional field measurements are required before modeling.
APPLICATIONS
  • 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
  • Identify small non-ferrous metallic objects such as ordnance
  • Map lateral and vertical distribution of soil type
  • Locate water resources
  • Identify karst bedrock features
  • Predict areas prone to slope failure
Operator conducting an EM survey with a Geonics EM-31

Electromagnetic readings are most commonly collected
along a series of closely-spaced parallel lines resulting
in a contourable data set.