Looking at the Facts About Electromagnetic Surveys

Facts About Electromagnetic Surveys

Electromagnetic (EM) geophysical surveys are based on the propagation of electric currents and electromagnetic fields in the earth.  EM surveys can be conducted as airborne, ground or borehole surveys, and is one of the most commonly used methods in mineral exploration.  In EM surveys, an electric current is transmitted through the Earth and the response of the current as it travels back out of the Earth is measured.  Conductivity and resistivity are the inverse of each other.  Resistive zones will return a weak response whereas conductive zones will return a strong response. These surveys are commonly used to identify large concentrations of base metal sulphides beneath bedrock without physically drilling into the ground.
There are two types of EM surveys:
  • Active (controlled source) EM Surveys - generate EM source with transmitter coil and measure induced field in subsurface
  • Passive EM Surveys - use naturally occurring EM currents in subsurface produced by ionospheric currents (magnetotelluric method)
There are two types of Active EM methods:
  • Frequency Domain - measure relative amplitude and phase of primary and secondary EM fields
  • Time Domain (transient field methods) - measure rate of decay of secondary field with time when the primary field is shut off


Types of Airborne Electromagnetic Systems 

The following types of airborne electromagnetic systems are available today:
  • Time Domain Electromagnetics
  • Frequency Domain Electromagnetics - multi-frequency "bird' towed by helicopter
  • AFMAG - airborne natural source Audio Frequency MAGnetotellurics
  • VLF-EM (very low frequency electromagnetics)
Applications for Airborne Electromagnetic Surveys
Mineral Exploration:
  • Base metals - massive sulphides, copper, zinc, manganese, nickel 
  • Gold - depth and character of regolith, structural and alteration mapping, sulphides, shear zones
  • Coal exploration
  • Graphite exploration
  • Diamonds - kimberlites and alluvial
Water and Environment:
  • Groundwater exploration, aquifer mapping, sand/clay distribution, groundwater flow
  • Exploration of buried valleys, paleochannels
  • Mapping salinity
  • Mapping fresh water and salt water encroachment
  • Environmental assessment measuring pollution plume related to groundwater
Energy Exploration:
  • Geothermal exploration
  • Uranium deposit exploration
Engineering Applications:
  • Studies for evaluation of landslide potential
  • Pre-construction engineering for pipeline corridors
Specialists in Electromagnetic Surveys
Terraquest was established in 1984 and since then, our dedicated team has flown over 1,500 airborne geophysical surveys using both fixed wing and helicopter platforms.  Our professional crews provide significant experience having carried out airborne magnetic, gravity, gamma ray spectrometry, and electromagnetics on five continents.  Our quality data sets have been utilized in the exploration for base and precious metals, kimberlite, hydrocarbons, uranium, rare earth minerals and water. The company has performed exemplary surveys for both small and large exploration groups as well as many government agencies.  References can be provided upon request.    Visit www.terraquest.ca.


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