Geophysical Mapping: Method Details
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Method Name: Frequency-domain electromagnetics - Profiling
Method Type:   Electromagnetic Methods
Assigned Problems:
+ Aquifer pollution Groundwater
+ Characteristics of hazardous waste Hazardous Waste
+ Contaminant plumes Hazardous Waste
+ Foundations of ancient structures Buildings and Structures
+ Fractures Groundwater
+ Gravel, clay, limestone, salt exploration Natural Resources
+ Groundwater table Groundwater
+ Host sediments, hydogeological settings Hazardous Waste
+ Location of Ancient Structures Cultural Heritage
+ Location of buried materials Hazardous Waste
+ Monitoring Hazardous Waste
+ Permafrost and ice detection Natural Hazards
0 Cavity detection Civil Engineering
0 Landslides Natural Hazards
0 Porosity / Permeability Groundwater
0 Quality / Thickness of aquifer/aquitard Groundwater
0 Quality and thickness (Natural resources) Natural Resources
0 UXO detection Hazardous Waste
0 Weapon Forsenic Investigations
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Frequency domain electromagnetic measures the lateral variations of electrical conductivity of the subsurface using the amplitude and phase of a magnetic field resulting from induced electromagnetic currents. In contrast to geoelectric profiling no galvanic ground coupling is required.
Keywords:   Frequency domain EM; FDEM Profiling; EM31; magnetic fields; resistivity profiles; resistivity contour maps
Prerequisites:  
  • Target must be characterized by a resistivity contrast.
  • Buried wires, metal pipes, metal fences may influence measurements.
  • Urban areas may cause high noise levels (e.g. stray currents).
  • High-voltage power lines, railways and antennas may influence measurements.
  • Coins, metallic belt buckle may influence measurements.
  • Changing waether conditions may influence measurements.
  • Topography / surface dips > 10 may require topographic corrections.
Resolution:   Lateral resolution is determined by the frequencies used, the spacing between the transmitter and receiver coil and the station spacing between measurements.

Typical depth of investigation: usually less than 30 m depth penetration. Resolution varies inversely with depth.

Expected Results:  
  • Measured parameter: magnetic fields resulting from induced currents are recorded with induction coils [mV].
  • Data analysis: voltages are plotted in form of apparent resistivities as profiles or contour maps.
  • Interpretation: often qualitative. Areas displaying anomalously high or low values, or anomalous patterns can be identified. Depth of objects can be roughly estimated. Additional geological or geophysical surface data may be required for reliable interpretation. A priori information (layer thickness and / or resistivity values) are helpful to constrain the models.
Combination with other Methods:  
  • Required additional information: rough estimates of target depths
  • Related add-on information:electrical data
  • Independent additional information: georadar data; magnetic data
Operating Expense:  
  • Crew size: 1 key person, 1-2 assistants
  • Acquisition speed: maximum of around 3 km profile length per day depending on coil separations and orientations, topography.
  • Processing: requires 1 - 2 days per acquisition day
  • Equipment rental costs: low
Parameters to specify:  
  • Array orientation: Should be perpendicular to the strike for a maximum response of geological structures (if only measured along profiles)
  • Spacing between measurements (few m to few tens of m).
  • Line spacing
  • Spacings between transmitter and receiver coil.
  • Spacing between measurements should be around half the coil spacing, line spacing should in the order of the coil spacing. A denser measuring grid above anomalies may improve the interpretation.
  • Coil orientation
  • Transmitter frequencies
QC Documents:  
  • Around 2 - 5 % of repeated measurements
  • Field notes (e.g., all activities, effective time schedule, present personnel)
  • Optional: Map of buried cables, roads
Products:  
  • Profile data
  • Contour maps
  • Interpretation
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