Geophysical Mapping: Method Details
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Method Name: Time-domain electromagnetics - Profiling
Method Type:   Electromagnetic Methods
Assigned Problems:
+ Depth of Overburden-bedrock interface Civil Engineering
+ Groundwater table Groundwater
+ Host sediments, hydogeological settings Hazardous Waste
+ Location of Ancient Structures Cultural Heritage
+ Permafrost and ice detection Natural Hazards
+ Quality and thickness (Natural resources) Natural Resources
+ UXO detection Hazardous Waste
+ Wall Construction Cultural Heritage
+ Weapon Forsenic Investigations
0 Aquifer pollution Groundwater
0 Characteristics of hazardous waste Hazardous Waste
0 Contaminant plumes Hazardous Waste
0 Dead body Forsenic Investigations
0 Fractures Groundwater
0 Gravel, clay, limestone, salt exploration Natural Resources
0 Ice thickness Natural Hazards
0 Landslides Natural Hazards
0 Location of buried materials Hazardous Waste
0 Porosity / Permeability Groundwater
0 Quality / Thickness of aquifer/aquitard Groundwater
0 Quantity/ Thickness Hazardous Waste
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Time domain electromagnetic measures the lateral variations of electrical conductivity of the subsurface using the temporal decay of a magnetic field resulting from induced electromagnetic currents. In contrast to geoelectric profiling no galvanic ground coupling is required.
Keywords:   Time domain EM Profiling; metal detector; magnetic fields; apparent resistivity; resistivity profiles; resistivity contour maps
  • 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
  • Interference from atmospheric storms
  • May not work well in very resistive materials
  • Induced polarization occurs clay-rich environments
  • Prohibited use: pronounced 2 - or 3 D dimensional subsurface geometry
Resolution:   Lateral resolution is determined by the spacing between the transmitter and receiver coil and the station spacing between measurements.

The variation of conductivity is detectable up to 1000 m.

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.
Combination with other Methods:  
  • Required additional information: rough estimates of target depths
  • Related add-on information: surface-based geoelectrical data; electrical data
  • Independent additional information: georadar data; magnetic data; seismics data
Operating Expense:  
  • Crew size: 1 key person, 1-2 assistants
  • Acquisition speed: maximum of around 3 km profile length per day depending on field conditions
  • Processing: requires 1 - 2 days per acquisition day
  • Equipment rental costs: low
Parameters to specify:  
  • Spacing between measurements (few m to few tens of m)
  • Line spacing
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
  • Profile data
  • Contour maps
  • Interpretation
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