Geophysical Mapping: Method Details
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Method Name: Time-domain electromagnetics - Sounding
Method Type:   Electromagnetic Methods
Assigned Problems:
+ Depth of Overburden-bedrock interface Civil Engineering
+ Groundwater table Groundwater
+ Host sediments, hydogeological settings Hazardous Waste
+ Permafrost and ice detection Natural Hazards
+ Quality and thickness (Natural resources) Natural Resources
+ UXO detection Hazardous Waste
+ 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:   The time-domain electromagnetic method measures the electrical resistivity of the subsurface by inducing pulsed currents into the ground. The decay of the induced currents results in a secondary magnetic field, which is measured.
Keywords:   Time domain EM; TDEM Souding; transient electromagnetic; magnetic fields; sounding curves; 1-D resistivity-depth functions
  • 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:   Time domain soundings provide average values over subsurface volumes. The lateral extension of these volumes increases with increasing depth from a few tens to several hundreds meters2. Normally, 3 - 4 layers can be resolved
Typical depth of investigation: from a minimum depth of a few meter to a few hundreds of meters (maximum a few km)
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:geological information for reliable interpretation
  • 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: depending on instrument and configurations few tens to few hundreds of measurements per day
  • Processing: requires 2 - 3 days per acquisition day
  • Equipment rental costs: intermediate
Parameters to specify:  
  • Side length of transmitter loop (accuracy of less than 1%)
  • Loop configuration
  • Measurement separations
  • Receiver gate location / Measuring period / Integration time (usuall less than 1 mnx for engineering - scale investigations)
QC Documents:  
  • At least 2 measurements per measuring point
  • Field notes (e.g., all activities, effective time schedule, present personnel)
  • Optional: Map of buried cables, roads
  • Raw data (reciever output voltage vs. Time)
  • Apparent resistivity vs. time functions
  • Inversion results: 1-D or 2-D resistivity vs. Depth functions
  • Interpretation
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