Geophysical Mapping: Method Details
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Method Name: Airbore electromagnetic methods
Method Type:   Electromagnetic Methods
Assigned Problems:
+ Regional Mapping Regional Mapping
0 Foundations of ancient structures Buildings and Structures
0 Location of buried materials Hazardous Waste
0 UXO detection Hazardous Waste
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Airborne electromagnetic methods cover several techniques (i.e., frequency domain, time domain methods, VLF) for investigating the subsurface conductivity. They can be applied from airplanes or helicopters and provide a quick aerial picture of the gross subsurface structures.
Keywords:   Airborne geophysics; airborne geophysical methods; electromagnetic field; frequency and time domain; VLF; map of electromagnetic field;
  • Target must be characterized by a resistivity contrast to surrounding
  • High-voltage power lines, train power lines, radio antennas
  • Metallic surface objects (e.g., vehicles)
  • Urban areas might cause high noise level
  • Steep topography
  • Weather conditions
  • Atmospheric disturbances / magnetic storms
Resolution:   Depth of investigation, accuracy and resolution depend on the aircraft speed, survey line spacing, and tolerances for altitude and positional deviations. Forward modelling may be required to determine the detectability of a particular anomaly.

Depth of investigation for frequency domain electromagnetic methods ranges up to 150 m, depending on the survey parameters (e.g., aircraft altitude) and the target physical properties. Airborne VLF investigations may be limited to the shallowest ~20 m.

Expected Results:  
  • Measured parameter: time varying magnetic fields are recorded with induction coils [mV]
  • Data analysis: data corrections (aircraft speed, aircraft altitude etc.). Data are plotted in form of apparent resistivities as profiles or contour maps. Conductive areas can be distinguished from less conductive areas. Refined, quantitative analysis can be performed by inverting the data
  • Interpretation: individual resistivities are related to different geological units. Additional geological or geophysical surface data may be required for reliable interpretation
Combination with other Methods:  
  • Required additional information: geological information for reliable interpretation
  • Related add-on information: airborne magnetic data; ground electromagnetic data
  • Independent additional information: seismic data; airborne gravity data
Operating Expense:  
  • Crew size: flight crew (normally 2 persons); processing and interpretation: 1 key person
  • Aquisition speed: 30 - 500 km2 per day, dependent on aircraft type, line spacing and instruments
  • Processing: 2 - 3 days per measuring day
  • Equipment rental costs: high
Parameters to specify:  
  • Line spacing (traverse and control lines)
  • Line direction
  • Sampling rate
  • Flight altitude or height above ground
  • Tolerances for altitude and positional deviations
  • Instrument parameters
  • General rules for survey design are: Choose the line spacing as small as possible (typical values are 50 to several hundreds of m); choose the flight height above ground as small as possible (few tens of m)
QC Documents:  
  • Field notes (e.g., all activities, effective time schedule, present personnel)
  • Profiles or contour maps
  • Inversion results (resistivity - depth functions)
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