Geophysical Mapping: Method Details
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Method Name: Frequency-domain electromagnetics - Sounding
Method Type:   Electromagnetic Methods
Assigned Problems:
+ Aquifer pollution Groundwater
+ 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 buried materials Hazardous Waste
+ Monitoring Hazardous Waste
+ Permafrost and ice detection Natural Hazards
0 Cavity detection Civil Engineering
0 Characteristics of hazardous waste Hazardous Waste
0 Dead body Forsenic Investigations
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 sounding measures the vertical 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 sounding no galvanic ground coupling is required.
Keywords:   Frequency domain EM; FDEM Sounding; EM31; magnetic fields; sounding curves; 1-D resistivity-depth functions
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:   The depth of investigation is a function of the spacing between the transmitter and receiver coil, frequency and coil orientation.
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. The sounding is applied in perpendicular direction to justify the 1-D models. Non-uniqueness problem: Several different underground models can be derived from one observed data set. Layer suppression: relatively thin layers between two layers of moderate resistivity may not contribute to the measured data and therefore remain hidden.
  • Interpretation: often qualitative. Areas displaying anomalously high or low values, or anomalous patterns can be identified. Depth of objects can be roughly estimated. Resistivity-depth functions are associated with geological units. A priori information (layer thickness and/or resistivity values) is helpful to constrain the models.
Combination with other Methods:  
  • Required additional information: rough estimate of target depths
  • Related add-on information: electromagnetic data; electrical data; knowledge of layer thicknesses (e.g., borehole logs, seismic methods) and/or resistivity values (e.g., electrical logs).
  • Independent additional information: georadar data; seismics data
Operating Expense:  
  • Crew size: 1 key person; 1-2 assistant
  • 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
  • Coil separation
  • 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:  
  • 1-D resistivity-depth functions
  • Profile data
  • Contour maps
  • Sounding curves
  • Interpretation
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