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Method Name: |
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Frequency-domain electromagnetics - Sounding |
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Method Type: |
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Electromagnetic Methods
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Assigned Problems: |
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Principle: |
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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. |
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Keywords: |
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Frequency domain EM; FDEM Sounding; EM31; magnetic fields; sounding curves; 1-D resistivity-depth functions |
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Prerequisites: |
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- 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
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Resolution: |
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The depth of investigation is a function of the spacing between the transmitter and receiver coil, frequency and coil orientation. |
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Expected Results: |
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- 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.
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Combination with other Methods: |
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- 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
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Operating Expense: |
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- 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
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Parameters to specify: |
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- 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
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QC Documents: |
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- Around 2 - 5 % of repeated measurements
- Field notes (e.g., all activities, effective time schedule, present personnel)
- Optional: Map of buried cables, roads
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Products: |
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- 1-D resistivity-depth functions
- Profile data
- Contour maps
- Sounding curves
- Interpretation
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