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Method Name: |
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VLF / VLF-R, Radiomagnetotellurics |
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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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VLF instruments measure the distortion of a VLF (very-low frequency) electromagnetic plane wave from a distant high-power military transmitter. Likewise, radiomagnetotellurics employs signals from local or regional radio stations. The plane wave distortions are caused by local changes of the electrical resistivity. |
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Keywords: |
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VLF; VLFR; radiomagnetotelluric, RMT; magnetic fields; electric fields; profiles and maps;
phase information; apparent resistivity |
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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.
- High-voltage power lines, railways and antennas may influence measurements.
- Measurements may not be possible around sunrise and sunset.
- Swamp edges and creeks, topographic highs may produce artefacts.
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Resolution: |
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Lateral resolution is usually a few meters.
Typical depth of investigation is a few meters to few tens of meters, depending on ground resistivity. |
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Expected Results: |
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- Measured parameter: vertical and horizontal components of magnetic fields with induction coils % or ° in measurement of tilt angles. Electric fields are measured with electrode pairs [mV]
- Data analysis: voltages are plotted in form of apparent resistivities as profiles or contour maps. Determination of tilt angles of magnetic fields. Determination of amplitude ratios and phase shifts between magnetic and electrical fields. Plotting of the quantities derived in forms of profiles or contour maps. Inversion of observed data (VLF-R, RMT).
- Interpretation:Qualitative: Detection of anomalous regions on profiles or contour maps. Quantitative: association of subsurface resistivities (inversion results) with geological units .
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Combination with other Methods: |
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- Required additional information: geological information for reliable interpretation.
- Related add-on information: electromagnetic data; electrical data.
- Independent additional information: georadar data; magnetic data; seismics data
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Operating Expense: |
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- Crew size: 1 key person, for VLF-R 1 assistant may be necessary
- Acquisition speed: 200 - 300 measurements per day, depending on surface topography and availability of VLF transmitters.
- Processing: requires 1 - 2 days per acquisition day
- Equipment rental costs: low
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Parameters to specify: |
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- Spacing between measurements (few m to few tens of m).
- Line spacing
- Line spacing (Electrodes; usually 2 - 10 m) .
- VLF-R: Spacing between electrodes (usually 1-10 m) .
- Measurements on a equally-spaced grid are recommended.
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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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- Profiles or maps (phase information; apparent resistivity)
- Interpretation
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