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Method Name: |
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Time-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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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. |
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Keywords: |
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Time domain EM; TDEM Souding; transient electromagnetic; 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
- 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
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Resolution: |
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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) |
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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.
- Interpretation: often qualitative. Areas displaying anomalously high or low values, or anomalous patterns can be identified. Depth of objects can be roughly estimated.
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Combination with other Methods: |
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- 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
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Operating Expense: |
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- 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
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Parameters to specify: |
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- 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)
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QC Documents: |
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- At least 2 measurements per measuring point
- 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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- 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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