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Method Name: |
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Magnetotellurics, Controlled source audiomagnetotellurics |
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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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Signals from natural (MT) or artificial (CSAMT) electromagnetic sources, located outside of the investigation area, are employed to determine the gross subsurface resistivity distribution. |
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Keywords: |
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Megnetotellurics; MT; Controlled Source Audiomagnetotellurics; CSAMT |
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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)
- Prohibited use in areas around high-voltage power line pylons, railways and antennas
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Resolution: |
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Spatial resolution scales with frequency of the source signal. MT provide a spatial resolution of a few hundreds m to km. CSAMT provide a spatial resolution of a few tens to several hundreds of m.
Investigation depths of MT surveys can be up to several tens of km. CSAMT surveys are usually limited to 1 km. |
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Expected Results: |
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- Measured parameter: electrical- and magnetic fields. Electrical fields are measured with electrodes and magnetic fields are recorded with induction coils. Therefore, both fields are represented by voltages.
- Data analysis: 1-D, 2-D or 3-D inversion of the data observed
- Interpretation: association of the resistivities with geological units
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Combination with other Methods: |
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- Required additional information: additional geological information may be helpful
- Related add-on information: vertical electrical sounding; electromagnetic methods
- Independent additional information: seismic methods; magnetic or gravimetric methods
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Operating Expense: |
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- Crew size: 1 key person; 1 - 2 assistants
- Acquisition speed: 20 - 40 measurements per day. Additionally around 1 day assembly and disassembly of the source (CSAMT)
- Processing: 1 - 2 days per measuring day
- Equipment rental costs: low to intermediate
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Parameters to specify: |
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- Station spacing (depends on lateral variability of the subsurface structure)
- Profiles should be perpendicular to strike of geological units
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QC Documents: |
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- Calibration tests
- Around 2 - 5 % of repeated measurements
- Field notes
- Resolution matrices
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Products: |
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- 1-D resistivity-depth functions
- 2-D resistivity distributions along a profile
- 3-D resistivity models
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