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Method Name: |
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Airborne magnetics |
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Method Type: |
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Magnetic Methods
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Assigned Problems: |
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Principle: |
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The goal of the measurements is to detect anomalies within the natural magnetic field of the earth. |
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Keywords: |
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Magnetics; airborne magnetics; magnetic susceptibility; magnetic remanence; magnetic gradient; subsurface magnetic anomaly |
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Prerequisites: |
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- Target must be characterized by magnetic (susceptibility) contrast with host material
- Additional survey station at ""magnetic quiet"" area for correction of temporal variations
- High-voltage power lines, railway power lines, or radio antennas may influence measurements
- Urban areas might cause high noise level
- Metallic surface objects (e.g., vehicles), steep topography, and weather conditions may influence measurements
- Atmospheric disturbances / magnetic storms may influence measurements
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Resolution: |
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Typically between several tens to several hundreds of m. Depth of investigation, accuracy and resolution depend on the aircraft speed, survey line spacing, and tolerances for altitude and positional deviations. Forward modelling may be required to determine the detectability of a magnetic anomaly.
Typical depth of investigation: several m (engineering applications). Depending on the target size (e.g., larger geological formations), magnetic anomalies up to tens of kilometers depth may be identified. |
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Expected Results: |
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- Measured parameter: total magnetic field, usually in nT, or the magnetic gradient in nT / m
- Data analysis: non-uniqueness problem: several different underground models can be derived from the same observed data set; additional geological or geophysical surface data may be required for reliable interpretation
- Interpretation: raw estimates on location, size, depth and magnetic properties of an anomaly. Modelling size, depth and magnetic properties on an anomaly. Additional a priori geological/geophysical information is necessary
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Combination with other Methods: |
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- Required additional information: geological / geophysical information for reliable interpretation (e.g., magnetic susceptibility values)
- Related add-on information: gravity electrical and electromagnetic data
- Independent additional information: georadar and seismic data
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Operating Expense: |
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- Crew size: Flight crew (normally 2 persons); processing and interpretation: 1 key person
- Acquisition speed: 30 - 500 km2 per day, dependent on aircraft type, line spacing and instruments
- Processing: 2 - 3 days per measuring day
- Equipment rental costs: low (without aircraft)
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Parameters to specify: |
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- Size of survey area (should be larger than expected anomaly
- Line spacing (traverse and control lines, often from 10 to 200 m)
- Line direction
- Sampling rate (magnetometer; usually 2 -10 Hz)
- Flight altitude or height above ground
- Tolerances for altitude and positional deviations
- Accuracy in surveying: horizontal around 10 m, vertical: around 3 m
- Orientation of profiles (perpendicular to strike / geological structures)
- Location of base station (at a magnetically quiet location)
- Measured parameter: total field and/or magnetic gradient
- Gradient measurements less noisy in areas around train power lines than total field measurements, contrary, total field measurements may be less affected by buried iron pipes
- To minimize disturbances: minimum distance to cars: 30m; minimum to metallic piles: 150 m
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QC Documents: |
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- Plot of diurnal variations (record of base station)
- Reoccupation of at least 20% of all stations
- Field notes (e.g., all activities, effective time schedule, personnel present)
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Products: |
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- Raw and processed data
- Corrected / reduced data (documentation of each correction step; diurnal variation correction)
- Profiles and / or maps of residual and regional anomalies
- Model of magnetic bodies (document accuracy and uncertainty due to non-uniqueness)
- Interpretation
- Optional: Modelling the expected anomaly / verify the hypothesis of susceptibility contrast
- Optional: test measurements
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