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Method Name: |
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Ground 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; ground magnetics; magnetic susceptibility; magnetic remanence; magnetic gradient; subsurface magnetic anomaly |
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Prerequisites: |
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- Target must be characterized by magnetisation contrast with host material
- Additional survey station at ""magnetic quiet"" area for correction of temporal variations
- High-voltage power lines, railway power lines, radio antennas may influence measurements
- Metallic surface objects (e.g., vehicles) may influence measurements
- Urban areas might cause high noise level
- Topography may influence measurements
- Atmospheric disturbances / magnetic storms may influence measurements
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Resolution: |
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Resolution is dependent on the distance between instrument and object, the magnetic properties of the object, its size, shape and orientation, and the background noise level. 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: estimates of location, size, depth and magnetic properties of 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: 1 person
- Acquisition speed: 400 - 1'500 points measurements or up to 15'000 continuous measured points per day
- Processing: 2 - 3 days per measuring day
- Equipment rental costs: low
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Parameters to specify: |
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- Size of survey area (should be larger than expected anomaly)
- Station spacing (often a few submeter to tens of m)
- Line spacing (should not be larger than 2 - 5 times the station spacing; desirable: equal-spaced grid)
- 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
- Required accuracy
- Sensor height (usually between 0.3 m and 4 m)
- 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: 30 m; 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 (document 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 remanence contrast
- Optional: test measurements
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