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Method Name: |
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Micro gravity |
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Method Type: |
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Gravity
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Assigned Problems: |
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Principle: |
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Precise ground-based measurement of relative lateral changes in gravity within a small area to establish subsurface distributions of densities. |
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Keywords: |
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Gravity; micro gravity; high-precision gravity; gravity anomaly; density contrast; subsurface density model |
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Prerequisites: |
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- Target must be characterized by a density contrast
- Requires very accurate elevation measurements (accuracy in the range of 0.02 m)
- Requires topographic, tidal and instrument-drift corrections
- High cultural noise (e.g., vibrations, traffic) may significantly reduce data quality
- Variable surface topography and variable near-surface densities may produce errors that are difficult to remove
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Resolution: |
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The measured anomaly produced by a target depends on its size, depth and density contrast. A general rule of thumb says that a body must be almost as big as it is deep to be detectable. Lateral resolution is a function of station spacing. Vertical resolution is a function of the ability to accurately estimate density of the geologic units. Typical depths of investigation are a few tens of meters, often less than 30 m. |
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Expected Results: |
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- Measured parameter: gravity anomaly in µGal (1 µGal = 10-8 ms-2 )
- Data analysis: non-uniqueness problem: numerous plausible underground models can be derived from the same observed data set
- Interpretation: a priori information (size and / or density of target) is required for modelling and reliable interpretation. Qualitative: Profiles or contour maps for horizontal and vertical localisation of anomalies. Quantitative: Modelling of size, depth and density of target
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Combination with other Methods: |
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- Required additional information: geological and other geophysical information (e.g., size and/or density contrast of target with host material)
- Related add-on information: magnetic data
- Independent additional information: georadar, seismic, electrical/electromagnetic data
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Operating Expense: |
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- Crew size: 1 key person
- Acquisition speed: 60 - 120 measurements per day (gravity survey); elevation measurements / surveying: 150 - 200 measurements per day
- Processing: 1-2 day per measuring week
- Equipment rental costs: intermediate
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Parameters to specify: |
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- Station spacing (often between 5 and 15 meters, may be 1 - 3 meters in very-high resolution surveys)
- Line spacing (should not be larger than 2 - 4 times the station spacing; desirable: equal-spaced grid)
- Reoccupation of base station: of the order of 30 - 90 minutes (corresponds to 8 - 10 measurements between each base-station occupation)
- Reoccupation of at least 20% of all stations; for very-high precision surveys each station should be reoccupied at least once during the survey
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QC Documents: |
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- Surveying: Documentation of accuracy of coordinates and gravity data (coordinates should be in the range of 0.02 m) and repeatability
- Plot of instrument drift (documentation of method of drift compensation)
- Field notes (e.g., all activities, effective time schedule, personnel present)
- Reoccupation of base station: every 30 - 90 minutes to control possible instrument drift
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Products: |
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- Raw and processed data
- Data including positioning (date, time, position), and gravity measurements (type of gravimeter, elevation, different applied corrections, Bouguer gravity anomaly)
- Grids and maps including free-air gravity anomaly, Bouguer gravity anomaly
- Corrected / reduced data (documentation of each correction step; document assumed density values)
- Profiles and / or maps of residual and regional anomalies
- Interpretation
- Optional: Model of density distribution (documentation of accuracy and uncertainty due to non-uniqueness)
- Optional: Models of the expected anomaly / verify the hypothesized density contrast
- Optional: Test measurements
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