Geophysical Mapping: Method Details
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Method Name: Land gravity
Method Type:   Gravity
Assigned Problems:
+ Density Civil Engineering
+ Regional Mapping Regional Mapping
0 Host sediments, hydogeological settings Hazardous Waste
0 Ice thickness Natural Hazards
0 Quality / Thickness of aquifer/aquitard Groundwater
   '+' = Technique applicable; '0' = Application possible/limited use
Principle:   Ground-based measurement of relative lateral changes in gravity over a relatively large area to establish subsurface distributions of densities.
Keywords:   Gravity; regional gravity; land gravity; gravity anomaly; density contrast; subsurface density model
Prerequisites:  
  • Target must be characterized by a density contrast
  • Requires accurate elevation measurements (accuracy in the range of 0.1 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
Resolution:   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.
The depth of investigation may extend to several km.
Expected Results:  
  • Measured parameter: gravity anomaly in mGal (1 mGal = 10-5 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 contrast of target.
Combination with other Methods:  
  • Required additional information: geological and other geophysical information (size and / or density contrast of target with host material)
  • Related add-on information: magnetic data
  • Independent additional information: seismic, electrical/electromagnetic data
Operating Expense:  
  • Crew size: 1 key person; 1 - 2 assistants
  • Acquisition speed: 30 - 120 measurements per day (gravity survey); elevation measurements and surveying: 70 - 200 measurements per day. Depends on ease of moving and locating measurement stations.
  • Processing: 1 day per measuring day
  • Equipment rental costs: intermediate
Parameters to specify:  
  • Station spacing (often between a few tens and a few hundred meters)
  • Line spacing (should not be larger than 2 - 4 times the station spacing; desirable: equal-spaced grid)
  • Reoccupation of base station: every of 30 - 90 minutes to control possible instrument drift
  • Reoccupation of at least 20% of all stations
QC Documents:  
  • Surveying: documentation of accuracy of coordinates and gravity data (coordinates should be in the range of 0.01 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
Products:  
  • 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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