Geophysical Mapping: Method Details
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Method Name: Aerogravity
Method Type:   Gravity
Assigned Problems:
+ Density Civil Engineering
+ Regional Mapping Regional Mapping
   '+' = Technique applicable
Principle:   Air gravity (airborne) measurement of relative lateral changes in gravity to establish subsurface distributions of densities.
Keywords:   Gravity; airborne gravity; airborne geophysics; gravity anomaly; density contrast; subsurface density model
  • Target must be characterized by a density contrast
  • Infromation for flight altitude, aircraft speed, flight direction, disturbing accelerations
  • Requires topographic, tidal and instrument-drift corrections
  • Variable surface topography and variable near-surface densities may produce errors that are difficult to remove
Resolution:   Typically between several hundred meters and 20 km. Depth of investigation (up to several kilometres), accuracy and resolution depend on the aircraft speed, survey line spacing, tolerances for altitude and positional deviations and disturbing accelerations.
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: (airborne) magnetic, (airborne) radiometric data
  • Independent additional information: seismic and electrical/electromagnetic data
Operating Expense:  
  • Crew size: flight crew (normally 2 persons); processing and interpretation: 1 key person
  • Acquisition speed: 30- 500 km2 per day, depending on line spacing
  • Processing: 3 - 4 days per measuring day, including GPS processing
  • Equipment rental costs: high
Parameters to specify:  
  • Line spacing (traverse and control lines; typically from 100 - 1'000 m)
  • Line direction
  • Sampling rate (gravimeter)
  • Flight altitude or height above ground
  • Tolerances for deviations in altitude, position and accelerations
  • Accuracy in surveying: horizontal around 10 m, vertical: around 3 m
QC Documents:  
  • Surveying: documentation of accuracy of coordinates and gravity data (coordinates should be in the range of 0.1 m) and repeatability
  • Plot of instrument drift (documentation of method of drift compensation)
  • Field notes (e.g., all activities, effective time schedule, personnel present)
  • Information on accuracy of navigation and accelerations and estimations of accuracy/reproducability of gravity measurements from observations of crossover points
  • Reoccupation of base station: every 30 - 90 minutes to control possible instrument drift
  • Raw and processed data
  • Line data including for positioning (Date, time, flight parameters), and gravity measurements (equipement parameter, accelerations, elevation, different applied corrections, Bouguer gravity anomaly)
  • Grids and maps including free-air gravity anomaly, Bouguer gravity anomaly, flight line locations
  • 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 (document 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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