GammaGrid Inversion


The GammaGrid Inversion tool performs a full inversion of K, U and Th line data to grids of elemental ground concentrations. It incorporates a comprehensive topographic correction. The advantage of this method is that the topographic correction is rigorous (all data points are considered in the inversion) and you also get improved interpolation between flight lines in the gridding. The disadvantage (relative to the point-based topographic correction described in the previous section) is that the grids are slightly smoothed (a constraint in the inversion).

The method is that of Minty and Brodie (2016). The method incorporates the height of the aircraft, the 3D terrain within the field of view of the spectrometer, the directional sensitivity of rectangular detectors, and a source model comprising vertical rectangular prisms with the same horizontal dimensions as the required grid cell size. The top of each prism is a plane surface derived from a best-fit plane to the digital elevation model of the earth’s surface within each grid cell area. 

The input to the inversion is the final radioelement concentration (either % K, ppm U or ppm Th) after levelling and micro-levelling. So the GammaGrid Inversion tool essentially "un-corrects" the data for height and sensitivities before doing the inversion. 

To use this tool:

    • Set the aliases and parameters – File|Set Aliases and Parameters. On exiting the aliases and parameters window, the tool will display the DEM in the left panel and the Reference Model grid in the right panel.
    • Select Run from the menu bar to run the tool.
    • View the log file – File|View User Log File to see the results.


Tool Tip:  After re-sizing the window, you have to click the Update button to re-draw the displayed grids.

Quick Link: Descriptions of the aliases and parameters used by this tool can be found in the GammaGrid Inversion aliases and parameters example control file.

See Tutorial 11: GammaGrid Inversion for a tutorial on the use of this tool.

Reference

Minty and Brodie, 2016. The 3D inversion of airborne gamma-ray spectrometric data. Exploration Geophysics, 47, 150-157.