Gradients to Total Magnetic Intensity

Use the 2D Filtering > Gradients to Total Magnetic Intensity menu option (geogxnet.dll(Geosoft.GX.FFT2D.GradientsToTMI;Run)*) to calculate the total magnetic intensity grid, given the coinciding X & Y horizontal derivatives of the magnetic field.

Gradients to Total Magnetic Intensity dialog options

X gradient grid

Select the name of the input X gradient grid.

Script Parameter: GRADIENTS_TO_TMI.XGRADIENT

Y gradient grid

Select the name of the coinciding input Y gradient grid.

Script Parameter: GRADIENTS_TO_TMI. YGRADIENT

Output TMI grid

Specify the name of the output total magnetic intensity grid calculated from the X, Y gradient grids.

Script Parameter: GRADIENTS_TO_TMI.OUTPUT

Save intermediate grids

Select to save the interim transform grids (optional). If you run this tool a subsequent time when the interim grids are in the working directory, they will be used rather than transforming the input grids again.

Script Parameter: GRADIENTS_TO_TMI.SAVE_INTERMEDIATE_GRIDS

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Expansion

Expansion method

Select the type of grid expansion. The options are:

  • Best match - Square or Rectangular (default)
  • If the number of points of X & Y coordinates are within 25% of each other, the grid is expanded to a square. Otherwise, it is expanded to a rectangle.
  • Square
  • Rectangular

Script Parameter: GRADIENTS_TO_TMI.EXPANSION_METHOD

% expansion

Specify the percent grid expansion (Default - 15). First the number of cells is augmented by the specified % expansion. Then it is further augmented to the next Winograd expansion number.

Script Parameter: GRADIENTS_TO_TMI.PERCENT_EXPANSION

Fill

Fill method

Select the grid fill method. The options are:

  • Maximum entropy (default)
  • Inverse distance

Script Parameter: GRADIENTS_TO_TMI.FILL_METHOD

Roll-off distance

Specify the number of cells beyond the valid area at which to roll-off to zero.

Default: Circular continuity is maintained between the opposite edges of the grid, and the data is not rolled off to zero.

Script Parameter: GRADIENTS_TO_TMI.ROLLOFF_DISTANCE

Global amplitude limit

Specify an amplitude limit. High amplitude anomalies can cause problems in FFT filtering. This option starts attenuating anomalies that exceed half the specified limit and caps them at this limit.

Script Parameter: GRADIENTS_TO_TMI.GLOBAL_AMPLITUDE_LIMIT

Edge amplitude limit

Specify an edge amplitude limit. High amplitude anomalies on the edges of the valid area can produce oscillations in the extrapolated areas. Limit anomalies along the edges of the grids to this value.

Script Parameter: GRADIENTS_TO_TMI.EDGE_AMPLITUDE_LIMIT

Application Notes

  • The yellow asterisk ( ) indicates a required parameter.

    • *The GX tool will search in the "...\Geosoft\Desktop Applications \gx" folder. The GX.Net tools, however, are embedded in the geogxnet.dll located in the "...\Geosoft\Desktop Applications \bin" folder. If running this GX interactively, bypassing the menu, first change the folder to point to the "bin" folder, then supply the GX.Net tool in the specified format.

    References

    • C. D. Hardwick (Formerly National Research Council of Canada), "Two Methods of Gradient-Enhanced Gridding – A Discussion", Technical Paper (2004).