How To: Calculate the horizontal and directional gradient of gridded data

Problem/Symptom:
How to calculate the total horizontal gradient of a grid, or gradient of amplitude of a grid in a specific direction.

Environment:
Applies to version 7 of Oasis Montaj. Previous versions follow same principle but refer to Geosoft Menumap (http://www.geosoft.com/support/faq/menumap.html) for equivalent menu options.

Resolution:

To calculate the gradient of amplitude of gridded data in a specific direction:

1 - Use Grid and Image > Filters > Horizontal gradient.
2 - Enter the desired angle. Note the definition of the angle. This is measured as positive from +X axis, so 90 degrees would be equivalent to gradient in the 000 bearing, or North direction (+Y axis).

To calculate the total horizontal gradient of amplitude of gridded data:

1 - Use the method above to calculate gradient in +X (0) and +Y (90) directions.
2 - Use Grid & Image > Grid Math to construct a simple equation... G0 = sqrt(G1*G1+G2*G2).
3 - Assign created G1, G2 grids using the browse buttons to locate the two gradient grids, and enter a suitable name for G0 grid.

Note: this technique uses a spatial convolution filter. FFT Horizontal gradient filtering is available in our Geophysics module.