Shaded relief question
manuelcatalán
Posts: 2
in Geosoft
I want to enhance some directional trending on a potential field map. I am using Grid Shaded Relief utility.
I have displayed the grid_s.grd only and I see that surface reflectance coefficients vary from 0.8 to 0.9.
According with HELP it means that the image surface almost completely faces the light source.
Nevertheless all these values shouldn't be accompany with negatives too regarding the opposite side of the source (as dipoles), or negative values should correspond to depressions only?
I have displayed the grid_s.grd only and I see that surface reflectance coefficients vary from 0.8 to 0.9.
According with HELP it means that the image surface almost completely faces the light source.
Nevertheless all these values shouldn't be accompany with negatives too regarding the opposite side of the source (as dipoles), or negative values should correspond to depressions only?
0
Comments

Maybe there are 2 questions here: 1) why are the values of your shadow grid distributed over such a narrow positive band, 2) how can you get a derivative that best emphasizes the trend in the data.
1) The shadow grid is calculated using the equation below (see the Application Notes of the grid display Help):
There are many variables in this equation. If these variables are selected so that
Inclination: approaches 90
Declination: If there is a predominant strong trend in the data and the declination is set along this trend direction that is dX ij. COS(declination) approaches dY ij . SIN(declination)
Scale Factor f: appraoches 1
You will get mostly positive values.
The range of the calculated shadow grid does not have to span equally over the negative and positive range.
2) The GX "Grid and Image> Filters > Generalized derivative" calculates a derivative grid using the Generalized Derivative Operator (Cooper 2011). This expression is similar to the equation above that calculates the shadow grid but takes into account the dZ component as well.
As an alternative to the shadow grid you could generate the GDO grid entering the optimal angles obtained from the interactive shadowing of the initial grid and use it to shadow your grid (enter it in the Shading Effect tab of the grid display tool) . Sometimes this approach tends to bring out more details. The example below is not the best example, but it illustrates the difference between using the input grid to shade the display and using the GDO grid.
Technical Product Manager, Geophysical1
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