Thursday, 3 November 2016

Azimuth, Fault Trends, Instantaneous Phase: how to convert these values into degrees?

GeoTeric users know that some of the attributes calculated with the software have “strange” values, which are excellent for further attribute calculations or providing visual clues, however, they are not immediately meaningful for the interpreter. This blog post deals with three of these attributes – azimuth, fault trends and instantaneous phase – and shows how the GeoTeric values can be converted into ones that are more familiar for the geoscientist.


Azimuth values in GeoTeric are calculated relative to top of the grid and the values are stretched over the full dynamic range to provide high visual resolution. However, they are very different from the azimuth values that we find in maps or in borehole images, therefore an immediate comparison is very difficult. Converting the GeoTeric values into degrees measured relative to North is a multi-step process, and we’ll rely on the Parser throughout.

First let’s establish the degree of rotation, relative to North. You can find it by following the next steps:

Tools → 2D Slice Viewer…
Select a volume from the 'Input Volume' dropdown in the top right.
Select View → View North.
Now select View → Rotate View, and in the new, small pop-up window you’ll see a number. That’s the value we need. We will call this alpha in the Parser equations below.

Now we convert the azimuths to a range of 0-360 and rotate so they are calculated from North, instead of top of the grid.

16 bit: (im1/182.03888888+180+alpha)%360
32 bit: (im1/11930464.70555555+180+alpha)%360

After these steps your output volume will have azimuth values in degrees (only integer numbers), measured clockwise from North. You can either use the Azimuth or, for a wider range of hues, the FaultTrendsRotary colourmap. Please make sure that you compress it so that the maximum value is 360 (by setting the range of compression between 0% and 70.45%). In case you use the latter, it’s worth turning the Interpolation off on the volume Properties panel, and please be aware that this colourmap has a very short black segment at zero.

Fault Trends

This conversion only requires a single step. The input is the FaultTrends volume (im1). All you need to know is whether your fault trend volume is 16 or 32 bit. We can then use the following Parser expression:

16 bit: (im1>0)*(im1/181.0331491712 – 1)
32 bit: (im1>0)*(im1/11864550.5359116022 – 1)

The scale factor is a bit different than in the previous case, because Fault Trends calculations reserve a short interval of values for the non-fault voxels (the black background). In the output the integer values will represent fault trends relative to North. Using the FaultTrendsRotary colourmap is recommended, with a compression between 0% and 70.6% (i.e. 0-180°).
Note: There’s one known issue with this conversion. Segments with FaultTrend values in the range of 0-0.5° will merge into the background. If those faults are in fact important for your project, please contact us and we’ll help you.

Instantaneous Phase

This is another single step conversion, with the output values rounded to the nearest integer. The input (im1) is the Instantaneous Phase volume calculated by GeoTeric’s trace attribute function. We can then use the following Parser expression:

16 bit: im1/182.03888888
32 bit: im1/11930464.70555555

Compression of the colourmap is also recommended for this volume (0-70.45%).

(Peter Szafian and Jacob Smith)

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