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Thursday
Aug292013

Colouring maps

Over the last fortnight, I've shared five things, and then five more things, about colour. Some of the main points:

  • Our non-linear, heuristic-soaked brains are easily fooled by colour.
  • Lots of the most common colour bars (linear ramps, bright spectrums) are not good choices.
  • You can learn a lot by reading Robert Simmon, Matteo Niccoli, and others.

Last time I finished on two questions:

  1. How many attributes can a seismic interpreter show with colour in a single display?
  2. On thickness maps should the thicks be blue or red?

One attribute, two attributes

The answer to the first question may be a matter of personal preference. Doubtless we could show lots and lots, but the meaning would be lost. Combined red-green-blue displays are a nice way to cram more into a map, but they work best on very closely related attributes, such as seismic amplitude of three particular frequencies

Here's some seismic reflection data — the open F3 dataset, offshore Netherlands, in OpendTect

Seismic section

A horizon — just below the prominent clinoforms — is displayed (below, left) and coloured according to elevation, using one of Matteo's perceptual colour bars (now included in OpendTect!). A colour scale like this varies monotonically in hue and luminance.

Seismic horizons

Some of the luminance channel (sometimes called brightness or value) is showing elevation, and a little is being used up by the 3D shading on the surface, but not much. I think the brain processes this automatically because the 3D illusion is quite good, especially when the scene is moving. Elevation and shape are sort of the same thing, so we've still only really got one attribute. Adding contours is quite nice (above, middle), and only uses a narrow slice of the luminance channel... but again, it's the same data. Much better to add new data. Similarity (a member of the family that includes coherence, semblance, and so on) is a natural fit: it emphasizes a particular aspect of the shape of the surface, but which was measured independently of the interpretaion, directly from the data itself. And it looks awesome (above, right).

Three attributes, four

OK, we have elevation and/or shape, and similarity. What else can we add? Another intuitive attribute of seismic is amplitude (below, left) — closely related to the strength of the reflected energy. Two things: we don't trust amplitudes in areas with low fold — so we can mask those (below, middle). And we're only really interested in bright spots, so we can edit the opacity profile of the attribute and make low values transparent (below, right). Two more attributes — amplitude (with a cut-off that reflects my opinion of what's interesting — is that an attribute?) and fold.

More seismic horizons

Since we have only used one hue for the amplitude, and it was not in Matteo's colour bar, we can layer it on the original map without clobbering anything. Unfortunately, there's no easy way for the low fold mask to modulate amplitude without interfering with elevation, because the elevation map needs to be almost completely opaque. What I need is a way to modulate a surface's opacity with an attribute it is not displaying with hue...

Thickness maps

The second question — what to colour thicks — is easy. Thicks should be towards the red end of the spectrum, sometimes not-necessarily-intuitively called 'warm' colours. (As I mentioned before in the comments, a quick Google image poll suggests that about 75% of people agree). If you colour your map otherwise, perhaps because you like the way it suggests palaeobathymetry in some depositional settings, be careful to make this very clear with labels and legends (which you always do anyway, right?). And think about just making a 'palaeobathymetry' map, not a thickness map.

I suspect there are lots of quite personal opinions out there. Like grammar, I do think much of this is a matter of taste. The only real test is clarity. Do you agree? Is there a right and wrong here? 

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Reader Comments (4)

Hi Matt

Terrific post. In fact, I venture to say this is probably the best thing on seismic attribute visualization I've seen
Great to see you are using the perceptual colour palettes in such a creative and experimental way.

About modulating the amplitude with low fold mask. I know it is not elegant but this may be one way to do it: display just the amplitude masked by the fold. on a 2D window or flat view from above with no vertical exaggeration, shading turned off, and perspective turned off. Save this as a geo-referenced raster, then drape it on the three-dimensional structure map co-rendered with opacity. I'd be curious to see the result.

August 30, 2013 | Unregistered CommenterMatteo

..or, crop in the area of low fold, save as a geo-referenced raster, then drape

August 30, 2013 | Unregistered CommenterMatteo

@Matteo: Thanks for the nice words!

I didn't think of just using a raster for the amplitudes. Potentially a bit fiddly in OpendTect, but I can bludgeon my way through a workflow like that.

Another thing occurred to me — these 'anomaly' datasets are quite sparse — since your colourmaps leave so much room in the hue and luminance spaces, it would be no problem to add further anomalies – as long as they all reside in that high luminance space. One could easily have blue and green (and perhaps magenta, yellow and cyan too) blobs floating around. One could even combine the the RGB channels additively for some added fun!

August 31, 2013 | Registered CommenterMatt Hall

Hi Matt

On second thought, the raster may not work in OpendTect. You'd have to have ability to apply amplitude cutoff and export this geo-referenced with transparent background, then drape it back and maintaining that transparency. But ACTUALLY cropping in OpenDtect the amplitude map to only include the extent of 100% fold may do. Still quite inelegant...

It is really smart how you are using the free hue spaces into an advantage! You are really giving us a lot of great ideas and food for thought.

One thing I did, but in Petrel, is to have the map with the structure and similarity, and the amplitudes, mapped as geobodies, LITERALLY floating on top of the map. If OpendTect allows goebodies - I confess I do not know - this way you could apply the fold mask to the geobody without affecting the structure map underneath.

September 1, 2013 | Unregistered CommenterMatteo

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