JANUARY 2001

The Geophysical Corner is aregular column in the EXPLORER,produced cooperatively by the AAPGGeophysical Integration and SEGInterpretation committees, andedited by R. Randy Ray. This month’scolumn is titled “Why We Need Colorfor Seismic Display.”

By ALISTAIR R. BROWNEverybody must have noticed that

seismic data is more colorful than itused to be. This is not just to makethe data pretty, nor because todaycolor is cheap; it is to conveyinformation.

Huge amounts of geology can beinterpreted from seismic data today –especially reasonable quality 3-D –and color, used properly, is anessential tool.

Wiggle traces started in the fieldon paper records. The playbackcenter of the 1950s added thevariable area display to help theinterpreter follow structure. But forinterpretation of stratigraphy,hydrocarbons, porosity and reservoirproperties we need something better.

Variable-intensity color is neededrather than variable-area wiggle(figure 1) for four reasons:

✔ Balanced appearance ofpositive and negative amplitudes.

✔ No overlap – and therefore, noclipping of higher amplitudes.

✔ No mislocation of higheramplitudes.

✔ Better visual dynamic range.All logic and intuition in color

usage comes from the color cube,and good color schemes are based

closely on it. Contrasting colorschemes are used for maps;gradational color schemes for data.

A double-gradational colorscheme, such as seen in figure 1,enhances high amplitude events and

is particularly applicable torecognizing hydrocarbon effects andstudying reservoir reflections.

A single-gradational colorscheme, on the other hand,enhances low amplitude events andis particularly applicable to faultrecognition and general structuralinterpretation. The best example hereis variable intensity gray.

* * *

The most common double-gradational color scheme and themost universal color scheme overallis the well-known blue-white-red(figure 1).

The normal and conventional useof this has blue for positiveamplitude, red for negative and whiteon zero. That makes it symmetricalwith respect to the color cube andsymmetrical with respect toamplitude numbers.

We can thus easily compare oneamplitude that is positive withanother one that is negative. We donot add any contrasting colorboundaries, because they makethose amplitude levels look special –so that they distract the eye from thestudy of amplitude trends, patternsand relationships.

This is the best data color scheme

GEOPHYSICALCORNER

Color Reveals Seismic Message

Figure 1 – Vertical seismic section displayed with gradational blue for peaks andgradational red for troughs, compared to same section (below), displayed in variablearea/wiggle trace.

Figures courtesy of Chevron USA

continued on next page

for the novice user. Pure primary blueand pure primary red is normallybest.

Natural pairing of adjacentreflections is a powerful interpretiveobservation that aids reflectionidentification and reservoirunderstanding. It is made possibleonly by the use of double-gradationalcolor.

Look again at figure 1, and notehow the upper high amplitude blueand red reflections very closely mimiceach other. This helps us identifythem as the reflections from the topand the base of one reservoir.

The lower high amplitude blue andred reflections also very closely mimiceach other. These are the top andbase of a separate lower reservoir.

* * *

Special enhanced dynamic rangecolor schemes permit even betterdefinition of stratigraphic detail.

A good example of this is the colorscheme cyan-blue-white-red-yellowillustrated in figure 2 (page 29), whichprovides even more visual dynamicrange than blue-white-red. Here, cyanand yellow highlight the maximumamplitudes.

Figure 2 shows a reservoir offshoreNigeria in which gas is over oil,which, in turn, is over water. You caneasily see that the gas-oil contact ishigher amplitude than the oil-watercontact, and that the gas bright spotis higher amplitude than the oil brightspot.

An interpreter with a detailedobjective will be looking for amplitudetrends and patterns, low amplitudeindications and high amplitudeindications. He will be looking forcharacter and lateral changes.

He will never see these importantsubtleties in wiggle trace displays.

He needs color for reflectionidentification using natural pairing. Heneeds color to help identify problemswith data phase and polarity (anearlier EXPLORER article).

But habits are difficult to break,and we are certainly all products ofour own experiences. Color isessential to modern interpretation,and all those who have been usingwiggle traces for years – andunderstandably like them – need tomake the transition so that they do notcontinue missing information.

Color is also valuable for other

JANUARY 2001

continued from previous page

See Colors, page 29

types of display. Structure mapsshould use a contrasting colorscheme. Horizon slices and mostattribute maps, on the other hand,require a gradational color scheme –again, this helps the interpreterrecognize important trends andpatterns.

Remember:� Color bars should be included

when plots are made so that thereader knows for sure what the colorsmean.

� Displays should always beclearly annotated, so that the readerknows exactly what he is looking at.

Please use color and select yourcolor scheme with care.

(Editor’s note: Alistair Brown is aconsulting reservoir geophysicistbased in Dallas. He was the first jointAAPG/SEG Distinguished Lecturer,and is the author of AAPG Memoir42, Interpretation of Three-Dimensional Seismic Data, which isnow in its fifth edition.) ❏

JANUARY 2001

29

Figure 2 – Gas-oil contact reflection and oil-water contact reflection, both shown bygreen arrows, in a producing reservoir offshore Nigeria.

Figure courtesy of ExxonMobil Nigeria

Colorsfrom page 27