In
the shaded relief image of figure 1
you see a rugged mountain with craters on its flanks.
Lunar topography?
A stretch of remote desert on Earth?
Hardly
— it is a time slice through a seismic volume.
Compare
this image with figure 2, a conventional
display of the same data, and you will agree that shaded relief
wonderfully complements standard displays by revealing details and
presenting them in a form that geologists readily appreciate.
But you
already knew that. Shaded relief has been employed for over a century
in displays of topography, and today it is ubiquitous in the display
of any geological or geophysical data that is presented as a map,
including digital elevations, gravity and magnetic data, interpreted
seismic horizons and geologic models.
Whatever
the application, the motivation for shaded relief displays is always
the same — to transform data into realistic-looking apparent topography
and thereby aid interpretation by revealing or suggesting true geology.
Why was
shaded relief not applied to the display of seismic data long ago?
After all, seismic reflections may fairly be taken as representing
buried topography, and so they seem natural for shaded relief.
Two reasons
stand out:
- Shaded relief requires
3-D seismic data, which has only become widely available in the
last decade.
- There is a conceptual
hurdle to jump over. Unlike most geophysical data, which define
a solitary surface to illuminate, 3-D seismic data must be thought
of as a collection of surfaces to illuminate simultaneously. This
concept is the key to seismic shaded relief.
To apply
shaded relief to a 3-D seismic data volume, then:
- Consider the entire
volume as a collection of reflection surfaces, and consider every
data point in the volume as lying on some particular reflection
surface (figure 3).
- Illuminate these
surfaces simultaneously with a single distant light source, the
"sun." Because the sun is distant, illumination is uniform in
direction and intensity throughout the seismic data volume.
- Compute the shaded
relief of a surface as its relative illumination, which is some
function of the angle between the incident light and the reflection
surface. The orientations of the reflection surfaces are found
through available reflection dip and azimuth attributes, which
measure the strike and dip of the buried rock layers.
- Decide whether the
surfaces appear dull or shiny, exaggerate the slopes in your data
to enhance the relief, and you are done.
Shaded relief
actually contains the same information as the common dip-azimuth
attribute (figure 4). This common information
is the reflection dip and azimuth. Shaded relief combines them into
apparent topography, whereas dip-azimuth shows them together with
dip determining the shading of the display and azimuth determining
its color. The difference is only in how it presents this information.
Of course,
as any marketer can attest, presentation is significant — and in
this case, geoscientists are likely to find that dip-azimuth confuses
as much as shaded relief enlightens.
While shaded
relief replaces dip-azimuth, it complements other attributes, such
as continuity (figure 5). Both shaded
relief and continuity reveal details hidden in the data, but continuity
highlights faults and other discontinuities, whereas shaded relief
shows changes in reflector orientation.
There is
also a difference of directionality, for most attributes reveal
structures in all directions, whereas shaded relief is directional,
enhancing features perpendicular to the illumination direction while
suppressing those that are parallel.
As a result,
shaded relief displays should be created in pairs with orthogonal
illumination directions so as to capture all features. This directionality
is useful, as it makes a powerful directional filter of shaded relief,
enabling a user to selectively highlight certain trends while hiding
others.
Seismic
shaded relief represents seismic reflections as apparent topography.
This facilitates geologic understanding by revealing structural
and stratigraphic details hidden in seismic data and presenting
them in a familiar and intuitive display.
Shaded
relief is effective, cheap and fun.
What a
relief!