IN 3-D SEISMIC INTERPRETATION
Today's geophysical workstations are splendid tools but they are only
tools. Unfortunately too many interpreters are expecting to find the
solution to their problem in the workstation! The skill remains the
thoughtful geological interpretation of geophysical data. As a
consultant, I am often in a position to review seismic interpretations
by others. It gives me the opportunity to reflect on how geoscientists
can improve interpretations and avoid pitfalls. All too often I am in
contact with seismic interpreters who have misidentified a horizon,
failed to understand the phase and polarity of their data, distorted
the result with a poor use of color, used an inappropriate attribute,
failed to recognize a significant data defect, or are still frightened
by machine autotracking.
On one occasion I was invited to listen to a
presentation on seismic attributes and my opinion was sought. We were
shown a map of Attribute no. 1, then we were shown a map of Attribute
no. 2, then we were shown a map of Attribute no. 3. At this point I
interjected: "What is the objective of this study and how do these maps
relate to that objective?" "I am gathering all the evidence for the
study of this reservoir" was the response. We were then shown Attribute
no. 4, Attribute no. 5, and Attribute no. 6.
I could not contain myself any longer: "Could you
please explain how you selected these particular attributes?" "Oh, they
are all very important!" Then the show continued with Attribute no. 7,
Attribute no. 8, Attribute no. 9. He was selecting attributes because
they existed on his workstation. Sadly too many workstation users today
are button pushers seeking the silver bullet rather than analytical
thinkers using the workstation as a tool.
On another occasion I was shown some rather
elaborate AVO and Converted Wave work, and the optimum drilling
location for this sand was being determined on this basis. I then
discovered that the sand was 5 meters thick at a depth of 5000 m. I did
some quick calculations and determined that the sand thickness was
about one fortieth of a wavelength! Experience dictates that a fortieth
of a wavelength is never seismically visible. For Tertiary gas sands
with large impedance contrasts, the Limit of Visibility can, at best,
be one thirtieth of a wavelength! We cannot benefit from the more
advanced techniques available today until some basic issues of seismic
resolution have been well understood.
The precision of machine autotrackers is typically
around one-quarter of a millisecond? In good data this precision
represents geology and must be exploited. Thus autotrackers are
indispensable tools of modern interpretation. Yet some interpreters are
frightened of them feeling that the human must stay in direct control
of the placement of the horizon. Others have not figured out how to
parameterize the tracker in moderate quality data. Manual tracking is
not only time consuming but it introduces imprecision that can obscure
detailed geology. Derivatives of autotracked time maps, such as
residual, dip and azimuth can yield vital structural detail not visible
in any other way.
Data phase and polarity critically determine
seismic character. Character is more important than amplitude in
directly identifying hydrocarbons with seismic data. Figure 1 shows the
four principal phase and polarity expressions (Zero Phase American
polarity, Zero Phase European polarity, and plus/minus 900 phase) of a low
impedance hydrocarbon sand. Two intermediate characters are also shown.
Once data phase and polarity is determined, hydrocarbon character can
be predicted, and this is of major importance in analyzing
prospectivity in younger sediments. Regrettably I observe interpreters
extracting amplitude and locating wells on the resultant map without
regard for the detailed character on the vertical section.
Figure 1. Phase and Polarity Circle
showing six of the possible seismic characters for a low impedance gas
Character is also key in making an effective well tie and thus
correctly identifying seismic horizons. Too many interpreters take a
well top (measured in depth) and a velocity (to convert to time) and
locate the horizon at that exact point on the seismic section. So why
do interpreters not think more deeply about phase and polarity, and
about tuning effects? I believe that every seismic interpreter,
particularly with an objective beyond structure, has the responsibility
to determine or verify the phase and polarity of his or her data. Many
dry holes have been drilled by those who failed to do so!
The choice between horizon amplitude and windowed
amplitude is another common pitfall. Windowed amplitude is more modern,
but this doesn't mean that we use it to the exclusion of horizon
amplitude that has been available for 20 years. RMS (root mean square)
amplitude seems to be the most popular type of windowed amplitude. This
has splendid application for various reconnaissance endeavors. The
squaring of the amplitude values within the window gives the high
amplitudes maximum opportunity to stand out above the background
contamination. RMS amplitude over a large flat or structured time
window can be used to identify many small bright spots at different
levels within a formation.
Horizon amplitude, extracted along the high-precision autotrack, is
preferable for studying a single reservoir. Most workstations use curve
fitting to interpolate a high-precision amplitude value at the exact
crest of the reflection.
Horizon amplitude suffers no contamination but
requires that the horizon has been correctly identified and tracked.
This also requires that phase and polarity have been properly
understood so that the well tie can be correctly made using character.
Horizon slices thus remain the best amplitude displays for selecting
the optimum drilling location or measuring the area of a reservoir. We
should make every effort to consider the amplitude on the top
reflection and the amplitude on the base reflection.
Figure 2 shows two high amplitudes targeted by an
exploratory well. This data is American polarity, so red-over-blue
(trough-over-peak) is the character of low impedance prospective sand.
The upper high amplitude has this character and has also high
amplitude-over-background. Both amplitudes were originally drilling
objectives but, on the basis of character, we can observe that the
lower amplitude is blue-over-red indicating that it is a hard bed and
thus most probably unprospective.
Figure 2. Two high amplitude reflection
pairs targeted by the same well. Note the different characters.
Seismic data can contain defects caused by the acquisition and
processing, and interpreters must attempt to understand these.
Amplitude is full of geologic information, so amplitude must be
preserved as thoroughly as possible in data processing. The presence of
surface obstacles or the lack of access (no permit) causes reduced and
variable seismic coverage. This tends to be the principal
acquisition-induced problem facing interpreters of land surveys.
Amplitude changes and pseudo-faults can both result from this type of
defect. Figure 3 shows a high amplitude considered to be very
prospective and the corresponding horizon slice on the top sand (red)
reflection. The reduction in amplitude to the south had been
interpreted as the limit of the hydrocarbon. In fact data disruption
caused by reduced surface coverage is the reason for the reduction in
amplitude. Compensating for this effect makes the prospect twice the
Figure 3. High amplitude prospective
reflection pair affected by reduced surface coverage, and corresponding
3-D seismic data should be collected and processed in a regular manner.
Irregularities in coverage can easily introduce effects that can be
confused with geology. Today's interpreter must appreciate the defects
in his data and understand what effect they have on his interpretation.
Recommendations to help today's interpreter get
more geology out of 3-D
seismic data in a reasonable period of time are outlined below. These
will also help avoid common interpretation pitfalls. Seismic
interpretation today involves a delicate balance between geophysics,
geology and computer science. As interpreters we must be continuously
learning to improve our understanding of geophysics, of geology and of
[The data examples are from Pemex in Mexico. I thank my colleagues
there for the release of this data and for the discussions that led to
the understanding of them.]
- Expect detailed subsurface information
- Don't rely on the workstation to find the answer
- Use all the data
- Understand the data and appreciate its defects
- Use time (or depth) slices / horizontal sections
- Visualize subsurface structure
- Use machine autotracking and snapping
- Select the color scheme with care
- Question data phase and polarity
- Tie seismic data to well data on character
- Try to believe seismic amplitude
- Understand the seismic attributes you use
- Prefer horizon attributes to windowed
attributes for detailed work
- Use techniques that maximize signal-to-noise