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EXPLORER
GEOPHYSICALCORNER
The Geophysical Corner is a regular column in the EXPLORER, edited by Satinder Chopra, chief
geophysicist for Arcis Seismic Solutions, Calgary, Canada, and a past AAPG-SEG Joint Distinguished
Lecturer. This month’s column deals with a basic question: What is seismic interpretation?
What Is Seismic Interpretation?
By ALISTAIR R. BROWN
Seismic Interpretation is the extraction
of subsurface geologic information
from seismic data. On that definition
we all are agreed.
However, if we seek a more penetrating
explanation, we find practitioners get
tongue-tied and talk around the subject in
a variety of ways.
In this article I attempt to give a longer,
more descriptive definition that will apply
to every interpretation project involving
reflection seismic data.
The danger in seismic interpretation
is in thinking that everything we see is
geology!
* * *
Reflection seismic data comprise:
u Continuity of reflections indicating
geologic structure.
u Variability of reflections indicating
stratigraphy, fluids and reservoir fabric.
u The seismic wavelet.
u Noise of various kinds and data
defects.
Seismic interpretation is the thoughtful
procedure of separating these effects.
The seismic wavelet starts as the pulse
of seismic energy, which, generated by
the energy source, travels down through
the earth, is reflected and travels back
up to the surface receivers carrying
the geological information with it. This
recorded wavelet is minimum phase of
some frequency bandwidth, and during
data processing it is converted (we
hope) into a zero-phase wavelet, making
interpretation easier and more accurate.
The interpreter is not directly interested
in the wavelet itself but rather in the
geological information that it carries.
Thus, understanding the wavelet and
distinguishing its characteristics from
details of the geology is one of the critical
tasks of today’s interpreter.
* * *
Noise is ever-present in seismic data. It
may be random noise, it may be multiple
reflections, it may be refracted energy, it
may be other energy of unknown source.
The data may suffer defects because of:
3 Irregular data acquisition showing as
footprint.
3 Obstacles to the data acquisition
crew.
3 Equipment difficulties in the field.
3 Processing problems.
The interpreter must know enough
about the acquisition and processing to
recognize these undesirable features, and
thus to not confuse them with the geology
he/she seeks.
* * *
Seismic energy is reflected from
interfaces where the acoustic properties
of the rocks change. These interfaces
follow sedimentary boundaries created at
the time of deposition of the sediments.
Following the continuity of these
reflections then defines for us the structure
imposed on these boundaries by the
tectonic forces of geologic history.
Following this continuity and making
structure maps is thus the most basic,
and most traditional, activity of seismic
interpretation.
To aid in this endeavor the seismic
The horizon track on Lines 57 and 60 defining the structure, and the Horizon Slice sliced
through the data volume 40ms below. (From Interpretation of Three-Dimensional Seismic Data,
AAPG Memoir 42, SEG Investigations in Geophysics No. 9, Seventh Edition, 2011.)
BROWN
interpreter can manipulate the data and
the display in various ways.
The time-honored approach to prepare
the data for structural interpretation is to
apply AGC (Automatic Gain Control) in
the late stages of data processing. This
reduces amplitude variability (where most
of the statigraphic information lies), and
hence increases visible data continuity.
The interpreter also may compress
the display color bar to optically saturate
and thus to render invisible more of the
amplitude variations. Other techniques
include the use of Instantaneous Phase
(which completely destroys amplitude
information) and Structurally Oriented
Filtering.
All these are good ideas – provided the
interpreter realizes that they are directed at
structural interpretation only, and that the
requirements of later, more advanced types
of seismic interpretation are quite different.
* * *
Once the structure has been
established, the interpreter turns his
Policy Watch
from page 58
u As a guide to the present and
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of all education funding in the United
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Much of the federal STEM funding
targeted economically disadvantaged
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60 MAY 2013 WWW.AAPG.ORG
The danger in seismic
interpretation is in thinking that
everything we see is geology!
attention to stratigraphic interpretation and
the detection of hydrocarbon fluids.
Overwhelming important here is
seismic amplitude – and the amplitude
may be presented to the interpreter or
extracted from the data in various ways.
The data loaded to the workstation must
be True Amplitude and Zero Phase, and
the interpreter must satisfy himself that the
data used are such.
Understanding the wavelet is
complicated and very important (part of
the fundamental separation of effects) but
outside the scope of this article.
In order to increase the visibility of
stratigraphic variations the interpreter will
remove the structure – and the best way
to do this is to make a Horizon Slice.
The concept behind the Horizon Slice
is the reconstitution of a depositional
surface at a key point in geologic history.
The structure used for the reconstitution
is most commonly defined at the level
of the objective. However, it is often
better to define the structure at one level
(conformable with the objective) and to
use this to remove the structure at the
curricula and teacher effectiveness
for K-12, and increasing students’
knowledge and interest in STEM.
Finally, yet importantly, many nonprofit
groups and volunteers work to
encourage students to choose STEM
careers – and support their education.
AAPG Foundation provides grants to
many STEM education initiatives, and
the Youth Education Activities committee
coordinates projects and volunteers.
A few example projects are:
u Continuing education courses and
course scholarships for middle school
and high school earth science teachers.
u The American Geoscience Institute
Earth Science Week.
u The E.F. Reid Scouting Program,
which supports Boy Scouts, Girl Scouts
and other youth organizations, and
objective level.
This very effectively separates
structure into step one and stratigraphy
into step two.
This procedure is illustrated in the
accompanying figure. The horizon
tracked on the two vertical sections
follows a reflection with good structural
continuity and little, if any, stratigraphic
variability. The horizon track is then
displaced downwards by 40 ms (a simple
horizon computation on the workstation)
to intersect the prominent red blob
visible below it, and the amplitude is then
extracted along the displaced track.
The resulting Horizon Slice, on the
right of the figure, shows a very clear
channel (the spatial pattern of the red
blob) with interesting amplitude variations
along it.
* * *
When the seismic interpreter extends
his analysis even further and enters the
field of reservoir evaluation, the data
requirements are even more stringent,
but the Horizon Slice concept is still
effective in removing the effects of
structure. Some form of Inversion may
be used here, and this process converts
interface information (amplitude)
into interval information (acoustic
Impedance).
The more advanced forms of inversion
seek to remove the wavelet, and this is
therefore part of the fundamental idea
of separating effects. However, the
challenge here is to exactly understand
the wavelet that has to be removed.
This is difficult, and many inversions
suffer and projects fail because of this
issue.
So seismic interpretation is the
thoughtful separation (with workstation
assistance) of the various effects that the
subsurface and the seismic acquisition
process have mixed together!
(Editor’s note: AAPG member
Alistair Brown is a consulting reservoir
geophysicist residing in Allen, Texas.
He was the first joint AAPG-SEG
Distinguished Lecturer, is the author
of AAPG Memoir 42, “Interpretation
of Three-Dimensional Seismic Data,”
recipient of an AAPG Distinguished
Service Award and a former editor of the
EXPLORER’s Geophysical Corner.)
helped develop the Boy Scout Geology
Merit Badge.
u Maps in Schools and Rocks in Your
Head Programs, which offer teacher
training and classroom materials.
u The Teacher of the Year award,
which spotlights quality geoscience
education.
Members can learn about these
activities, contribute to the AAPG
Foundation and join the Youth Education
Activities committee by visiting AAPG’s
website.
The GEO-DC blog will alert AAPG
members on the progress of legislation
reauthorizing the American COMPETES
Act. Readers can subscribe to the blog
on the GEO-DC web page,
www.aapg.org/geoDC/.
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