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Spectral
decomposition applications and thin-bed reflectivity
inversion
Presented by Satinder
Chopra
April 15th or April 17th, 2008
11:55 to 1 pm (lunch provided)
Suite 2600, 111 - 5th Ave SW
East Petro-Canada Tower
Limited seating,
click here to
RSVP
This
is the fifth in a series of lunch & learn seminars, see
the sidebar for information on the other topics.
Spectral decomposition of seismic traces has existed
since the earliest days of seismology. A new paradigm
in the use of spectral information has developed over
the last five years beginning with the pioneering work
of Greg Partyka, Kurt Marfurt and others at Amoco
Research. A fundamental change in thinking has resulted
from workstation technology that has allowed the rapid
computation and visualization of Fourier spectra
calculated with small windows as a continuous
attribute. Such spectra are dominated by local
reflectivity patterns. Anomalous geological features
such as channels and hydrocarbon-filled reservoirs can
have anomalous frequency responses. Thus, visualizing
the data at discrete frequencies may reveal anomalous or
diagnostic behavior not readily apparent on the
broad-band seismic data. Recently, wavelet transform
techniques have been used to reveal spectral
characteristics of individual composite reflections. A
number of examples will be shown during the seminar to
illustrate differences in frequency response of
composite reflections caused by thin hydrocarbon
reservoirs. It is interesting to note that for thin
reservoirs, seismic attenuation is a secondary effect,
and frequency spectra are dominated by the reflectivity
spectra. The result is that low impedance gas reservoirs
often have anomalously high peak frequencies that can be
used as a diagnostic hydrocarbon indicator.
Enhancing the frequency bandwidth of surface seismic
data has always been a quest for geophysicists. In
fact, seismic resolution is the key to extraction of
stratigraphic detail from seismic data and this has
become more important over the last decade or so.
Seismic resolution refers to both the vertical and
horizontal resolution. While both aspects are important
for interpreting small features on seismic data, here,
we focus our attention to the vertical resolution,
recognizing that migration procedures are usually put in
place for collapsing the Fresnel zones that enhance
spatial resolution.
If the
average spectrum of a seismic wavelet is centered around
30Hz, which is usually the case, reservoirs having a
thickness less than 25 m, may not have top and base
reflectors resolved. Stratigraphic targets are usually
set to look for reservoirs 10 m or less in thickness.
Attempts to achieve such objectives often lead to
frequency enhancement procedures to be followed on
surface seismic data.
Seismic sections are expected to bear a striking
resemblance to geological cross-sections. When this
happens, interpretation becomes straight-forward. If
this does not happen, seismic interpreters resort to the
art of geological interpretation of the seismic data
based on their knowledge of the basic principles of
geology and geophysics and their experience. In the
practical scenario a certain amount of imagination is
relied on when the bandwidth of the data is not
supportive of the interpretation. Thin-bed reflectivity
inversion helps the seismic interpreters by narrowing
the gap between the two extremes mentioned above.
The
highly resolved seismic data retrieved in the form of
reflectivity data is not only very useful for making
accurate interpretations but proves to be advantageous
in many ways, two of which can be stated as: (1)
convolving the extracted reflectivity with a band pass
wavelet of say 5-120 Hz provides a high frequency
section yielding more information to the seismic
interpreter, (2) attributes derived from high-frequency
data provide enhanced lateral resolution of the features
that are of interest and need to be interpreted.
Examples will be shown that illustrate such
applications.
To learn more, attend the
lunch & learn session on April 15th or 17th.
Click here to RSVP.
This course is
available as a free in-house seminar. For more
information contact Florence Janzen, 781-1437 or email
fjanzen@arcis.com. |
