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Spectral
decomposition applications beyond
the seismic bandwidth
Presented by Satinder
Chopra
November 6th or November 8th, 2007
11:45 to 1 pm (lunch provided)
Limited seating, click here to
RSVP
This
is the fourth 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.
According to the Widess
model (which consists of an isolated thin bed) the peak
frequency of the seismic response is higher than that of
the wavelet; below 1/8th of a wavelength the seismic
response becomes the derivative of the wavelet and does
not change shape with changing thickness. Our experience
with spectral decomposition has led to the surprising
conclusion that the Widess model of thin bed response is
a very special case that is very different from most
combinations of reflection coefficients.
When the reflection
coefficients at the top and base of a thin bed are not
exactly equal and opposite, a more general behavior is
observed where the peak frequency decreases as thickness
decreases below the tuning frequency. This tells us
that the seismic response is more sensitive to thin beds
than thought previously. In fact, we find that encoded
in the spectral decomposition of a seismic trace is
information that exceeds the bandwidth of the actual
seismic signal and allows us to make inferences about
thin beds that are far thinner than classical limits of
seismic resolution.
Such knowledge can be
used to remove the seismic wavelet without magnifying
noise and can thus be used to produce high resolution
reflectivity sections that are far superior to
conventional seismic sections in resolution and
interpretability.
To learn more, attend the
lunch & learn session on November 6 or November 8.
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. |
