2273 spectral decomp
TRANSCRIPT
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Spectral Decomposition in HRSKevin Gerlitz
This PowerPoint presentation illustrates a method of implementing
spectral decomposition within HRS by utilizing the Trace Maths utility.
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The Problem:
Given a 3D seismic volume,
use spectral decomposition to
create a tuning cube and
create data slices of thefrequencies
A sand channel in the
Blackfoot seismic dataset
Spectral Decomposition in Hampson-Russell
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The Solution:
A tuning cube can be created using aTrace Maths script and slices
extracted from this cube.
16 Hz Amplitude Map from aTuning cube.
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For comparison, a conventional
amplitude envelope extraction of the
mean value in a window 30 ms below
the Lower Mannville horizon from
slide 1.
The channel is oriented north-south
in the center of the window.
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Ensuring that the 3D seismic volume is displayed, click on Process -> Utility -> Trace Maths
You need the seismic
dataset as an input
variable for TraceMaths. Ive renamed
the input dataset to a
Variable Name of in
and set its Usage to
used
Creating a Tuning Cube in Hampson-Russell
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Call the output volume something meaningful. I am going to calculate the spectra over a 63 ms window in
this example.
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Copy and paste the
DFT_Hamp.prs Trace Maths
script into the Trace Mathswindow. You will have to edit
the start time (t1 = ), the
window length (wLen = ) and
the output time. In this case, Im
starting at the Lower_Mannville
horizon and using a 63 ms
window. The spectrum will be
output starting from 300 ms.
Dft_hamp.prs
Click on the icon below to copy and
paste the DFT Trace Maths script to
your system.
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Youll also need to know something about the DFT and the script
The Trace Maths script will plot the amplitude spectrum starting from 0 Hz up to the positive
Nyquist frequency. The frequency resolution is given by the inverse of the time window.
In my case, my dataset has a 2 ms sampling rate which corresponds to a Nyquist of 250 Hz (=
1/(2*0.002) ). For a 63 ms window, this corresponds to a frequency resolution of ( 1/0.063 = )
16 Hz. My first sample point will correspond to 0 Hz and my 16th data point will correspondto 250 Hz. The amplitude spectrum is placed starting at the output time of 300 ms, which
corresponds to the start of the dataset.
(from the File > Export Trace option)(single trace showing amplitude spectrum)
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After Trace Maths has created the Tuning cube, create slices of the various frequencies.
In my example, the slice at 300 ms is 0 Hz, 302 ms is 16 Hz, 304 ms is 32 Hz, etc
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0 Hz
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16 Hz
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32 Hz
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48 Hz
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64 Hz
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80 Hz
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By using a similarprocess with the
DFT_Hphase.prs Trace
Maths script, you can
create maps of the phase
angle for the appropriate
frequencies.
16 Hz phase map
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Trace Maths scripts are slower to run than compiled code. The time to process 800 traces with a 63
ms window on my 1 GHz PC was 6 minutes.
There is a trade-off between the length of the data window and the spectral resolution. Using a longer
window will provide better resolution in the frequency domain. On the other hand, a long windowmay be contaminated by the response from the underlying and overlying events of the zone of
interest. Having a high sampling rate may improve the situation but simply resampling the data will
not add any new information.
Viewing the frequency slices and interpreting the results as zone thickness can be misleading due to
wavelet effects. Like most geophysical imaging tools, care must be taken in the interpretation of theresults. A synthetic wedge model and the results of spectral decomposition are described in the
following slides.
Drawbacks of the Spectral Decomposition method
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Created a wedge model synthetic using a 5/10 50/70 Hz bandpass
wavelet (dominant period of 33 ms). The channel thickness was changed
from 1 m to 100 m with a 1 m increment => each Inline corresponds to
the thickness of the channel.
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Created the tuning cube using a 80 ms window from the top horizon.
This yields a spectral resolution of 12.5 Hz
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Wavelet effect / DoubletStrong low frequency
component below channel0 Hz = 80 ms period
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12.5 Hz = 80 ms periodSeparation of top & base
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25 Hz = 40 ms period
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37.5 Hz = 26 ms period
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50 Hz = 20 ms period
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62.5 Hz = 16 ms period
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12.5 Hz
37.5 Hz
0 Hz
Normal Polarity (positive frequencies)37.5 Hz 26 ms
50 Hz
50 Hz
50 Hz 62.5 Hz
62.5 Hz
75 Hz
Interpretation
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37.5 Hz
50 Hz
62.5 Hz 25 Hz25 Hz
62.5 Hz
75 Hz
75 Hz
Reversed Polarity (negative frequencies)