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Offset Scaling

Irfan SaputraHRS J k tHRS Jakarta

December 2008/CE8R3

Why offset scaling is needed ?

To correct systematic offset-dependent amplitude distortion in the gathers. This error is common because many processing methods assume a because many processing methods assume a constant RMS amplitude level from near to far offset, even though the amplitude tends to decay from near to far.To pre-condition gathers for other processes.

Offset Scaling Concept

The overall concept is to determine the standard AVO response for the non-hydrocarbon-bearing, wet rock to derive a background set of parameters for scaling to derive a background set of parameters for scaling. Potential reservoirs, as well as areas of strong structure (e.g. faulting), complex lithology or salt features should be excluded. Offset Scaling Analysis then used to define the test windows and then develop scaling parameters to windows and then develop scaling parameters to restore the actual data to that behaviour and apply that to the data.

Offset Scaling Concept

Sample case : (Scale Background trend of Seismic Data to a Wet Well Synthetics)

Wet well synthetic gather Actual Seismic gathergenerated from well log data. We want to develop the scaling parameters from here and then apply them to the and then apply them to the actual data.

Start offset scaling analyse menu

Generally, offset scaling is performed in two steps. First, we develop the scaling parameters and second we apply the scaling to the appropriate volume.Open the Offset Scaling Open the Offset Scaling Analysis window by selecting Process>AVO(Prestack)> Offset Scaling>Analyse .g y

Analysis Options

1. Select the reference volume to analyze.

2. Adjust the range of offset, inline,

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3xline. You can set the increment.

3. Select how the time window will be defined:• 1st option is a single time • 1st option is a single time

window. This is the default. The time range is specified in the Time text box.

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• 2nd option is relative to the horizon; when you select this, the Advanced Analysis Optionwill come up (next slide).will come up (next slide).

Analysis Options > Advanced Analysis Options

a. Use this part to specify the reference horizon, window size and exclusion windows position

a

relative to the horizon.b. If you wish to include azimuth

in your analysis, then specify the number of azimuths from 1 bthe number of azimuths from 1 to 16.

c. You can view the history of parameters and the analysis

b

results, and reset that history by using the buttons in this box. c

Analysis Options

4. Time : Specify ranges for the time window.Analysis bins : Number of analysis bins (defaulted to half of the number

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3bins (defaulted to half of the number of offsets).Order of fit : Specify analysis order of fit (1 – 5).Type : type of analysis performed Type : type of analysis performed on each specified analysis window, gathers and bins :• RMS

⎟⎟⎞

⎜⎜⎛∑ xN

i2

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• Mean absolute value

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⎠⎜⎜⎜⎜

⎝

∑=

Ni

i1

⎟⎟⎞

⎜⎜⎛∑ x

N

i

• Geometric mean

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⎠⎜⎜⎜

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=

Ni 1

⎟⎟⎞

⎜⎜⎛∏N

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ix ⎟⎠

⎜⎝∏=i

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Analysis Options

5. After all parameter have been specified, then click the Perform Background Analysis

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3button. The results will be displayed graphically in the Analysis Results section.

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Analysis Results

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Analysis results are displayed graphically where Analysis results are displayed graphically where the offset is plotted against the Absolute Amplitude (default), and a calculated curve as fitted according to the specified order is printed at the topthe top.

The Display setting can be changed by clicking Display Option [1]. We can set the X and Y axis view Y axis scaling and Least Square Fit We can view, Y axis scaling and Least Square Fit. We can also specify whether to plot the modeled curve.

Output Parameters

1 S if th t t l 11. Specify the target volume.2. Order fit and coefficients here are

referring to the analysis. If we change the type of analysis, then the coefficient will

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be updated.3. Weight factor (optional): A value of 0

means the full effect of the scaling applied, while 1 will leave the offset

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4app ed, e ea e t e o setscaled data identical to the input data.

4. Mean calculation time range is defaulted the same as the analysis time range.

5 Select calculation method: trace by trace

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5. Select calculation method: trace by trace or least square fit (independent of trace to trace variation).

6. Click Apply & QC Parameters to see the scaled volume on a temporary window

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scaled volume on a temporary window. You can change the parameters and redo this process.

Click OK to end the analysis. All parameters will be saved.

Offset Scale Apply

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1. After the analysis is done, the next task is applying the scale to the target volume by selecting Process>AVO (Prestack)>Offset Process>AVO (Prestack)>Offset Scaling>Apply.

2. A popup message will notify what parameters were selected in the analysis

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analysis.

Offset Scale Apply

Specify the input volume to be scaled, name and path of theoutput volume , range of time offset and CDPof time, offset and CDP.

Offset Scale Apply

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1. This section shows parameters calculated from the last analysis

2. For scaling, there is a new option h ll d “U d fi d I t here called “User defined Input Global Intercept & Gradient”.

3. When the third scaling option selected, the Input Global

2Coefficient box will be active to allow input parameters.

4. There is an option here to print the applied scalar in the runtime

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ppmessage, and you can set the condition for the maximum scalar to be applied.

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Result

Scaled seismic gatherScaled seismic gather

Raw seismic gathers