Offset Scaling. Irfan Saputra. December 2008/CE8R3 SAMPLE IMAGE

Transcription

1 Offset Scaling SAMPLE IMAGE Irfan Saputra HRS Jakarta December 2008/CE8R3

2 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 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.

3 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. 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 restore the actual data to that behaviour and apply that to the data.

4 Offset Scaling Concept

5 Sample case : (Scale Background trend of Seismic Data to a Wet Well Synthetics) Wet well synthetic gather generated from well log data. We want to develop the scaling parameters from here and then apply them to the actual data. Actual Seismic gather

6 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 Analysis window by selecting Process>AVO(Prestack)> Offset Scaling>Analyse.

7 Analysis Options 1. Select the reference volume to analyze. 2. Adjust the range of offset, inline, xline. You can set the increment. 3. Select how the time window will be defined: 1 st option is a single time window. This is the default. The time range is specified in the Time text box. 2 nd option is relative to the horizon; when you select this, the Advanced Analysis Option will come up (next slide)

8 Analysis Options > Advanced Analysis Options a. Use this part to specify the reference horizon, window size and exclusion windows position relative to the horizon. b. If you wish to include azimuth in your analysis, then specify the number of azimuths from 1 to 16. c. You can view the history of parameters and the analysis results, and reset that history by using the buttons in this box. a b c

9 Analysis Options 4. Time : Specify ranges for the time window. Analysis bins : Number of analysis bins (defaulted to half of the number of offsets). Order of fit : Specify analysis order of fit (1 5). Type : type of analysis performed on each specified analysis window, gathers and bins : RMS N x i i= 1 2 Mean absolute value N Geometric mean N N i= 1 N x i= 1 N x i i

10 Analysis Options 5. After all parameter have been specified, then click the Perform Background Analysis button. The results will be displayed graphically in the Analysis Results section

11 Analysis Results 1 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 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 also specify whether to plot the modeled curve.

12 Output Parameters 1. Specify the target t volume Order fit and coefficients here are referring to the analysis. If we change the type of analysis, then the coefficient will 2 be updated. 3. Weight factor (optional): A value of 0 3 means the full effect of the scaling applied, while 1 will leave e the offset scaled data identical to the input data Mean calculation time range is defaulted the same as the analysis time range. 5. Select calculation method: trace by trace or least square fit (independent of trace 6 5 to trace variation). 6. Click Apply & QC Parameters to see the 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.

13 Offset Scale Apply 1 1. After the analysis is done, the next task is applying the scale to the target volume by selecting Process>AVO (Prestack)>Offset Scaling>Apply. 2. A popup message will notify what parameters were selected in the analysis. 2

14 Offset Scale Apply Specify the input volume to be scaled, name and path of theoutput volume, range of time, offset and CDP.

15 Offset Scale Apply 1. This section shows parameters calculated from the last analysis 2. For scaling, there is a new option here called User defined d Input Global Intercept & Gradient. 3. When the third scaling option selected, the Input Global Coefficient box will be active to allow input parameters. 4. There is an option here to print the applied scalar in the runtime message, and you can set the condition for the maximum scalar to be applied

16 Result Scaled seismic gather Raw seismic gathers