Wedge Modeling is part of the AVO modeling suite of tools. It creates seismograms but lets you alter a petro-physical or fluid parameter to see the predicted changes. Wedge Modeling is used to understand vertical (thin bed) resolution limited by seismic. Changes in bed thickness are introduced into the model along with changes in acoustic properties determined through rock physics modeling. In this document describes the process of creating a wedge model and viewing the tuning analysis chart. This process requires a well, a wavelet, and a zone for creating the wedge. 2
3 Tuning makes two reflected signals appear as either one signal or even as no signal, this thickness therefore represents the minimum bed resolution that the wavelet can identify. The tuning effect illustrated for a wedge; the material above and below it is the same, but the wedge has different acoustic impedance. When the wedge is a quarter-wavelength (1/4) thick, the second half-cycle of the reflection from the top interferes constructively with the first half-cycle from the bottom, resulting in an increase in amplitude. The magnitude of the increase and how many amplitude maxima there are depend on the shape of the embedded wavelet. (a) The wedge; (b) seismic section across a linear wedge that has lower acoustic impedance than the surrounding sediments for a minimum-phase wavelet; (c) amplitude versus thickness graph.
Opening Wedge Modeling Initiate the wedge model process as indicated. The wedge model feature can be generated by opening the AVO Modeling option in the processes tab. With the left mouse button double click the Wedge Modeling option. This will open the parameters to generate a wedge model. 4
5 Select the Well to you would like to wedge. Select the P-wave, S-wave, and Density The reservoir Interval is the zone of interest for the reservoir. This Zone of Interest forms the wedge. The Zone of Interest can be formed using tops or by depths from the surface.
To set the parameters Start Value will start your wedge at the stated value and the End Value will end your wedge model at the stated value. The Increment - Defines increments between the start and end parameter values. Zone Options: The Fix Zone Top is kept level and the bottom changes. The Fix Zone Bottom of the zone is fixed but only in the Depth domain. Wedge Model: Shift values outside the zone: The log values will be preserved below and above the zone of interest, so the log curves are shifted up or down. Replace values outside the zone: The samples below the zone will be overwritten when the zone becomes thicker than the original log. 6
Advanced Tab: Depth-time curve options: Shift - Preserves correlations between the log and seismic. This method uses the original time-depth curve to predict the times for the pseudo wells. Re-compute from the zone top - Uses the time-depth curve for the original well for the section above the top of the zone and recalculates a new time-depth relationship within and below the zone by using the new sonic log. Recreate depth-time table - This calculates a new time-depth relationship for each pseudo well using only the new sonic log. Output base survey - Creates and saves a 2D volume of the model. Output Wells - Saves all of the pseudo wells generated by the wedge modeling process. Use this if you want to verify the processes. 7
The Synthetic tab sets the synthetic seismogram options: Algorithm – is the actual algorithm that will be used to perform the wedge modeling. Note: That the Number of Offsets sets the number of traces for each pseudo-well in the wedge model. Wavelet - is the chosen wavelet that will be used In the above chosen algorithm. Note: The elastic wave algorithm if selected opens an additional tab allowing the response wavelet, wave recording, conversion modes, and additional features. 8
The Synthetic Advanced tab sets the target zone. The windowed area is identical to the create AVO Synthetic dialog. Note: The Time Window to Model should be larger then your seismic data if using the Shift values outside the zone option in the Wedge Model Basic tab. Synthetic Type: The Synthetic Type is the control options for the synthetic generation options. Modeling Options: The Modeling Options are the users modeling options to use when generating the wedge model. 9
10 The wedge is created and now we move to the tuning analysis. Pick horizons and pick your horizon on the base of the wedge. Click OK to save the picks. Right click within your seismic tab and select View – Show Pick Analysis
By right clicking on your seismic pick the amplitudes of the new horizons and the pick attribute options. 11
In this case, the dominant wavelength wedge model with tuning results displaying a tuning affect at a thickness of 182 meters. 12
Conclusion Wedge Modeling is used to understand vertical (thin bed) resolution limited by seismic. Changes in bed thickness are introduced into the model along with changes in acoustic properties determined through rock physics modeling. You can find more information on Wedge Modeling and other tools on our knowledgebase. Knowledgebase: http://www.cggveritas.com/knowledgebase_main.aspx?cid=861http://www.cggveritas.com/knowledgebase_main.aspx?cid=861 13