AVO Analysis Guide

Guide to AVO Analysis 1

May, 2011

Guide to AVO Analysis Introduction Geoview is the program from Hampson-Russell which you can use to evaluate and model Amplitude Versus Offset anomalies. For this you will need:

• One or more well logs. • A pre-stack seismic volume, either 2D or 3D. This volume has usually been

processed to the final CDP gather stage. AVO analysis can be divided into two parts: synthetic modeling and seismic data analysis. This tutorial takes you through the seismic data analysis phase of two AVO projects. The first project analyzes a single 2D line, with a strong AVO anomaly. The second project analyzes a small 3D volume. For the synthetic modeling phase, see the tutorial entitled Guide to AVO Modeling.

2 Guide to AVO Analysis

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Starting Geoview Geoview is the starting program for all the applications in the Hampson-Russell suite, including AVO Analysis. To start this tutorial, first start the Geoview program. On a Unix workstation, go to a command window and type:

geoview On a PC, click the Start button and select the Geoview option on the Programs > HRS applications menu. When you launch Geoview, the first window that you see contains a list of projects previously opened in Geoview. For example, the figure below shows a single previous project, which could be opened now. Your list will be blank if this is the first time you are running Geoview.


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For this tutorial, we will start a new project. Before doing that, you can set all the data paths to point to the location where you have stored the tutorial data. To do that, click the Settings tab:

Now you can see a series of default locations for the Data Directory, Project Directory, and Database Directory. We would like to change all of these to point to the directory where the tutorial data is stored. To change all of the directories to the same location, click on the option Set all default directories and then click the button to the right:

Then, in the File Selection Dialog, select the folder which contains the tutorial data:

After setting all three paths, the Geoview window will now show the selected directories (note that yours may be different):


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When you have finished setting all the paths, click Apply to store these paths:

Now select the Projects tab and click the New Project button:

A dialog appears, where we set the project name. We will call it AVO Analysis Guide, as shown below.


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Enter the project name and click OK on that dialog:

Now a dialog appears, asking for the name of the database to use for this project:

The database stores all the wells used in this project. By default, Geoview creates a new database, with the same name as the project and located in the same directory. For example, this project is called AVO Analysis Guide.prj, so the default database would be called AVO Analysis Guide.wdb. That would be desirable if we were starting a new project, intending to read in well logs from external files.


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For this tutorial, to save time, we have already created a database, which has a well already loaded. To use that database, click Specify database:

On the pop-up menu which appears, select Open. Then, select the database AVO Analysis Database, as shown, and click OK:

Now the previous dialog shows the selected database and the new project name. Click OK to accept this:


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The Geoview Start Window now looks like this:


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Displaying the well The left part of the Geoview window (called the Project Manager) shows all the project data so far. The tabs along the left side select the type of project data. Right now, the Well tab is selected and we can see the single well from the external data base. Click the “+” sign beside that well to see a list of curves in that well:

To see more details about the well, click the Data Explorer tab to the right:

The Geoview window now changes as shown:


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Click the arrow next to the well name to get more information about the curves in that well:


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Finally, to see the most complete view of the log curves within a well, go to the icon for that well within the Project Data window and double-click:

This creates a new tab within the workspace, called the Wells tab, which displays the selected well curves:

We can see that this well contains three log curves which are used in AVO Modeling: the P-wave velocity log, the density log, and the S-wave velocity log. The S-wave velocity log for this well was actually created from the existing P-wave and density logs, using mathematical transforms. In addition, the depth-time curve for this well has been modified using the process of log correlation. For details on those processes, including loading logs into the Geoview database, see the tutorial AVO Modeling Guide.


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Loading the Seismic Data We have now loaded the well which will be used in the AVO Analysis process. The next step is to load the seismic volume, which we will analyze for AVO anomalies. In the Project Manager, on the far left side of the Geoview window, click the Seismic tab:

The window to the right of this tab shows all seismic data loaded so far. This is empty. Go to the bottom of the window and click the Import Seismic button:

On the pull-down menu, select From SEG-Y File:


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On the dialog that appears, select the file gathers.sgy, by either double-clicking on the file name or by selecting the file and clicking on Select >, as shown below:

Click Next at the base of the dialog:

Set the Geometry Type to 2D and click Next:

On the third page, we are telling the program what information it can use from the trace headers. In fact, in this data set, there are no X and Y coordinates. That is why we answer No to this question:


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After modifying that item, as shown, click Next to see the SEG-Y Format page:

By default, this page assumes that the seismic data is a SEG-Y file with all header values filled in as per the standard SEG-Y convention. For example, it expects to find the CDP numbers at the byte location shown above. If you are not sure that is true, you can click Header Editor to see what is in the trace headers. In our case, we believe the format information is correct, so click Next to move to the next page. Now the following warning message appears because the program is about to scan the entire SEG-Y file:

Click Yes to begin the scanning process.


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When the scanning has finished, the Geometry Grid page appears:

Because we have not read X-Y information from the headers, the program assumes this is a single straight line, which is correct. Click OK. After building the geometry files, a new window appears, showing how the well is mapped into this seismic volume:

In this case, the mapping is not correct because we did not supply the X-Y location of the well, and there were no X-Y coordinates in the seismic trace headers.


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We can manually locate the well at the know CDP location (330). Type that number in the location shown:

Then, click OK to accept the new locations shown on this window. Now the seismic data appears within the workspace:

Modifying the Seismic Display The workspace currently shows the single Inline from this dataset. To see other parts of the line, slide the scroll bar at the base of the display.


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To see the display positioned at the well location, go to the Well icon and click the down arrow as show below:

The drop-down menu shows the one well in the project. Select the well and the Geoview window shows the seismic data in the vicinity of that well location:

We can also modify other plotting parameters by using the Seismic View Parameters window. To make that window appear, click the “eyeball” icon as shown:

The Seismic View Parameters window contains a series of pages which control various aspects of the plotting. To see the parameters for a specific item, select that item from the list at the left side.


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For example, here we have selected the Inserted Wells item:

Let us (temporarily) insert the density log by selecting that item as shown:

Now click Apply on the Seismic View Parameters window. The display is modified accordingly:


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We will continue the tutorial with the sonic log inserted. To do this, click Reset Page and OK on the View Parameters window. This redraws the Geoview window as before:

Applying the AVO Analysis processes: CDP Stack Now that we have read in all the data necessary for the AVO Analysis, we are ready to start the process. First, look at the tabs to the left of the Geoview window. You will see that one of these tabs is called Workflows. Click that tab to see a list of pre-defined workflows for various processes, including AVO Attribute Analysis:

One strategy for doing the AVO Analysis is to use one of the workflows specified here. We will do that later in the tutorial. First, we will use the standard approach. To do that, click the tab called Processes. You will see a list of all the operations which are available in Geoview. Each of the processes can be expanded.


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For example, if you click on both the Seismic Processing and AVO Analysis options, the following expanded list is seen:

To start, we will create a CDP stack. To do that, click next to the Stack option in the Seismic Processing submenu to see the two types of stack available, and double-click CDP Stack:

Now the parameters for this process appear on the right:


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There are some features of this dialog which are common to all Process Parameter dialogs. For example, there is a location to specify the input and output files names:

The input file must be one that has already been loaded. In this case, there is only one file available, gathers. The output file can have any name. The program has suggested cdp_stack. There is a section to specify the data range to process. By default, it is the entire volume:

For example, we could choose to stack only a limited range of offsets:

By default, only the most critical parameters for this process are specified on this page. To see the more advanced option, click the button at the base of the menu:


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This produces a series of extra tabs, which control in detail the process to be performed. Note that these will differ from one process to the next. At the top of the menu, there is a small icon showing an “airplane”:

Click that icon and the Parameter Dialog detaches from the Geoview window to allow it to be moved aside, making the data more visible. Clicking the “airplane” again re-attaches the dialog.:

At the base of the Parameter Dialog, we see a series of buttons:

If we click the Run Batch button, that will start the process in background, leaving the main Geoview program free to carry on with other work. That is often helpful for long, computer-intensive processes. For now, click OK to start the CDP stack process as usual. When the process finishes, the Geoview window looks like this – a split window showing both the input and output volumes:


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Modifying the seismic display The default split-screen display is very useful for looking at the results, but there are many modifications possible. For example, you can increase the available plot space by clicking the “x” on the Project Manager window, as shown, to temporarily hide that window:

To restore the Project Manager window, click its name to the left:

You can also temporarily hide one of the views. For example, click on the first icon shown below to temporarily hide View 1, which shows the input data:


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To restore View 1, click it again:

The fourth button sets the orientation horizontally:

Click the fourth button again to restore the vertical orientation:

Finally, to see the most complete control of the seismic display, right-click on either of the seismic windows. A pop-up menu appears:


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One of the items is View>Seismic View Parameters:

If you click this item, a dialog appears, allowing complete control of the display:

To continue this tutorial, click Cancel on this dialog.


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Super Gather The next process we will apply is Super Gather. Super Gather is the process of forming average CDPs to enhance the signal-to-noise ratio. We do the averaging by collecting similar offset traces within adjacent CDPs and adding them together. This process reduces random noise, while maintaining amplitude versus offset relationships. Double-click Seismic Processing > Gather > Super Gather, as shown:

On the Super Gather Parameter dialog, the only change we will make is to change the Size of Rolling Window to 5:

This means that five adjacent CDP’s will be summed to give each output CDP. This will reduce random noise. Notice that the program has defaulted to create output bins with 11 offsets each. This was chosen because that is the average fold of the input gathers. When you have changed the single parameter, click OK to run the process. The result looks like this:


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Notice that the gathers look cleaner and more consistent, with a pronounced AVO anomaly at around 630 ms. Displaying the angle range Now we will display the range of incident angles as a color display.

On the window showing the Super Gather, right-click and select Color Data Volume > Incident Angle, as shown below:


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The dialog which appears specifies the velocity field used to transform from offset to angle:

We have three choices for specifying this velocity field – as a table of control points, using a single log or providing a volume in SEG-Y format:

For large volumes, the first choice is most appropriate, typically importing that table using the Import button at the base of the dialog. For a small volume like this one, we will use the Single Log which ties the volume.


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Select Single Log and the display changes to show the velocities calculated from the check-shot corrected sonic log within the AVO_WELL.

Note that, by default, a 500ms smoother has been applied. Turn OFF the smoothing by removing the check mark:

Now the original unsmoothed sonic log appears. It is usually very appropriate to smooth the velocities used for the angle gather calculation. However, the default 500 ms smoother is rather long for this short well. Set the Vertical Smoother back ON, but set the smoothing at 200 ms. The velocity field now looks like this:


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Finally, click OK to store this velocity field. The Super Gather now shows the incident angle as a color display:

We can see from this display that the maximum incident angle at the zone of interest (630 ms) is around 30 degrees. That information will be used in a later step. Picking the pre-stack data In this step, we will pick an event at the zone of interest and display those picks to observe the AVO anomaly. We will pick the Super Gather volume. First, turn off the color display by right-clicking on that display and selecting Color Data Volume > none:


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Then, hide View 1, so that the Super Gather display fills the window:

Now select Horizon > Pick Horizons:

On the dialog which appears, we must specify which data set we are picking. We are picking the Super Gather in View 2, so this field must be modified:

The final dialog looks like this. Click OK to start the picking process:


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A series of controls appear at the base of the seismic window. These are used for the picking process:

One item, called Mode, determines how picks will be created using mouse clicks:

The Rubber Band Mode means that if you click somewhere, then hold the left mouse button down, move the mouse along the section and release the button, picks will be created in the region of the “rubber band” which appears between the mouse clicks. That is very useful for detailed picking. For a very clean data set like this one, a convenient mode is Left & Right Repeat. In this case, you would click a point that you interpret as being part of the horizon. This becomes the seed point. Picks will be created throughout the entire line based on this point. We wish to pick the Trough which shows the AVO anomaly at around 630 ms. Change both the Mode and Snap parameters as shown:

Then position the mouse cursor anywhere near the trough at 630 ms and click once:


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The entire event should be picked like this:

If your display looks different, check your Mode and Snap parameters and click again. There is no need to delete the original picks. They will be automatically replaced. If we wished to pick a second event, we would select Horizon > New Horizon from the picking dialog:

In this case, we are happy with the single event, so click OK to complete the picking process:

Now that we have picked the event, we would like to see a display of the picked amplitudes.


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To do that, right-click and select View > Show Pick Analysis:

On the Pick Attribute Option dialog which appears, choose the option to Show Pre-stack Picks With Gradient Analysis. This option is based on the two term Aki-Richards equation:

Click Next several times to accept all the defaults for this analysis.


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The second to last page confirms that we are using the Two Term Aki-Richards equation and the velocity field we have defined previously:

Finally, click Next and OK to get the Pick Analysis display:

This display shows the original pick values (in blue) and the calculated Aki-Richards curves (in red). By scrolling through the data volume, we can see that the AVO behavior is most pronounced in the vicinity of the well and flattens out as we move away.


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To remove the Pick Analysis display, right click on the display window and toggle OFF the display:

Angle Gather In this step, we transform from the offset to angle domain. From the Processes tab, double-click Seismic Processing > Gather > Angle Gather:

On the Angle Gather Parameters dialog, we see that we are transforming the volume super_gather into the new volume angle_gather:


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We know from a previous display that the maximum angle is about 30 degrees, so we will change the maximum Angle To to 30, as shown above. Also, note that we are using the velocity field set up in a previous step. When you have modified the Angle Gather Parameters dialog as shown, click OK to run the Angle Gather process. When the process has completed, the Geoview window shows the calculated Angle Gathers:


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AVO Gradient Analysis The next step we will perform is AVO Gradient Analysis. The purpose of this process is to analyze the AVO behavior of one or more events at a particular CDP. To start that process, double click AVO Analysis > AVO Gradient Analysis:

On the dialog which appears, we specify the Input Volume as the super_gather. We also tell the program that we are analyzing the CDP near the well, which is CDP 330:

Default all the remaining parameters by clicking OK at the base of the dialog:

The display which appears shows the seismic gather at CDP 330, along with AVO pick values for the default initial time, which is at the centre of the gather time scale:


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The first thing we can do is improve the time scale of the gather data. One quick way to do that is to select the Fit to View check box:

To zoom in more, click the Zoom In button one or more times:

Right now, the analysis is being performed at the arbitrary time of 550 ms:


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We would like to analyze the AVO anomaly at around 630 ms. So, position the mouse cursor near the trough at 630 ms and click the left mouse key:

The display should now look like this:

The red line on the seismic display shows the time location at which the amplitudes have been extracted. Those amplitudes are plotted as red squares on the right-hand graph. The curve which has been fit through the picks is a plot of Aki-Richards two-term equation. We can confirm this by the information at the top of the graph:

By clicking various time locations on the gather, we could see the equivalent picks and curve for any other event on the gather. Actually, it can often be helpful to see two events at the same time.


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To do this click the Two Events toggle ON:

Now, to make the second event appear, click near the strong peak below the target trough:

Now the display should look like this:

Notice that we are seeing a classic class 3 AVO anomaly with amplitudes increasing for both the trough at the top of the sand (red) and the peak at the base of the sand (green). Notice also that the fit of the AVO curves is extremely good. Mathematically, this is expressed by the normalized correlation between the picked amplitudes and the curves, printed at the top of the graph:


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If we wish to see the same information at other neighboring CDP’s we can modify this selection item:

Notice, also, that the AVO curves are plotted as a function of Offset, because we have used the super gather as input. We can see the same plot as a function of angle this way: Go to this selection box at the top of the graph and change to Angle:

Now we see that the maximum angle for this event is about 30 degrees, as we observed when creating the angle gather:

At the base of the graph we see a series of tabs. One of them, for example, allows us to access the Parameters, which control the calculation of the Aki-Richards curves:


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Another interesting display is the Cross Plot of calculated Gradient against Intercept. This is accessed by clicking the Cross Plot tab:


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The background trend points have been extracted from sample values near the event time around the well location. The red and green squares correspond to the calculated Intercept/Gradient values for the selected events. Note that the locations of these squares are consistent with the interpretation of this anomaly as a class 3 AVO anomaly. AVO Attribute Volume Now that we have examined the AVO anomaly using AVO Gradient analysis, we will apply the calculation to the entire volume to see the distribution of AVO anomalies. To start that, double-click AVO Analysis > AVO Attribute Volume:


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The AVO Attribute process uses the two or three term Aki-Richards equation to extract AVO attributes from the seismic data. The attributes are based on combinations of intercept, gradient and curvature, as defined by the Aki-Richards equation. On the Parameter dialog on the right, we see the input and output volumes for this process:

Because we have created an angle gather volume, this will be used as input. Note that the original offset gathers or super gathers could also be used, but then a velocity field would be needed to convert from offset to angle during this calculation. As output, the program will create several volumes, depending on the Type of Analysis. For the default case of two-term Aki-Richards analysis, the volumes will be called avo_a and avo_b, corresponding to the intercept and gradient. Looking further down the Parameters dialog, we see that the default Type of Analysis is the Two Term Aki-Richards:

That is appropriate for this case because we only have incident angles less than 30 degrees. In order to reliably extract three terms we need high angle data, usually exceeding 45 degrees. Click OK to extract the AVO Attributes using the default parameters. When the process completes, the calculated attributes appear in a split screen:


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The window showing the AVO Attributes actually contains two separate volumes. The annotation at the top of the window shows what is currently plotted:

The wiggle trace data is the calculated Intercept (A). The color data is currently the product of intercept and gradient (A*B). Since this is a class 3 AVO anomaly, we can see a strong positive response at the top and base of the reservoir at 630 ms. Actually, the response is currently obscured a little by the horizon which is drawn over it. Temporarily remove that horizon from the display by right clicking and selecting View > Seismic View Parameters:

On the Seismic View Attributes dialog, select Horizons and No Horizons, as shown.

Then click OK. The seismic display now clearly shows the positive AVO response at the top and base of the reservoir:


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To see another combination of attributes in color, right-click in that window as shown below:

Select Scaled Poisson’s Ratio Change. This is the sum (A+B), which is roughly proportional to the change in Poisson’s Ratio. This produces this attribute plot:

At the top of the reservoir, we can see a strong negative response (orange), indicating a drop in Poisson’s Ratio, while at the base of the reservoir we see a positive response (yellow), indicating an increase in Poisson’s Ratio.


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AVO Cross Plot The final step we will perform on this 2-D AVO example is to create a cross plot of the derived attributes. The purpose of the cross plot is to further investigate the type of AVO anomaly and to delineate cross plot zones which can be mapped to the volume. Double-click Cross Plotting > Cross plot seismic:

The parameter dialog which appears has a number of items which need to be filled in. We are specifying the Cross Plot Type as AVO attributes and the input volume is the avo volume just created in the previous step:

We will analyze a range of CDP’s from 300 to 360:


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We will set the analysis window around the picked horizon, with a window size of 100ms:

When you have filled in the dialog as shown, click OK. The cross plot which appears shows the expected background trend through the origin, with anomalous events in quadrants 1 and 3, consistent with class 3 AVO anomalies.

We can improve this plot by focusing attention on only the peaks and troughs.


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To do that, right-click on the plot area and select Set Data Sample Filter:

Change the Data point filter option to Peaks and Troughs as shown, and click OK:

The new cross plot shows a much simpler character, with anomalies clearly separated from the background trend:


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Now we will highlight the two anomalous zones and project those zones onto the seismic section.

To draw a zone, first click once on the Polygon icon: Then move to the cross plot and draw the shape roughly as shown below, using a series of left-mouse clicks at each of the corners of the polygon and double-click on the last corner to finish the polygon. When you are done, the screen should look similar to this:

Note that the polygon can be modified by grabbing the “handles” and dragging them. Note also that the data area inscribed by the polygon has been highlighted on the seismic section which is now visible in the Seismic tab. If the red zones appear too small, expand your zone by dragging the handles.


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This nicely delineates the top of the sand reservoir (you may want to remove the horizon to see it better). We can name this zone by going back to the cross plot window and typing in a new name:

Now repeat this process for the base of the sand. Click once on the polygon icon. Then draw a polygon around the anomalous points in quadrant 1:

Call this zone Base of sand:

The seismic cross section now shows both the top and base of the sand reservoir delineated:

The cross plot window is now floating over the Geoview window. We can dock it into its tab by clicking the Cross Plots button at the lower right. This will now be saved in the project. If you wish to see this cross plot later, go to the Cross Plots tab, and find it there.


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Part 2: AVO Analysis on a 3D volume We have completed the AVO Analysis of the 2D volume. We will now perform AVO Analysis on a 3D volume. First start a new project. To do that, select the Start tab at the top of the Geoview window:

Then click New Project:

Call this project AVO Analysis Guide 2:


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Once again, we will use an existing well log database. Click Specify database > Open:

On the dialog, select the database avo3d_database and click OK

Finally, click OK on the Specify Database dialog. When you select the Data Explorer tab, the Geoview window now shows the new project with the single well loaded:


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To see the curves in the well, double-click the Well icon in the Project Manager:

This will bring up the following well log curve display on the right hand side of the window. Note that the Wells tab will now be selected:

Using the AVO Analysis workflow In the first part of this tutorial, we used the individual processes, which we called the standard approach. For this second part, we will use an alternate procedure: the pre-defined Workflows. Click the Workflows tab. The window changes like this:


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Each of the items on this tab contains a complete workflow for the specified process. Click the item called AVO Attribute Analysis. The window changes again:

We now see the suggested series of steps to be followed for AVO Attribute Analysis. The steps are colored red to indicate that the parameters have not yet been supplied. These are the “default” steps, but the list can be edited and customized, as we will see later.


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Double-click the first item, Select Pre-stack Seismic:

The dialog which appears on the right allows us to select a pre-stack seismic data set for analysis, which has already been loaded into Geoview. This, of course, is empty. To import a seismic volume now, click the button at the bottom right of the window and select from SEG-Y file:

On the dialog that appears, select the file avo3d_seismic.sgy:


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Click Next at the base of the dialog:

Set the Geometry Type to 3D and click Next:

On the third page, we are telling the program what information it can use from the trace headers. In fact, in this data set, there are Inline and Xline numbers, and X and Y coordinates. That is why we answer Yes to both questions shown below :


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Now, click Next to see the SEG-Y Format page:

By default, this page assumes that the seismic data is a SEG-Y file with all header values filled in as per the standard SEG-Y convention. For example, it expects to find the Inline and Xline numbers at the byte locations shown above. If you are not sure that is true, click Header Editor to see what is in the trace headers. In our case, we believe the format information is correct, so click Next to move to the next page. Now the following warning message appears because the program is about to scan the entire SEG-Y file:

Click Yes to begin the scanning process.


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When the scanning has finished, the Geometry Grid page appears:

Because we have read the required information from the SEG-Y headers, the geometry is correct. Click OK. After building the geometry files, a new window appears, showing how the single well is mapped into this seismic volume:

In this case, the well is mapped to the correct Inline / Xline location because the X and Y locations have been properly set within the Geoview database. If this had not been done previously, you would type in correct values for the Inline and Xline numbers. Click OK to accept the location shown on this window.


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Now the seismic data appears within the Geoview window:

To see the seismic data near the well location, click the Down arrow by the Well icon at the top of the seismic display and select the well:

We can now see a very strong AVO anomaly:


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Now double-click the second item on the workflow, Select Horizons:

As with the seismic case, a dialog appears to select previously imported horizons. Since we have not imported any, click the button Import Horizons > From File:


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From the File Selection Window, highlight the file called avo3d_horizons.txt and click Select:

Note that, at the lower left corner of the dialog, we are specifying this to be a Free Format file. Click Next:

The next page of the dialog specifies how the file is organized:


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Click the View Files button to see the ASCII file:

The file display shows that there is 1 horizon in the file, and that we do not need to skip any information lines. Fill in the format dialog as shown below:

When you have modified the dialog as shown above, click OK and the imported horizon will be displayed on the seismic window:


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Modifying the workflow We have now loaded all the data required for the AVO analysis. The next series of steps consist of data enhancement processes, which often need to be customized for the particular data set. For example, the next suggested step is Mute. While this data looks like a mute might be helpful, we might choose to delay that decision until after the Super Gather and Trim Statics. To do that, select the item Mute and right-click:

Select Move Process Down.


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This dialog now appears, which tells you that a new tab will be created which will contain the new edited workflow:

Click OK on this dialog, and the workflow now looks like this, with the item Mute moved down one level:

Repeat this process one more time - select Mute, right-click and select Move Process Down. Now the item Mute appears after the Trim Statics item:

In addition to moving processes around, we can also add or remove processes from the list.


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For example, let’s remove the last item, Create Cross Plot. Select that item and right click:

On the pull-down menu, select Remove process. The final workflow will look like this:


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Super Gather Now that the workflow has been modified, we are ready to continue with the data conditioning processes. The next process to apply is Super Gather. Super Gather is the process of forming average CDPs to enhance the signal-to-noise ratio. The averaging is done by collecting similar offset traces within adjacent CDPs and adding them together. This process reduces random noise, while maintaining amplitude versus offset relationships. Double-click Super Gather, as shown:

On the Process Parameter dialog, we will accept all the defaults:

This means that 3 adjacent CDP’s will be summed to give each output CDP. This will reduce random noise. Notice that the program has defaulted to create output bins with 20 offsets each. This was chosen because that is the average fold of the input gathers. Click Run to run the process. The result looks like this:


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Notice that the gathers look cleaner and more consistent, but there is still a strong residual NMO which will be corrected in the next step. Trim Statics Double-click the next item on the workflow, which is Trim Statics:

Trim Statics is the process which attempts to correct for residual moveout errors and align the events on the gathers. There are a number of reasons why this process is necessary, including errors in velocity analysis, and non-hyperbolic moveout. Whatever the cause, these residual moveout errors can cause significant distortions to the calculated AVO attributes, so they need to


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be corrected before proceeding. In the Trim Statics process, a pilot trace is formed by stacking each CDP gather. Then, each gather trace is correlated with the pilot trace, using a series of sliding windows. The cross correlations are used to calculate an optimal time shift for that window. Finally, the shifts for the windows are interpolated to produce a time-variant stretch of the trace. The result is to align events with the pilot trace. The dialog which appears shows that the default is to apply the process to the entire volume and create a new output volume called trim_statics:

We will use a series of windows of size 200ms with a maximum shift of 20 ms:

When you have filled in the menu as shown above, click Run to start the process. The result shows a much improved alignment at the target zone around 1200 ms:


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Mute Double-click the next item on the workflow, which is Mute:

The purpose of the Mute process is to remove the far trace amplitudes which are noisy and will contaminate the AVO attribute calculation. In this case, we could probably achieve this result by simply limiting the range of offsets to process. However, generally speaking, this requires a mute which varies with offset, time and CDP location. The dialog which appears allows you to type these mute values into a table. A more convenient option is to graphically draw the mute onto the displayed CDP gathers.


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Right now the Mute dialog is covering up the gather display:

There are two options for temporarily removing the dialog. One is to click on the Parameters name to the right of the dialog:

This causes the dialog to be hidden on the right side of the screen:


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To bring the dialog back, position the mouse pointer over the word Parameters and click again:

An alternate way to move the dialog aside is to click on the “airplane” icon at the top of the dialog:

This causes the dialog to “float” over the main window, allowing it to be moved anywhere, including to another screen:


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When you want to restore the dialog to its docked location on the right, click the “airplane” icon once again. We will keep it floating for this part of the tutorial Now that we have removed the dialog, go to the right-hand seismic display, showing the trim_statics result:

Position the mouse pointer somewhere over the display and notice that the pointer changes to the shape of a pencil, indicating it is now ready to draw the mute:

Using a series of left mouse clicks, draw the desired mute, something like this:


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If you make a mistake and wish to start over, click on this icon:

If the drawing icons are not visible on your screen you must either increase the width of the screen or click on the double arrow shown below on the right:

This will bring up the icons below the main menu options:


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Notice that by drawing the mute pattern you automatically fill in the mute table on the dialog:

When you have drawn the mute, click Run on the Parameters dialog to apply the process. The result should look something like this:


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Setup Velocity Model Double-click the next item on the workflow, Setup Velocity Model:

The purpose of this step is to define a velocity model, which will be used for all subsequent angle calculations. The dialog which appears is one that we have seen before. Once again, we will choose the Single Log from the well, with a 500 ms smoother:


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When you have modified the menu as shown above, click Save to accept this velocity model. To see the effect of this velocity model, we will display the incidence angles. To do that, go to the window containing the mute display and right click as shown below:

The resulting color display shows usable angles at the zone of interest out to about 45 degrees:


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Angle Gather Double-click the next item on the workflow, which is Angle Gather:

The Angle Gather process transforms the input CDP gathers from the offset domain to the incident angle domain, using the defined velocity field. The dialog shows that, by default, we will create 15 angle traces ranging from 0 to 45 degrees:

Since the incident angle display showed that this is the range of usable angles, the defaults are appropriate for this case. Click Run to perform the Angle Gather. The result will look like this:


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Note that the trace headers for the angle-gather are in degrees rather than in meters.

Gradient Analysis Double-click the next item on the workflow, which is Gradient Analysis:

The main parameter to change on the dialog specifies the location for the analysis:


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Normally, we would like to start the analysis at the well location. To get there on the dialog, click on the Well icon as shown above and select the well:

The dialog will now look like this:

Now, click Run to start the Gradient Analysis process. When the window appears, the analysis event is first positioned at the center of the time window. Click on the event above 1200 ms to see the AVO response at the zone of interest:

As expected, the analysis shows a strong class 2 AVO response.


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We can see both the top and base of the anomaly by selecting Two Events and positioning the second event as shown below:

AVO Attribute Volume Double-click the next item on the workflow, which is AVO Attribute Volume:


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In this process, we will analyze the entire 3D volume to calculate the classic intercept/gradient attributes, using the Aki-Richards equation:

Click Run to apply the default parameters. When the process finishes, the display look like this:

By default, the color display shows the product of intercept and gradient (A*B). This is not very appropriate for the class 2 AVO anomaly. To see a better display, right click and choose Scaled Poisson’s Ratio Change, as shown below. This is sum of intercept and gradient (A+B):


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The resulting color display now shows a strong AVO anomaly at the zone of interest, around the location of the imported horizon:

Creating the AVO Attribute data slice We will now create a data slice of the derived AVO attributes around the zone of interest. Double click Create AVO Attribute data slice:


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On the dialog, we choose to display Scaled Poisson’s Ratio Change:

We are also specifying a 50ms window around the imported horizon:

One final change we would like to make is to calculate the Amplitude Envelope of the derived AVO Attribute. To select this, we need to see the Advanced Options. Click that button at the bottom of the dialog:


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This causes a series of extra tabs to appear on the dialog. Select the Advanced tab:

One of the options is to select the type of transform to apply to the data before creating the data slice. Note that we could select several transforms and have all the slices created at the same time. Select Amplitude Envelope as shown below:

Finally, click OK to create the data slice. The resulting data slice looks like this:


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Saving the new workflow We have now completed the workflow and the AVO Analysis project. Notice that the current Workflow list shows two tabs. The User tab indicates that this is a customized workflow, created in this project.

As it stands, the new customized workflow is only available within this project. To make it available to other projects and other users, we need to export the workflow. To do that, right-click anywhere on the workflow:


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On the pull-down menu, select Export Workflow and Parameters. On the dialog which appears, give the new workflow a name, like AVO Analysis Guide, and click OK:

We have now saved the new workflow, and the parameters used in this project, to two separate files. To import the saved workflow and parameters into a new project, click on the Import Workflow button at the top of the Workflow dialog:


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On the dialog which appears, we see the two files which have been created:

The file with the shorter name, AVOAnalysisGuide_workflow.xml, is the list of process names in the new workflow. This is the file we need to import if we wish to use the chosen steps in a new project. The other file, AVOAnalysisGuideAVOAttributeAnalysisParameter_parameter.xml, is the complete list of parameters used in this current project. If we import this second file, as well as the first, the dialogs which are created will have exactly the same parameters as used previously. Thus, the combination of both files together will be a reproducible history of the project. Double-click each of the files named. The right side of the dialog now changes to this:

If we now click OK, we will import both the list of processes and their parameters. For this tutorial, click Cancel on this dialog.


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Exiting the Project We have now completed the AVO Attribute Analysis tutorial. To exit the Geoview program, click on File > Exit on the upper left hand corner:

There is no need to save the project, as Geoview has automatically saved it for you.

Documents

AVO Analysis Guide