AVO Fluid Inversion Guide Introduction..................................................................................................................................... 1 Starting GEOVIEW ........................................................................................................................ 5 Starting AFI .................................................................................................................................... 7 Loading the Seismic Data ............................................................................................................... 9 Creating the Data Slices................................................................................................................ 15 Performing the Trend Analysis..................................................................................................... 29 Editing the Stochastic Model Parameters ..................................................................................... 35 Running the Simulations............................................................................................................... 39 Calibration of the Real Data ......................................................................................................... 44 Applying to the Real Data............................................................................................................. 50 Using Angle Stacks in AFI: .......................................................................................................... 56

AFI 1

Guide to AFI Introduction AVO Fluid Inversion (AFI) is a program which is used to analyze AVO responses, compare them with theoretically derived responses, and predict fluid properties. This process can be thought of as an extension of conventional AVO analysis in this way: Conventional AVO analysis is typically used to determine the fluid properties of a target reservoir. AFI attempts to determine, in addition, the probability or likelihood that this determination is reliable. It is a tool for analyzing and understanding the uncertainty in the AVO process. AFI starts with the assumption that the target reservoir can be represented by a 3-layer model, with a sand layer enclosed by shales:

Each of the parameters in this model is actually described by a probability distribution, which encapsulates our uncertainty about the value of that parameter. The shales are described by distributions for the basic parameters, PV , , and . The target sand is described by a range of more basic petrophysical parameters as shown below:

SV Density

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The first task in the AFI analysis is to set up these probability distributions. One convenient way of doing this is by using trend analysis on well logs in the area. As we will see in the sections to follow, these distributions will usually vary with burial depth. Once we have established probability distributions for the model, the next step is to generate a large number of possible “realizations”, i.e., particular 3-layer model examples consistent with these distributions. This is also called Monte-Carlo analysis. From each of these models, synthetic traces are calculated internally for the purpose of predicting the Intercept and Gradient consistent with that model:

By repeating this process numerous times, we generate a simulation analysis, which shows the type of response expected for each of the fluids:

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The cluster analysis provides two types of information. First, the degree of separation of the clusters tells us how well AVO can be expected to distinguish those fluid types under ideal circumstances. If two clusters overlap significantly, that means we can probably not resolve them. Of course, the degree of overlap or separation depends on the probability distributions, which depend on things like the burial depth, average velocities, densities, etc. The second use of the cluster analysis is to compare real data points with the predicted points, and make probability predictions. The real data which AFI deals with are amplitude slices from 3D pre-stack volumes. By superimposing the real data points over the simulated points, we can visually determine the likely fluid for those points:

Before doing this, we will have to determine scalers, which account for the overall scaling differences between the synthetic and real data.

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Finally, using Bayes’ theorem, we can calculate the most likely fluid, as well as the probability of hydrocarbon for each of the points in our real data slices:

In summary, the steps in the AFI process are:

(1) Read in the 3D pre-stack volume. (2) Create data slices at the horizon of interest. (3) Extract a wavelet. (4) Perform trend analysis of wells in the area to determine probability distributions. (5) Run simulations to produce model clusters. (6) Perform calibration with real data to determine scalers. (7) Calculate indicator and probability maps.

In this guide, we will perform all these steps on an AVO data set.

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Starting GEOVIEW The data for this AFI guide consists of the following:

(1) A set of logs for a single well, already loaded into a GEOVIEW database. (2) A 3D pre-stack SEGY volume. (3) A single horizon which has already been picked for that volume.

To start the process, start the GEOVIEW program. On a Unix workstation, do this by going to a command window and typing:

GEOVIEW On a PC, GEOVIEW is initialized by clicking on the Start button and selecting the GEOVIEW option on the Programs / HRS applications menu. On the Open Database menu, click Open and click OK:

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On the Directory Chooser window, select the AFI_database.wdb database and click OK.

When the database has been read in, the Well Explorer appears, showing you a list of all the wells in the database (there is only one in this project):

Select the avo3d_well well in the Well Data List, and click Display Well.

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This shows all the curves loaded in this well:

To close the Well Display, select File>Exit on the window showing the log curves. Starting AFI Now that we have loaded the GEOVIEW database, we will start the AFI program. To do that, go to the GEOVIEW main window, click the AVO button, and select AFI:

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When this window appears, select the default, which is to start a new project, and click OK:

Name the new project AFI_guide, as shown below, and click OK:

The AFI window appears:

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Loading the Seismic Data Now that we have started the AFI program, the next step is to load the 3D SEGY volume and the picked horizon. To load the seismic volume, select Data Manager>Import Data>Open Seismic>From SEG-Y File:

On the File Selection window, select the AFI_seismic.sgy file and click Next >>:

The next page asks whether this is a 3D volume or a single line:

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Accept the default on this page and click Next >> to get this page:

Since the SEGY file contains complete header information, including X & Y coordinates, just continue to click Next >> on this and the subsequent page to accept the defaults. The file will then be scanned. Then the Geometry Grid Page appears.

After clicking OK on this final page, the file will be read in. The Seismic List window shows the sole volume read in . You can close this window.

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Also, the following window appears, allowing you to verify that the well log is being inserted in the proper location:

Click OK on this window and the AFI window will look like this:

The well has been inserted at Inline 31. To see this displayed, type 31 in the Inline Display window and press <ENTER> on the keyboard:

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After scrolling over to Xline 100, the window will look like this:

Now we will load the picked horizon for this data. Select Horizon>Import Horizons>From File:

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Select the AFI_horizon.txt file and click OK:

Fill in the next two pages as shown below:

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When the pick file has been loaded, the display now looks like this:

Note that since we have imported a post-stack pick file, the picks are set as constant within each CDP gather.

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Creating the Data Slices Now we have all the data required to begin the AFI analysis. At this point, we have to decide between two possible workflows as shown in this figure:

First, we will do workflow 1, i.e., we will pick the pre-stack data and create attribute maps using those picked values. The easiest way to do the pre-stack picking is to start with the imported post-stack picks and ask the program to automatically pick the nearest events on the pre-stack data. To start this, select Horizon>Pick Horizons:

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When the window appears, select Pick Existing Horizon and click OK:

On the Options menu which appears, choose the Automatic Picking option:

On the Automatic Picking window, change two fields – one says that we are picking a Trough and the other that we wish to replace the Actual Picks:

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This Replace item tells the program not to fill in picks for any CDP’s which do not have a post-stack pick as input. Click OK on this window and the pre-stack data will be picked. When it has finished, the AFI window looks like this:

Click OK on the Picking section at the bottom to accept these picks. Now we will create a set of AVO attribute maps, using the amplitudes at these picked locations. To do that, select Process>AVO (Prestack)>AVO Attribute Map:

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On the first page of the window, accept the default, which sets the name of the map:

Click Next to get the second page:

On this page, accept the default which is to do a 2-term A/B or Intercept/Gradient analysis. Click Next >>. The last page sets all parameters for calculating the A/B maps. One of the requirements is a velocity field, used to calculate incidence angles. This has not yet been set, so we will use a well log for this process. Click Open Well Log:

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Click first on the Well Name, avo3d_well, in the Well list to select that well:

Then, when the log appears in the Log list, click on the P-wave1 log:

The window will now be complete, so click OK on this window:

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The original Create Attribute window now looks like this, showing that we have now set up the velocity field:

We will accept all the defaults on this page for calculating the Intercept and Gradient. Click Next >> to get the last page, which allows you to choose which attribute maps to create:

Select the 3 maps shown above. We need at least the A and B maps to do the AFI analysis. We will also use the Scaled Poisson’s Ratio map (A+B) as a handy analysis display for highlighting regions of special interest. Click OK and the maps will be produced. We now have 3 maps, which can be thought of as the result of conventional AVO analysis. Our normal procedure, at this point, would be to interpret the 3 maps to determine likely hydrocarbon locations. What AFI will do further is to interpret the maps automatically and provide us probabilities of hydrocarbons at each location. To proceed with this it is useful to isolate specific locations on the maps which are most interesting. We will define zones on the Scaled Poisson’s Ratio (A+B) map ("AFI_horizon SPR"). This is an especially good map for this purpose, since, under ideal circumstances, A+B should be proportional to the change in Poisson’s Ratio. This makes it a good indicator of hydrocarbons. One problem with the Scaled Poisson’s Ratio map we have just created is that the color range is not ideal.

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To change that, go to this map and select View>Change Color Amp:

On the window which appears, click Recalculate Color Amplitudes:

This will force the numbers to be automatically recalculated. Now click OK on the Color Amplitude Range window. The resulting plot looks like this:

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Now we would like to highlight four zones which are especially interesting. The purpose of this will be to calibrate the real seismic data slices with synthetic data, which we will calculate later. For this purpose, it is useful to highlight both potential hydrocarbons as well as likely wet zones. The four zones which we will highlight are shown on this figure below:

There are two zones, HC Zone 1 and HC Zone 2, which we believe are possible hydrocarbons, and two zones, Wet Zone 1 and Wet Zone 2, which are likely wet. Note that the exact definition of these zones is not important, nor are the names. They are just convenient locations, which we will display later on the Intercept / Gradient cross plot. Now, create the first zone this way. (You may want to zoom your plot and fill as much of the screen as possible to make this easy).

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On the window showing the Scaled Poisson’s Ratio, select View>Zones>Define a zone graphically using>Polygon:

Now position the mouse cursor over the region you wish to draw for the first zone. The way to draw the polygon is this: • Click the left mouse button to create the first point. You will then see a rubber band

connecting the cursor as you move it around.

• Then move the cursor to the next corner and click the left mouse button again. This will tie down that corner.

• Keep doing this until you have created all the corners of the desired polygon.

• Finally, click the right mouse button to “close the loop”.

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When you do this, a window will appear in which you will fill in the name of this zone:

Note that your numbers will be different since they depend on exactly how you have drawn the zone. Fill in the name as shown above, and click OK. The plot should now look similar to this:

Don’t worry if it is not exactly the same. If you are unhappy with the zone and wish to draw it again, select View>Zones>List Zones. That will bring up a window which allows you to delete a zone and draw it again. Continue with this process until you have drawn the four zones and named them as shown above. Create the other zones: HC Zone 2, Wet Zone 1 and Wet Zone 2, using the View>Zones menu each time. Note that even though we defined these zones on the Scaled Poisson’s Ratio plot, they can also be displayed on the other plots by selecting View>Zones>Display All Zones.

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For example, the A or Intercept display will look like this:

After creating the four zones as shown above, remove all three data slices from the screen by selecting File>Exit on each display. There is one more data slice which we will create: this is the input picked horizon converted to depth. This step is optional, and is only useful when we consider the target horizon to have significant depth variation. In this case, we will do it to show how it is done. On the main AFI window, select Horizon>Time-depth Conversion:

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On the first page of the window, select this input horizon, AFI_horizon, to be converted:

Click Next >> to get the next page:

By default, the program will use the velocity which was used previously to calculate the AVO Attribute Maps. Click Next >> to accept these defaults. The last page of the window sets the name of the output depth horizon:

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Click OK to calculate the horizon. The program will ask whether you want to display the map. Answer Yes to display it.

The final step in the real data analysis is to extract a wavelet from the seismic. This is necessary because it is used to determine the influence of layer thickness and event tuning on the model. For this purpose, the exact phase of the wavelet is not required, just the amplitude spectrum. So, we will extract a statistical wavelet from the seismic data alone. Select Process>Wavelet>Extract Wavelet>Statistical:

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On the first page of the Extract Wavelet window, set the parameters to use only a single Inline (31):

On all subsequent pages, accept all the default parameters by clicking Next >> twice and OK. The extracted wavelet will look like this:

Close the Wavelet and Map windows through File>Exit or the X at the top right corner, but do not close the AFI Seismic window.

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Performing the Trend Analysis We have now imported and prepared all the data required for the AFI analysis. We are ready to do the first calculation step in that process. As we described at the beginning of this Guide, the major steps in the analysis are:

(1) Define probability distributions. (2) Run simulations (3) Calibrate the real data (4) Create probability maps from the real data.

In the first step, we need to define probability distributions for the 3-layer model at one or more depth locations. A convenient way to derive many of the required parameters is to analyze well logs from the area. This is the process called Trend Analysis. To start this process, select Model>Analyze Trends from the AFI Seismic window:

A multi-page window appears, which sets the initial parameters for this analysis. Some of these parameters we will set later after we see the initial display. On the first page, we select which logs to use:

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The program can use these logs for setting parameters (if available): - sonic log (P-wave) - density - porosity

In addition, the program can use one extra log to differentiate sand from shale. Typically, this is a Gamma Ray log, but could also be a Vshale log, or in fact, any other log. In this case, we use the default P-wave, Density and Gamma Ray. In addition, we must select which wells to use:

The first box, All usable wells in database, shows all wells which have the logs selected in the previous box. In our case there is only one well, and the default is to use it. There is one other item on this page:

This item is only useful if there is more than one log. In that case, we would like to line up the logs in depth before displaying them on a common cross plot. The option specifies a particular formation top to be aligned at a specified depth. Since there is only one log, we leave this option unselected. Click Next >> to get the next page. On the second page, we set the condition for distinguishing sand from shale:

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Change the condition numbers to 55, as shown above. Note that any available log curves could be used here. The other parameters on this page set the blocking interval, which is used to limit the amount of data being plotted on the cross plot, and the option to Normalize the condition log, i.e., the Gamma Ray. We will leave these defaults.

Click Next >> to get the next page. The last page is used to set the depth locations at which the stochastic model will be created:

To start, there are no locations set. This is fine for now. We will change this later, after looking at the display.

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Click OK to see the Trend Analysis Display:

The display initially shows two trend plots: the shale velocity and the shale density. The scattering of points have come from the P-wave and density logs from the chosen wells. The red points are the shale points (as determined by the Gamma Ray log) and the grey points are “not-shale”, i.e., sand. The blue lines show the automatically generated trend curves. The smoothness of these lines has been controlled by one of the parameters on the input window. Click the Next button:

The sand velocity and density trends are now displayed. Now change the log which is shown in View 2 and click Redraw:

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This will show the Gamma Ray, which is the Condition Log in this case.

Reset View 2 to Density. We wish to set depths at which to extract the velocity and density distributions from these curves. To do this, select Parameters>Set Parameters:

This brings up the Analyze Log Trends window once again. Click Next >> twice to get to the third page. Now we will set up a series of depth control points around the depth of interest, which is 1500 m. Change the window as shown here:

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Click OK to see the new display:

We can now see four vertical blue lines indicating the locations at which we will calculate stochastic models. To begin this process, select Parameters>Update Model on the Analyze Trends window:

This will save the depth information on the stochastic models (the four blue vertical lines in the plot).

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This window appears, confirming that we wish to create these control points:

Click Yes. Finally, select File>Exit to dismiss the Analyze Trends display.

Editing the Stochastic Model Parameters Now we wish to examine the parameters which have automatically been set up for us, and edit some of them. To do this, return to the AFI Seismic window and select Model>Edit Existing Depth:

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This window will appear, asking you which of the 4 depth levels you wish to edit:

Actually, we will make changes which affect all the levels. Click OK to edit the deepest one. The window which appears is a multi-page window containing all the parameters for the statistical distributions at this depth level:

For detailed information on all these parameters, see the online AFI Help documentation. In this guide, we will give an overview of the most significant parameters.

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The currently visible first page contains the parameters for the base wet sand. Some of these parameters, (Density, VP, and Porosity), have been inserted into this window from the trend analysis information at this level. Other parameters, (Shale Volume, Water Saturation, Biot-Gassmann parameters), have been defaulted. Finally, there are 3 parameters at the bottom left (Matrix Density, Dry Rock Bulk Modulus, and Dry Rock Poisson Ratio), which have been calculated from the others. These parameters are used in the subsequent calculations to replace brine with oil and gas. For a detailed explanation of this theory, see the online AFI Help documentation. Normally, we do not change the values on this page, and we will accept the defaults this time. Click the Shale Parameters tab to see the next page:

The Shale Parameters page sets the probability distributions for the shale layers above and below the sand layer. These parameters have been set automatically from the trend analysis and show a Normal distribution for VP and Density. Since we did not have any VS logs in the trend analysis, the option has been selected to use Castagna’s relationship to calculate VS from the VP value. Also, a 10% random error has been added to the calculated VS value. We will accept these defaults.

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Click the Brine tab to see the next page:

This page sets the parameters for Brine which are used in the Biot-Gassmann analysis. The two parameters are Brine Modulus and Brine Density. By default, these are constant values derived from the literature. They may also be set as probability distributions, if desired. However, we will leave the default constant values. The next pages, Oil, Gas and Matrix, are all similar to the Brine page. We will leave the default constant values. Finally click the Reservoir tab to bring up the last page:

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The Reservoir page is used to set the range of parameters in the sand reservoir to model. Note that we will model a Uniform range of thicknesses, from 0 to 50 meters. Also, click this option:

This is because we want these model parameters to be used at all depth levels in the model, and not just the current level. We have now completed the changes for this depth level, so click OK on this window. When this message appears, click Yes:

If you wish, you could examine the parameters for the other 3 depth levels, but this is not necessary since we have already changed the thickness parameter for all levels. Running the Simulations Now that the stochastic parameters have been defined at 4 depth levels, we are ready to run the simulations. To do this, select Simulations>Run Simulations:

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The Run Simulations window appears:

This window sets the parameters which will be used to generate simulations or model points in Intercept/Gradient from the stochastic 3-layer model. For a detailed description of this process, see the AFI online Help. Usually the default values on this window are acceptable. In particular, we are choosing to generate 200 points each of gas, oil, and brine. We are using Zoeppritz’ equations to calculate synthetics at 15 and 30 degrees, from which the Intercept and Gradient will be extracted. These angles do not have to correspond to the real data angles, since we are only using them to calculate the Intercept and Gradient. These attributes refer to the Top of Sand. We are modeling the sand thickness, using the extracted wavelet, wave1. Click OK to run the simulations.

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When the simulation calculation has completed, this window appears:

Click OK on this window to see the simulation display at the first depth level (1000m):

This shows the expected values of Intercept/Gradient for gas (green), oil (red), and brine (blue). To scroll through the other depth values, click Next >> and << Prev. It is also convenient to display more than one cross plot at a time. To do this, first dismiss this window by selecting File>Exit.

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Then select Simulations>Display Simulation Result:

When the window appears, change the number of windows to Four and select Automatic Scaling:

The plot now looks like this:

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We can clearly see from this display a gradual change in behavior with depth. Another interesting option is to see the effect of changes in a single variable. To do that select Calibration>Base Menu On from the Simulations window:

This section now appears at the base of the display:

This section allows you to change one of the parameters and see immediately the new clustering of points. For example, change the Porosity to a Uniform distribution and change the Max from .05 to .20. Then click Update Model:

The new plot shows the clusters moving to the lower right hand quadrant:

Finally, select File>Exit to dismiss this window.

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Calibration of the Real Data Now that we have created the stochastic model at four depth levels, we would like to compare the synthetic points with the real data points extracted previously. Before that can be done, we must calibrate the data. This means we must derive scalers which can be applied to the real data points to convert them to the same amplitude range as the model data. The reason why scalers are required is that, while the simulated data accurately reflect the theoretical range of intercept and gradient for reflection coefficients, the real data values typically have an arbitrary scaling. There may also be an error in the gradient due to processing limitations. AFI assumes that the correction of the real data is effected by two numbers:

globalS = a number which multiplies both the intercept and gradient values

gradientS = a number which multiplies only the gradient values If the input (unscaled) real data values are inputI and , then the output scaled values will be: inputG

*output global inputI S I= * *output global gradient inputG S S G=

To begin the process of determining these scalers, click the Calibration button:

On the first part of the window, we specify how many cross plots we want and which zones to display:

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Note that these are the zones defined previously on the real data slices. Change the window as shown above. Note that all of the zones in the Selected zone list will be included, regardless of which zone is currently highlighted. The highlighting in the Selected zone list is only used for deleting selected zones, and not for calibration. On the second part of the window, we specify the name which was given to the Simulation result previously generated:

In addition, we tell the program to use the depth slice AFI_horizon_depth in the process. This depth slice is used this way: all the points for the Selected zone in a particular cross plot are compared with the depth slice to determine the average depth for those points. Then the depth level nearest the average is used for that cross plot. Change the window as shown above. Finally, the lower part of the window specifies how the real data slices were generated:

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Change the window as shown above and click OK. The display looks like this:

The black dots are the real data points from each zone. The colored dots are the simulated model points. There is a large difference in scaling between these two groups because we have not calculated the proper scalers yet. In fact, both scalers are set to 1, as shown on the lower left of the display. As a result, most of the real data (black dots) are outside the plot range for these cross plots. The first thing to do is calculate the scalers automatically. This process works with the following steps:

(1) Select a series of wells which tie the current volume. (2) For each well, select the default p-wave, s-wave, and density logs. If p-wave is not

available, ignore this well. If s-wave is not available, calculate it using Castagna’s equation. If density is not available, calculate it using Gardner’s equation.

(3) Using the Current Wavelet in the project, calculate two traces, one at incident angle 0 degrees and one at 45 degrees. From these traces, calculate the model intercept and gradient, MI and MG . Note that each of these is a trace over the entire available time range.

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(4) Now, from the real data volume, collect pre-stack traces from bins around each well location. This is very similar to the Composite Trace extraction in other AVO processes. From these pre-stack traces, calculate the real intercept and gradient, RI and . RG

(5) Using a user-defined time window calculate the root-mean-square values for each of these: , , , RMS

MI RMSMG RMS

RI RMSRG

(6) Finally, calculate the desired scalers

RMSR

global

RMSM

gradient

RMSR

RMSM

global

GSGS

IIS

∗=

=

To start the process, click Apply Auto Scaling From Volume:

On the first page of the window, select the well to use in the analysis:

Click Next >> to get the second page:

This page specifies how the real data traces around each well location will be extracted. We will accept these defaults. Click Next >> to get the next page:

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We will set a time window above the main horizon we are attempting to interpret, thus ensuring that the calculation of the scalers is independent of the target horizon. Set the window as shown above and click Next >> to get the last page:

This page sets the parameters for the Intercept / Gradient calculation. These are the same as we used in the calculation of AVO Attribute Maps previously. Accept these defaults by clicking OK.

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When the calculation is finished, this message appears, showing you the calculated scalers:

Click Yes to see the result applied to the cross plots:

We can see that both wet zones fall on the wet clusters, while both hydrocarbon zones fall on the gas clusters. If we want, we can modify either of the scalers manually, by typing in new values and clicking on Apply Manual Scaling. For now, we are happy to accept this result.

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Click OK at the bottom of the display:

This message appears, confirming that you want to save these derived scalers for the next step in the process:

Click Yes to save these scalers. Applying to the Real Data The last step in the AFI process is to apply the probability calculations to the entire real data map and create probability maps. To start that process, click Apply on the AFI Seismic window:

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The window which appears consists of three parts. The first part lists information about the Real Data Slices:

This information is already correct, since we supplied it when doing the calibration. Do not change this part. The second part of the window specifies the name of the Simulation Data set which we are using for the analysis:

This is also correct. The third part of the window lists the outputs we wish to create:

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The last item, Minimum Acceptable Probability specifies that unless a fluid has a higher probability than 60%, we do not want to see it plotted on the Indicator Map. Change the window as shown above, and click OK. Four maps are now produced. One of them is the Indicator Map:

This map shows the most likely fluid at each location. Many points on the map contain no color at all – these are points where the probability is less than 0.6. The cluster of yellow points indicates most likely gas locations. The other maps show the probability associated with each of the fluids.

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For example, the Gas Probability map looks like this:

From this map, we see that the probability associated with the gas region is around 0.6. This is the Probability of Hydrocarbon map:

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From this plot, we see that the probability of Hydrocarbon (Oil or Gas) is above 80%. Finally, we can verify that all the new Data Slices are stored in the Data Slice Archives for future use. To see this select Data Manager>Data Explorer from the AFI Seismic window:

The Data Explorer allows you to see all the data in the project, conveniently grouped. Under the Data Type item, you can see these types:

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If the Data Type is set to Data Slice, we can see all the newly created Data Slices, along with the previously generated AVO Attribute Maps:

Double-clicking on any one of these items causes it to be displayed. Close the Data Explorer with File>Exit. Close the map windows, but not the AFI Seismic display.

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Using Angle Stacks in AFI: In the last part of this AFI guide, we will use the alternate workflow for the real data analysis. As described previously, there are two possible workflows:

So far, we have completed workflow 1, on the left side. One potential problem with this choice is that it requires the picking of pre-stack data. This may often be difficult or impossible. An alternative is to use workflow 2, on the right side.

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To start this process, go back to the main AFI window and select Process>Stack>Common Angle Stack:

On the first page of the window, specify the name of this stack as 15_degree_stack:

Click Next >> to get the next page. On that page, default all the parameters, except to specify that we want 1 angle range from 5 to 25 degrees:

By this choice of parameters, we are actually stacking all samples with incidence angles between 5 and 25 degrees. We call this a 15-degree stack because that is the value in the middle of the range.

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Click Next >> and OK to produce the stack:

Now we will pick the amplitude near the picked horizon on this stack. Select Process>Slice>Create Data Slice on the 15 degree Stack Seismic display:

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On the first page of the window, accept the defaults:

Click Next >> to get the second page:

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Here, we are changing a number of parameters. We select the data slice Near a Picked Event. The picked event is AFI_horizon. We are looking at a window of size 10 ms centered on the horizon. The amplitude is the Minimum value (unselect all others), because we are tracking the trough which is the top of the sand layer. After you have filled in the window exactly as shown above, click Next >> and OK to produce the amplitude slice:

Now we will create the second angle stack. Go back to the main AFI Pre-stack Seismic window, which shows the input gathers, and again select Process>Stack>Common Angle Stack:

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On the first page of the window, give this result the name 30_degree_stack:

Click Next >> to get the second page:

On this page, change the angle range to 20 to 40 degrees, as shown above. Click Next >> twice and OK to get this stack:

Now, using this stack result, create the amplitude slice exactly as before.

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Select Process>Slice>Create Data Slice on the 30 degree Stack Seismic window:

The first page specifies the data to use in this analysis. This should be correct as shown:

Click Next >> to get the second page.

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Once again, change the parameters on this page, exactly as shown below, with only Minimum selected in that menu:

Click Next >> twice and OK to create the amplitude map for 30 degrees:

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Note that we can still display the previously selected zones on these maps. For example, go to the last map created (30_degree_stack) and select View>Zones>Display All Zones:

We see that the zones are still roughly right, although we may choose to redefine HC Zone 2, based on this new map:

Since the precise definition of the zones is not critical for this analysis, we will leave them as originally defined. Select File>Exit on the map display to remove it from the screen. We are now ready to apply the AFI probability analysis to the new maps. In theory, we do not need to perform the calibration again, since we have done this in the early part of the guide. However, we can check how the previously derived scalers fit the new data maps. To do this, click the Calibration button. Note that it does not matter whether you use the menu from the original pre-stack, the 15 degree gather or the 30 degree gather seismic display windows, as the input is the same.

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On the Calibration window, the only changes are to the parameters which specify the Real Data Slice:

Fill in the window as shown above, and click OK. The new Calibration Plot appears:

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As expected, the previously derived scalers fit the new data values very well. Click Cancel to remove this display. To apply the probability calculations to the new data slices, click Apply on the 30 Degree Stack window:

Because we have just done the Calibration step, the Apply window has already been changed to reflect the new data slices:

Also, looking at the bottom of the window, we see that, by default, this application will overwrite the previously created probability maps:

You may change these names if you wish, but as the new maps should be more accurate because of the calibration, overwriting is no problem.

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Click OK to create the new maps:

As expected, the indicator and probability maps are very similar to those created using Workflow 1. We have now completed the AFI Tutorial. To close down the AFI program, go to the main Pre-stack Seismic window and select File>Exit Project:

On the two windows which appear, click Yes to save the project before exiting:

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