MESA Expert This series of exercises will introduce you to many of the options that are available in MESA for the design and QC of surveys and subsurface models. It is a good idea to refer to the MESA user’s manual for more details about the features described in these exercises.
GMG Expert Files.....................................................2 Ex #1: Basic MESA Expert Usage...........................3 Ex #2: Building Models..........................................14 Ex #3 Expert Attributes.........................................26 Ex #4 Converted Waves.......................................45 Ex #5 Converted Waves (Part II)..........................63 Ex #6 Smart Aperture Tool....................................81
GMG Expert Files The files in the following list are generated by MESA Expert. These files are a combination of ASCII and binary. Not all of these files will be found with every database.
File Extension
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Format
Description
*.gmf
ASCII
Model information
*.idd
Binary
Image ray attribute information
*.ird
Binary
Image ray information
*.mdd
Binary
Model attribute information
*.ndd
Binary
Normal ray attribute information
*.nrd
Binary
Normal ray information
*.odb
Binary
Offset ray information
*.rdd
Binary
Offset ray attribute information
*.srd
Binary
Smart ray information
EXERCISE #1 -- Basic MESA Expert Usage This exercise shows the basic sequence of steps for using MESA Expert. You will be laying out a geometry, creating a subsurface model, raytracing the model, calculating attributes, and then creating a synthetic from the raytracing results. This exercise uses a 2D survey and a “pseudo” 2D model as a simple example. 1)
Start MESA or select New Database from the File menu. Lay out the receivers and sources using the values shown below. Save this survey as “expert01”.
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2)
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Shoot the survey with a 1x240 template. Create a bin grid with the following parameters and calculate the fold, offsets, and azimuths information.
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Choose Launch Model Builder from the Expert menu. Press the File New button to create a new model. Fill in the “Model Definition” dialog as shown to define your model space. Save the model as “expert01”.
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The display shows a map view of the model space. Draw a cross section on the model that has a Y coordinate of 0 for the start and end of the section. There must be just one cross section that only has two points and it must extend all of the way across the model space. This will allow you to use the 2.5D gridding option.
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5)
Switch to Horizon View. Press the New Horizon button and draw a flat horizon at a depth of -45.
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Draw four more horizons. Horizon2 should go from a depth of -345 on the left side of the model to a depth of -450 on the right. Horizon3 should go from -545 to -650. Horizon4 should go from -695 to -800. Horizon5 should be flat at -1250. As you add new horizons, you will be prompted to define the position of the horizon relative to existing horizons. If you define the model from top to bottom, then select the last item in the order list— otherwise, insert the new horizon into the proper, logical place in the sequence.
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7)
Select Gridding from the Model Grid button pulldown menu. You have to grid all of the horizons to extend them across the 3D model space. Choose Top in the “Horizon Selection for Gridding” list. Select Model Extent in the “Grid Method” list. This sets the Top horizon to the top of the model space.
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Select the other five horizons in the list and choose 2.5D from the “Grid Method” list. Press the (Un)Select All Horizons button to highlight the five horizons and the top horizon. This will activate the “Inc. (dx=dy) edit box. Change the “Inc. (dx=dy)” field to 6.25. Press OK to grid the model.
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Select Display – 3D Window to look at the results of the gridding.
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Switch to Velocity View. Choose each layer in the list and set the velocities and density. Layer
Vp
Vs
Density
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Top – Horizon1 Horizon1 – Horizon2 Horizon2 – Horizon3 Horizon3 – Horizon4 Horizon4 – Horizon5 Horizon5 – Bottom
800 2800 4600 4200 4820 5500
440 1540 2750 2310 2651 3025
1.789 2.448 2.829 2.709 2.803 2.898
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Save the model and exit Model Builder.
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Select Load Model File from the Expert menu. Load the “expert01” model. The model name will be listed in the Legend after is loaded. You can view the model and the survey in the 3D Window. You could also calculate Model Attributes at this point.
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Choose Expert – Launch Enhanced Raytracer to start the raytracer. The model and survey files will be automatically loaded.
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14)
Select Raytracing – Parameterize. The model and geometry will already be specified. Press the Create New Database radio button. Press the Output Database button and name your raytracing results file “expert01_enhanced”.
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Press the Parameterize button and choose the ray types to create and which horizons to use in the raytracing. Select P-P Waves and Head Wave from the P Wave options. Select all five interfaces in the Select Interface(s) listbox. Press OK and then press the Trace Rays button to create the ray files. Exit the program after raytracing is completed.
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Load the expert01_enhanced_pp.odb ray file into MESA by selecting Load Offset Ray File in the Expert menu. The ray file name will be listed in the Legend to show that it has been loaded.
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Select 3D Window from the Display menu. Press the Scene Information button. Go to the “Offset Rays” tab and choose all of the horizons to display rays from all of the horizons. Go to the “Sources” tab and select a source in the list to display the rays for that source. The rays are colored by their source – receiver offset.
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Choose Expert Attributes from the Expert Attributes button pulldown menu. This opens the Expert Attributes Manager Window. Press the Create Offset Ray Attributes button. Select the Calculate All Attributes radio button and enter a name for the attribute set such as “horizon 5”. Uncheck the Save CRP Fold to Named Fold Calculation checkbox and choose Horizon5 from the “Target Horizon” list. Press OK to calculate all of the attributes for Horizon5.
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There are eight raytracing attributes that can be displayed in map view. Select the CRP Fold radio button from the Display listbox. Press the Expert Attribute button in the Design Window to display the CRP Fold. You can leave the Expert Attributes Manager dialog open while you view the attributes in the Design Window. Choose a different radio button in the Display listbox to view that attribute.
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20)
You can create synthetic trace gathers based on this rayset. Select Gather Window from the Advisor menu. Choose Expert Synthetic Parameters from the Display Expert Synthetic Gather button pulldown menu. Set the Wavelet Type to Ricker. Uncheck all of the Noise Events. Uncheck Top Horizon and Horizon1 in the Horizons list. Set the Trace Length(ms) to 1000. Press OK to generate traces from the specified horizons.
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Choose the Select Source option from the Select Sources button pulldown menu. The Design Window will come to the front. Click on a source in the center of the survey. The trace gather for that source will be displayed. Apply a gain to the traces by pressing the
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Gain and Filtering Parameters button. Select a Mean AGC with an AGC Length of 50 ms.
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Next we’ll examine a few bin gathers. Choose Numeric Entry from the Select Bins button pulldown menu. Enter 1100 for the Starting Bin and 1500 for the Ending Bin. Select Trace Processing under the Display Expert Synthetic Gather button pulldown menu.
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Check the Stretch Mute, Apply NMO, and Stack Bin Gathers checkboxes. Set the stretch mute Percent to 0.25. Press OK to see the stacked gathers.
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EXERCISE #2 -- Building Models This exercise demonstrates how to build depth or time models in Model Builder. You will use several different methods to create horizons, including ASCII import, a background image, and a SEGY file. The second model in this exercise is the Laurain model and it uses the parameters and model image presented in Laurain, R., and Vinje, V., 2001, PreStack Depth Migration and illumination maps: Expanded Abstracts, SEG 71st Annual Meeting, San Antonio, MIG2.7. 1)
Select Launch Model Builder from the Expert menu in MESA. Select New Model from the File menu in Model Builder. Change the Z value from meters to milliseconds. Set a Min X value of -2000, a min Y of -1000, a min Z of 0, a Max X of 4000, a Max Y of 4000, and a Max Z of 4000. Press OK and save the model as “expert02_time”.
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Go into the Import menu and choose SEGY -> Load. For the trace format, use the file “gather_window.tdf” and for the trace file use the file “4HorizonStack.sgy”. Both files are included with the example data. Model Builder automatically creates a cross section using the coordinates in the SEGY header. Switch to Horizon View to see the traces displayed on that cross section. You can use the Show Panel Edges option in the Horizon View button pulldown menu to toggle the green panel lines on and off.
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For each of the four events in the seismic data, create a new horizon by pressing the New Horizon button and defining two endpoints on each side of the section. The first horizon should be at 1000 ms, the second horizon should be at 1356 ms, the third horizon should
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be at 1660 ms, and the fourth horizon goes from 2636 ms to 2140 ms. Your model should now appear as shown:
4)
Grid the model using Model Extent for the Top horizon and 2.5D gridding for all of the other horizons with a grid increment of 60.
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Go to the Velocity View and set the velocities for the each layer as shown: Layer Top – Horizon1 Horizon1 – Horizon2 Horizon2 – Horizon3 Horizon3 – Horizon4 Horizon4 – Bottom
6)
Vp 2000 2800 3300 4000 5000
Vs 1100 1650 1650 1100 2750
Density 2.25 2.49 2.49 2.25 2.829
Once the layer velocities have been defined, the model must be converted from time to depth. Only depth models can be used for raytracing. Simply press the Convert to Depth button to perform the conversion. Model Builder will prompt you to save the time model before it is converted to depth. After the conversion finishes, select Save As from the File menu and name the depth model “expert02_depth”.
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7)
Choose File – New Model and create a model space (units in meters) with a Min X of 0, a Min Y of -750, a Min Z of -5000, a Max X of 15,000, a Max Y of 750, and a Max Z of 0. Save the model as “expert02_laurain”.
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Choose Horizon from the Import menu and press the ASCII button in the resulting dialog. Load the file called “LaurainHorizons.txt” into the Import Window. In order, the four columns of this file represent the horizon name, the x coordinate, the y coordinate, and the z coordinate for the model horizons. Set the first data line (line 2) and define the columns for importing these four data fields. Press the Go button to import the horizons. Use a grid interval of 50 for the imported horizons when you are prompted. You can view the imported horizons by selecting 3D Window from the Display menu.
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9)
Switch back to the cross section view. Use the Manual Entry button and define a single two-point cross section line that runs from point (0,0) to point (15000,0). Switch to the Horizon View to see the imported horizons on this cross section.
10)
Select Image – Load from the Import menu. Choose the Fit to Cross Section option and then load the file “LaurainModel.lyr”. You can now use this image to define the salt lens in the model.
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11)
Press the New Horizon button and give it a name of “Salt Top”. Click anywhere on the cross section. This will bring up the “Horizon Order” dialog. This dialog is used to place the new horizon between two existing horizons. This new horizon needs to be between the “Water Bottom” and the “Target” horizons. Select Water Bottom – Target in the list and press OK.
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Click on the leftmost tip of the salt lens to place the first horizon point. All horizons in the model must extend through the entire model space. Click on the far left edge of the cross section to extend the horizon in that direction. Adjust the points so that the horizon is as flat as possible and so that it does not intersect the imported “Target” horizon.
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13)
Use the right mouse button to zoom in on the salt lens. Click points along the top edge of the salt lens to define the horizon around that feature. Extend the horizon straight across from the rightmost tip of the salt lens to the right edge of the cross section.
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Create a new horizon with the name “Salt Bottom”. Again click anywhere on the cross section to bring up the “Horizon Order” dialog. Select Salt Top – Target. Click on the “Salt Top” horizon point at the leftmost tip of the salt lens. This links the “Salt Bottom” horizon to the “Salt Top” horizon at that point. Model Builder . Press the Left button to automatically link the “Salt Bottom” horizon and the “Salt Top” horizon to the left of the selected point.
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15)
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Zoom in on the salt lens again and click points along the bottom edge of the salt lens. Click on the “Salt Top” point on the rightmost tip of the lens to link the horizons again. Press the Right button to automatically link the rest of the horizon.
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Select Gridding from the Model Grid button pulldown menu. Choose Top in the horizon list and set its gridding method to Model Extent. Choose the two salt horizons and set their gridding method to 2.5D. Press the (Un)Select All Horizons button to select all of the horizons, change the model grid increment to 50, and press OK to grid the model.
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Go to the Velocity View and set the velocities for each layer: Top – Water Bottom: Vp = 1500 Water Bottom – Salt Top: Vp = 2400 Salt Top – Salt Bottom: Vp = 4000 Salt Bottom – Target: Vp = 3000 Target – Bottom: Vp = 3600
The Vs and Density values for each layer will be automatically calculated from the Vp. Save the model. Save the model again as expert02_laurain.gmf.
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DO IT YOURSELF MODEL BUILDING From the File menu, select the Save As option. Save your current model when prompted to do so. Save the new model as “expert02_self”. You can use the Edit menu options to move horizon information between cross sections. If you are in Horizon view, you can cut or copy the currently selected horizon. If you are in Cross Section view, you can cut or copy all of the horizons on the currently selected cross section. Switch to the Cross Section view on your model and select Copy from the Edit menu. Draw a new cross section, parallel to the existing cross section, towards the bottom of the model space. Use Paste in the Edit menu to put the copied horizons on the new cross section. Create another parallel cross section towards the top of the model space and paste the horizons again. Go into Horizon view and edit the horizons on the different cross sections. Try adding points, moving points, deleting points, and linking points. Open the Gridding dialog and try some of the different gridding methods on your edited model. You can view the results by selecting 3D Window from the Display menu.
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EXERCISE #3 -- Expert Attributes This exercise reviews the basic workflow for Mesa Expert. You will create a simple geometry, use a model from the previous exercise, raytrace the model, and analyze the calculated attributes. This exercise uses the pseudo 3D laurain model from the previous chapter and one source line from a transition-zone style shooting geometry. 1) Start Mesa and open the Unit Template window from the Layout menu. Set a grid size of 25 meters when prompted. 2) Lay out a very small transition zone geometry with two short receiver cables spanned orthogonally by a segment of a source line. Fill out the Template Layout dialog parameters as shown in the following diagram and press the Exit button.
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3) The next step is to replicate the unit template to create our source and receiver locations in the Design Window. Press the Shoot Options button in the Unit Template Window (gun icon) to bring up the Unit Template Repeat dialog box. Fill out the parameters as shown in the following diagram and press OK to generate the survey. Note: Since we are only generating one source line, the Crossline Spacing is irrelevant.
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4) By default, the Unit Template Window creates a default 25 m by 25 m bin grid. Perform a fold calculation (Bin Analysis-Fold Calculation) to verify that you have created a 2-fold survey. We have intentionally made this survey very small in order to quickly complete the raytracing calculation in the following steps. Save your survey design as “expert03”. 5) Select Expert-Launch Enhance Raytracer. When the Enhanced Raytracer program comes up, select Raytracing-Parameterize. Load the “expert02_laurain.gmf” model and the “expert03.mas” geometry. Select the Create New Database radio button. Press the Output Database button and name your raytracing database “expert03_all”.
6) Press the Parameterize button. Fill out the parameters as shown in the following diagram. By default, all sources will be fired. Select the P Wave radio button and select the P-P
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Waves checkbox. Select all four interfaces in the Select Interface(s) listbox. Press OK when you are finished.
7) The raytracer is now completely parameterized. Press the Trace Rays button to start the process. The main program window will update the status of the raytracing after completing each source point.
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8) After the raytracing is completed, select File-Exit and return to Mesa.
9) In Mesa, select Expert-Load Model File and load “expert02_laurain.gmf”. Select ExpertLoad Offset Ray File and load “expert03_all_pp.odb”. Select Expert Attributes from the pulldown menu of the Expert Attributes pushbutton in the Design Window. The Expert Attributes Manager Window will appear.
10) Press the Create Model Attributes button to open the Model Analysis dialog. This feature allows you to analyze some characteristics of your model (independent of raytracing) such as maximum dip or the maximum bin size you can use without spatially aliasing your data. In this example, we’ll generate model attributes from two different horizons in the model. In the Model Analysis dialog, set the Attribute Set Name to “Top of Salt”. Select “Salt Top” from the Horizon list and press OK.
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11) Repeat this process, but set the Attribute Set Name to “Target Horizon” and select “Target” from the Horizon list. Press OK to complete the calculation. You should now have two sets of model-based attributes listed in the Expert Attributes Manager window.
12) Select “Top of Salt” from the list of Calculated Attributes. Press the Model Attribute Display Setup button. The Model Attribute Display Options dialog box allows you to set parameters for viewing dip angles, bin sizes, and resolution maps. Select the Maximum Dip radio button and the Dip Angle Map radio button and press OK. Press the Expert Attributes button in the Design Window to view the dip angle map for the top of the salt body.
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13) Next, select “Target Horizon” from the list of calculated model attributes in the Expert Attributes Manager Window. Press the Model Attribute Display Setup button. As before, select the Maximum Dip radio button and the Dip Angle Map radio button and press OK to display the dip angle map for the target horizon. Note the steeply dipping portion of the model on the west end of the project.
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Now, press the Model Attribute Display Setup button again and change the display. Select the Bin Size radio button and set the frequency range to 8-80 Hz.
Press OK and you will see a map of the maximum bin size without spatial aliasing along the target horizon. In this example, some of the non-aliased bin sizes are as small as 9.63 meters for 80 hz. Given that our chosen bin size for this survey is 25 meters, we can expect some aliasing for this event unless we either modify our design or change the bin size.
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14) Next, we’ll generate attribute data based on the offset raytracing results. In the Expert Attributes Manager, press the Create Offset Ray Attributes button. This opens the Calculate Raytracing Attributes dialog box. As with the model-based attributes, raytracing attribute sets are tied to a specific horizon. If you are only interested in common reflection point (CRP) fold, select the CRP Fold Only radio button. If you want to calculate the CRP fold plus all the other offset ray attributes, select the Calculate All Attributes radio button. For this example, calculate all of the attributes and name the attribute set “Offset-Target”. Optionally, you can also save the CRP fold calculation to a “named” fold calculation. This provides some additional flexibility for comparing CRP fold to standard CMP fold. Check the Save CRP Fold to Named Fold Calculation checkbox and give the calculation the name “CRP Fold”. Select “Target” from the Target Horizon listbox. Press OK to calculate the attributes for this horizon.
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15) The attribute set named “Offset-Target” now appears in the Expert Attributes Manager list. Select this attribute set from the list. The contents of the Display group box change depending on which type of attribute set you select. When you select an attribute set calculated from offset raytracing, eight radio buttons appear. Select the CRP Fold radio button and make sure the Expert Attributes button is still pressed in the Design Window. This will produce a map of CRP fold on the “Target” horizon in the model.
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16) Keep the Expert Attributes Manager Window open and examine the rest of the offset raytracing attributes by selecting the other radio buttons. The following diagram shows a map of the two-way travel time for the target horizon.
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17) Press OK to close the Expert Attributes Manager. Select Generate Normal Rays from the Expert menu. Normal rays are zero-offset rays that simulate the raypaths after stacking the traces in a CMP bin. Select the Shoot Rays from Bin Grid Centers radio button and name the output file “expert03_normal”. Press OK to generate the normal rays.
18) Select Generate Image Rays from the Expert menu. Image rays are zero-offset rays that simulate the raypaths after time migration. Select the Shoot Rays from Bin Grid Centers radio button and name the output file “expert03_image”. Press OK to generate the image rays.
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19) Select Expert-Load Normal Ray File and load “expert03_normal.nrd”. Select Expert-Load Image Ray File and load “expert03_image.ird”. 20) Re-open the Expert Attributes Manager window from the Expert Attributes button pulldown menu in the Design Window. Press the Create Image Ray Attributes button to create image rays attributes from the target horizon. Select “Target” from the Horizon listbox and name the attribute set “Image-Target”. Press OK to calculate the attribute set.
21) Press the Create Normal Ray Attributes button to create normal rays attributes from the target horizon. Select “Target” from the Horizon listbox and name the attribute set “NormalTarget”. Press OK to calculate the attribute set.
22) You now have five sets of attributes listed in the Expert Attribute Manager list. For zero-offset attribute sets, there are four display choices. Select “Normal-Target” from the list. Select the Displacement radio button and view the result in the Design Window (make sure the Expert Attributes button is pressed). The normal ray displacement is a first order approximation of
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the migration aperture. For this example, the display shows some displacements of ~2000m on the western side of the model. Since the bins with these displacements are roughly 4000m from the edge of the survey, our survey extents are probably adequate to image this horizon. If we had large displacement values close to the edge of the model, we might consider extending the extents of our survey.
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23) Select “Image-Target” from the list. Select the Displacement radio button and view the result in the Design Window (make sure the Expert Attributes button is pressed). The image ray displacement is the xy-difference between the reflection point (after time migration) and the emergence point on the surface. This plot may give some insight into how well time migration will position this horizon. In this example, there are some bins with relatively large displacements on the eastern side of the project.
24) In order to examine the cause of these large displacements in more detail, open the 3D Window from the Display menu. Open the Scene Information dialog box and select the Image Rays tab. Select “Target” from the listbox to turn on image rays for the target horizon. With the image rays displayed, it is obvious that the salt body is affecting the path of the image rays from the target horizon.
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25) As a last step, we will compare the CRP fold to the CMP fold. Perform another CMP fold calculation and save the results as a named fold calculation (expert 03 – full data).
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. 26) Select Bin Analysis-Fold Compare. Calculate the difference between expert 03 full data and CRP Fold. Store the result in ‘Fold Difference”.
27) The fold difference between the CMP and CRP fold should now be visible in the Design Window. Since some of the difference values will be positive and some will be negative, it may be beneficial to use one of the alternate color scales. Press the Set Options button below the color scale in the Design Window to open the Color Scale Settings dialog. Use the
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next and previous buttons to cycle through the standard color scales until you load the redblue color scale. Grab the “white” slider tab on the color scale and slide it up until is has a value of zero. This will set your color scale so that red values are positive and blue values are negative.
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28) Finish this exercise by examining the other model and raytracing attributes available in Mesa expert.
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EXERCISE #4 -- Converted Waves This exercise reviews some of the functionality for modeling the behavior of PS converted waves in Mesa Professional and Expert. 1. Create a survey with a single receiver line and a single orthogonal source line using the following parameters. First, select Layout-Receivers-Lines/Bricks and lay out a single EW receiver line with 100 receivers.
Next, select Layout-Sources-Lines/Bricks and lay out a single NS source line with 100 sources.
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The simple survey should appear as follows:
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2. Shoot the survey such that every shot fires into all 100 receivers. Press the Shoot button and create a 1x100 template. Select the Automatic Template Centering option and make sure to uncheck the Template Roll On/Off checkbox. Verify that your settings are the same as shown in the following diagram and press the Shoot button.
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You will get prompted to set the template roll limits. Press the Survey Edges button to keep the template from rolling off and press the Shoot button. All 100 receivers will be active for each source points.
Save the database as “single_cross”. 3. The next part of the exercise will demonstrate how the locations of CMP midpoints differ from P to S CCPs (common conversion points). Press the Midpoint Scatter Display toggle button to display the CMP locations. As expected for this survey, the pattern of CMP locations will form a square.
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For the purpose of creating a reference marker, select Display-Annotations to open the annotation toolbar. Select the option to Create Polygon Annotation. Select a line style and a line color. In this example, the user chose to draw a dashed, red line. Click out a square polygon around the boundary of the CMP locations. Double click on the last vertex to close the polygon. Select Display-Annotations to exit annotation definition mode. 4. Select Bin Analysis-Converted Waves from the menu.
Check the Use Converted Wave Midpoints checkbox. Set the Vp/Vs ratio equal to 2.0. While the Use Converted Wave Midpoints checkbox is selected, all bin attribute calculations in MESA will use CCP locations instead of CMP locations. Conversion points migrate closer to the receiver locations due to the slower velocity of the S wave on the upgoing travel path. MESA has two different calculations for determining the locations of PS conversion points. The traditional method for calculating conversion points uses an asymptotic assumption of a very deep conversion point according to the equation:
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CCPxy = SRCxy + (RECxy - SRCxy) / (1 + ( Vs / Vp ) ) The second method uses a depth-dependent algorithm for determining the location of the CCPs. Initially, we are going to use the Asymptotic CCP calculation, so select the Asymptotic radio button and press OK. 5. Change the midpoint scatter display options by selecting the drop down menu from the Midpoint Scatter Display button.
This will open the following dialog box.
Select the CCP radio button so that the midpoint scatter display will show CCP locations instead of CMP locations and press the OK button. 6. The Design Window will now show the CCP scatter using the asymptotic conversion point assumption for a Vp/Vs = 2.0. The following diagram is annotated with arrows to show how the conversion points have migrated closer to the receiver locations.
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Again, for the sake of reference, select Display-Annotations to open the annotation toolbar. Select a line style and color and draw a box around the extents of the CCPs for a Vp/Vs = 2.0.
Select Display-Annotations to close the annotation toolbar. 7. As a final test for the asymptotic CCP calculation, select Bin Analysis-Converted Waves again and change the Vp/Vs ratio to 4.0. Press OK and the Design Window will update with the
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new locations of the conversion points. As you can see, the CCPs have migrated even further toward the receiver locations with the increase in Vp/Vs.
Select Display-Annotations to draw another box around this set of CCP locations. You may also wish to add some text annotations to clarify the display as shown in the following diagram.
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8. The next step is this exercise is to demonstrate the difference between the asymptotic and depth dependent conversion point calculations. Select Bin Analysis-Converted Waves again. Set the Vp/Vs ratio back to 2.0. This time select the Specify Depth radio button and set the depth to 1000.0. Press OK when you are finished.
9. The Design Window now shows the locations of the CCPs using the depth-dependent algorithm.
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Compare the scatter of these conversion points with the outline of the scatter using the asymptotic algorithm and a Vp/Vs = 2.0. It should be obvious that for a shallow reflector, the conversion points are much closer to the receivers using the depth dependent algorithm than using the asymptotic algorithm. In fact, the locations of the conversion points using the depth dependent algorithm are actually closer to the results of the asymptotic algorithm using a Vp/Vs = 4.0. Once again, for reference, open the annotation toolbar (Display-Annotations) and draw a polygon around the conversion point area. Select Display-Annotations a second time to close the annotation toolbar. 10. Select Bin Analysis-Converted Waves and change the depth of the reflector to 3000. Leave the Vp/Vs = 2.0 and press OK. The Design Window will update with the new positions of the conversion points.
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Note that with the deeper reflector, the positions of the conversion points are now much closer to the conversion point region using the asymptotic calculation. However, if you use the Range/Bearing tool to measure the distance between the corner of the asymptotic region and the corner of the depth dependent region, you will find that there is still a difference of about 150 feet between the locations. This is a large enough difference to affect the fold calculations. As an exercise, return to Bin Analysis-Converted Waves and change the depth of the reflector until you get a satisfactory match with the asymptotic algorithm. Try the same experiment with using a Vp/Vs = 4.0. The take-away point from this section of the exercise is that both Vp/Vs and depth play a role in the location of the conversion points. If your target is sufficiently deep, either algorithm should provide a satisfactory answer. For shallow targets, the depth dependent algorithm is preferable. So why provide both methods in MESA? The asymptotic algorithm is more common in the industry and in some processing systems it may be the only method available for binning conversion points. The depth dependent tool allows you to analyze the possible ramifications of this difference. 11. Turn off the annotations. Select Display-Annotations to open the annotation toolbar. Press the Display Annotations toggle button to hide the annotations. Select Display-Annotations a second time to close the toolbar. 12. In the next phases of this exercise, we will demonstrate how to directly compare CMP and CCP bin attributes. Define a bin grid with 25 x 25 foot bins. 13. Select Bin Analysis-Converted Waves and uncheck the Use Converted Wave Midpoints checkbox. Press OK to close the dialog box.
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14. Select Bin Analysis-Fold Calculation. Select the Fold, Offsets, and Azimuths radio button. Check the Named Fold Calculation checkbox and name the calculation “p-wave.”
Press Ok. After the fold calculation completes, display the fold in the Design Window. You should see a single fold region centered on the intersection of the lines.
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15. Select Bin Analysis-Converted Waves. Check the Use Converted Wave Midpoints checkbox. Set the Vp/Vs = 2.0. Select the Specify Depth radio button and set the depth to 3000.0. Press OK when you are finished.
16. Select Bin Analysis-Fold Calculation. Select the Fold, Offsets, and Azimuths radio button. Check the Named Fold Calculation checkbox and name the calculation “converted-wave.” Press OK when you are finished (you may get prompted to close the bin plots—select Close).
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17. Display the converted wave fold. Since the natural bin size for the PS conversion points is different than the natural bin size for the CMPs, you should see a distribution of bins with zero, one, or two fold.
18. To directly compare the difference between the CMP and CCP fold distribution, select Bin Analysis-Fold Compare. In the Compare drop-down list, select “converted’wave”. In the To
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drop-down list, select “p-wave”. In the Calc name edit box, type “fold difference.” Press OK when you are finished.
19. Display the fold difference in the Design Window. Open the Color Scale Settings and change the scale to a Discrete Range color scale with a minimum of -1 and a maximum of 3 (5 levels). An example is shown below.
The plot should highlight the coverage differences between CMP and CCP (using a depth of 3000) when using the same binning grid. By comparing the differences in the fold maps, you may be able to converge on design parameters (i.e. line spacings) which produce optimal coverage for both PP and PS data.
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20. If you have the freedom to use a different binning grid for the PS data, how do you choose the appropriate grid sizes? First, display the CCP scatter. Make sure that you still have the CCP radio button selected in the Midpoint Scatter Options (refer back to step 5 in this exercise). Also, make sure that in Bin Analysis-Converted Waves you still have the Use Converted Wave Midpoints checkbox selected, the Vp/Vs set to 2.0, and have a Specified Depth of 3000. 21. Zoom in tightly on the CCP scatter. Use the Range/Bearing tool to measure the inline and crossline distances between the CCPs. Near the intersection of the source and receiver line, the measurement tool shows (roughly) an inline separation of 33.33 ft and a crossline separation of 16.66 ft.
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Clear all of the Range/Bearing symbols (do this from the drop down menu) and exit Range/Bearing mode by pressing the toggle button. 22. Define a new bin grid with an inline bin size of 33.33 and a crossline size of 16.66. Center the bin grid around the conversion points near the intersection of the source and receiver line.
23. Perform a fold calculation. Select Fold, Offsets, and Azimuths. You can uncheck the Named Fold Calculation checkbox and simply store the results as the default calculation. Display the results in the Design Window.
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As you can see, the converted wave fold coverage is now much more consistent when using an appropriate bin grid. There are a few stripes of zero fold in the coverage map since the conversion point spacing is not constant when using the depth dependent algorithm (spacing changes with increasing offset). As an exercise, go back to the converted wave options and select the asymptotic algorithm. Re-do the fold calculation. You will be able to get a uniform single fold map using this algorithm.
24. Save your database. The “single_cross” survey will be used in Part II of the converted wave exercise.
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EXERCISE #5 -- Converted Waves (Part II) This exercise continues with the examination of converted wave tools from Exercise #4. 1. With the single_cross survey already loaded in MESA, launch Model Builder from the Expert menu. Define a model space with the following parameters.
2. Press the Manual Entry button to define a single cross section (west to east) across the model space. Add points with coordinates (-1000, 0) and (6000, 0). Name the cross section “CrossSection1”.
3. Press the Horizon View button so you can begin defining the horizon layers. We are going to make a simple model with two dipping horizons. Press the New Horizon button. Use the default name (Horizon1). Click the mouse just outside the left edge of the box at a depth near -2000 feet. Click the mouse just outside the right edge of the box a depth near -1000 feet.
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4. Press the New Horizon button again. Use the default name (Horizon2). Click the mouse just outside the left edge of the box at a depth near -4000 feet. Click the mouse just outside the right edge of the box a depth near -2000 feet.
5. Press the Model Grid button and select Gridding from the drop down menu. Select all three horizons from the list box (Top, Horizon1, Horizon2). Set the grid increment to 50.0 (this is the Inc. (dx=dy) edit box).
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Now select only the “Top” horizon from the list. From the Grid Method drop down list, select Model Extent.
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Select “Horizon1” and “Horizon2” from the list. From the Grid Method drop down list, select 2.5D.
Press OK to grid the model. 6. Press the Velocity View button. You should see a model with two dipping horizons but without a defined velocity structure.
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Select “Top – Horizon1” from the list box. Enter a Vp = 4000 ft/s. Enter a Vs = 2000 ft/s. Let the rest of the values default. Select “Horizon1 – Horizon2” from the list box. Enter a Vp = 9000 ft/s. Enter a Vs = 3000 ft/s. Let the rest of the values default. Select “Horizon2 – Bottom” from the list box. Enter a Vp = 12000 ft/s. Enter a Vs = 6600 ft/s. Let the rest of the values default.
7. As a final check, select Display-3D Window to view your model. If the model looks correct, save the model as “single_cross” and exit Model Builder.
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8. Launch the Enhanced Raytracer (either directly or from the Expert menu in MESA). Select Raytracing-Parameterize. Load the model “single_cross.gmf”. Load the geometry “single_cross.mas” (depending on how you launched the raytracer, these files may already be loaded). Press the Output Database button and name the file “single_cross_enhanced”. Select the Create New Database radio button.
Press the Parameterize button. Select the P-P Waves and P-SV Waves checkboxes. Make sure both horizons are selected from the Select Interface(s) list box. Press OK when you are finished (refer to the following diagram for all settings).
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9. The setup dialog box should now report that the project is “Parameterized for Execution.” Press the Trace Rays button.
When raytracing is completed for all 100 sources, exit the Enhanced Raytracer. 10. Before you can use models or raytracing results in MESA, the files must be explicitly loaded. Even though the model may already be loaded, select Expert-Load Model File and select
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“single_cross.gmf.” When the raytracing was performed, the results for the p-p and p-s raytracing was written into separate files. Select Expert-Load Offset Ray File and select “single_cross_enhanced_pp.odb.” We will deal with the p-s data momentarily. 11. Press the Expert Attribute button on the toolbar in the Design Window.
When the Expert Attributes Manager window appears, press the Create Offset Ray Attributes button. In the Calculate Raytracing Attributes dialog box, select the radio button to Calculate All Attributes. Name the attribute set “pp_horizon1”. Check Save CRP Fold to Named Fold Calculation and name the calculation “CRP Fold-pp1”. From the Target Horizon drop down list, select “Horizon 1”. Refer to the following diagram.
Press OK to calculate the attributes using the pp reflections from Horizon 1. An attribute set named “pp_horizon1” will appear in the Expert Attributes Manager list box. 12. Press the Create Offset Ray Attributes button again. In the Calculate Raytracing Attributes dialog box, select the radio button to Calculate All Attributes. Name the attribute set “pp_horizon2”. Check Save CRP Fold to Named Fold Calculation and name the calculation “CRP Fold-pp2”. From the Target Horizon drop down list, select “Horizon 2”. Refer to the following diagram.
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Press OK to calculate the attributes using the pp reflections from Horizon 2. An attribute set named “pp_horizon2” will appear in the Expert Attributes Manager list.
13. Press the OK button to close the Expert Attributes Manager window. Select Expert-Load Offset Ray File from the menu. Open the file “single_cross_enhanced_ps.odb”. This file contains the results of the converted wave (p-s) raytracing.
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14. Open the Expert Attributes Manager again. We are now going to create attribute sets for the first and second horizons using the converted wave data. Press the Create Offset Ray Attributes button. In the Calculate Raytracing Attributes dialog box, select the radio button to Calculate All Attributes. Name the attribute set “ps_horizon1”. Check Save CRP Fold to Named Fold Calculation and name the calculation “CRP Fold-ps1”. From the Target Horizon drop down list, select “Horizon 1”. Press OK to calculate the attributes using the ps reflections from Horizon 1. An attribute set named “ps_horizon1” will appear in the Expert Attributes Manager list. Repeat the process to create ps attributes for Horizon 2. Refer to the following diagrams to get the parameterizations correct.
15. There are now four sets of raytracing attributes in the list box. Select “pp_horizon1”. Select CRP Fold from the set of radio buttons in the Display group box.
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If the CRP Fold for Horizon 1 is not already visible in the Design Window, press the Expert Attributes toggle button on the Design Window toolbar. You should see the CRP Fold map.
As expected, the reflection points have migrated up-dip for our model and we no longer have a symmetrical, uniform single-fold square centered on the intersection of the source line and receiver line. 16. Select the “pp_horizon2” from the list box. The display will be updated with the CRP Fold map for Horizon 2.
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The reflection points have migrated even farther up-dip for the second horizon. 17. Select the “ps_horizon1” from the list box. The display will be updated with the CRP Fold map for Horizon 1 using the converted wave raytracing.
Note that the distribution of conversion points is similar to the distribution using the depth dependent conversion algorithm (exercise 4, step 9). The differences are due to the conversion points migrating up-dip and the difference in depth from the west side of the model to the east side of the model.
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18. Select the “ps_horizon2” from the list box. The display will be updated with the CRP Fold map for Horizon 2 using the converted wave raytracing.
19. Take a moment to examine some of the other displays for the four attribute sets. For example, select attribute set “ps_horizon2” and choose Two-way Travel Time. You should see the following display.
Note: There is one caveat as you examine the attributes for the converted wave raytracing. Both CMP to CRP Displacement and CMP to CRP Azimuth show the difference between the CMP location and the conversion point calculated from raytracing. These attributes do not show the
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difference from the depth dependent (or asymptotic), flat-Earth conversion point position calculated in MESA and conversion point calculated from raytracing. 20. Toggle off the display of the Expert Attributes. Select Bin Analysis-Fold Selection. In the dialog box, you will see a list of all the fold calculations you have saved.
Select “CRP Fold-ps1” from the list and press OK. This will load the fold calculation information for the converted wave raytracing (horizon 1) into the standard fold, offset, and azimuth plots. Press the toggle button to display Fold in the Design Window.
As expected, this should replicate the CRP Fold map that we saw for Horizon 1 when viewing attributes through the Expert Attributes Manager. 21. Select any of the standard bin attribute diagrams for analysis of your converted wave results. For example, the following diagram shows the near offset plot based on the converted wave raytracing from Horizon 1.
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22. In the next step in this exercise, we will view the raypaths in the 3D Window. Select Display3D Window. Press the Scene Information button in the lower left corner of the 3D Window.
Select the Offset Rays tab and select Horizon2 from the list.
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Select the Sources tab and select source 10001 from the top of the list.
You should now see the converted wave paths for Horizon 2 for this source point. The rays are color-coded by offset.
Experiment for a few moments by selecting different source points. You can also return to the Offset Rays tab and select Horizon 1. The list boxes for selecting the source points and horizons are multi-select if you wish to display the rays for more than one source or for both horizons simultaneously.
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23. As a final step for this exercise, we will review creating synthetic traces. Select AdvisorGather Window. The Gather Display Window will open. From the drop menu of the Display Expert Synthetic Gather toggle button, select Expert Synthetic Parameters.
In the Expert Synthetic Parameters dialog box, select a Ricker wavelet from the drop down list of Wavelet Types. Uncheck the checkboxes four all types of Noise Events. Make sure that both Horizon1 and Horizon2 are selected from the Horizons list box. Refer to the follow diagram for the settings.
24. Choose Select Source from the drop down menu.
This action will bring the Design Window to the front. Select the northernmost source point in the survey by clicking on it with the mouse. The synthetic gather for this source point will be loaded.
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Since we turned off all of the noise events, only the converted wave reflections for the two horizons appear on the gather.
25. For more information on creating synthetic shot gathers or creating bin gathers, refer back to step 20 of exercise 1.
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EXERCISE #6 -- Smart Aperture Tool This exercise will review the building of models from external gridded surfaces and demonstrate how the Smart Aperture tool can be used to determine the extents of your survey design. 1. Launch MESA. From the Expert Menu, select Expert-Launch Model Builder to start the model building application (if a prompt comes up asking you to ‘Save current changes?,” answer ‘No.” 2. In Model Builder, select File-New Model. Dimension the model as shown in the following diagram. The units should be ‘Feet’. The model should be a 20000 foot square with a depth of 3500 feet. Press OK when you are finished.
3. We are going to import horizons to make the model, but you still need a single cross section defined across the model. Press Manual Entry and define the endpoints of the cross section as (0,10000) and (20000,10000).
Accept the default name of ‘CrossSection1.’ The window should look as shown in the following diagram.
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4. Select Import-Horizon from the main menu. This will open the Import Horizon dialog box. We will be importing an ASCII file containing two gridded horizons. Press the ASCII button.
The ASCII Import Window will appear. (Note: We will not discuss the details of importing ASCII files here. Refer back to exercise five of the MESA Training Manual for more information on the operation of the ASCII Import Window.) 5. Open the file “sask_maple_creek.txt.”
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There is one line of header information, so mark line 2 as the first data line for importing. Select ‘Horizon Name’ from the list box and highlight columns 1-12. Define this field as a ‘String.’ Columns 2, 3, and 4 in this file are X Coordinate, Y Coordinate, and Z Coordinate respectively. Define each of these fields as ‘FLOAT.’ The maximum width of xy-coordinate column is the value 20000.00, so make sure you pad the column width appropriately. The maximum width of the zcoordinate is -3100.00. After you have defined the four fields for importing, press the GO button. You will be prompted (twice) for a grid interval. Enter a value of 100.0 each time.
6. You should see the following diagram in the Horizon View.
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Select Gridding from the drop down menu of the Model Grid button. In the Gridding dialog box, select the ‘Top’ horizon. Select Model Extent from the Grid Method drop down list and press OK.
Select Gridding from the drop down menu of the Model Grid button a second time. In the Gridding dialog box, select both ‘White_Shale’ and ‘Medicine_Hat’ from the horizon list. Select Resample from the Grid Method drop down list and press OK.
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7. Press the Velocity View button to complete the model. Enter the following p-wave velocities and let the rest of the parameters default: Top-White Shale: White Shale-Medicine Hat: Medicine Hat-Bottom:
4500 ft/sec 7500 ft/sec 9000 ft/sec
8. Save the model as “sask_maple_creek.gmf”. Select Display-3D Window to view your model.
Exit Model builder and return to MESA.
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9. From the Expert Menu in MESA, select Expert-Load Model File. Open “sask_maple_creek.gmf”. From the drop down menu of the contour display toggle button, select Display CYR from Model.
This interface allows you to select a horizon from your model and display it as a contour map in the Design Window. Select ‘Medicine_Hat’ from the list box and press OK.
The contour map will now display in the Design Window (press the full unzoom button to refresh the display). Go back to the drop down menu for the contour display toggle button and select Options. Set the Start Elevation to -3000 and press OK.
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10. The Smart Aperture tool is used to shoot a cone of rays from a point on your model to the surface. This exploding-reflector raytracing can be used to determine the migration aperture required for your survey to image your target. Select Expert-Smart Aperture Tool from the main menu. A toolbar will be added to bottom of the Design Window.
11. The important first step in using the Smart Aperture tool is to choose the correct horizon from which to shoot the rays. The contour map being displayed is just for reference—it has no functional tie to the Smart Aperture tool whatsoever. If you are using the contour map for reference, make sure you are currently displaying the proper horizon. In this case, we are displaying the Medicine_Hat horizon from the model. Select ‘Medicine_Hat’ from the horizon list on the Smart Aperture toolbar as well. The standard rule of thumb for migration aperture is to capture 30 degree rays from the reflector. This is the default setting in MESA. Press the Options button on the toolbar to bring up the following dialog box.
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The Smart Aperture raytracer shoots a cone of rays from each point you select on the subsurface horizon. If you wish to shoot with an angle other than 30 degrees, change the setting for Maximum Incidence Angle. By default, the azimuthal angle between rays in the cone is 2 degrees, which produces 180 rays per target. It is rare that you will ever need to change these default values. Press OK to close the dialog. 12. Zoom in slightly on the primary structure on the map. Press the Add Point Target button. When you click on a point on the map, the local dip is calculated. A cone of rays with an incidence angle of 30 degrees on the dipping horizon is shot to the surface. Refer to the following diagram.
For each target you select, a dot displays the location of the target and an oval is drawn around the surface emergence points of the 180 rays traced. The oval represents the migration aperture required to properly image the selected point. If you select a location on the model with little to no dip, the emergence points will form a circle centered on the target. If you select a location on a
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steeply dipping flank of a structure, the emergence points will be skewed relative to the target location. 13. If you wish to see the full-fold area required to properly migrate all of the target locations you have selected, press the Create Aperture Hull button.
A convex hull encompassing all of the emergence points will be drawn.
Press the Clear button to remove all of the targets and the aperture hull. 14. Instead of defining individual target points for shooting rays, a more common use of the Smart Aperture tool is to define a target region. In this example, the objective of the seismic survey is to image the NW flank of the prominent structure. Press the Add Polygonal Target button. Click out a polygon that follows the (-2200) foot contour line as show in the following diagram. A point target will be created at each grid node along the perimeter of the polygon. A cone of rays will be shot from each of these targets and an aperture hull will be created that encompasses the emergence points of all the rays. The area of this hull represents the fullfold area your survey will need to properly image this target.
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15. Press the Save to Exclusion file button. Select the Polygonal Target Regions checkbox. Select the Save to External Exclusion File radio button. Press the File button and name the file ‘sask_maple_creek.xcl’.
16. Press the Save Smart Rays button. Save the rays to a file named ‘sask_maple_creek.srd’. Close the Smart Aperture tool by selecting Expert-Smart Aperture Tool from the main menu. 17. Open the exclusion toolbar by selecting Edit Exclusions from the drop down menu of the Exclusion Zone display toggle button. Press the Read Exclusion File button.
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Open ‘sask_maple_creek.xcl’. You should see the smart aperture polygon in the list box. Press OK.
Select Edit Exclusions from the drop down menu to close the exclusion editing toolbar.
18. The last step in the exercise is to create a survey which properly fills the full-fold region for imaging the objective. Select Layout-Unit Template. Set a grid size of 25.0 feet. Create a unit template using the parameters in the following diagram.
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Press the Shoot button. The Unit Template has a feature for filling regions with sources and receivers. Select the Fill Polygon radio button. By default, the Layer should be set to Smart Apt – Selection Polygon and the Zone should be set to Polygon 1. In the Clipping group box, select Clip Fold to Bounds. Also, select the Use Full Template checkbox. Refer to the following diagram for the settings.
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Press OK. This will create a survey that fills the smart aperture polygon with full-fold with the appropriate taper zone.
19. Calculate fold for the survey and display the results. The following diagram shows that the smart aperture region is filled with 48 fold.
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Save the survey as “sask_maple_creek”. 20. You can view Smart Aperture raypaths in the 3D Window. Select Expert-Load Smart Aperture Ray File. Open ‘sask_maple_creek.srd’. Select Display-3D Window. Press the Scene Information button. Select the Smart Rays tab. Select ‘Medicine_Hat’ from the list box and press OK.
The cone of rays from each of the perimeter target points will be displayed.
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