Processing Lab 9

Objectives: - View a timeslice- Consider binning effects on migration- Post stack Migration

Sumit Verma Bryce Hutchinson

Creating a time slice

1. For the input use a stacked volume with your 5x5 velocity that you picked in the last lab.

2. Define the time slice to be calculated from 1000 to 2000 ms. This will reduce computational expense.

A timeslice represents a horizontal slice through a data volume at a specific time. Dr. Marfurt particularly enjoys using time slices to display attributes.

Source: www.zetica.com

Time slice

Linear features

Upon first glance of the time slice, linear features are apparent. Where are these from?

The corresponding time for the timeslice you are looking at is displayed here. Click through different times using the scroll bar at the top of the window.

Adjust the color bar4. Change the min and max values of the color bar to better represent the range of amplitude values.

We can adjust the color bar and display to make the timeslices more closely resemble classic amplitude data.

1. Click here to display the color bar.

2. Right click on the color bar and click Color-Bar Dialog

3. In the settings tab, choose a Red-White-Blue color bar

5. The linear features persistently appear, with a different color bar and through different timeslices.

Understanding the geometry applied 1. To better understand this problem, let’s take a look at the geometry of the raw data.

2. Calculate and display the fold as we’ve done in previous labs

2. Notice entire inlines displaying no fold information.

Understanding the geometry applied

Empty bins seem like a problem. However, square bins are necessary for migration. If bins are rectangular, the migration algorithm will interpret this as having a greater velocity in one direction than the other, thus being anisotropic.

We will use Kirchhoff 2D/3D Time Migration

in this lab

Advantages of Kirchhoff Migration

Multipathing and phase changes through caustics can be handled, but are so complicated most production codes do not do so

Insufficient quality of image in areas of complex geological structures (areas beneath salt domes or close to salt flanks)

Special correction needed to preserve amplitudes and phases

Versatile and Efficient Easy to run from topography Kirchhoff migration can be performed for target lines or target sub

volumes No need to regularize sparse irregularly sampled data

Disadvantages of Kirchhoff Migration

10-8 Slide courtesy Dr. Kurt Marfurt

Defining the migration aperture and emergence angle

z

r

Migration aperture(the first radial distance from the midpointThis is NOT the source-receiver offset!)

tan = z/

h

reflections

Head waves

Emergence angle

The cost of migration increases as πr2z, the volume of voxels imaged. Increasing r, increases the cost. If your velocities are inaccurate, increasing r hurts your image! If you need to image steep dips, r may need to be large enough to allow the rays to “turn into” the target.

Horizontally traveling energy is almost always noise. Data coming in at shallow emergence angles is often associated with head waves (say with a critical angle of 30 degrees. We can simply ask the migration aperture to only image data greater than a given emergence angle. In the Vista software, this angle is defined as a function of depth and is measured from the …

Slide courtesy Dr. Kurt Marfurt

Running poststack migration

1. Migration is an incredibly simple flow in Vista, however, some parameters must be tweaked.

1. Input is selected within the migration flow icon. Set yours to be the 5x5 velocity applied data. Same as you used for creating the timeslice.

2. Choose velocity to be your 5x5 velocity.

3. Set the aperture size to be 7000 ft and add to the angle list 1000 ms and 40 degrees

Running poststack migration

2. Select one inline, preferably in the middle of the data (an area with high fold) to test the migration. Make sure you set the X-Line range to the entire dataset otherwise you will only be migrating one trace!

1. Test the aperture and angle to be certain it works for the data.

Something about aperture here

3. If the aperture parameters work, then run the migration on the entire volume.

Input stacked data volume

after migration

Note the removal of high angle vertical noise and consistency of reflectors with depth.

Note : if you have difficulty in changing the annotation and color scale , please follow the previous labs.

Real

Faults are visible Real or not

Acknowledgements

• Dr. Kurt Marfurt• Thang Ha

Lets move to the right climate summer is close !!!

Did geophysicist learned migration from us?