Normal Moveout Correction

Description Apply or remove NMO correction to gathers.
Module(s) Image Gather Processing
Requirements Gathers, Velocity model
Related Stacking, Mute Gathers
Works with Gathers

Normal Moveout (NMO) Correction applies or removes moveout calculated from a supplied velocity model. 

Some processes, such as Q-Filter, are travel-time dependent and require NMO uncorrected gathers. Use this process to remove NMO correction prior to running the filter, then reapply NMO correction afterwards.

Create an NMO correction process

Create an NMO correction process
  1. In the Control Panel, open the Process tab.
  2. At the tab header, click the Add icon and select New Process.
  3. Double-click Normal Moveout Correction.
  4. Type a name for the process and click OK.

Define NMO correction settings

Define NMO correction settings
  1. Input: The input volume to be corrected
  2. Mode: Apply or Remove the NMO correction
  3. Offset Source: Whether offset should be taken from the OFFSET header, or calculated from the source and receiver positions (SX/SY/GX/GY headers)
  4. Model: Either Isotropic or Anisotropic VTI correction. For the isotropic case, you have the choice of 2nd- or 4th-order correction.
  5. Velocity: Either a velocity volume or a constant velocity value. If the velocity provided is insufficiently deep, it will automatically be extrapolated downward.
  6. Eta (VTI anisotropic, Azimuthal 4th order): Either a volume or a constant value for the anisotropic parameter
  7. Ellipticity: Either a ellipticity volume or a constant ellipticity value.
  8. Fast-direction: Either a fast-direction volume or a constant fast-direction value.
  9. Input azimuthal range: Narrow or rich depending on the narrow or rich azimuthal content of the data used to derive the velocity model.
  10. Picking-azimuth: Velocity picking azimuth value
  11. Azimuth source: Available for 2D volumes only. Selects whether the Azimuth value should be calculated from the source and receiver positions (SX/SY/GX/GY headers), or taken from the OTRAV header. 3D volumes will always use OTRAV.
  12. Honour receiver depth: When selected, this option will take into account the depth of the receiver when calculating NMO corrections. For use with OBN data or deep tow (slant cable) data prior to any receiver redatuming.
  13. Honour source depth: When selected, this option will take into account source depths (SELEV) when calculating NMO corrections.
  14. Stretch Method: Choose the stretch processing. No stretch processing does nothing, Use stretch mute performs a stretch mute using the sample stretch cut-off, Use stretch limit to limit the NMO stretch to the cut-off value.

Note: If you use a stretch limit when applying an NMO correction, you must use a stretch limit with the same cut-off when you remove the NMO correction.

  1. Cut-off (samples): If applying a stretch mute the amount of stretch beyond which a sample will be muted. If using a stretch limit, this is the maximum value of stretch.
  2. Ramp (ms): If applying a stretch mute, the length of the linear mute ramp.

As a result of this process, a new normal moveout correction volume is available in the Volume tab.

Azimuthal Anisotropic NMO

Azimuthal anisotropic NMO is applied using a generalised version of the Corrigan (1996) equation (below). Where velocity analyses have been performed using narrow azimuth input data (such as conventional marine streamer), the Input azimuthal range = Narrow option should be selected. In this instance the following four inputs are required:

  • a velocity model (V pick)
  • the nominal (picking) azimuth from which those velocities were derived
  • the azimuth of the fast direction
  • the ellipticity of the velocity

For velocities derived from data with a rich azimuthal coverage (as is typical for 3D land or OBN surveys), the nominal picking azimuth is assumed to be halfway between the fast and slow directions = as per Corrigan’s original equation. In this case, the Input azimuthal range = Rich option should be selected.

The azimuth of the fast direction and ellipticity can be set to constant values or volumes can be derived using the HTI Auto Picker acting on output from either HTI migration scans or the Azimuthal NMO Scanner.

Generalised Corrigan equation

Anisotropic azimuthal 2nd order equation

Anisotropic azimuthal 4th order equation

Tip: To quickly open the output volume, right-click the process item in the Process tab and click Open Child Volume. Vice versa, you can also open the process from the Volume tab directly. To do this, right-click the volume which was created from a process (indicated in italics) and click Open Parent Process.

Reference

Corrigan, D., Withers, R., Darnall, J. and Skopinski, T., 1996, Fracture mapping km azimuthal velocity analysis using 3-D surface seismic data: 66th Meething, SEG, Expanded Abstracts, 1834-1837.