Normal moveout

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Seismic data is sorted by common midpoint and then corrected for normal moveout

In reflection seismology, normal moveout (NMO) describes the effect that the distance between a seismic source and a receiver (the offset) has on the arrival time of a reflection in the form of an increase of time with offset.[1] The relationship between arrival time and offset is hyperbolic and it is the principal criterion that a geophysicist uses to decide whether an event is a reflection or not.[2] It is distinguished from dip moveout (DMO), the systematic change in arrival time due to a dipping layer.

The normal moveout depends on complex combination of factors including the velocity above the reflector, offset, dip of the reflector and the source receiver azimuth in relation to the dip of the reflector.[3] For a flat, horizontal reflector, the traveltime equation is:

t^{2}=t_{0}^{2}+{\frac {x^{2}}{v^{2}}}

where x = offset; v = velocity of the medium above the reflecting interface; t_{0} = travel time at zero offset, when the source and receiver are in the same place.

NMO correction

NMO in principle

From the equation, it is possible to calculate the velocity when the offset and two-way times at zero and non-zero offset are known. This velocity is the NMO velocity and can be used to remove the effect of offset on the traveltimes (as shown in the diagram). NMO correction can be used as a seismic processing tool to powerfully distinguish between reflections and other events such as refractions, diffractions and multiples. If an accurate NMO correction has been applied, reflections will appear as straight horizontal lines (see diagram), refractions will now appear as inverse curves and diffraction and multiple arrivals will retain some curvature.[4] These unwanted arrivals can now be removed with a properly designed filter.

References

  1. ↑ Schlumberger Oilfield Glossary. NMO.
  2. ↑ Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press. p. 86. ISBN 0-521-46826-4. 
  3. ↑ Yilmaz, Öz (2001). Seismic data analysis. Society of Exploration Geophysicists. p. 274. ISBN 1-56080-094-1. 
  4. ↑ Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press. p. 146. ISBN 0-521-46826-4. 


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