Dictionary:DMO (dip moveout) processing

A seismic processing operation to correct for the fact that, for dipping reflections, the component traces of a CMP gather do not involve a common reflecting point.

Dip-moveout processing creates apparent common-reflection-point gathers by a convolution applied to adjacent common-midpoint gathers, with the feature that the moveout with offset for reflections from a dipping bed no longer depend on the dip angle (see Figure C-9b).

FIG. C-9. Common-midpoint method. (a) In six-fold shooting with 24-geophone groups and the source point moved two group intervals between successive shots; the same subsurface is sampled six times (A⇒23, B⇒21, C⇒19, D⇒17, E⇒15, F⇒13). (b) A reflector that dips does not have a common reflecting point and common-midpoint stacking results in reflection-point smearing unless DMO (q.v.) processing or migration precedes stacking. (c) To achieve a common-reflection point in the case of dip requires unequal surface spacing. (d) A common-source gather is a collection of traces having the same source; (e) common-receiver gather; (f) common-offset gather. (g) If there are horizontal velocity variations, prestack migration is required to form a common-imaging-point gather. Compare Figure C-13. All diagrams assume constant velocity.

DMO effectively corrects for the reflection-point smear that results when dipping reflectors are stacked by the CMP method. After DMO is applied, events with various dips stack with the same velocity.

DMO stands for dip moveout, but it is different from the classical dip moveout that is simply the effect of dip on arrival times.

DMO can be performed in a number of ways, including prestack partial migration, time-domain, finite-difference methods (offset continuation), Fourier-domain implementation, integral (Kirchhoff) methods.

FIG. D-20. DMO. (a) Depth section showing the updip movement of the reflecting point for an offset geophone for constant velocity; $\Delta =(h^{2}/D)cos\xi sin\xi$, where $\xi$; is the dip (Levin, 1971). To avoid reflection point smearing, an offset trace should be gathered with the updip zero-offset trace at a distance $G=(-h^{2}/D)sin\xi$, but such a gather is not hyperbolic; the DMO correction makes this gather hyperbolic. (b) A diffraction in location-offset space, a Cheops pyramid, is not a hyperboloid. (c) Applying NMO changes the Cheops pyramid into a saddle-shaped surface. (d) Applying DMO along with NMO yields data that can be stacked without reflection-point smear. (e) NMO corrects for the time delay on an offset trace assuming horizontality, DMO moves the data to the correct zero-offset trace for a dipping reflection, and migration further moves it to its subsurface location.

Velocity-dependent DMO is usually applied after velocity-dependent NMO. Gardner’s DMO applies velocity-independent DMO prior to velocity-dependent NMO. See Figure D-20 and double square-root equation.

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