The need for DMO

The need for Dip Moveout appeared from the problem of stacking traces that share the same Common-Mid-Point (CMP). The CMP Stacking assumes that reflections come from horizontal layers. Under this assumption the Normal Moveout correction (NMO) flattens the hyperbolas in a CMP gather to horizontal lines. After NMO, traces belonging to the same CMP are summed together (stacked) to enhance the reflected signal and to attenuate the noise.

Problems appear when dipping events are present in a section. A dipping reflector in a CMP section also appears as a hyperbola (see Figure a). The NMO velocity necessary to flatten the hyperbolas originating from reflections on the dipping layer is higher than the real velocity. Applying the NMO correction with the velocity necessary to flatten this hyperbola will misplace the position of the dipping reflector.

Figure a shows several CMP gathers containing reflections from a dipping layer and from a horizontal layer. After NMO with the real velocity the hyperbolas corresponding to the flat layer become straight lines (Figure b) and after stacking, the horizontal event is preserved at the expense of the dipping event. In Figure b NMO is performed with a higher velocity which is necessary to flatten the hyperbolas corresponding to the dipping event. After stacking (Figure c) the dipping event is preserved at the expense of the horizontal layer. It is obvious that the NMO correction with a single velocity combined with CMP stacking acts as a dip filter. The result is a degradation of the final image when conflicting dips are present.

 

CMPnmoEx1
Figure 1 The effect of CMP stacking with real velocity on dipping reflectors.
a. Several CMP gathers for a flat layer and a dipping layer.
b. The CMP gathers after NMO with medium velocity (v_real); the hyperbolas from the horizontal event are flattened.
c. Traces after stacking for the same CMP.

 

CMPnmoEx2
Figure 2 The effect of CMP stacking with velocity $(v_{nmo}= {v_{real} / {\cos \theta}})$ on horizontal reflectors.
a. Several CMP gathers for a flat layer and a dipping layer.
b. The CMP gathers after NMO with higher velocity; the hyperbolas from the dipping event are flattened.
c. Traces after stacking for the same CMP.