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Normal Moveout Correction to Align Offset Reflections with ZO Reflections.

The CMG traces in Figure 1.9 are misaligned with one another so that a brute stack will produce cancellation of the signal. To avoid this cancellation we flatten out the reflections by using equation 1.6 to apply a normal moveout (NMO) time shift tNMO(x) to the data, where
tNMO(x) = t(x)-t(0),  
  = $\displaystyle t(0)[\sqrt{1 + (x/v_{NMO}t(0))^2 }-1],$ (1.10)

and we have replaced the vRMS by vNMO. Once flattened, the traces in a NMO-corrected gather can be stacked together for constructive reinforcement of the reflection events.

But how do we in practice determine the vNMO values? A systematic means for determining Vnmo is described in the next section on velocity analysis.

The MATLAB script for NMO corrected data is given below:

%
% data(x,t)    = CMG data
% datanmo(x,t) = CMG data with NMO correction
% t0           = 2-way normal incidence time
% v(t0)        = Stacking velocity as a function of 2-way normal incidence time
%
for x=1:nx;
for t0=1:nt;
 tx = sqrt(t0^2+(x*2/v(t0))^2);
 datanmo(x,t0) = data(x,tx);
end;
end;

Application of a script like this to the LHS of Figure 1.16 will "flatten" the primary reflections to give the NMO corrected traces shown on the RHS.

  
Figure 1.16: Mobil's Gulf of Mexico CMG (LHS) before and (RHS) after NMO correction. Note, the absence of surface waves (why?) and the cleaner appearance (no static problems) of these marine records compared to the messy land data from Utah. Marine data, typically, are cheaper to acquire and cleaner than land data. The primary problem with marine data, however, is usually the presence of sea-floor multiples.

\psfig{figure=gomdata.ps,width=2.5in,height=2.5in}



\psfig{figure=gomdatanmo.ps,width=2.5in,height=2.5in}



next up previous contents
Next: Velocity Analysis to Determine Up: Basic Processing Steps Previous: Velocity Filter Data to
Gerard Schuster
1998-07-29