The velocity field (Figure 1.5-12) is used in normal moveout (NMO) correction of CMP gathers. Based on the assumption that, in a CMP gather, reflection traveltimes as a function of offset follow hyperbolic trajectories, the process of NMO correction removes the moveout effect on traveltimes. Figure 1.5-13 shows the CMP gathers in Figure 1.5-9 after moveout correction. Note that events are mostly flattened across the offset range — the offset effect has been removed from traveltimes. Traces in each CMP gather are then summed to form a stacked trace at each midpoint location. The stacked section comprises the stacked traces at all midpoint locations along the line traverse.
As a result of moveout correction, traces are stretched in a time-varying manner, causing their frequency content to shift toward the low end of the spectrum. Frequency distortion increases at shallow times and large offsets (Figure 1.5-13). To prevent the degradation of especially shallow events, the amplitudes in the distorted zone are zeroed out (muted) before stacking (Figure 1.5-14).
The CMP recording technique, which was invented in the 1950s and published later, uses redundant recording to improve the signal-to-noise ratio during stacking. To achieve redundancy, multiple sources per trace ns, multiple receivers per trace nr, and multiple offset coverage of the same subsurface point nf, are used in the field. Given the total number of elements in the recording system, N = ns × nr × nf, the signal amplitude-to-rms noise ratio theoretically is improved by a factor of [math]\sqrt N[/math]. This improvement factor is based on the assumptions that the reflection signal on traces of a CMP gather is identical and the random noise is mutually uncorrelated from trace to trace. Because these assumptions do not strictly hold in practice, the signal-to-noise ratio improvement gained by stacking is somewhat less than [math]\sqrt N[/math]. Common-midpoint stacking also attenuates coherent noise such as multiples, guided waves, and ground roll. This is because reflected signal and coherent noise usually have different stacking velocities.
In areas with complex overburden structure that gives rise to strong lateral velocity variations, the hyperbolic moveout assumption associated with reflection traveltimes in CMP gathers is no longer valid. As a result, hyperbolic moveout correction and CMP stacking do not always yield a stacked section in which reflections from the underlying strata are faithfully preserved. In such circumstances, imaging in depth and before stack becomes imperative.
- Mayne, 1962, Mayne, W. H., 1962, Common-reflection-point horizontal data stacking techniques: Geophysics, 27, 927–938.
- Sengbush, 1983, Sengbush, R. L., 1983, Seismic exploration methods: Internat. Human Res. Dev. Corp.
- CMP sorting
- Velocity analysis
- Multiple attenuation
- Dip-moveout correction
- CMP stacking
- Poststack processing
- Residual statics corrections
- Quality control in processing
- Parsimony in processing