Practicalities of CMP Processing

For each CMP gather, the velocity analysis is done at many times. A common method of displaying this work can be seen in the example to the right (click the image for a larger version). The velocity spectrum defines the stacking velocity as a function of reflection time. The energy (cross-power ) function used to define the stacking velocity is calculated for a large number of narrow time windows along a seismic section. The velocity spectrum is typically displayed alongside the draped section. In the example shown here, stacking velocities for all times less than about 3 seconds are reasonably clear. Beyond 3 seconds there are multiple peaks that are caused by overlapping hyperbolic trajectories. The low velocity arrivals are due to multiple reflections in the near surface. They arrive at the same time as the reflection events from deep horizons.

A stacking velocity is defined for each time sample. The normal moveout correction and stacking can be carried out by summing along a different hyperbola for each time on the seismogram. Equivalently, the CMP gather may first be NMO corrected and then stacked. Both approaches are done in practise.