Figure 1.17 illustrates the procedure for automatically determining the stacking/NMO velocities from the CMG. Here, CMG amplitudes are summed together along different hyperbolas described by the traveltime equation 1.6. The summation is carried out for different values of (t0, VNMO), and the result is contoured in (t0,VNMO) space (see the RHS of Figure 1.17). The correct values of (t0, VNMO) will describe an hyperbola that coincides with the reflection event to give a large summation value; otherwise the summation values (or semblance) are small. The summation value is called a semblance value because we really sum over a thick hyperbolic line, weight the summation with a normalization factor, and insure positive summation values by taking the absolute value of the sum.
Identifying the bullseyes on the RHS of this figure, and connecting lines
between these bullseyes describes the optimal vNMO vs t0curve. The vNMO can also
be thought of as the stacking velocity for a layered medium,
and is considered to be good estimate of vRMS.