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
(*t*0, *V*_{NMO}), and the result is contoured
in
(*t*0,*V*_{NMO}) space (see
the RHS of Figure 1.17).
The correct values of
(*t*0, *V*_{NMO}) 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 *v*_{NMO} vs *t*0curve. The *v*_{NMO} can also
be thought of as the stacking velocity for a layered medium,
and is considered to be good estimate of *v*_{RMS}.