Velocity Analysis to Determine Vstack.

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, V_NMO), and the result is contoured in (t0,V_NMO) space (see the RHS of Figure 1.17). The correct values of (t0, 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 t0curve. 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.

  

Figure 1.17: CMG panel and corresponding semblance panel. There are 2 groups of reflection events, one corresponding to reflections from a shallow reflector with a velocity of about 5,000 ft/s, and the other from a deeper reflector with a moveout velocity of about 10,000 ft/s. Note that the bullseyes in the semblance panels correspond to the correct T0 and NMO velocities.