Horizon velocity analysis
One way to estimate velocities with the accuracy required for detailed structural or stratigraphic studies is to analyze the particular horizon of interest, continuously. Such a detailed velocity estimation is called horizon velocity analysis (HVA). Horizon velocity analysis is an efficient way to get velocity information at every CMP location along selected key horizons, as opposed to the conventional velocity analysis that provides velocity information at every time gate at selected CMP locations. The underlying principle is the same as that of the velocity spectrum. The output coherency values derived from hyperbolic time gates are displayed as a function of velocity and CMP position. Correlation values are computed from a gate that includes the horizon of interest. Horizon times are digitized and input to the horizon velocity analysis. Figures 3.2-32 shows a stacked section and Figure 3.2-33 shows HVA semblance spectra over five horizons. Note the short-wavelength variations of stacking velocities along the line traverse — such variations ordinarily are not captured by velocity analyses conducted at sparse CMP intervals. These semblance spectra can be picked to obtain horizon-consistent rms velocities which are then used to derive interval velocities (equation 24). Similar types of computational details, such as smoothing and biasing, are considered applicable to velocity spectrum.
Whenever there are structural discontinuities on a stacked section, HVA is carried out on segments of the horizon that are separated by faults. Horizon velocity analysis can improve a stacked section in areas with complex overburden structure that may cause nonhy-perbolic moveout. This is somewhat surprising, since HVA still is based on hyperbolic moveout. Nevertheless, in practice, HVA provides the detailed lateral velocity variations along a marker horizon, which may be missed by conventional velocity analysis locations that are sparsely spaced along the line. Consider horizon A in Figure 3.2-34, which is below the salt dome S. The salt dome behaves as a complex overburden, causing the raypaths that are associated with the underlying reflectors to bend. Note the rapid lateral changes in velocity and the improvement in the CMP stack after using the HVA picks. The rapid change in stacking velocity associated with the base of the salt is typical. Starting on the left, the reflector is deep in time and flat. Then it dips; hence, the higher stacking velocity. Then it gets shallower; hence, the lower stacking velocity. Then it dips again, yielding a higher stacking velocity. Finally, it becomes flat; hence, a decrease in velocity.
A velocity section derived from HVA is structure-consistent, whereas a velocity section derived from vertical velocity functions picked at selected analysis locations along a line traverse is, in general, structure-independent. Consider the stacked section in Figure 3.2-35 with interpreted time-horizon segments associated with subsurface geological markers. The semblance spectra associated with the eight horizons picked from the stacked section computed by HVA are shown in Figure 3.2-36. The top spectrum corresponds to the shallowest time horizon (dark blue) after the water-bottom horizon. These spectra were picked to obtain the horizon-consistent rms velocity profiles, which were then used to derive the rms velocity section shown in Figure 3.2-37 (top). The time horizons picked from the stacked section (Figure 3.2-35) are superimposed on the velocity section. Compare the velocity sections in Figure 3.2-37 derived from HVA (top) and vertical functions (bottom), and note that the HVA-based velocity section is more consistent with the subsurface structure. Hence, the HVA results are more appropriate to use in Dix conversion to derive structure-consistent interval velocities with meaningful magnitudes (equation 24).
Figure 3.2-34 A CMP-stacked section obtained by sparsely spaced conventional velocity analysis (top) and the stacked section (bottom) obtained by using velocities derived from horizon velocity analysis (HVA) (middle). The HVA for horizon A below the salt dome S is shown in the center.
Figure 3.2-37 Top: the velocity section derived from the HVA-based semblance spectra shown in Figure 3.2-36; bottom: the velocity section derived from the interpolation of the vertical rms velocity functions picked at selected CMP locations along the line traverse. Superimposed on these sections are the time horizon segments interpreted from the stacked section in Figure 3.2-35.
Figure 3.2-38 Top: the CMP-stacked section derived from the HVA-based velocity section shown in Figure 3.2-37 (top); bottom: the CMP-stacked section derived from the velocity section shown in Figure 3.2-37 (bottom) derived from the interpolation of the vertical rms velocity functions picked at selected CMP locations along the line traverse.
What about the quality of stacks (Figure 3.2-38) obtained from the velocity sections (Figure 3.2-37) derived from HVA and vertical functions? Two sets of details from the stacked sections shown in Figures 3.2-39 and 3.2-40 reveal that, indeed, there can be more than marginal differences. While the HVA-based stack shows better continuity of reflections, it is inferior to the conventional stack with regard to diffractions. This should be expected since the HVA analysis is done along reflection events and not diffractions (Figure 3.2-35).
In conclusion, if the objective is to obtain an optimum CMP stack with the highest stack power possible, conventional velocity analysis at selected CMP locations along the line traverse or over the 3-D survey area yields a robust velocity section. If, on the other hand, the objective is to derive interval velocities from Dix conversion, then horizon-consistent velocity analysis yields structure-consistent results that are geologically plausable.
- The velocity spectrum
- Measure of coherency
- Factors affecting velocity estimates
- Interactive velocity analysis
- Coherency attribute stacks
- Topics in moveout and statics corrections