Page 462 - Acquisition and Processing of Marine Seismic Data

9.4 QC IN VELOCITY ANALYSIS 453

9.4 QC IN VELOCITY ANALYSIS the structural trend of the seafloor morphology
as well as the target geological structures, that
The important points that must be considered is, from the oil industry approach, the prospect.
during the computation of semblance plots have A denser distribution of supergather locations
been explained so far. In this section, some of the along a 2D line must be selected over the target
substantial issues that may occur during the structure. For instance, there are two regional
picking of velocity functions on the semblance highs on the 2D seismic line in Fig. 9.28A. It is
plots and their QC analyses are explained. recommended that denser analysis locations
Although not a certain rule, the velocity gener- over these structures be considered as compared
ally increases with depth (or with the zero-offset to the rest of the seismic line, so that the velocity
time) mainly because of the compaction due to structure of these highs can suitably be repre-
the increasing overburden pressure. Therefore, sented in the final 2D velocity field.
the enclosures on the semblance plots generally A similar approach must also be considered
indicate an increasing velocity trend with the for complex seafloor morphology such that, if
depth, and a velocity function that ensures the a regional or morphological trend exists in the
general trend of the enclosures must be pre- seismic data, it almost always requires a denser
ferred. In general, if the geology is not complex, velocity picking to obtain a correct 2D velocity
the proper velocity value of a reflection hyper- distribution. In 3D surveys, the analysis loca-
bola coincides with approximately the central tions must also be selected by regarding the
point of an enclosure occurring at the same 3D structural perspective of the subsurface geol-
zero-offset time of that specific reflection. ogy. Fig. 9.28B shows the structural map of a
Modern seismic processing software systems regional anticline. A much denser 3D velocity
allow us to incorporate different velocity deter- grid around this anticline, as in Fig. 9.28C,
mination approaches at the same time during may result in a better representation of the veloc-
velocity picking, which provides a more accu- ity distribution of this structure in the final 3D
rate picking of the velocity. For instance, velocity cube.
constant velocity stack panels for different pre- During velocity picking, it is crucial to avoid
determined velocity functions can be displayed the picking velocities of multiples from the
along with the semblance plot of a supergather, enclosures appearing at relatively lower velocity
and a picked velocity value can be double- values on the semblance plots. In some cases, the
checked if it corresponds to an enclosure on amplitudes of the multiples are so high that they
the semblance plot and a high-amplitude stack produce their own enclosures on the semblance
stripe on a constant velocity stack panel simulta- plots, which almost completely mask the enclo-
neously. Furthermore, a real-time NMO correc- sures of primary reflections (Figs. 9.19 and 9.20).
tion may be applied to the supergather using the If selected, the amplitudes of the multiples on
picked velocity value to ensure that the primary the stack data become more prominent, since
reflection hyperbolas are perfectly flattened the efficiency of the stacking on the suppression
after NMO correction. of multiples is bypassed, which strongly
The locations of supergathers both in 2D and depends on the hyperbolic moveout behavior
3D surveys must be carefully selected based of the multiple reflections after NMO correction.
on the shapes and locations of geological struc- During velocity picking, it is recommended to
tures. On 2D lines, velocity analysis locations take advantage of the real-time NMO correction
can be selected on brute stack sections regarding option for quality control purposes. Fig. 9.29