Page 469 - Acquisition and Processing of Marine Seismic Data

460 10. NORMAL MOVEOUT CORRECTION AND STACKING

arrival time, providing a correct velocity field is specific noise types such as side effects, strong
used. Fig. 10.1 shows example CDP gathers multiples, etc. Fig. 10.3 shows the effect of
before and after NMO correction, and the veloc- NMO velocity on the flattening of a reflection
ity function used. Practically, NMO correction is hyperbola on a synthetic CDP gather. After a
a process that removes the offset distance successful NMO correction, all of the primary
between source and receivers so that all the reflection hyperbolas on the CDP gathers along
reflection arrivals are reduced to t(0) vertical the whole seismic line must be perfectly flat-
two-way zero-offset time on a CDP gather. tened (Fig. 10.3B), providing an isotropic sub-
Fig. 10.2 shows the geometrical sense of the cor- surface composed of horizontal layers. If we
rection both on ray paths and on a synthetic use a slower velocity, reflection hyperbola is
CDP gather. overcorrected (Fig. 10.3C) whereas it is under-
The success of the NMO correction in flatten- corrected if a faster NMO velocity is used
ing of a primary reflection depends on a number (Fig. 10.3D). The most suitable velocity is the
of factors, including the accuracy of the velocity one that flattens the primary reflection hyperbo-
used, anisotropic effects of the subsurface, las after NMO correction, which equals the true
spread length, structural dip of the reflector, velocity of the upperlying medium for a single
nonhyperbolic reflections, existence of some horizontal reflector case.

FIG. 10.1 Two successive CDP gathers (A) before, and (B) after NMO correction. (C) RMS velocity function (blue line) used
for NMO correction as well as interval velocity function (red line) calculated from the Dix equation (Eq. 9.9).