Page 436 - Acquisition and Processing of Marine Seismic Data

9.2 VELOCITY DETERMINATION FROM SEISMIC DATA 427

small-spread data always has lower resolution the seismic processing industry, and efficiency
and accuracy than the data from longer streamer and accuracy of the analysis can be significantly
acquisitions, especially for deeper parts. increased by a simultaneous application of more
The basic assumption in velocity analysis of than one method.
the seismic data is that the reflections are hyper-
bolic on CDP gathers. However, the existence of 9.2.1 Constant Velocity Scan
complex seafloor morphology, anisotropy or
extreme dips in the subsurface such as salt Constant velocity scan is a simple method
flanks can result in nonhyperbolic reflections basedontheflatteningofthereflectionhyperbolas
(Fig. 1.24). In such cases, the theory behind the on a particular CDP gather after NMO correction
velocity analysis fails and it may become quite applied with a range of velocities. NMO correc-
difficult to find out the accurate velocity func- tion is applied to a selected CDP gather using dif-
tion at the desired resolution. In practice, there ferent velocities and the results are plotted. The
are a number of velocity analysis methods in procedure is schematically shown in Fig. 9.3.

FIG. 9.3 Schematic derivation of velocity function (dashed blue line) for a CDP gather using constant velocity scan. NMO
correction is applied to the input CDP for a range of constant velocities (here, from 1480 to 2400 m/s). The suitable velocity for
a specific reflection hyperbola is the one that perfectly flattens the hyperbola.