Page 321 - Acquisition and Processing of Marine Seismic Data

312 5. PREPROCESSING

• Common offset sections are obtained by • The number of traces in a common offset
displaying the first channel of every shot, and section is relatively low when compared to
thus, their horizontal axis is shot (or FFID) brute and full stack sections. Since each
number, because each shot contributes one shot gather provides one trace, the number
single trace to the common offset section. of traces in the common offset section exactly
Brute and full stacks, however, are obtained equals the number of shots along the line,
by stacking after a CDP sort process; and the distance between these traces
therefore, they have the order of CDP as the equals the shot interval. However, the
horizontal axis. number of traces in a full stack section is
• The S/N ratio of the sections is quite different: [(2 shot interval)/receiver interval] times
The quality of the common offset section is higher than those in a common offset section,
relatively low since it is single trace data and and the distance between these traces
does not benefit from stacking. Although the equals half of the group interval. This
S/N ratio is better for deeper parts of the brute indicates that the lateral resolution of a
stack, it is considerably low for shallow areas; common offset section is considerably
this is because the shallow parts of a section lower than that of a brute stack or a full
need a precise velocity field for an accurate stack section.
NMO correction, yet we use a single velocity • Common offset and brute stack sections
function for an entire section, which seems generally suffer from multiple reflections.
not to represent the correct velocity of This is because the common offset sections do
shallow subsurface sediments in sufficient not benefit from the advantage of stacking in
accuracy. As expected, the full stack section eliminating the multiple reflections. In brute
has the highest S/N ratio and resolution both stack sections, multiples may also be quite
in shallow and deeper parts. distinct since there is no velocity analysis and
• The reflections are not located at their correct typically water velocity is used in NMO
horizontal locations in the common offset correction. Multiples are of relatively high
sections because each trace in the common amplitudes on the brute stacks because they
offset section is assigned at a shot point also propagate mainly with water velocity
location for each shot. However, assuming a (Chapter 7), which results in the flattening of
horizontal subsurface reflector, the multiple reflection hyperbolas during the
reflections are recorded from the midpoints NMO correction. However, stacking itself is a
of the source-receiver pairs, and this is powerful method to suppress multiples, and
ensured by the CDP sort process in brute full stack sections benefit from this property
stack and full stack sections. of the stacking.