Page 484 - Acquisition and Processing of Marine Seismic Data

10.4 SPECIFIC STACKING METHODS 475

FIG. 10.16 Effect of stacking on the random noise suppression. (A) An NMO corrected synthetic CDP with 120 traces con-
taminated by 50% random noise. (B) Stacked traces obtained using first 4, 12, 24, 36, 48, 60, 72, 96, and 120 traces of CDP gather
in (A), respectively. Increasing number of traces in stacking improves S/N ratio significantly.

geometry requires nonzero offset data record- both near and far offset traces are treated simi-
ing. During the processing, source-receiver off- larly and stacked all together. Amplitude varia-
set is removed by NMO correction based on tions depending on the offset or incidence
the assumption that the reflections are of per- angle of reflections in CDP traces are, however,
fectly hyperbolic moveout. However, in some considered to be a significant hydrocarbon indi-
areas with irregular seafloor morphology and cator and may incorporate important information
complex subsurface geology as well as lateral for the oil and gas industry. Estimation of pore
velocity variations, reflections may not display fluids in the possible subsurface reservoirs from
hyperbolic behavior. In addition, due to the mul- surface seismics requires knowledge of how the
tiple reflections and conflicting dips, stack sec- pore fluid saturation affects the signal ampli-
tions and zero-offset sections are commonly tudes. The variation of reflection amplitude with
not equivalent. angle of incidence is known as amplitude versus
offset (AVO) and is explained in Section 12.6.Pos-
sible AVO effects on seismic data can quickly be
10.4 SPECIFIC STACKING analyzed by partial stacking of different offset
METHODS ranges independently. A common method to
obtain AVO information from surface seismic
data is to stack near and far offset traces in
10.4.1 Partial Stack
CDP gathers separately, which generates angle
Classical stacking of traces in a CDP gather dependent stack sections. This method is known
into the zero offset destroys the amplitude varia- as partial stack or near/far offset stack, and if
tions of the reflections with respect to the source- computed in terms of the incidence angle, then
receiver offset or incidence angle. This is because it is termed angle stack.