Page 348 - Acquisition and Processing of Marine Seismic Data

6.5 DETERMINATION OF DECONVOLUTION PARAMETERS 339

autocorrelation time gate and deconvolution shelf and its autocorrelation section. Since
design gate must be suitably defined. 1500 m/s water velocity is used in NMO correc-
tion during the preparation of the brute stack,
multiple reflections are quite distinct both on
6.5.1 Autocorrelation Time Gate
the brute stack and autocorrelation sections.
Autocorrelation of the input seismic trace The autocorrelation time gate corresponds to
provides substantial information used to deter- the window over which the autocorrelations of
mine the deconvolution parameters and to the traces are calculated. It is selected from the
evaluate the quality of deconvolution outputs. zones of usable reflections and the noisy areas
Analyzing the autocorrelation function, recur- should be avoided. On the shot gathers, the gate
ring events, such as long- or short-period generally starts just above the seafloor reflection
reverberations, in the seismic data can be and typically has a length of 500–1000 ms.
determined. Long-period reverberations appear Length of the autocorrelation gate is another
as isolated amplitude packages in the autocorre- aspect and its effects are illustrated on the shot
lation series, and, in ideal conditions, it is gather and its autocorrelograms in Fig. 6.25.If
separated from other amplitudes with an the gate is too short, it may not be possible to
amplitude-free zone. Fig. 6.24 shows an example determine the length of the first transient zone
brute stack section from a shallow continental on the autocorrelograms (Fig. 6.25B). The time

FIG. 6.24 A shallow brute stack section (top) with several seafloor multiples and its autocorrelation traces (bottom).