Page 319 - Acquisition and Processing of Marine Seismic Data

310 5. PREPROCESSING

to the vertical frequency axis recovers the ampli- as band-pass filtering or deconvolution, can
tudes of deeper reflections in near offsets and be initiated from seabed arrival time instead
produces better f-k filter results (Fig. 5.73C). of applying them to whole time samples of
the data. This prevents us from processing the
water column noise and may significantly
5.10 BRUTE STACK save data processing time, especially for large
data volumes.
Brute stacks are the sections prepared by
During the preparation of brute stacks, only
applying only essential processing steps to raw
essential steps are applied and time-consuming
seismic data. They are used for several reasons:
processing applications such as deconvolution,
• Brute stacks provide subsurface geological muting, velocity analysis, or migration are omit-
information immediately after a survey line is ted. Basically, a brute stack can be obtained
completed. using the steps given in Fig. 5.74. In data load-
• Several acquisition QC tests can be performed ing, it is not necessary to include all available
on the brute stacks instead of applying them channels in shot gathers, and in general, the first
to shot or CDP gathers, which can provide a 24 channels are sufficient for a proper brute
better understanding of acquisition issues. stack. One of the most important points is that
• Preparing brute stack sections also ensures a constant velocity of approximately 1500 m/s
effective QC analysis of processing is used for NMO correction for an entire line.
applications. For instance, quality of a In some cases, a single velocity function linearly
deconvolution or band-pass filter output can increasing with recording time is preferred
be better guaranteed by analyzing the results instead of a constant velocity. In each case,
on the brute stacks instead of shot gathers. velocity analysis is ignored. If necessary, a fast
• If the survey line contains extensive seabed migration algorithm, such as Stolt migration
morphology variations, or large-scale (Section 11.4), can also be applied to brute stack
subsurface geological structures such as salt sections. In 3D seismic data, brute stack cubes
domes, diapirs, or potential reservoirs, extra
velocity analysis locations may be preferred
in the vicinity to delineate the possible lateral
velocity variations around these structures.
Such specific velocity analysis locations along
the seismic line can be precisely determined
on the brute stacks.
• Some of the multiple suppression algorithms
require water bottom arrival time to be
entered into the database and into the headers
of the seismic data. Water bottom time along
the survey line can be picked on the brute
stack sections if no single-beam echo-sounder
data is available.
• Some data processing software enables us to
perform an automatic application of top
mutes using water bottom times picked from
the brute stack sections. FIG. 5.74 Basic processing steps used to obtain a brute
• Calculations of some processing steps, stack section. Complex steps such as velocity analysis or
implemented on a trace-by-trace basis, such migration are omitted in brute stack flows.