Page 260 - Acquisition and Processing of Marine Seismic Data

5.4 GEOMETRY DEFINITION 251

CDP gathers so that we can apply velocity 5.4.1 QC in Geometry Loading
analysis and stacking. In the CDP gathers,
midpoint coordinates of each shot and receiver Several data processing steps such as NMO
pair are fixed, while both shot and receiver correction, CDP sort, or velocity analysis require
coordinates change. Trace spacing between that the geometry of the seismic data be already
the successive traces in a CDP gather equals defined in the database and in the trace headers
before the application of these steps, and incor-
the double of the shot or receiver spacing,
rect geometry definition results in incorrect out-
depending on which of these is larger.
4. Common-offset section is the group of traces puts from forthcoming processing steps. Even
from exactly the same recording channel (and simple mistakes during the geometry loading
hence with the same offset) of all shots along may result in deadly errors on the final stack sec-
the line. It is also known as a single trace tions. Therefore, quality control implications are
section and is generally prepared using the always necessary after loading the geometry
first or second channel of each shot gather into the trace headers, and this can be achieved
in different ways:
(Section 5.3). A common-offset section
resembles the final stack section, but its S/N i. Checking the trace headers:Trace headers of
ratio and resolution are quite poor (Fig. 5.5). randomly selected traces from raw shot
In the common-offset sections, the offset value gathers can be displayed and some specific
between shot and receivers is fixed while both header words can be analyzed to ensure that
shot and receiver coordinates change. Trace they all have the correct geometry and
spacing between the successive traces in a navigation information, such as offset,
common-offset section equals the shot interval shot/receiver x coordinate, shot/receiver y
and the trace-by-trace consistency of the coordinate, CDP numbers, and CDP fold.
reflections on the common-offset sections Fig.5.9Ashowsthefirst90bytesofanexample
demonstrates the subsurface geology. seismic trace in SegY format (the first channel
5. Stack section is a group of traces obtained by from shot number 101) before the definition of
stacking all traces in each CDP gather geometry. After geometry loading, some
following the normal moveout (NMO) specific headers such as CDP number at byte
correction. The traces in a stack section are 21, offset value at byte 37, shot and receiver
reduced to the zero-offset time since the coordinatesinmetersbetweenbytes73and88,
required propagation time of the signals from etc., are correctly filled in (Fig. 5.9B).
shots to receivers along the subsurface is ii. Calculating the propagation velocities from shot
removed by NMO correction. After stacking, gathers: After loading the geometry, seismic
each CDP gather produces only one single velocities can be computed on the shot
“stacked trace” and stack sections are gathers, since the offset values of each trace
prepared plotting these stacked traces side are defined by the geometry loading.
by side (Section 10.3). Trace spacing between Checking the propagation velocities of well-
the successive traces in a stack section equals known arrivals, such as direct wave and
half of the group interval and the reflections seabed reflection, can provide information
on the stack sections reveal the subsurface about the accuracy of the geometry loading.
geology. Although a number of additional For this purpose, seismic traces of the
processing steps can be applied to the data filtered shot gathers can be displayed and
after stacking, such as migration, scaling, or the propagation velocity of direct wave as
time variant filtering, a stack section is the well as seabed and subsurface reflections
ultimate objective of the seismic surveys. can be computed (Fig. 5.10) to ensure that