Page 258 - Acquisition and Processing of Marine Seismic Data

5.4 GEOMETRY DEFINITION 249

define a generalized approach describing the ref- other information stored in the trace headers
erence latitude and longitudes or origin of the prepared during the acquisition or processing
metric coordinate system. This can be achieved stages. The main purpose in reconstructing the
using map projections that transform the geo- different trace gathers is to apply the different
graphical coordinates (e.g., latitude and longi- processing steps in different domains where
tude in degrees-minutes-seconds) into metric the noise is best separated from the signal.
coordinates (e.g., easting and northing in The ray paths and schematic displays of the
meters). most common trace gathers are shown in
Although a number of projection systems are Fig. 5.8A. In addition, if we plot the shot coordi-
used today, the most common one is known as nates against the receiver coordinates using the
the Universal Transverse Mercator (UTM), which geometrical relationships given in Fig. 5.8B, we
provides systematic representation of geograph- get a specific source/receiver location chart,
ical coordinates as metric coordinates all over the termed the stacking chart, schematically repre-
world. In UTM projection, the world is divided sented in Fig. 5.8C for the end-on spread, which
into 60 segments (named UTM zones) in the is standard for marine surveys. Each blue point
north to south direction, where each zone is 6° on this chart represents a seismic trace from a 2D
wide (Fig. 5.7C) and has a central meridian survey with a time axis perpendicular to the
located at the center of each segment. Fig. 5.7D paper plain. Definitions of the trace groups indi-
shows an example of this segmentation for the cated by the numbers from 1 to 5 are as follows:
Gulf of Mexico which extends through three suc-
cessive UTM zones: Zone 14, 15, and 16 North. 1. Common-shot gather is the group of traces
Each UTM zone has its own central meridian: propagated from a single shot and
for instance, it is W99° for Zone 14, W93° for Zone recorded by all available receivers. It is also
15, and W87° for Zone 16. Based on this layout, known as a shot gather, and raw seismic field
easting (x) and northing (y)valuesofany point data is recorded as shot gathers. The number
in the survey area, like point A in Fig. 5.7Cand of traces it contains equals the number of
D, can be obtained in meters as channels available on each streamer. Trace
x(m) ¼ [Horizontal distance from the central spacing between the successive traces in a
meridian to point A] + [False easting/2]. shot gather equals the group interval. In the
y(m) ¼ [Vertical distance from Equator]. shot gathers, receiver coordinates change
Here, false easting is a constant to prevent while the shot coordinate is fixed. 3D seismic
negative x coordinate values in areas close to data consists of several shot gathers even for
the western border of any UTM zone and is one single shot, termed shot groups, which
taken as 500,000. consist of the traces from one shot and
After loading the geometry into the database recorded at different streamers.
and trace headers of the data, seismic traces can 2. Common-receiver gather is the group of traces
be grouped and processed/displayed as differ- propagated from different shots but recorded
ent trace groups. These different trace groups at a common receiver located exactly at the
sorted by different geometric criterion are same station point along the seismic line.
known as trace gathers, or ensembles. Con- According to the reciprocity theory (i.e.,
structing such different trace gathers commonly interchanging source and receivers reveals
changes the domain of the data, such as from exactly the same data), a common-receiver
shot to the CDP domain, or to common offset gather contains exactly the same information
domain, etc. Sorting the traces into different as that in a shot gather with the same
gathers requires the use of the geometry and geometry. In offshore seismic refraction