Page 419 - Acquisition and Processing of Marine Seismic Data

410 8. CDP SORT AND BINNING

FIG. 8.4 Schematic representation of traces forming the CDP gathers using reflections from only one horizontal reflector
along a 2D seismic line. Δx is group interval, s is shot interval.

FIG. 8.5 (A) CDP and CMP terms are identical providing that the earth is composed of horizontal reflecting interfaces.
(B) If the interface is inclined, these two terms are no longer equivalent to each other.

distribution for the whole survey area is pre- calculated as the case in 2D surveys given by
pared (Fig. 8.6B). Eq. (8.1), and the crossline fold depends on the
Maximum fold value is controlled by the sur- number of streamers and the distance between
vey geometry and acquisition parameters. For them. If only one vessel tows several streamers,
2D lines, only the inline fold F I is pronounced as is the case for conventional 3D acquisition, no
and is calculated by overlap in midpoint coverage occurs. Therefore,
in narrow azimuth marine acquisitions, cross-
number of channels group interval
F I ¼ (8.1) line fold is generally 1.
2 shot interval
For instance, the nominal inline fold is
For 3D surveys, maximum or nominal fold of the F I ¼ 80 for the following parameters in a conven-
survey is obtained by multiplying the inline and tional 3D flip-flop acquisition:
crossline folds: Number of recording channels: 640.
Group interval: 12.5 m.
(8.2)
F N ¼ F I F C
Shot interval: 25 m (flip-flop).
where F I and F C are the inline and crossline fold Source separation: 50 m.
numbers, respectively. For 3D, the inline fold is Streamer separation: 100 m.