Page 130 - Acquisition and Processing of Marine Seismic Data

2.5 DATA ACQUISITION PARAMETERS 121

The azimuth of the seismic profiles is impor- of the total streamer length of the active sections
tant in the optimization of the survey. In a rect- (Fig. 2.74). If the number of recording channels
angular working area, it is always preferred to or length of the streamer(s) increase, inline cov-
shoot on the line parallel to the longer border erage also increases.
of the area for a cost-effective survey, if the struc- In 3D surveys, multistreamer acquisition pro-
tural dip permits. This allows running of longer vides a wide coverage area for a single sail line.
profiles with respect to the orthogonal direction Inline coverage is the same as that for 2D sur-
with a smaller number of line changes, which veys; however, increased number of streamers
cause significant dead times during the turns, simply extends the total crossline coverage. An
especially with long spreads. In addition, sail eight-streamer acquisition yields two times
lines should be sufficiently long to obtain full- wider coverage area than that from a four-
fold data from target depth and a reliable migra- streamer acquisition (Fig. 2.75), which, there-
tion result. fore, results in a more optimized operation time,
since the sail line interval can be increased. The
2.5.1.6 2D and 3D Coverage number of total sail lines in 3D surveys is
Coverage is the subsurface area where the inversely proportional to the number of
seismic data is recorded, depending on the size streamers deployed. Acquisition with higher
of the seismic spread. Only inline coverage is number of streamers also provides better azi-
pronounced for 2D surveys, while 3D surveys muth distribution.
provide both inline and crossline coverage. Total Lateral distance between the streamers in 3D
coverage for one single shot depends on differ- surveys determines the distance between CMP
ent factors, such as lines from which the seismic traces are actually
reflected. For a single source array, the distance
• Streamer separation (3D)
• Number of streamers (3D) between CMP lines is half of the streamer sepa-
• Number of sources (3D) ration (Fig. 2.76A). In a flip-flop acquisition with
two identical source arrays, it is reduced to one
• Number of recording channels (2D and 3D)
quarter of the streamer separation (Fig. 2.76B) if
For 2D surveys, the seismic data is recorded the source separation is half of the streamer sep-
only from the zone just beneath the seismic line, aration as a common survey layout in 3D towed
and the total inline coverage area is always half streamer acquisition. As a result, smaller

FIG. 2.74 Coverage area for one single shot in a 2D case. For 2D surveys, only inline coverage is pronounced, which equals
to the half of the overall streamer length (a).