Page 100 - Acquisition and Processing of Marine Seismic Data

2.3 3D MARINE SEISMIC ACQUISITION 91

the head sections of the streamers), mid-net to 3 degrees and they can assure to maintain
(incorporates only the specific nodes on the mid- the zero feathering soon after activation of the
dle sections of the streamers), and tail-net (incor- real-time streamer steering control system
porates the nodes at tail buoys and the specific (Fig. 2.47A). For higher feathering angles, the
nodes on the tail sections of the streamers). As separation between the streamers can be
a specific case, if the network runs the overall assured by steering systems, although straight-
length of all available streamers within the ness of the streamers behind the vessel cannot
spread, then it is termed a fully braced acoustic be retained parallel to the sail line (Fig. 2.47B).
network, which provides a complete solution Without automatic steering systems, however,
without deploying the compass birds to arrive it is not possible to maintain the streamer sepa-
at a solution (Fig. 2.46). The IRMA acoustic posi- ration along the crossline offset (Fig. 2.47C).
tioning system of Western Geco and the com-
plete Nautilus streamer steering solutions, 2.3.5 Flip-Flop Shooting
along with the Sentinel streamers of Sercel, pro-
vide fully braced acoustic networks. Conventional 3D seismic acquisition is per-
Separation between the streamers is automat- formedbyfiringtwoidenticalairgunarraysalter-
ically maintained by dedicated software, which nately termed flip-flop acquisition. The
integrates the data from streamer steering separation between the arrays is generally half
devices and acoustic positioning units in the of the streamer separation of the spread, which
areas of moderate surficial currents. These sys- doubles the crossline sampling and reduces the
tems can compensate streamer feathering up acquisition footprint. In flip-flop acquisition, the

FIG. 2.47 (A) Feather angles after the activation of automated streamer steering control. Schematic illustrations of
streamers (B) with and (C) without automatic steering system.