3-D seismic data acquisition and processing - 3-D Seismic Interpretation - page 24

means that the seismic vessel requires a large turning area and needs to remain clear
of other shipping in the area. Often, smaller boats known as picket vessels are used
during the seismic survey to warn other boats to keep their distance while operations
are underway. The actual length of the receiver cables depends on the depth to the
target and also on the expected velocity profile in the earth. A deeper target requires a
longer cable in order to pick velocities accurately and remove multiples. A good rule
of thumb is that the cable length should be at least as long as the depth to the main
target level, although longer cables may be required under special circumstances. The
ends of the cables are attached to a tail buoy that contains a radar reflector to warn
shipping of its presence and to enable the position of the cables to be monitored and
recorded.
At the end of the required surface line the boat needs as much as2htoturn safely ready
to shoot the next swathe of lines. This is non-productive time and so data are usually
acquired with the longest axis of the survey area as the shooting direction for the sake
of efficient operations. Other considerations such as dominant dip of the subsurface and
current strength and direction are also taken into account when designing marine seismic
operations. Data are usually shot in the dominant subsurface dip direction (fig. 2.9 ) as
this is usually the direction with the smallest distance between recorded traces. There
may be circumstances such as those illustrated in fig. 2.10 where strike or another
shooting direction is preferred. For some targets, such as those under irregular salt
bodies exhibiting large velocity contrast with the surrounding sediments, there may be
no single optimum acquisition direction; surveys may have to be shot in several different
directions to achieve good results. Currently this is rare, but is likely to become more
common as companies experiment with multi-direction shooting. In an area with strong
Dip
Direction
Strike Direction
Fig. 2.9 Dip and strike directions. Over the years there have been several debates on the relative
merits of shooting in the dip and strike directions. The argument generally hinges on velocity
complexity. Velocities are complex and affected by dip in the dip direction, whereas they are not
affected by dip in the strike direction. However, processing can unravel this complexity. In practice,
most acquisition is in the dip direction, largely because the direction of shooting is usually the most
densely sampled and therefore less prone to data aliasing.