3-D seismic data acquisition and processing - 3-D Seismic Interpretation - page 19

Fig. 2.2 Examples of modern marine 3-D seismic vessels.

impedance, part of the sound wave is reflected back to the surface where it is captured
by the receivers and transmitted to the boat for further processing or writing to tape for
storage (fig. 2.3 ) .
In the early days (mid-1980s) of 3-D data acquisition the boats were not power-
ful enough to tow more than one cable and one set of guns, so the 3-D acquisition
geometry was just a series of closely spaced 2-D lines (fig. 2.4) . Owing to the high op-
erating expense of this design, the surveys tended to be rather small and used only over
developing fields. To reduce time wasted during operations several novel geometries
were tried including circle and spiral shooting (fig. 2.5) . These had varying degrees
of success but certainly increased the difficulty of data processing. They do not lend
themselves to modern multi-cable surveys. The benefit of the much clearer subsurface
picture obtained from 3-D data led to a growing demand for such surveys and rapid
advances in technology. Today, specially designed seismic vessels are powerful and
sophisticated enough to tow multiple cables and deploy two or more gun arrays that
are fired alternately. This allows multiple subsurface lines to be collected for each pass
of the boat, significantly increasing the efficiency and reducing data acquisition costs.
Figure 2.6 shows a typical mid-1990s layout of four cables and two gun arrays, giving