acquire 2D seismic data, and that they would use 3D for both exploration and production. During that decade, 3D surveys took off, helped by workstations that provided new ways to interpret and visualise seismic data in the 3D world. 4D seismic is 3D seismic repeated over time over the same
Figure 2.11: MV Ara, a cargo ship, sailing along the coast of Norway in the 1950–60s. She was rebuilt as the Seis Mariner in 1973 (see Figure 2.12). In 1993 she was converted into a fish-processing barge under the name Cruz Del Sur, and caught fire and sunk in Chile in 1995.
oil or gas field in order to monitor production. 4D seismic started in the early 1980s, but only became commercial in the late 1990s. In the North Sea, 4D seismic was first investigated on a full field scale at the Gullfaks field in a joint Statoil- Schlumberger project around 1995. Detectable time-lapse signals were indeed measured, and soon proved to be of economic value in identifying drained and undrained areas (see Figure 1.51). Soon afterwards, 4D surveys were acquired over several Norwegian and UK North Sea fields, including Schiehallion, Foinaven, Draugen, Troll, Oseberg, Norne, Statford, Forties and Gannet. In the early 1990s, Berg (see Berg et al., 1994) at Statoil led
the development of the SUMIC (SUbsea seisMIC) system, whereby both shear and pressure waves were recorded by sensors implanted in the seabed. Te 1993 pilot survey over the Tommeliten structure in the North Sea demonstrated that the SUMIC technique could successfully image subsurface structures through and below gas chimneys through the use of shear waves (Figures 2.14 and 2.19). Shortly after the introduction of the SUMIC technique,
Figure 2.12: MV Seis Mariner in Immingham on the east coast of England in March 1981. The banner on the stern says ‘Two mile cable in tow’ (but not in the harbour, we believe!).
imposing special requirements both on the vessels and the crew. Drilling in the North Sea became more expensive, and the oil companies needed the best seismic data possible for de- risking prospects. As more and more exploration licences were offered, the ship owners saw that the seismic market was a way of making money. Tey invested in more advanced vessels, and around 1980 the first contracts were made for purpose-built seismic vessels.
2.1.3 1980–2000
In the early 1980s, most 3D surveys were acquired by towing one cable and making many parallel passes through the survey area. However, soon it became common to use two cables and two sources with the benefit of shorter acquisition time and reduced cost. Capabilities steadily increased, and multiple streamers and multiple sources made 3D more effective. In 1991, Petroleum Geo-Services (PGS) was founded in
Norway. Te company had two seismic vessels, and its vision was to provide the most efficient acquisition of 3D marine seismic data. In 1995, its Ramform seismic vessel technology was introduced with the Ramform Explorer, which was the first vessel to tow eight streamers (1995), subsequently becoming the first to tow 12 streamers (1997). In the early nineties, Shell stated that they would no longer
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it was discovered that ocean-bottom surveying (OBS) was useful for detailed seismic imaging of complex geology due to sampling of all azimuths, analogous to taking photographs of an object from all directions to image all sides of it. To use a football analogy: the problem of collecting seismic data is like attending a football match. Your view of the game depends not only on the lighting system of the stadium but also on where you are sitting. For example, a journalist may prefer to be in the stands where he or she will have a good view of the entire game, which is necessary for analysing and reporting all of the moves and tactics. A photographer, however, may prefer to be near the touchline where he or she can immortalise the goals, even at the expense of not seeing the rest of the game. Te ticket prices for these special positions may be more than that of a standard seat, but the extra cost will pay off handsomely. As in football matches, the view of the subsurface is determined by the location of the sound sources for ‘illuminating’ the area of interest. Equally important is
Figure 2.13: The trawler Longva II was hired by Geco in 1975 and rigged for seismic acquisition. Later it was bought by Geco and renamed Geco Kappa.
John Jones
Per Sundfær
www.skipet.no
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