týsdagur 27. november 2001síða 21

Nr. 229 - 27. november 2001 41 41 40 Nr. 229 - 27. november 2001 Faroes Rock to new seismic source From Veritas DGC Have you ever been dis- turbed by loud music from noisy neighbours or errant teenagers? If so, did you notice that the music sounded »duller« than it would if you had been in the same room as where it was being played? In fact, the reason the music sounded dull is not just because your tastes are likely very different from that of the culprits, but because sound is affected by the way it travels be- tween a musical source and your ears. Specifically, deep notes from drums and other bass instruments that normally carry the beat (so- called »low frequencies«), are able to pass through walls more easily than the higher pitched notes (»high frequencies«) that usually carry the tune. As a result, the high notes are relatively weak, and music coming from another house or room typically sounds like a deep, »thud, thud«. Although the context is somewhat different, Pro- fessor Anton Ziolkowski of Edinburgh University in Scotland has recently suggested that the fact that low frequency sound can penetrate barriers more easily than high fre- quencies could have a pro- found impact for oil ex- ploration in areas such as the Faroes. This is because modern exploration is heavily reliant on seismic information, which in turn is obtained by measuring echoes from artificial sound sources generated at the surface. Unfortunately, most of the prospective areas in the Faroes are cov- ered in basalt, and this acts as a barrier that prevents sounds from penetrating to the deeper formations that hopefully contain valuable oil fields. As a result, seis- mic studies have had limi- ted success, and this has discouraged oil companies from exploring the area more fully. However, Pro- fessor Ziolkowski has pointed out that, although basalt is relatively »opaque« to conventional seismic sources, it should be more »transparent« at low frequencies. Hence it may be possible to obtain reflections from beneath the basalt by using special sources that produce more of this type of energy. Unfortunately, imple- menting Professor Ziol- kowski’s idea is far from easy — especially in the marine environment. Marine seismic surveys use so-called »airguns« which generate sound by releasing high-pressure air into the surrounding seawater, and thereby producing a “bang” (the effect is similar to that of popping a balloon). As with musical instruments, airguns can be tuned to different frequencies, and this done by varying the amount of air they release and changing the depth at which they are deployed in the water. However, Pro- fessor Ziolkowski’s calcu- lations suggest that the source needs to have frequencies below 10 Hz (i.e. ten vibrations a second), and this in turn means that the airguns have to be both deeper in the water, and four or five times the normal size used for exploration. As a result, the equipment required for this type of survey is far from standard, and, from a practical point of view, much more difficult to handle. Notwithstanding the challenges inherent in configuring a set of airguns to produce ultra-low fre- quencies, seismic contrac- tor Veritas DGC recently acquired a set of test data over prospective areas in Faroese waters with a view to testing Professor Ziol- kowski’s idea. The survey was nicknamed “BLAST” (Basalt Large Airgun Seismic Test), and was carried out in conjunction with the members of Faroes Licence Groups 6 (Statoil, Anadarko, Enterprise, Phillips Petroleum and Veba Oil) using Veritas’ seismic vessel, M/V New Venture. In addition, the survey deployed a new type of recording cable, or »streamer«, made of solid material, that is able to detect weaker sounds than conventional streamers which are made of hollow tubes filled with liquid. Veritas is also a pioneer in using ultra-long streamers to help provide additional information on deep geo- logy, and the streamer towed in the experiment was 12 km long - twice the length normally used for exploration. Detailed analysis of the BLAST data is still in pro- gress at Veritas’ office in Crawley, England. How- ever, the initial results are encouraging, and com- parisons with data obtained from older surveys show strong evidence that the new source configuration can produce reflections beneath the basalt that are not obtained in convent- ional surveys (see com- parison below). Veritas’ Manager of Geological Services, Richard Morgan says, »The BLAST results are extremely exciting, and could lead to important advances in our geological understanding of basalt environments. I am sure this technology will have an important role in future exploration in the Faroes.« Meanwhile, Professor Ziolkowski is continuing to refine his ideas, and Veritas is trying to make an even better low-frequency source using novel airgun designs. If everything pro- ceeds according to plan, the new source will be available for further sur- veys by the middle of next year. So, the next time you are disturbed by loud music from noisy neighbours, you might like to think of BLAST, and how under- standing why the music sounds the way it does is helping discover new oil fields. It is just another form of rock music! Veritas DGC’s marine seismic survey vessel M/V New Venture. Comparison of conventional seismic data (left) and results from BLAST (right). The pictures are like slices through the Earth, revealing different rock layers, and show that the BLAST data contain much more information on the deep structures, for example in the area highlighted by the circle. (Data courtesy of the BLAST group: Statoil, Anadarko, Enterprise, Phillips Petroleum, Veba Oil and Veritas DGC). Professor Anton Ziolkowsdki C M Y K 40