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Self-landing and ascending OBS: opportunity for commercial seismics in the ultra deep sea
- Source: First Break, Volume 20, Issue 12, Dec 2002,
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- 01 Dec 2002
Abstract
Offshore seismic exploration focuses more and more on water depths below 1000 m Nowadays, tenders for seismic surveys down to 3000 m water depths are on the increase. As the water gets deeper, so the resolution of measurements made by surface streamer tends to decrease. To obtain enhanced resolution and an indication of the fluid content of a hydrocarbon reservoir, the acquisition of shear waves is becoming of interest. This requires the data to be acquired on the seabed. To date, such surveys have predominantly been conducted using OBCs (ocean bottom cables). But there are a number of shortcomings with this type of survey: • The cable itself substantially distorts the horizontal shear wave components. • In rugged terrain, several 3C geophones may not be coupled to the ground at all. • In jagged areas, the cable may be damaged or even cut. • In rough sea, the vessel may run into trouble. • Below 2000 m the weight/strain relation of the feeder cable itself becomes uneconomical. This is why self-landing and ascending ocean bottom seismic (SLA-OBS) systems, which have been used by the scientific community for the past 20 years, offer a promising alternative, especially in the ultra-deep sea environment.