Ocean-bottom node

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Ocean bottom acquisition is a marine seismic acquisition approaches popular in the latest decades. In ocean bottom acquisition, receivers can be either ocean bottom cables or ocean bottom nodes. Ocean bottom cables are usually positioned by receiver boat, while ocean bottom nodes are completely free positioned in deep ocean bottom. Ocean bottom cables only work in shallow water area, usually less than 1km in depth; while ocean bottom nodes can be applied in the water from tens of meters to thousands of meter deep.

Compared to conventional streamer acquisition on the water surface, ocean bottom acquisition has lots of benefits:

  1. streamer acquisition with hydrophone receivers can only record pressure wave field; while ocean bottom acquisition with geophones can record 3C or 4C elastic wave field. Thus, shear wave information can be retrieved in ocean bottom seismic analysis. Electro-Magnetometers are also helpful in ocean bottom acquisition.
  2. ocean bottom acquisition can provide a much wider azimuth range than streamer acquisition, which limited by number of towed streamers.
  3. ocean bottom node acquisition plays a significant role in scientific observation in special area, where steamers can not be there or can not stay there for a long time.
  4. seismic data recorded by ocean bottom acquisition usually is much cleaner than surface acquisition, which usually affected by the noise of water waves, streamer boats and other boats (fishing boats, platforms).

Moreover, ocean bottom acquisition has more advantages in 4D survey monitoring.

  1. ocean bottom acquisition is not affected by oil platforms or tankers working in the oilfield, which is a big problem for towed steamers;
  2. ocean bottom acquisition can repositioned more easily than steamers, or even continuously record for days or months.

History

  1. the first 2D case studies using OBNs were carried out in the North Sea in the 1990s;
  2. substantial 2D & 3D pilot surveys followed in the early 2000s in the Gulf of Mexico, the North Sea, and in West Africa
  3. the first full 3D OBN survey was carried out in 2004/2005 in the southern Gulf of Mexico, and until 2008 only one or maximum two 3D OBN survey per year were acquired world-wide
  4. Since 2008, about 12 OBN surveys have been acquired world-wide, and demand for 2011 onwards is increasing.