Advanced Survey Geometries
Seismic survey design must be increasingly creative to meet the demands of modern imaging. In complex geology, illumination shadows and complex noise modes remain a challenge. These call for alternative acquisition geometries. Survey modeling is increasingly important to find the optimal acquisition and imaging solutions for specific geophysical objectives.
Even in established areas such as the North Sea, West Africa, Gulf of Mexico, Brazil and the North West Shelf of Australia, salt, volcanics, carbonates and chalk in the overburden impede seismic penetration, and have negatively affected both data quality and exploration success for many decades.
Whilst broadband seismic methods have certainly improved the quality of data acquired in challenging hydrocarbon basins, much remains to be done to optimize signal penetration, the attenuation of complex noise modes, as well as spatial sampling and subsurface illumination.
The industry has learned that complex subsurface imaging challenges benefit from advanced acquisition geometries. Consequently, survey design has become an important consideration. Given a specific subsurface complexity and certain geophysical objectives, the acquisition and imaging solutions should produce the best quality subsurface image in the most cost effective way.
The illustration below shows a number of survey geometries that can be deployed depending on the expected complexity of the geology. These range from single vessel Narrow-Azimuth (NAZ) operations to single vessel Multi-Azimuth (MAZ), and multi-vessel Wide-Azimuth (WAZ) illumination.
Acquisition geometries range from single vessel narrow azimuth (NAZ) to multi-vessel wide azimuth (WAZ). The most appropriate geometry will produce the best quality subsurface image in the most cost effective way. Rose diagrams indicate the offset and azimuth illumination provided by each option.
Sometimes a very large source-to-receiver offset is needed, which exceeds the maximum length possible or practical with towed marine streamers. Using Continuous Long Offset (CLO) a streamer vessel works together with an additional source vessel that is positioned a full streamer-length in front. Signals from the rear vessel provide the ‘near’ half of the full offset range, whereas the lead source vessel provides the ‘far’ half of the offset range.
Operationally, towing 6 km or 8 km streamers is safer and simpler than controlling a spread of 12 km or 16 km streamers. Shorter cables are less susceptible to feathering and line turns are more efficient. The risk of tangling is reduced, barnacle growth can be controlled more easily and drag is significantly less, which speeds up acquisition.
Deploying a randomized firing scheme, both source vessels can shoot simultaneously to produce much denser shot sampling and higher fold. This acquisition set-up is referred to as Simultaneous Long Offset (SLO).
Compare illumination of near and far offsets with traditional survey design and with SLO
Example from West Africa acquired using Simultaneous Long Offset (SLO). Data was acquired with 6 km streamer and a full offset range of 12 km (near + far). Gathers are displayed with a 45 degree angle mute.
Continuous offset and azimuth coverage can be achieved in areas with the most complex subsurface geology by combining multi-azimuth and wide-azimuth acquisition into a full-azimuth design. PGS used such solutions in the Gulf of Mexico on its Triton program. This survey deployed a specially designed acquisition template that comprised five vessels in a combined wide-azimuth and SLO configuration. Data was acquired in three different azimuthal directions, resulting in full azimuth coverage for offsets up to 16 km.
The rose diagram for the Triton program illustrates how complete source-receiver offset and azimuth coverage was achieved, providing the advanced subsurface illumination required for sub-salt exploration in this very complex area.
Full-azimuth acquisition template used for the Triton survey in the Gulf of Mexico. Five vessel were combined in a wide-azimuth configuration with two of the vessels shooting in SLO mode.