Acquiring additional azimuths with a single vessel is not the only means of adding new data. Advanced survey geometries may also use combinations of two or more seismic vessels to improve azimuth and offset coverage.
Salt, volcanics, carbonates and chalk in the overburden are known challenges which can impede seismic penetration to negatively affect both image quality and exploration success. Adding additional angles of illumination can help to get around these obstacles.
- Improved signal penetration
- Increased illumination from more azimuths
- Cleaner data with better attenuation of complex noise
Wide-azimuth (WAZ) surveys acquire an expanded range of azimuths by using additional recording and/or source vessels.
- Two or more vessels acquire a survey in a single direction
- Azimuth diversity increases with increased acquisition effort
- Additional azimuth coverage improves illumination and facilitates removal of complex noise
In the example shown below, the WAZ configuration deploys the streamer and source vessels in a fixed relative geometry during acquisition of the survey area; this is referred to as a tile. The first tile has the minimum lateral source-streamer separation (close to zero). Each succeeding tile increases this lateral offset by a distance equal to the width of the streamer spread. Thus two tiles of acquisition simulate acquisition with a streamer spread twice as wide as the one actually being towed. Three tiles simulate acquisition with a streamer spread three-times as wide as the one actually being towed, and so on. The source positions are exactly repeated for each tile.
The tiles are later combined in processing and imaging.
WAZ surveying involves two or more vessels acquiring 3D seismic data over the same survey area, typically with one survey azimuth, but with two or more different relative vessel configurations. In this illustration, one streamer vessel is complemented by two additional source vessels. The survey area is acquired two or more times, each with a different lateral separation between the streamer and the source vessels as the source vessel trajectories are repeated.
In the most challenging survey areas, even greater acquisition effort may be required. A FAZ acquisition applies a WAZ vessel configuration to a MAZ shooting template to acquire the most continuous range of azimuths for all shot positions.
- Two or more vessels acquire data in multiple sail-line directions
- Better illumination of the most challenging targets
- Ultimate attenuation of complex coherent noise
By repeating the shot locations, using ever larger crossline source-receiver offsets, the range of azimuths available for each shot expands. By acquiring WAZ surveys with a MAZ shooting template we can achieve the ultimate combination of target illumination and complex coherent noise attenuation.
Rose diagrams for a three-azimuth MAZ survey (left), a WAZ survey (center), and a FAZ survey (right). A MAZ survey combines two or more NAZ surveys acquired by a single vessel, but with different survey azimuths. A WAZ survey uses two or more vessels to acquire a larger range of azimuths for each shot. Conventional WAZ shooting acquires parallel lines with one survey azimuth. A FAZ survey applies a WAZ vessel configuration to a MAZ shooting template and acquires the most continuous range of azimuths for all shots.
Simultaneous Long Offset
Sometimes offset, from source to receiver, is the parameter that must be adjusted to achieve the desired illumination. It is possible to achieve this by using multiple vessels.
- Increased range of offsets for improved subsalt imaging
- Better operational efficiency with reduced feathering and infill
- Reduced operational risk
- Faster turnaround
A standard seismic vessel, towing both source and receivers, is combined with an additional source vessel that is typically positioned up to a full streamer-length in front. Signals from the rear vessel can provide the ‘near’ offsets, while the additional source vessel provides the ‘far’ offsets. The result is a significantly extended full-offset range, equivalent to towing very long streamers.
Operationally, this is safer and simpler than controlling a very long spread. Shorter cables are less susceptible to feathering and line turns are more efficient. The risk of tangling is reduced, barnacle growth can be more easily mitigated, acquisition is faster and drag is significantly less.
Deploying a randomized firing scheme, combined with advanced deblending in signal processing, 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).
Having it All
On the Triton FAZ survey in the Garden Banks and Keathley Canyon areas of the Gulf of Mexico, two vessels towed source arrays and large streamer spreads, supplemented by three additional source vessels. This five-vessel simultaneous shooting configuration acquired three survey azimuths and yielded ultra-long offsets of 0-16 km with uniform full-azimuth sampling.
Azimuth-sectored RTM images from the Triton FAZ survey. Note how the structural imaging varies in quality below the salt and at the salt boundaries with different source-receiver azimuth contributions. Full-azimuth (FAZ) acquisition provides many opportunities for optimized velocity model building in structurally complex settings.
The results achievable with multivessel solutions are exciting but project economics are clearly challenging. The scale of the projected asset development will be important when determining the appropriate level of investment. Asset teams must also consider whether MAZ or WAZ surveys should be ‘upgradeable’ to meet anticipated future production objectives. Such complex acquisition projects may have an extended time dimension.
Pre-survey planning will be essential and should include both technical and commercial disciplines to fully understand the life of prospect and field implications.