Single Vessel Configurations

The most common approach to marine seismic acquisition is to use a single vessel towing a wide spread of many streamers and one, two or more source arrays. 

The streamer and source configuration will determine data quality, illumination, and acquisition efficiency. The number of streamers, their length and separation, the number and positioning of sources have to be selected based on the geophysical and geological objectives.

 

Both NAZ (dual or triple source) and MAZ surveys are acquired with single vessel configurationsBoth NAZ (dual or triple source) and MAZ surveys are acquired with single vessel configurations

 

Triple Source

Most marine seismic surveys use a dual-source setup, however, triple-source can provide better spatial crossline sampling for high-resolution imaging, or greater streamer separation and thus greater acquisition efficiency. 

  • Provides better crossline common mid-point (CMP) sampling, for a given streamer separation, provides images with an improved spatial resolution
  • Permits wider streamer separation and overall spread width with no reduction in crossline sampling
  • Enables a reduction in deployed streamer equipment (without changing spread width or crossline sampling) 

The table below shows alternative options, with two or three source arrays, to achieve a given spatial sampling in the crossline direction.  

Crossline
bin width (m)

Sources

Streamer separation (m)

12.5

dual

50

12.5

triple

75

18.75

dual

75

18.75

triple

112.5

25

dual

100

25

triple

150

  

comparison of crossline bin width and illumination using dual and triple source configurationsThe illustration shows the source-streamer ray path contributions to each subline (i.e. CMP location) for dual-source shooting with 100 m streamer separation vs. triple-source shooting with 150 m streamer separation. In both scenarios, source separation is 50 m and crossline bin width (subline separation) is 25 m. The black diamonds represent the streamers and the blue rectangles represent sources. The crossline subsurface illumination is illustrated by the horizontal illumination bar at the bottom.

 

Of course, a trade-off is required. To achieve the same in-line CMP fold, the shot interval for triple-source must be decreased by 33%, compared to a dual-source acquisition. This means a part of each shot record may include energy from the next shot. PGS has extensive experience with simultaneous and overlap shooting and has patented a variety of techniques to enable optimal source deblending in data processing.  

Multi-Azimuth

In conventional, narrow-azimuth 3D marine surveys, a single vessel tows a streamer spread and the subsurface geology is illuminated from the acquisition direction.  The multi-azimuth (MAZ) method also uses a single vessel but acquires data over a survey area in two or more directions. 

In areas with complex geology, adding more azimuths can positively affect the subsurface image. The benefits of additional azimuths extend beyond illumination to include better signal-to-noise ratio and improved spatial sampling.

A standard MAZ survey comprises two or three survey azimuths. This may involve acquiring new data to complement a legacy survey. 

MAZ surveying involves a single vessel acquiring 3D seismic data over the same survey area with two or more survey azimuthsWhere the subsurface above the target area is complex, data quality and resolution may be improved by illuminating the area of interest from several different orientations. This can be achieved by repeating the acquisition from several sail-line directions using a technique called multi-azimuth acquisition (MAZ).