2.8 Simultaneous Source Separation


Marine seismic data acquisition is changing rapidly. One of these developments is the use of so-called ‘simultaneous sources’, where multiple seismic sources that intentionally interfere with one another are activated simultaneously. Proposed and demonstrated over 15 years ago, the subject of marine simultaneous sources saw long fallow periods when nothing seemed to happen until the uptake of wide-azimuth surveys and, more recently, when the industry needed to improve the way in which geophysicists deliver high quality data while maintaining viability in a cost-focused market. The goal is to challenge conventional thinking and explore what could be the solutions of the future.


Truth is ever to be found in simplicity, and not in the multiplicity and confusion of things. Sir Isaac Newton


Traditionally, seismic data have been acquired sequentially: an impulsive source is fired and data are recorded until the energy that comes back has diminished and all reflections of interest have been captured, after which a new shot at a different location is fired. Being able to acquire data from several sources at the same time is clearly highly desirable, as the example acquisition configuration in Figure 2.49 suggests. Not only would it allow us to cut expensive acquisition time drastically, but it would better sample the wavefield on the source side, which typically is much more sparsely sampled than the distribution of receiver positions. It would also allow for better illumination of the target from a wide range of azimuths


Guest Contributors: Dirk-Jan van Manen and Johan Robertsson, ETH Zurich


and would acquire more data from the wavefield in areas with surface obstructions. In addition, for some applications such as 3D VSP acquisition, or marine seismic surveying in environmentally sensitive areas, reducing the duration of the survey is critical in order to save costs external to the seismic acquisition itself (e.g., down-time of a producing well) or to minimise the impact on marine life (e.g., avoiding mating or spawning seasons of fish species).


2.8.1 Simultaneous Source Seismic Data Acquisition


Simultaneous sources have a long history in land seismic acquisition, dating back at least to the early 1980s. Commonly


Figure 2.49: Example of using multiple sources on one vessel to increase cross-line sampling density. Top: In conventional sequential shooting, one source (green) is fired at a first location yielding the subsurface points dotted green and the second source (yellow) is fired at a second location, typically 25m away, giving the subsurface points dotted yellow. In simultaneous shooting, both sources are fired together, thus improving the cross-line trace density. Bottom: By extending the concept to a five source configuration a significant increase in data density is achieved. The challenge for the geophysicist is to intelligently ‘encode’ the sources so that the overlapping data measurements can be ‘decoded’ into data that would be acquired from each individual source.


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Polarcus


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