Recent advances in processing algorithms and new cooperative management techniques developed in conjunction with Statoil and Reservoir Imaging Ltd. (RIL), have effectively eliminated downtime or time sharing due to seismic interference (SI). This not only enables us to acquire data while other contractors are acquiring in the same area, but also allows us to deploy two (or more) vessels on a survey if data needs to be acquired quickly. This synchronized acquisition enables a 100% increase in productivity without compromising quality so that deadlines can be met and asset teams have more time to evaluate the data.
Ideally we need to avoid SI that is both broadside and arrives at the same time from shot to shot, however new subsurface imaging techniques enable even shot-to-shot coherent broadside noise to be attenuated. It has been shown (Dhelie et al, 2013; Laurain et al, 2013; Elboth et al, 2015) that controlling the moveout of SI and the randomness of the arrival time on consecutive seismic records is significantly more important for its attenuation than the amplitude of the SI noise.
By analyzing SI-contaminated data a series of noise thresholds have been established for zones of acceptable SI and zones where attenuation methods need to be tested (Laurain et al, 2014 and 2016). These describe a cone around the acquiring vessel, perpendicular to the sail line direction where SI might be problematic. Vessel speeds are actively adjusted to avoid vessels from entering any noise cone and also to ensure sufficient time differences in the arrival of SI on shot gathers.
The lateral extension is based on the assumption that SI ceases to be a problem at distances greater than ~100km, although this assumption can be updated depending on water depths and sea bottom characteristics. In general, only the interference from the direct arrival propagating through the water is used for the purpose of planning acquisition cooperation schedules, as this is easy and quick to compute.
By using an independent third party to coordinate all the acquisition in the Norwegian Sea to avoid shot-to-shot coherent (and where possible broadside) SI, significant improvements in acquisition efficiency were achieved during the 2015 and 2016 acquisition seasons. The cost of the coordination is that some vessels have to slow down during acquisition of some lines. However, the cost of these speed adjustments is typically less than 2%, compared with an average 12% loss for standby when time-sharing. Combining this with the flexibility of being able to operate as close as 8 km apart enables overall operational efficiency to be considerably increased.
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