The challenging combination of shallow water and shallow targets cannot be sufficiently solved by the application of conventional marine seismic acquisition and imaging techniques. The need for temporal and spatial high-resolution imaging requires multicomponent streamer acquisition systems as well as high-density spatial sampling. Typical surveys lack near offsets due to relatively large minimum distances between seismic source arrays and outer streamers and can result in significant acquisition footprints at shallow target levels. While perfectly sampled data can be recorded by increasing the acquisition effort, the cost and operational complexity will rapidly increase and become the limiting factor. Several acquisition strategies have recently addressed these challenges in areas such as the Barents Sea.

Existing solutions for shallow-water acquisition

Recent PGS MultiClient 3-D campaigns in shallow-water areas were acquired with spreads of 10 and 12 dual-sensor streamers, with a 75-m (246-ft) separation and dual sources (Table 1). These configurations were chosen as optimal trade-offs between geophysical illumination and sampling requirements for the exploration targets and acquisition efficiency. The data were processed using separated wavefield imaging to deliver high-quality shallow target images with a spectral bandwidth of 2 Hz to 200 Hz. This processing solution provides near offsets and reduces footprint issues to deliver improved angle gathered for amplitude versus offset/amplitude versus azimuth (AVO/AVA) analysis.

The P-Cable system was developed for near-surface imaging and consists of many very short and densely separated streamers and a single source. A 16-m-by- 12.5-m (52-ft-by-41-ft) configuration provides good near-offset coverage and a crossline bin size of 6.25 m (21 ft). However, the lack of longer offsets and fold restricts the use of the data to shallow targets and does not enable full integrity imaging and AVO/AVA analysis. In addition, the necessary sail line separation of 100 m (328 ft) makes data acquisition for larger areas ineffective compared to a towed-streamer spread that is 12 m (39 ft) by 75 m with a nominal sail line separation of 450 m (1,476 ft).

Towing deeply to preserve bandwidth

Recent case studies have shown that seismic images of shallow plays can have a spectral content in the range of 2 Hz to 200 Hz and higher. Towing streamers deeply can ensure the recording of a high-quality low-frequency signal, but deeper tow of hydrophoneonly streamers sacrifices the integrity of the higher frequencies. Multicomponent systems, with complementary ghost responses, can be towed deeply without compromising the integrity of the entire frequency range and can significantly reduce deghosting complexity and uncertainty.

Dense sampling and efficient acquisition

Preservation of the recorded spectral bandwidth throughout a 3-D imaging workflow requires denser crossline sampling compared to what is usually acquired. A 12-m-by- 75-m configuration combined with a triple source would provide a crossline bin size of 12.5 m.

Modern high-capacity seismic vessels can tow many streamers with a dense separation without sacrificing efficiency. A 16-m-by-56.25-m (185-ft) spread reduces the nominal crossline bin size to 9.375 m (31 ft, dual and triple source) and also improves receiver side sampling. By combining triple sources with a streamer separation of 37.5 m (123 ft), a nominal crossline bin size of 6.25 m can be achieved, the same spatial sampling as P-Cable.

The introduction of additional sources can potentially further reduce the crossline bin size. However, additional sources may result in increased shot point intervals (and reduced fold) or must be assisted by overlap shooting and source blending and deblending techniques. These techniques may have an impact on image quality and quantitative interpretation when geological targets at several depth levels are being considered or prestack data have to be analyzed.

Improving near-angle coverage

This approach provides improved spatial sampling, but the near-offset/angle challenge remains to be solved. An obvious way to improve the near-offset coverage is to reduce the streamer count and thus the spread width for the configurations previously discussed. This would, however, result in more sail lines and increased turnaround and cost. In the context of the shallow imaging challenge, wider source towing may be utilized to improve the near-offset distribution for streamer acquisition. In the case of a dual-source setup, widening the source separation moves the two seismic sources out of their centered locations behind the seismic vessel toward the outermost cables on their respective side of the spread. Widening the sources reduces the crossline distance to the streamers in parts of the spread but increases the crossline distance to the remainder. With this configuration, larger areas can be populated with near-offset traces compared to standard dual-source acquisition. In combination with the crossline sampling provided by high-density acquisition, this also provides a much-improved starting point for near-trace interpolation and regularization. The same concept can be applied to triple-source or higher source count configurations.

Further optimization

Seismic exploration surveys usually target more than one specific geological formation and should provide a good image of the larger geological setting covering both shallow and deep structures. High-density acquisition (as required for shallow plays) is not as necessary for imaging deeper geological targets. However, imaging and quantitative interpretation of deeper targets requires longer offsets. As all seismic vessels have limited towing capacity and streamer inventory, streamer spreads with varying cable length and separation can be a pragmatic and cost-effective way to provide optimal sampling in the shallows while also including longer offsets for deeper velocity updates (Figure 1). High-resolution data, dense spatial sampling and near-offset/near-angle information are key elements for success. Wide towing of sources along with variable streamer lengths can provide near offsets and high-resolution velocity models that can deliver reliable imaging solutions to de-risk exploration.

References available. Contact Rhonda Duey at rduey@hartenergy.com for more information.

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