SUMMARY

TOPSEIS is CGG’s latest addition to its marine broadband acquisition portfolio and is the most recent outcome from eight years of technical collaboration between CGG and Lundin Norway AS.

The TopSeis system is a tailored solution, specifically designed to improve imaging of shallow and intermediate-depth targets by overcoming the lack of near-offset fold inherent in towed-streamer seismic. It achieves this by using an innovative source-over-spread acquisition configuration and specifically designed processing algorithms.

Based on an original idea from Lundin, TopSeis has been developed by a multi-disciplinary team of CGG and Sercel Equipment, Marine Acquisition and Geoscience experts, capitalising upon CGG’s long-standing broadband capabilities, including deep-tow experience from acquiring over 350,000 km2 of BroadSeis, deployment of our broadband source (BroadSource) and the company’s recognised Subsurface Imaging experience.

TopSeis has been developed over the last two years by undertaking synthetic modeling, field trials and conducting both a 2D and 3D pilot survey. It is now a commercially-ready solution and the first survey is scheduled for launch in the Barents Sea with Lundin’s support in 2017.

The challenge and the TopSeis solution

Effectively imaging shallow reservoirs is hampered by the lack of near-offset data from conventional towed-streamer acquisition. In a conventional spread, only the inner streamers record near offsets, creating holes in the coverage at the edges of sail lines. This problem has been exacerbated by the industry move towards wider spreads to increase efficiency, as, the wider the spread, the larger the holes in the near-offset coverage. Attempts to solve this problem using processing solutions have had only limited success.

TopSeis places the sources over the spread to create a split-spread configuration, similar to that used in onshore surveys. This results in far higher near-offset fold, as can be seen in Figure 3, where the circles show the near-offset data.  TopSeis records real zero-offset and very near-offset data, to give more reliable results than can be obtained by processing solutions such as interpolation, extrapolation or imaging from multiples.

Shallow data has always been lower fold than the rest of the seismic section due to the fact that only near-offset data, with incident angles less than the mute angle in the image stack, can be used in imaging. The critical angle depends on the velocity difference between the two layers either side of the interface.  NMO stretch also limits the length of offset that can be used. Figure 2 illustrates the additional fold achieved by using TopSeis.

Ocean bottom seismic (OBS) may also record zero-offset data, but the receiver sampling tends to be very sparse although shot sampling may be dense. Additionally, as the receivers are on the sea floor in OBS they also suffer from increased noise which affects the very shallow image. For shallow to intermediate targets, TopSeis offers a better, more cost-effective solution than OBS, delivering denser coverage with an improved signal-to-noise ratio for much clearer images.

Figure 2. Schematic side view of TopSeis versus typical BroadSeis configuration illustrates the benefit of TopSeis recording all the energy at the near surface.

Figure 2. Schematic side view of TopSeis versus typical BroadSeis configuration illustrates the benefit of TopSeis recording all the energy at the near surface.

Benefits

By placing the sources over the spread, TopSeis records zero-offset data and delivers a significant increase in the near-offset coverage.

TopSeis delivers high fold for high resolution with full-azimuth near offsets. TopSeis also delivers better crossline sampling from more densely spaced streamers.

The benefits of TopSeis include:

  • Enhanced imaging of shallow and intermediate-depth reservoirs
  • Better signal-to-noise ratios from increased fold
  • Improved AVO from increased fold and recorded zero-offset data
  • Anticipated better multiple attenuation from increased near-offset coverage
  • Anticipated improved resolution and bandwidth
  • A more cost-effective solution than OBS (Ocean bottom seismic) methods
Figure 3. Schematic plan view of TopSeis versus a conventional configuration - The highlighted circles show the near-ofset data acquired.

Figure 3. Schematic plan view of TopSeis versus a conventional configuration – The highlighted circles show the near-ofset data acquired.

Market readiness

Over the last two years CGG has developed TopSeis from an idea to a practical solution, using synthetic data and field trials to derive optimum configurations for specific targets and has developed methods to deploy these configurations safely.

Extensive geophysical and HSE field trials have demonstrated that TopSeis can be deployed safely and without damaging in-sea equipment.  The company has many years’ experience of towing streamers deep from deploying the BroadSeis streamer spread down to 50m. CGG has also conducted thorough risk analyses and developed protocols to deal with all anticipated scenarios, including loss of power from either vessel or streamers surfacing.

Figure 4. Comparison of TopSeis and a traditional spread on a synthetic model of a shallow field in the Barents Sea. Image File:OG-Aust-July-2016-31

Figure 4. Comparison of TopSeis and a traditional spread on a synthetic model of a shallow field in the Barents Sea.

Figure 5. Comparison of TopSeis and BroadSeis 2D line, Offshore Gabon

Figure 5. Comparison of TopSeis and BroadSeis 2D line, Offshore Gabon

CGG has developed processing algorithms to deal with split-spread deghosting, the increased direct arrival noise and any propulsion noise from having a source vessel over the streamers.

The company’s proven broadband source, BroadSource, with Full Source Characterisation (FSC) and advanced proprietary deghosting algorithms, combined with careful design of the cable shape to be tailored for the target, enables us to deliver broadband ghost-free data with good signal-to-noise ratios throughout the bandwidth.

The data comparisons produced so far from the pilot surveys are very encouraging and CGG is looking forward to delivering a step-change in Barents Sea exploration and development from the first commercial survey in 2017.

CGG would like to thank OMV for input on the geological model on the Wisting area and discussions on the original modelling project design.