CGG: Broadband Brings New Life to North Sea

The Central North Sea (CNS) is a mature basin, yet still rich in opportunities for the discovery and development of new fields. Although this seems somewhat contradictory, targets continue to be elusive as the area is notoriously difficult for seismic imaging, especially the deep High-Pressure High-Temperature (HPHT) part of the Central Graben.

CGG has been active in the CNS for many years and has continuously invested in a high-end multi-client data library which is the reference in this area. This extensive dataset, known as Cornerstone, has benefited over the years from consistent application of the latest acquisition and processing technology to ensure it remains state-of-the-art.

Cornerstone has now entered an exciting new phase and is benefiting from an entire portfolio of new technology to address the imaging challenges of the CNS and create new opportunities for operators:

• BroadSeis™ and BroadSource™ acquisition, adding full bandwidth for the highest-resolution shallow imaging and ultra-low frequencies for deep imaging below the Base Cretaceous Unconformity (BCU)
• Broadband dual-azimuth coverage in the central HPHT area to improve sampling, illumination and imaging with fast-track data available now
• Re-imaging of the full 35,000-km² dataset with new broadband processing and completely new velocity modeling for anisotropic Pre-Stack Depth Migration, with data available from August
• Value-added products, including seismic Pore Pressure Prediction (PPP), to provide volumetric estimates of pore pressure across the deep part of the Central Graben to identify potential drilling hazards and improve well positioning, and Facies Finder to provide easy-to-interpret attribute volumes for lithology and fluid identification, with availability towards the end of the year.

With four octaves of signal below 40Hz, BroadSeis provides the best resolution for imaging below the BCU. Combining BroadSeis with BroadSource also delivers high frequencies up to the sampling Nyquist frequency for incredible resolution of the shallow channels and geohazards from full-bandwidth data. BroadSource is a synchronized multi-level source which uses source depth diversity to attenuate the source ghost notch, complementing the receiver ghost diversity employed by BroadSeis to remove the receiver ghost. This acquisition technique is becoming standard for our multi-client data in this area and is also being employed on the 19,000-km² Horda survey in the Norwegian North Sea.

Advanced Ghost Wavefield Elimination has been applied to all the legacy Cornerstone data to extend the bandwidth as near as possible to the new BroadSeis data, so that it can be merged to create a single contiguous broadband pre-stack depth migrated (PSDM) dataset. The bandwidth that can be achieved depends on the signal-to-noise ratio in the recorded data and so the ultra-low frequencies of BroadSeis true broadband data cannot be obtained on the legacy data set. Nevertheless, considerable improvements are delivered, providing an incredible 35,000-km² broadband PSDM dataset, believed to be the largest in the world. The fast-track dual-azimuth dataset over the HPHT region is already showing significant improvements in imaging below the BCU, and may indicate the future path of North Sea acquisition.

The geophysical challenges of the Central North Sea – such as shallow anomalies, heavy multiple contamination and sharp velocity contrasts – can now be overcome with a combination of broadband data and the latest processing and imaging techniques.
The near-surface of the CNS features large-scale Quaternary channeling that strongly influences the imaging of deeper data. Accurate modeling of these shallow features is difficult using reflection tomography, due to the short offset range available. Traditionally, this type of anomaly has been compensated by 1D updates based on the depth distortion of a horizon picked below the anomaly. Recent advances in Full Waveform Inversion (FWI) and Dip-Constrained Tomography now allow us to build highly detailed velocity models incorporating (and compensating for) these channels, resulting in more accurate imaging of deeper events.

A plethora of multiples afflict CNS datasets. These vary in source and character, from high-energy, short-period seabed multiples to internal multiples generated within the thick, high-velocity chalk. Success in attenuating these with conventional demultiple approaches, such as Tau-P deconvolution or SRME, was limited. Recent methods such as Model-based Water-layer Demultiple (MWD) and Internal Multiple Modelling (IMA) offer vast reductions in multiple contamination, especially when applied in a cascaded fashion, targeting each specific multiple class individually.

Complex structures with strong velocity contrasts are found in this region. They can often be located at great depths, where limitations of available data – such as angle range, frequency bandwidth and signal-to-noise ratio – can restrict the ability to model velocity and anisotropy. Multi-layer Tomography (TomoML) brings stability and accuracy to the velocity modeling in such areas, producing a more geologically plausible velocity model, honouring geologic constraints and providing improved imaging results.

Press Release, June 19, 2014; Image: CGG