French seismic firm CGG says the UK’s central North Sea is still rich in opportunities for the development and discovery of new fields, despite its relative maturity and the challenges is poses around seismic imaging, especially in the deep, high-pressure, high-temperature (HPHT) part of the Central Graben.
CGG has recently released its reprocessed merged Cornerstone dataset, which it says delivers an unprecedented volume of high-quality anisotropic broadband PSDM data over a huge area of the Central Graben of the central North Sea, including the HPHT area.
Image: Comparison of new regional Cornerstone PSDM data with previous locally tailored data. From CGG.
The Cornerstone dataset has benefited from new technology to address the imaging challenges posed in the central North Sea. Specifically, developments in broadband technology, velocity modeling and demultiple have been applied to the dataset. In addition, recent phases of acquisition have used BroadSeis variable-depth streamers, and the most recent acquisition employed the BroadSource broadband source, delivering ghost-free data with over six octaves of bandwidth.
“With four octaves of signal below 40Hz, BroadSeis provides the best resolution for imaging below the Base Cretaceous Unconformity (BCU),” says CGG. “Combining BroadSeis with BroadSource also delivers high frequencies up to the sampling Nyquist frequency for incredible resolution of shallow channels and geohazards.”
Advanced Ghost Wavefield Elimination (GWE) has been applied to 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 contiguous broadband pre-stack depth-migrated (PSDM) dataset. CGG says the bandwidth achievable depends on the signal-to-noise ratio in the recorded data, so the ultra-low frequencies of BroadSeis true broadband data cannot be obtained. Nevertheless, considerable improvements have been delivered, providing an incredible 35,000sq km broadband PSDM dataset, it says.
New demultiple techniques such as Model-based Water-layer Demultiple (MWD) and Internal Multiple Modeling (IMA) have been applied to this dataset and deliver vast reductions in the multiple contamination that has historically afflicted central North Sea datasets.
Large-scale Quaternary channelling in the near surface strongly influences the imaging of deeper data. Recent advances in Full Waveform Inversion (FWI) and Dip-Constrained Tomography have enabled highly detailed velocity models to be built incorporating these channels, resulting in more accurate imaging of deeper events. There are also complex structures with strong velocity contrasts found here, often located at great depths, where limitations of available data may restrict velocity and anisotropy modeling. Multi-layer Tomography (TomoML) brings stability and accuracy to velocity modeling in such areas, producing more geologically plausible models, and providing improved imaging results. Applying these techniques to the entire Cornerstone dataset with extensive well control has produced a regional anisotropic PSDM dataset with better local detail and definition than previous proprietary datasets tailored to specific local targets.
The new E-W oriented Cornerstone Phase 8 BroadSeis-BroadSource survey overshoots the existing conventional long-offset, N-S oriented data. This new survey has been combined with the reprocessed earlier data to provide a dual-azimuth volume over the HPHT area of the Central Graben, with all the inherent benefits of dual-azimuth data: increased sampling, options to apply true 3D processing algorithms for better noise and multiple attenuation, improved illumination beneath the Base Cretaceous and optimal imaging of subtle features such as four-way closures due to more accurate velocity and anisotropy models. The dataset shows significant improvements in imaging below the BCU, and may indicate the future path of North Sea acquisition.
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