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Spanning decades of exploration and production in the United Kingdom Continental Shelf, many programs of towed streamer data have shaped our knowledge of the Central North Sea. However, the fundamental lack of illumination and azimuth/offset coverage provided by towed streamer geometries, remains a blocker to resolving the imaging challenges associated with many higher-risk Jurassic and Triassic plays. This means that existing streamer data is rapidly approaching the limit in the value it can add to our understanding of this mature basin. The Cornerstone ocean bottom node (OBN) program looks at using the well-known benefits of OBN data; full azimuths, long offsets and rich low frequencies, to provide a step change in imaging of this important region of the North Sea. This is achieved through improved model building, in particular the detail unlocked by full waveform inversion using the latest Time Lag cost function ( Zhang et al.,2018 ).Utilizing TL-FWI on this OBN data aimed at improving the entire section of the velocity model: the complex overburden, intra chalk and sub-chalk layers. In addition to the added illumination achieved from OBN data, the use of the multiples, further illuminates areas of the subsurface not captured in the primary wavefield.

Full waveform inversion (FWI) using diving waves has in recent years become a standard model-building tool in the Central North Sea (CNS). Below the maximum depth of diving wave penetration however, we have remained reliant on ray based tomographic methods, which although powerful, have many limitations.In this paper, we will describe the use of time-lag FWI (TLFWI). TLFWI uses a modified cost function which aims to minimize travel time differences between recorded and modelled shots. This mitigates many of the issues encountered in other FWI solutions when reflection information is included. The ability to reliably use reflection information make it possible to reduce our dependence on ray-based approaches at depth. This is of particular benefit in areas where these ray-based tomography methods are inherently limited such as fast, layered chalk layers. The application of this technology to a large survey in the Central North Sea will be described.

The Greater Castberg survey was acquired in 2019 using a source-over-spread acquisition design with an additional source at the front of the streamers, towed by the receiver boat, to permit recording of longer offsets. Starting from an initial anisotropic model, time-lag full waveform inversion (TLFWI) was used to compute a 13 Hz velocity model over the 5000 km2 area, which led to a much improved migration image (e.g., better imaged reservoir flat spots). When pushing the inversion frequency up to 90 Hz, the resultant TLFWI velocity model enabled more detailed delineation of reservoir boundaries compared to the migration image. Furthermore, the FWI Image as an alternative view of the FWI velocity model, provided access to new reflectivity information, overcoming some limitations of current migration tools. The use of the full wavefield, including refraction, reflection of primary, multiple and ghost, greatly enhanced image without needing the complex data pre-processing required by conventional imaging. In the context of thick gas clouds, this new imaging technique provided accurate sub-gas reflectivity, which effectively enhanced event continuity compared to reverse time migration (RTM) results.

Offshore Gabon has several challenges for seismic exploration. The major difficulty is obtaining a detailed and accurate velocity model. Complicated salt structures with overhangs, variability within the salt and carbonate rafts with Karst features pose difficult challenges to conventional velocity model building. Moreover, these introduce illumination issues which are not addressed by traditional migration. This is particularly important in the pre-salt areas where the target generally lies. With this paper we show how the recent technological advances in velocity model building through Full Waveform Inversion (FWI) and particularly Time-Lag FWI (TL-FWI), followed by imaging through the Least-Squares (LS) migration have provided a step change improvement in the pre-salt image of narrow azimuth data acquired in the West African Atlantic margin of Gabon. To overcome cycle-skipping and amplitude discrepancy between synthetic and recorded data in the presence of sharp velocity contrasts and large scale geo-bodies, typical for Gabon context, we propose the use of a robust FWI cost function like the one employed in TL-FWI. The alignment of the resultant estimated velocity model with the geology of the margin allows the generation of considerable imaging uplifts. Additionally, the LS migration mitigates the noise generated by the migration operator and illumination deficiencies.

We discuss the processing and imaging challenges relating to a marine seismic survey acquired northwest of the Shetland Islands. The proximity of the survey to the islands forced the acquisition direction to be strike to the subsurface geology. A shooting vessel provided wide azimuth data to illuminate in the dip direction, and triple sources increased crossline sampling. We show how 3D source designature and source/receiver deghosting were required for this wide azimuth data. For demultiple we illustrate how utilizing narrow azimuth data in water layer multiple modelling was crucial for the wide azimuth multiple prediction. In addition we highlight the benefits of a wave equation deconvolution based approach in this shallow water setting. For velocity model building we show how using time-lag full waveform inversion helped resolve the specific challenges created by the geological setting. Finally we demonstrate how that imaging was aided by the use of least squares migration and the inclusion of the wide azimuth aspect.

The combination of ever-increasing computational power and more robust algorithms have made it possible to run full-waveform inversion (FWI) to higher frequencies and, also, offer more possibilities to take advantage of the reflections in the inversion. Through a process known as FWI Imaging, the detailed velocity models produced can be used to generate a reflectivity normal to the reflector plane. We outline the methodology and advantages of FWI Imaging, and introduce the concept of a dip-coherency image as an additional interpretation tool, using information parallel to the reflector plane. We show examples from the densely sampled source-over-spread Greater Castberg survey in the Barents Sea, demonstrating the uplift in the FWI Image over conventional imaging methods in terms of more balanced illumination and richer low frequencies. We performed decimation tests to assess the acquisition geometry impact on FWI imaging. Although the benefit of FWI imaging can still be observed on less well-sampled data, the best result remains with the original, densely sampled source-over-spread acquisition.

The Messinian interval in the West Nile Delta, offshore Egypt, is a thin evaporite layer characterized by highly irregular velocities with rapid spatial variations. Its complexity poses unique challenges for sub-Messinian reservoir imaging, resulting in erratic gather curvatures, distorted structures, and nonuniform illumination. Conventional ray-based tomography suffers from unreliable curvature picking due to poor gather quality, complex gather move-out, and inaccurate ray tracing through the fast and complex Messinian layer. Previous full-waveform inversion (FWI) had little success in this area because of strong amplitude mismatch between recorded data and modelled data at the Messinian layer and the limitations of the existing multi narrow azimuth (multi-NAZ) streamer data, which lacks good low frequencies and has limited offsets. We present a case study that utilizes a model building flow driven primarily by Time-lag FWI (TLFWI) starting from a tomography model with a reasonable long-wavelength velocity. TLFWI using both refraction and reflection data resolved the velocity errors in and below the Messinian interval and provided good uplifts to sub-Messinian reservoir imaging. In addition, least-squares Q-Kirchhoff (LSQ-Kir) migration with the improved velocity model compensated for irregular illumination and earth attenuation without over-boosting noise, thus further improved the S/N and resolution of the reservoir image.

The North Sea is a mature basin where it is increasingly challenging to make new and economically viable discoveries. Even after discoveries have been made, it is vitally important to have high confidence in the reservoir’s extent and spatial location before committing to a development plan. Here we detail the case history of a multi-azimuth pre-stack depth migration re-processing and re-imaging project over the Oda (formerly Butch) discovery in the Norwegian sector of the North Sea. The Oda structure is a complex salt diapir making it crucial to have as much confidence as possible in the position of the reservoir before making a drilling commitment. This study documents how the use of the latest imaging technologies, which include source and receiver deghosting, multi-azimuth full waveform inversion, multi-layer tomography and tomographic uncertainty analysis can aid in de-risking and improving reservoir definition, thereby increasing confidence in development well planning.

The geology of northern Oman presents significant challenges for land velocity model building. We show in this paper that these challenges can be overcome by using an integrated high-resolution velocity model workflow, through the combination of different types of waves, that allow resolving different parts of the velocity model. This dedicated workflow consists of Multi-Wave Inversion (MWI) for the near-surface, followed by Optimal Transport Full Waveform Inversion (OT-FWI) and then by ray-based joint reflected and diving wave tomography inversion. It resolves challenges imposed by complex shallow geology and allows for proper imaging of deeper structures. Compared to a conventional ray-based only model building flow, the integrated high-resolution workflow enabled generating a geologically plausible velocity model which minimizes depth positioning errors and greatly enhances structural and stratigraphic trends.