Summary A towed streamer 3D seismic survey of ∼500 km2 has been acquired over a potential Carbon Capture and Storage (CCS) area south in the North Sea. The full-fold polygon area happened to include a subsea power cable, crossing the area diagonally. The cable, in addition to delivering electric power from shore to an offshore platform also have a set of fibres inside. Most of them are in use, but the cable also had a few spare dark fibres available. These were hooked up to a DAS laser-interrogator onshore and used to monitor and record the complete seismic 3D survey. The distance from the interrogator unit and out to the seismic survey area, was in the order of 100–150km. Despite this extreme distance, the signal recorded on the fibre, from the offshore seismic survey, is of high quality. A key challenge related to the CCS fields is the requirement to monitor for potential leakage over a period of minimum 20 years after end of the injection. This will require new, and cheaper technology compared to OBS acquisition, and DAS using cheap subsea fibres as receivers has the potential to be a game-changer in this scenario.

Summary This Norwegian North Sea case study presents two applications of least-squares migration and its improvement to 4D data quality on multiple vintages of the Edvard Grieg ocean bottom dataset. The first example showcases a well repeated dense inline shooting baseline and monitor pair, where for individual 3D volumes, least squares migration can be utilised to resolve for variability in illumination and broaden seismic bandwidth to enhance subsurface structures. This 3D uplift is also benefits seismic resolution at the target reservoir depth and consequently improves interpretation of the 4D difference volumes when compared to a Kirchoff pre-stack depth migration only. The second example demonstrates how least-squares migration can be used to remove the pre-migration requirement to heavily decimate two non-repeatable acquisitions to a common, 4D co-located geometry in the case of a sparse cross spread baseline and a denser inline shot monitor. Here, the post-stack least-squares reverse time migration used survey independent point spread functions to model and compensate for the illumination pattern specific to each acquisition configuration. The resultant 4D attributes extracted at the target reservoir interval show significant reduction in 4D noise when compared to the conventional decimated volume.

Summary Following the conventional 50 x 25m dual-source acquisition of the 2022 monitor survey over the Edvard Grieg field, a 1.8 by 8km area to the north of the field was acquired again with a denser hexa-source (six source array) configuration. Shot interval was reduced to 1.7 seconds resulting in the subsequent shot energy (N+1) directly overlying the target reservoir interval. Iterative Source Separation with Priors (ISSP) technology was deployed to evaluate whether the primary signal could be successfully recovered and deliver comparable 4D results and metrics to the production dual source 4D configuration. 4D differences were generated with the conventional 2016 baseline survey and attribute extractions at the target reservoir interval showed good replication of the key 4D signal event and low NRMS values through the overburden and at target depth, <7%. Due to the denser shot spacing (25 x 25 m) and closer to point source configuration of the hexa-source gun layout, some uplift to 3D image was also observed in the signal content at frequencies above 90Hz, providing some additional benefits for the shallow event resolution above 800ms.

Summary The Nordkapp Basin is a large under-explored salt basin of the Barents Sea. Despite several exploration campaigns over the past decades, no successful drilling was achieved. A new hybrid survey combining streamers and nodes was acquired in 2021 to unlock this new play. Sparse nodes recording continuously during a 3 months period, with a nominal spacing of 1200m in both inline and crossline directions, supplemented a natively dense source over streamer data acquired with 7 simultaneous sources and 18 cables. We present here a fully data-driven FWI flow designed to exploit and combine the different types of data recorded by this survey to obtain optimal velocity model. The flow combines the ultra-low frequencies diving waves obtained from node interferometry and the ultra-wide offsets of node active seismic gathers to obtain a background velocity model for accurately imaging salt flanks. For higher frequency FWI, the streamers dataset with its dense spatial sampling including more near offsets traces complemented the sparse OBN data. The final 200Hz FWI product allows to directly distinguish in the velocity model the Carnian sands target and reveals details in shallow as small as 3 to 4m, which opens up new possibilities for hydrocarbon and shallow hazard detections.

Summary Generating reliable and repeatable 4D time lapse results within an accelerated seismic processing timeline can greatly impact the value of the final product and its contribution to adapting the field development strategy. The Edvard Grieg field, Norwegian North Sea, has been acquired every two years since 2016 with a densely shot (50 x 25 m) repeatable inline geometry ocean bottom cable survey, with receiver spacing 200 x 25 m. After six years of monitoring, an updated streamlined workflow, applicable across all four seismic vintages and suitable for PP and PS converted wave data, was deployed, and facilitated delivery of an interpretable product within ten days of data receipt. Delivering within this timeframe has been made possible through strategic project management, strong collaboration between the field operator, processing and acquisition contractors and significant workflow developments including the application of up/down deconvolution and reverse time migration. Joint interpretation of the PP and PS datasets allows for an enhanced understanding of saturation versus pressure dynamic changes across the reservoir around key production and injection sites.

Summary The Edvard Grieg field is an offshore oil field located 180km off the west coast of Norway that has been in production since 2015. Fluid flow and production effects in the reservoir have been monitored using OBC seismic full-field 4D surveys repeated every second year. As the field is now mature, it may no longer be economically viable to continue with these large and costly surveys. To address this issue, a feasibility study of using an ultra-light 4D detection system has been performed. The aim is to detect changes in a specific area of the subsurface using a small number of optimally placed source and receiver pairs. Using the legacy data selected areas have been identified as "trigger-spots" that could be monitored more frequently as a way of activating a new survey if necessary. The feasibility of the ultralight 4D monitoring system has been assessed through two initial tests on the Edvard Grieg field and the results are in line with the expected 4D results from the full-field surveys.

Summary This paper presents a novel acquisition design and image processing using the latest Multi-Parameter Full Waveform Inversion technology (MP-FWI). The acquisition was optimized to capture the required wavefield sampling, in terms of primaries, ghosts, multiples and diving-wave refracted energy. A key point was to design a densely and well sampled acquisition that indeed captured all of the wavefields and as much reflected energy as possible in all azimuths with both short- and long-offsets. The final setup involved four sources towed inside a 12-streamer dense setup with negative −250m offset. This design is capable of delivering an almost even sampled shot-carpet as well as provide full azimuth zero -offset coverage. As the streamers were 4km long, offsets of up to 3750m was captures which are important for AVO work in this area of the Barents Sea. MP-FWI imaging has been used to simultaneously determine velocity and a variety of subsurface attributes, such as anisotropy, relative density, reflectivity and even angle-dependant reflectivity for AVA analysis.

Summary Additional insight into the interaction between pressure and fluid saturation changes within the reservoir can be extracted through discrimination of time lapse (4D) signal from PP and PS data. Previous processing of the 4D PP dataset over the Edvard Grieg field from 2016 to 2022 yielded interpretive saturation related 4D changes due to production and injection. The PS radial data followed, centred on a co-processed up/down deconvolution (referred to here as radial/down deconvolution for PS) workflow in alignment with the PP route. The result was a unique 4D PS response to that of PP due to changes being pressure related. The UDD workflow involved spectral division of the radial with the downgoing wavefield in the 3D tau,p,q domain. The output was shaped to a user-defined wavelet and 3D deghosted. Although the condensed and optimised deconvolution process requires minimal processing, the result is sensitive to instrument calibration, source variability, and spatial sampling. Performing joint PP-PS tomography provided an uplift in depth-domain event alignment, which helped to correlate structural boundaries with the extracted 4D attributes. The 4D PS result correlates well with the predicted model and the low frequency response was observed to be significantly improved over the legacy processing route.

Summary The Haugaland High, in the Norwegian North Sea, consists of a layered overburden of sub-horizontal sediments almost 2km thick that sits on the chalk basement. The background velocity regime of these top layers has a low vertical gradient down to the chalk interface. This velocity behavior is particularly poorly suited for diving wave FWI, and the strong multiple content present in the data does not allow for an efficient tomographic update. Using all reflections and diving waves recorded, Time-Lag FWI can provide a high-resolution velocity field that explains the complex velocity variation present in the overburden and simplifies the reservoir structure. With the use of narrow-azimuth towed-streamer data covering 2000km2, the velocity was updated up to 40Hz, both helping structural imaging and bringing additional information to better understand the rock properties of the basement over the entire region.

Summary The Nordkapp Basin in the Barents Sea is considered an underexplored basin with limited amount of good quality seismic for exploration. The salt diapirism is prolific with diapirs penetrating all the way up to the seafloor. In order to image the complex salt flanks, accurate 3D velocity models for imaging is required. Experience from TopSeis acquisition both in the Barents Sea and the North Sea has provided sufficient evidence to continue a similar acquisition setup for recording of the seismic wavefield. In addition to a high quality seismic wavefield, a high-resolution 3D velocity model is required to image around and up against the irregular salt bodies. This can be achieved by deploying Ocean Bottom Seismic nodes on the seafloor. Recent Full Waveform Inversion (FWI) technology can obtain very accurate models to high frequencies even from very sparse node geometries. A split-spread source-over-streamer acquisition geometry using six wide-tow sources in addition to a long offset FWI front source has been used to acquire 3700 km2 of high-quality data. Seafloor nodes in a sparse grid of 1200 x 1200 m was deployed to record the necessary low frequencies and long offset full azimuth data required to build an accurate velocity model.

Summary Multi-component data recording from ocean-bottom seismic (OBS) surveys captures both PP and PS (converted wave) events. Processing such data can produce superior images compared to those obtained from conventional streamer acquisitions. In addition, PP and PS images can provide valuable insights into reservoir properties. However, PS imaging needs high-quality and high-resolution P- and S-wave velocity models in depth. While full-waveform inversion (FWI) for P-wave velocity model building is well established, an equivalent tool for updating the S-wave velocity (Vs) is still a challenge. A recent FWI methodology based on PS reflection data (PS-RFWI) has been proposed for the Vs model building. Updates from this technique are typically low wavenumber in nature. In this abstract, we show an application of PS-RFWI to OBS data from the Central North Sea and demonstrate an approach to update the high-wavenumber Vs components. Our real data application produces a high-quality 30 Hz Vs model that reduces the image undulations at the reservoir level and allows to generate a subsequent high-resolution Vs FWI Image.

Summary The 3700km2 Nordkapp Basin area, Barents Sea, was recently acquired with a wide-spread source-over-spread design. With its 6 sources sitting on top of 18 multi-sensor streamers, one sail-line can record a dense carpet of 108 sublines separated by only 6.25m. By thinly sampling the near offset over the full azimuth, this new source-over-spread setup is particularly well suited to image the steeply dipping salt flanks that extend up to the water-bottom. After application of a dedicated processing sequence carefully designed to honour the full resolution of the recorded data, the obtained high-resolution image is able to distinguish even small-scale geological features. This large 3D volume was also compared with an NFH image. Using the full benefits of the hexa-source, a high-end processing sequence was applied to the NFH data to overcome the usual weak signal-to-noise ratio of such records. The comparison between the two final images confirms the high-resolution quality of the source-overspread volume, which includes enhanced lateral resolution, especially along the crossline direction, and access to AVO and RMO information. On the other hand, the very thin vertical sampling of the NFH data extends the recorded bandwidth by two octaves, making it a possible complement to the source-over-spread image.

Summary To improve the imaging of the Barents Sea’s Nordkapp basin, either in terms of resolution or geological structure, a new seismic acquisition design was proposed, using a widespread hexa-source sitting on top of 18 non-flat streamers. Imaging this new recorded data required specific and complex processing steps using advanced algorithms to maintain high spatial resolution. This were not easy to handle onboard or through a fast-processing flow, so we implemented an Ultra-Fast Track processing by leveraging the capability of Deep Neural Networks to perform the pre-processing stages, ensuring quality in a limited timeframe. Pseudo-synthetic training sets were built from the first batch of received data and data augmentation was applied, with different setting for each processing steps. This result showed an improved resolution with respect to the legacy volume and was used to start interpreting potential hydrocarbon prospects and initiate the velocity model building. The Ultra-Fast Track also served as a helpful tool to assess the remaining challenges to be faced during the full processing.

Summary Logging of the vertical distance to top reservoir during horizontal drilling on the Edvard Grieg field has revealed that the reservoir zone and top seal is more planar with less undulations than conventional seismic images in time and depth reveals. Recent developments of Full Waveform Inversion (FWI) utilizing travel time misfits measured in frequency-dependent time windows (Time Lag FWI) have led to significant improvements in the depth control and the ability to provide more geologically consistent seismic images in depth, with less undulations of top reservoir. The new technology is able to capture greater velocity variation in the overburden which translates to an improved image of the reservoir zone. The new updated image with the flatter top reservoir section is a breakthrough in terms of finally being able to match the real observations from the horizontal production well-logs to the seismic sections in depth, without the spurious up- and down- undulations. The combination of low noise, densely sampled, full azimuth 4C OBC data should enable us to extend the FWI algorithms further into the elastic domain.

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Summary Conventional deblending using random time delays has been the most popular technique in marine simultaneous source acquisition and processing for some time. Signal apparition using periodic time delays has recently emerged as an attractive alternative to conventional deblending. In this new technique, periodic modulation times are used to encode multiple sources during the acquisition of simultaneous source data. These data can later be decoded using the known modulation times to separate the simultaneous sources into the individual sources. This method has the potential to increase the density of seismic sources, which can improve subsurface sampling and reduce the acquisition time. In this paper, we present a new source separation method for seismic data encoded with periodic time delays and use it to process hexasource variable depth streamer data from the Utsira region in the North Sea. A comparison of our new method with a conventional deblending solution indicates similar separation quality.

Summary Over the last years, wave-equation based AVO inversion (WEB-AVO) has become an established geophysical tool to extract elastic subsurface properties from migrated seismic data. In this paper we take full advantage of ocean-bottom 4D seismic by performing a joint WEB-AVO inversion using both PP and PS converted wave data. The 4D data was acquired across the Edvard Grieg field in 2016 and 2018 and both PP and PS migrated gathers have been used as input to the study. As shear wave data is insensitive to fluid changes in the reservoir, inclusion of this datatype allows improved separation between pressure effects and water flooding effects. The WEB-AVO technique is particularly suitable to handle time-lapse seismic data, because the non-linearity caused by reflectivity changes together with travel-time changes is automatically handled by the wave-equation. Inclusion of PS data for the 4D time-laps inversion has provided improved detection and separation of pressure and fluid effects and the work will soon be extended to include the 3rd 4D survey acquired in 2020.

Summary This paper presents a new seismic acquisition method designed and optimized to acquire very high-resolution towed marine seismic data across PL965 and PL962 in the Barents Sea. Fairly recent legacy 3D seismic existed in the area, however the acquisition parameters were not optimized for the shallow reservoir targets in the region. The new acquisition method was specifically designed to acquire close to zero-offset traces as well as obtaining very high spatial sampling of the subsurface. This was achieved by utilizing four single string, short, focused sources, towed wide, to keep the offsets to the outer streamer as low as possible. In addition to the novel source setup, the acquisition utilized six multi-measurement streamers towed flat and deep to reduce noise levels and allow for deghosting of the receiver wavefield. A key goal of the acquisition was to be able to extract wide angle ranges for accurate AVO analysis. By redesigning the source arrays, we achieved less than 20m near-offset and by towing 2km long streamers we have been able to capture angles of 0 to 45 degrees across all target levels.