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With many production reservoirs located at a depth greater than 1km, the near surface is often overlooked during seismic processing. Therefore, valuable information relating to shallow geohazards, faults and changes to lithology are lost or unused. We present a new processing methodology to improve the spatial resolution of the near-surface seismic image. The resulting seismic image has high spatial resolution making shallow features highly resolved. The derived velocity model is sufficiently highly resolved to be considered as a tool to aid in seismic interpretation and sediment classification. This comprehensive workflow was essential to overcome the challenges in shallow water acquisition.

The presence of absorption (Q) anomalies in the overburden, typically associated with gas accumulations, cause seismic obscured areas in our images and reduce our ability to see and interpret events inside the resulting “shadow zone”. In this paper we present our recent developments for addressing these challenges. We review progress made in the area of Q-compensating prestack depth migration (Q-PSDM) in order to deal with the co-existing multi-pathing and absorption effects for imaging through or under complex gas clouds using P-waves. In addition, to mitigate the problem associated with over-boosting of noise and migration artefacts introduced by Q-PSDM, more advanced imaging methods, such as least-squares Q-migration, have been developed to maximize the benefit of Q-PSDM. We then highlight a recently developed visco-acoustic full-waveform inversion (Q-FWI) model building technique for joint estimation of Q and velocity models. This has been applied to a production example from the Norwegian North Sea, where we see that the Q-FWI detects attenuating bodies of varying strength and scale throughout the survey and provides a clear uplift in the subsequent imaging process.

Reservoir imaging under the triangular Conger salt remains very challenging even after significant velocity model building efforts in recent years. Continuity and focusing of reservoir reflectors are sub-optimal due to the subtle velocity errors from the Conger salt and its neighboring carapace, which are very difficult for conventional methods, such as interpretation-guided salt scenarios and ray-based tomography, to resolve. Diving-wave full-waveform inversion (FWI) has difficulty updating the velocity at this depth due to the limit of maximum offset, and thus penetration depth, of input data. In this study, we performed reflection FWI (RFWI) using ocean-bottom node (OBN) data for velocity model updates. Our results showed that RFWI can effectively resolve the subtle low-wavenumber velocity errors in the overburden and substantially improve reservoir imaging. We also demonstrated that RFWI using OBN data can result in a better model than using wide-azimuth towed-streamer data due to its full azimuth and much longer offset coverage.

Reflection FWI is effective at providing low-wavenumber velocity updates for deep areas beyond the penetration depth of diving waves and significantly improves seismic imaging. However, the tomographic term of the FWI gradient that is good for low-wavenumber velocity updates can be contaminated by the much stronger high-wavenumber migration term. We present a reflection FWI workflow that is based on Born modeling and thus is free from the contamination of the migration term. In addition, we propose to use a set of partial stacks as reflectivity models for Born modeling to reduce the risks of cycle-skipping and incorrect update sign and to use a traveltime cost function to mitigate the negative impact from amplitude mismatch between input data and modeled synthetic data. Finally, we demonstrate the benefit and effectiveness of our approach using one field data set in the Gulf of Mexico.

We show how the method recently proposed in Messud et al. (2017; 2017b) allows to consider “resolved space” tomographic uncertainties in complement to total tomographic uncertainties. Resolved space uncertainties are obtained by restricting the tomography model space to the one that can be resolved by tomography. Total uncertainties mostly quantify the illumination uncertainties, whereas resolved space uncertainties tend to be more correlated to the final tomography model. We illustrate how those two uncertainties give complementary information for the subsequent seismic interpretation.

This work highlights visco-acoustic full-waveform inversion (Q-FWI) as a method for estimating high-resolution velocity and attenuation models. We present a very large, real data, case study where Q-FWI has been applied to ~36,000 km2 of 3D, narrow azimuth, variable-depth streamer data over the North Viking Graben region of the northern North Sea. The results show that Q-FWI can invert for both velocity and attenuation with a high degree of spatial and vertical resolution. Our visco-acoustic FWI results, in comparison to those from conventional acoustic FWI, are supported by superior imaging in an attenuation compensating pre-stack depth migration.

Land surface seismic data are usually strongly affected by the presence of surface waves, (ground-roll), which can completely obscure the underlying primary reflection signal. It is therefore crucial to isolate the reflection signal for both imaging and amplitude analysis needs. Complex near-surface conditions complicate the ground-roll attenuation and may introduce other sources of noise such as multiples. We present an application of interferometric ground-roll removal and 3D convolutional de-multiple method on a seismic dataset acquired onshore North Kenya, where the shallow subsurface was characterized by strong heterogeneity and structural complexity.

Hydrocarbon prospection with seismic data on Barents Sea can be difficult to interpret because of severe contaminations of the sections by diffracted multiples residual noises often located above the objectives. Those diffracted multiples mainly result from significant scars of the sea floor produced by paleo-iceberg drifts during glacial ages. To mitigate this problem, a new marine seismic design has been developed which consist into a split-spread geometry where three different shot locations are located on the middle of the spread while acquiring broadband seismic signals. It allows for improved subsurface illuminations allowing for sharp and detailed representation of ground reflectors, including the sea floor. We used several multiples models in simultaneous adaption procedures, including 3D SRME models, but also 3D wave-equation based multiple modeling to fully benefit from those available rich subsurface reflectivity representations. On top of that, an innovative and specific implementation of WE modelling allowed for building diffracted multiples “only” models, into separated datasets for feeding the multi-model adaptive subtractions while preserving primary information (specially low frequencies) with AVO-driven primary models. The ensemble lead to unseen and improved demultiple results on those difficult areas.

This paper is the second part of two papers presenting the new source-over-cable marine acquisition solution. It presents the actual large-scale ~1950 km2 marine seismic acquisition as well as discusses the novel processing and imaging involved with this unique split-spread towed marine data. The acquisition setup consisted of a streamer vessel towing 14 streamers trailed by a shooting vessel in the middle of the streamer spread allowing for both positive-, zero- and negative-offsets to be recorded. Fast track migrations along with early QC and initial demultiple results from the new source-over-cable acquisition data, indicates that the new data delivers on its promises of superior image quality. The benefits are drawn from a multitude of new aspects such as but not limited to split-spread source-over-cable acquisition, dense streamer spacing and deep towed zero- and near-offset recording of energy from the new triple wide-tow-sources with very dense shot point interval using dithered overlapping shots.