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Imaging through near-surface absorption bodies with visco-acoustic least-squares migration: a case study from the Northern Viking Graben

Amplitude attenuation and phase distortion of seismic data are byproducts of the Earth’s anelasticity (Q). These effects are exacerbated under regions of anomalously high absorption, such as shallow gas, causing uneven illumination and migration artifacts. We present a case study from the Norwegian North Sea where we utilize single-iteration, least-squares, visco-acoustic, prestack depth migration to produce a stable Q-compensated image. This used high-resolution, p-wave velocity and attenuation models, derived from visco-acoustic FWI. We show this workflow compensates locally for amplitude loss, eliminates the need for a Q-compensation amplitude gain limit, and enhances illumination, event coherency and AVO attributes, while reducing noise

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3D VSP imaging under a complex salt finger at Atlantis, Gulf of Mexico

To improve image resolution in the vicinity of the well, 3D VSP data was acquired at the Atlantis field in the Gulf of Mexico to complement the existing towed-streamer and OBN data. Two unique data challenges require special consideration and innovative techniques in order to unlock the full imaging potential at the Atlantis field: (1) illumination issues related to the receivers being placed in a well that goes through a complicated salt finger; (2) receiver reorientation issues related to the survey’s two-phase acquisition due to receivers being pulled out and placed back into well. We demonstrate how we mitigate the impact of these challenges through improved XYZ vector fidelity reorientation and least-squares RTM.

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Improving reflection FWI reflectivity using LSRTM in curvelet-domain

We investigate the benefits of considering the effects of the Hessian matrix in the reflection FWI (RFWI) reflectivity, i.e., using least-squares RTM (LSRTM) to estimate the short-wavelength component of the model. In addition, a more efficient approach using single-iteration LSRTM, more specifically, using curvelet-domain Hessian filters is proposed. Using synthetic and field data sets, we show how this approach can improve the reflectivity model and, therefore, the synthetic reflection data, which ultimately benefits RFWI. Finally, we discuss some of the limitations of this approach and some of the challenges that are still not addressed by it.

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Overcoming the challenges of a shallow-water sparse wide-azimuth survey to improve deep reservoir imaging in the East China Sea

A new broadband Wide-Azimuth Towed-Streamer (WATS) survey was acquired in a shallow water region of offshore China to better resolve reservoir compartments. Two side boats were added as additional source boats to form the WATS acquisition geometry to resolve the shortcomings of narrow-azimuth acquisition along strike direction. This WATS acquisition is much sparser than common WATS surveys in deep water environments due to only one-side WATS configuration. The combination of sparse acquisition, shallow water and deep targets imposes challenges on how to optimally utilize the side-gun data as the key adding on to improve the final image. The 3D effect and severe aliasing expected in the Crossline direction pose tremendous difficulties for side gun data processing. A comprehensive flow for resolving these challenges especially in deghosting, demultiple and regularization for sparse and shallow wide-azimuth data is presented in the paper. A tilted orthorhombic (TORT) velocity model is also built with better constraints from the wide azimuth information for better fault positioning and imaging. Side gun data clearly enhances the final target reservoir imaging and better ties with the well due to better illuminations. A new channel is discovered based on the interpretation from the inverted Vp/Vs ratio, which clearly explains the previous misleading prediction that an exceptional well was drilled to a thinner and shallower channel not connected to the main reservoir.

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The Zambezi Delta Basin: A Complex Puzzle with Missing Pieces

Exploration on the African continent has traditionally been concentrated in prolific basins on the Sub-Saharan Western margin, in detriment to the East. Proven prolific basins, existing infrastructure, favourable and progressively transparent regulation; along with the analysis of prospective conjugate basins of Brazil are just some of the reasons for creating a positive environment for international investors. A report submitted in 2012 by the USGS, predicts undiscovered mean gas resources of more than 370 TCFG in offshore East Africa, including Tanzania, Mozambique, Madagascar and Seychelles. World-class gas discoveries have been made between 2010 and 2013 in the offshore regions of Tanzania and Mozambique, along with onshore discoveries in Uganda and Kenya thus reinvigorating exploration interest along the Eastern African margin. The Zambezi channel, offshore Mozambique, has traditionally been the locus of intense academic research, with a number of 2D seismic and gravimetric and magnetic acquisition campaigns deployed in recent years. However, the region still remains poorly understood. With this in mind, in 2017 CGG acquired a high-resolution 3D seismic survey located in the outer Zambezi Delta Basin, west of the Beira High. This seismic survey, in conjunction with newly acquired high-resolution shipborne grav/mag survey and access to data from multiple wells in the area, aims to bring a new and fresh dimension on the geological understanding of the basin.

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Sparse nodes for velocity: Learnings from Atlantis OBN full-waveform inversion test

As the key step in subsalt imaging, conventional salt model building has typically relied on manual interpretation of the salt geometry, which proves to be difficult for resolving complex salt models. As a result, subsalt imaging has been approaching a plateau in the last few years, largely due to our inability to improve the accuracy of salt models. Fast forward to today, where the recent success of using full-waveform inversion (FWI) to automatically update salt models and significantly improve subsalt images has opened the door to a new era of subsalt imaging. As FWI for salt model updates prefers data with good low frequencies, long offsets, and full azimuth, sparse node-for-velocity surveys were proposed to serve as an economic yet suitable acquisition solution for large-scale subsalt exploration. Due to FWI’s prior inability to update salt models for field data, the feasibility of sparse node-for-velocity surveys was previously only studied based on synthetic data, from which the conclusions might not be immediately applicable to field data. Using our recently-developed FWI algorithm that proves to work on salt in field data and by decimating the densely-acquired Atlantis ocean bottom node (OBN) data, we studied the impact of sparse node data for FWI salt model updates. Based on the understanding gained from this, we further proposed and validated methods to improve FWI results with sparse node-for-velocity data.

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Internal multiple attenuation for OBN data with overburden/target separation

Areas with complex overburdens pose a major challenge for seismic imaging of deep targets. Reduced illumination can compromise velocity estimation and the image at the target level. A solution to this problem is given by ocean bottom node (OBN) acquisitions. They provide increased illumination through larger offsets and full azimuthal coverage. In these complex cases, another issue that might arise is that of internal multiples. Strong reflectors in the overburden can generate internal multiples, which will be imaged incorrectly as artifacts. These artifacts may have detrimental effects on interpretation and amplitude analysis. The pre-salt in the Santos Basin, offshore Brazil is a good example of this scenario, where both illumination and internal multiples contribute to affect imaging in the target areas. While internal multiple attenuation (IMA) methods have been widely studied in the case of streamer acquisitions, a full solution tailored to OBN acquisitions is still lacking. We propose a solution based on a method recently proposed for streamer acquisitions.

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Illuminating Santos Basin’s pre-salt with OBN data: Potential and challenges of FWI

Santos Basin has become one of the most prospective oil provinces in the world. Due to the geological complexity, OBN acquisition has emerged as an imaging solution. It provides the full-azimuth and long-offset illumination FWI needs to realize its potential. Given the right data, a reliable FWI engine and a good starting model, we are able to derive accurate velocities to impact the imaging of the salt and pre-salt. Two major challenges remain. FWI is dependent on the initial model and anisotropy remains a challenge. Better acquisitions together with advanced multi-parameter FWI technology are still needed to overcome these challenges.

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Least-squares RTM with ocean bottom nodes: potentials and challenges

Stampede field is a faulted subsalt four-way reservoir in Green Canyon, Gulf of Mexico. Imaging for part of the field has remained challenging due to interference from the complex overburden, which carries large velocity errors and creates non-uniform illumination for the subsalt. Before correcting the velocity error, least-squares reverse time migration (LSRTM) does not produce desirable subsalt image, even when using newly acquired ocean bottom node (OBN) data. With an improved OBN full-waveform inversion (FWI) model, combined with the benefits from the full-azimuth and long-offset coverage of OBN data, LSRTM greatly improves the subsalt image. However, further improving LSRTM is still challenging due to the remaining velocity uncertainty and un-modeled physics, as well as un-attenuated multiples and converted waves.

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