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Velocity model building challenges and solutions for seabed- and paleo-canyons: a case study in Campos Basin, Brazil

SEG - Society of Exploration Geophysicists, September, 2017
Kai Zhang | Javier Subia | Chevron; Chanjuan Sun | Hao Shen | Nuree Han
©2017 SEG

The Campos Basin, offshore Brazil, features complex shallow geology in the forms of pronounced seabed canyons and paleo-canyons. The rapid variations in the velocity field due to these complex shallow geologic features can be difficult for ray-based tomography techniques to resolve, resulting in distorted images in deeper section. Full waveform inversion (FWI) is able to utilize the recorded diving–wave energy to resolve the high-resolution velocity model in these geologically complex areas. Additionally, dip-constrained non-linear slope tomography introduces dip constraints to ray-based residual move-out tomography and is able to capture small-scale velocity anomalies associated with these shallow heterogeneities. A combined workflow of FWI and dip-constrained tomography enabled Chevron to build accurate and detailed velocity models in the Campos Basin, resulting in fewer seismic image distortions. We demonstrate the method using a Campos Basin, Brazil narrow-azimuth streamer dataset.

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Improved iterative least-squares migration using curvelet-domain Hessian filters

SEG - Society of Exploration Geophysicists, September, 2017
Ming Wang | Shouting Huang | Ping Wang
©2017 SEG

Least-squares migration (LSM) can potentially provide better amplitude fidelity, higher image resolution, and fewer migration artifacts than standard migration. Conventional LSM is often solved iteratively through linearized inversion, and therefore is often referred to as iterative LSM. In recent years, various single-iteration LSM approaches have been proposed as a cost-effective approximation of iterative LSM and have produced promising results. To exploit the full potential of LSM, we propose to employ the curvelet-domain Hessian filter (CHF), useful in single-iteration LSM, as a preconditioner for conventional iterative LSM. We call this approach CHF-preconditioned LSM (CPLSM). We first validate our CPLSM approach using SEAM I synthetic data and show that it produces better amplitude fidelity over the single-iteration CHF approach and converges faster than conventional iterative LSM. Furthermore, we demonstrate with an application to field data that CPLSM produces fewer migration artifacts and less noise than conventional iterative LSM. This addresses a known problem of iterative LSM that is caused by the use of inaccurate modeling algorithms followed by overfitting the modeled synthetic data to the recorded data.

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Applications of single-iteration Kirchhoff least-squares migration

SEG - Society of Exploration Geophysicists, September, 2017
Lorenzo Casasanta | Francesco Perrone | Graham Roberts | Andrew Ratcliffe | Karen Purcell | Arash JafarGandomi | Gordon Poole
©2017 SEG

Least-squares depth migration approximates the inverse of the forward modeling. We show two real data applications of a single iteration (non-iterative) Kirchhoff least-squares depth migration process, generically referred to as migration deconvolution, to highlight the benefits of this process. Our first example demonstrates improved amplitude behavior of least-squares migration deconvolution images in an offshore Gabon data set. In the second example we propose an efficient way to include attenuation in the least-squares migration process and, using a Central North Sea data set, highlight a stable uplift in both resolution and illumination of the final image.

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Improving fractured basement imaging through WAZ OBC acquisition

SEG - Society of Exploration Geophysicists, September, 2017
Chenghai Jiao | FongCheen Loh | Yitao Chen | Xiaobo Li | Xusong Wang | Yi Xie | Joe Zhou | CGG; Nguyen Lam | Anh and Vu Van Khuong | Vietsovpetro
©2017 SEG

The oil-bearing fractured granite basement rocks form a very important and complicated hydrocarbon reservoir in Cuu Long basin offshore Vietnam. However, the poor fractured basement imaging in the conventional tow streamer data makes it hard for detailed interpretation and future well placement. To improve the seismic imaging, the first 3D/4C OBC acquisition over the field was carried out to provide better illumination, better elimination of multiples and broader spectrum with better signal to noise ratio. However, the presence of strong azimuthal anisotropy poses a serious challenge in imaging this OBC data with the full azimuth (FAZ) nature. The steeply dipping fracture imaging can be smeared if the subsurface orthorhombic (ORT) velocity model is not properly derived. In this paper, we present a new orthorhombic velocity model building workflow to estimate the azimuthal anisotropic velocity by incorporating shear wave splitting analysis, well formation microimager (FMI) information and 3D RTM subsurface angle gather based velocity sweeping inside basement. Two geological layers with strong azimuthal anisotropy are identified and incorporated into the final ORT model which results in much shaper imaging not only in shallow classic sediment layers but also in the fractured basement.

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A 3-stage approach to derive key elastic properties for marine reservoir with faulted overburden

SEG - Society of Exploration Geophysicists, September, 2017
Yang Song | Yan Liu | Bing Bai | Lingli Hu | CGG; Michelle Abraham | Yafei Wu | Anadarko Petroleum Corporation
©2017 SEG

Seismic inversion transforms seismic reflection data into quantitative rock-property descriptions of a reservoir. Seismic data bandwidth is limited by signal-to-noise ratio, absorption, source wavelet, and shot and receiver ghosts. A typical deterministic seismic inversion workflow fills the low frequencies by extrapolating or interpolating existing well logs along stratigraphic layers. The interpolation result is often biased by the well locations and quality of the well logs and can be affected by the interpolation method. We propose a 3-stage method to minimize the dependency of seismic inversion on a well-log based initial model and improving confidence in the final result.

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