Imaging through gas clouds: The application of CSI and FWI in Bohai, China

2021 ◽  
Vol 40 (5) ◽  
pp. 365-373
Author(s):  
Zhengxue Li ◽  
Yong Ma ◽  
Chengbo Li ◽  
Charles C. Mosher ◽  
Jun Ming ◽  
...  
Keyword(s):  
Model Building ◽  
Seismic Imaging ◽  
Oil Field ◽  
Growth Potential ◽  
Bohai Bay ◽  
Ocean Bottom ◽  
Time Migration ◽  
Area 5 ◽  
Seismic Image ◽  
Gas Cloud

Oil field A, situated in Bohai Bay, was discovered in 1999 and has been developed as one of the most productive oil assets in China. It continues to hold significant growth potential for the future. Though the field contains a large amount of resources remaining to be developed, seismic imaging has been challenging in area 5, resulting in structural uncertainty for reservoir interpretation and well planning. In the past three decades, several 2D and 3D seismic surveys have been acquired, processed, and reprocessed in this area. However, due to the existence of complicated gas clouds, which are shallow, multilayered, and extensive, obscured sub-gas-cloud images appear in all legacy seismic results, making fault interpretation under the gas clouds almost impossible. To improve the sub-gas-cloud image and overall structural interpretability, a narrow-azimuth full-field ocean-bottom cable (OBC) acquisition was conducted in field A during 2018 and 2019, and later, a compressive seismic imaging (CSI)-based full-azimuth and large-offset OBC infill survey was acquired in area 5, covering the widest gas cloud. Through high-fidelity signal processing, full-waveform inversion (FWI)-driven velocity model building, and imaging using both Kirchhoff migration and reverse time migration (RTM), the seismic image quality beneath complicated gas clouds is improved significantly. It is the first time that sub-gas-cloud faults and the Base of Guantao event have been imaged by seismic without significant dim zones. CSI acquisition, FWI, and RTM are the key elements to resolve gas-cloud-related challenges in area 5.

Shenzi OBN: An imaging step change

2021 ◽  
Vol 40 (5) ◽  
pp. 348-356
Author(s):  
Cheryl Mifflin ◽  
Drew Eddy ◽  
Brad Wray ◽  
Lin Zheng ◽  
Nicolas Chazalnoel ◽  
...  
Keyword(s):  
Model Building ◽  
Seismic Imaging ◽  
Survey Design ◽  
Waveform Inversion ◽  
Complex Salt ◽  
Step Change ◽  
Ocean Bottom ◽  
Maximum Frequency ◽  
Reverse Time ◽  
Time Migration

The story of seismic imaging over BHP's Shenzi Gulf of Mexico production field follows the history of offshore seismic imaging, from 2D to 3D narrow-azimuth streamer acquisition and to its leading the wide-azimuth movement with the Shenzi rich-azimuth (RAZ) survey. Each RAZ reprocessing project over the last 15 years applied the latest processing technology, culminating in hundreds of scenario tests to refine the salt model, but eventually the RAZ data reached a technical limit. A new ocean-bottom-node (OBN) survey acquired in 2020 has produced a step-change improvement over the legacy RAZ image. The uplift can be attributed to several factors. First, an OBN feasibility and survey design study demonstrated that a core of dense nodes combined with sparse nodes would improve the accuracy and resolution of the full-waveform inversion (FWI) solution. Second, the OBN data acquired following the survey design and employing FWI as the main model-building tool realized the predicted improvement. The result was a substantial change to the complex salt model, verified by a salt proximity survey as well as other salt markers, and improvement in imaging over the entire field. In addition to the improvement arising from a more accurate FWI velocity model, the steep-dip imaging also benefited from the new full-azimuth and long-offset data. However, the best steep-dip and fault imaging comes from the FWI image, a direct estimation of reflectivity from the FWI velocity. As the maximum frequency used by FWI moves toward the maximum frequency of the final reverse time migration (RTM), the FWI image approaches the resolution necessary to compete as the primary interpretation volume. Its subsalt illumination surpassed that of the RTM and even the least-squares RTM volumes. These imaging improvements are providing a new understanding of the faults and stratigraphic relationships of the field.

Velocity model building by waveform inversion of early arrivals and reflections: A 2D case study with gas-cloud effects

Geophysics ◽  
2018 ◽  
Vol 83 (2) ◽  
pp. R141-R157 ◽  
Author(s):  
Wei Zhou ◽  
Romain Brossier ◽  
Stéphane Operto ◽  
Jean Virieux ◽  
Pengliang Yang
Keyword(s):  
Large Scale ◽  
Model Building ◽  
Waveform Inversion ◽  
Real Data ◽  
Velocity Model ◽  
Ocean Bottom ◽  
Initial Model ◽  
Early Arrival ◽  
Gas Cloud

Joint full-waveform inversion (JFWI) combines reflection waveform inversion (RWI) and early-arrival waveform inversion to build a large-scale velocity model of the subsurface from long-offset data. The misfit function of JFWI requires an explicit separation between the short-spread reflections and early arrivals, the feasibility of which is illustrated with a real data case study. JFWI is alternated with a waveform inversion/migration of short-spread reflections to provide a short-scale impedance model. This model is needed for building the sensitivity kernel of RWI along the two-way reflection paths. The large-scale velocity macromodel built by JFWI can be used as the initial model for classic FWI to enrich the high-wavenumber content of the subsurface model. We have developed an application of this workflow to a real 2D ocean bottom cable (OBC) profile across a gas cloud in the North Sea to review its main promises and pitfalls. Viscoacoustic VTI seismic modeling allows us to account for attenuation and anisotropy effects in a passive way during JFWI and FWI. Using a smoothed version of an existing traveltime tomographic model as the initial model, we first find that the JFWI velocity macromodel is more accurate than the RWI counterpart thanks to the key contribution of the diving waves. Second, we find that the large-scale velocity model updated by JFWI provides a more accurate initial model for classic FWI than does the original smoothed tomographic model. However, because a data difference-based misfit function is used, 2D JFWI still suffers from cycle skipping when a crude 1D velocity model is used as the initial model; therefore, more robust misfit function should be designed to mitigate cycle skipping.

Reducing seismic reflector distortion beneath gas clouds in the Malay Basin using full-wavefield imaging approaches

2020 ◽  
Vol 39 (8) ◽  
pp. 591a1-591a8
Author(s):  
Ahmad Riza Ghazali ◽  
Muhammad Hafizal Mad Zahir ◽  
Muhammad Faizal Abdul Rahim ◽  
Kefeng Xin ◽  
Farah Syazana Dzulkefli ◽  
...  
Keyword(s):  
Seismic Imaging ◽  
Waveform Inversion ◽  
Implementation Strategies ◽  
Effective Medium ◽  
Full Waveform ◽  
Seismic Image ◽  
Inversion Techniques ◽  
Gas Cloud ◽  
Complex Transmission

A seismic example from the Malay Basin is presented, demonstrating improved seismic imaging beneath gas clouds using full-wavefield imaging approaches. Overall imaging concepts, synthetic examples, and field implementation strategies are discussed, and results that tie with well information are presented. Seismic imaging beneath gas clouds using the full-wavefield redatuming technique improves the image by estimating the waveform transmission operators via equivalent-medium representation of the overburden from the gas cloud reflection response for use in a form of multidimensional deconvolution of the wavefield. The other example shown uses the full-wavefield migration seismic imaging technique, which utilizes primaries and higher-order multiples as signals to improve the reflectivity estimation in imaging. The demonstrated full-wavefield imaging approach uses information carried by the gas cloud reflection response to correct seismic image distortion. Removing the internal multiple using conventional demultiple processing in the gas cloud area will also remove the valuable information of the subsurface that it carries. Such multiples must be preserved for this method to be successful. The information is translated into transmission operators that are estimated by simulating the reflection response through an effective medium of the gas cloud overburden. The effective medium is obtained via nonlinear full-waveform inversion techniques from the reflections of the gas cloud overburden area. Finally, a deconvolutional process removes the transmission operators from the gas cloud reflections and recovers the underlying reflectors. Full-wavefield imaging can reconstruct the amplitudes of the reflection response below a gas cloud overburden zone so that the complex transmission imprint on the area underneath is removed properly. The Malay Basin field case study shows that implementation of this approach can provide a reliable amplitude image of the subsurface affected by gas clouds, calibrated and verified by the well information.

Fast model building using demigration and single‐step ray migration

Geophysics ◽  
1994 ◽  
Vol 59 (3) ◽  
pp. 439-449 ◽  
Author(s):  
David N. Whitcombe
Keyword(s):  
Model Building ◽  
Relative Size ◽  
Single Step ◽  
Layer By Layer ◽  
Vertical Layer ◽  
Data Set ◽  
Depth Migration ◽  
Layer Cake ◽  
Time Migration ◽  
Seismic Image

This work provides explorationists with simple procedures to perform depth conversion more accurately than can be achieved with simple vertical layer cake depth conversion. The use of image rays, which are inadequate in structurally complex areas, is avoided. Migrated time interpretations are still used and are "demigrated" using the Kirchhoff time migration equations. This backs out the effect of the time migration prior to a ray depth migration and enables the lateral shifts between the time migrated image and a depth migrated image to be quantified. These shifts can be separated into a mismigration component and a refraction component. The relative size of the components define whether time or depth migration is required and may be used to justify a remigration of the seismic image. Furthermore, the tedious layer by layer approach to ray depth migration may be avoided by using the velocity depth model from the vertical layer cake depth conversion of the time‐migrated data for ray depth migration of the unmigrated data for all horizons in a single step. A satisfactory result is usually achieved without the need to iterate. These methods are illustrated with both a synthetic example and a real 3-D data set from the Norwegian North Sea.

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