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

SEG - Society of Exploration Geophysicists, September, 2019
Yan Liu | Yi Chen | Hongda Ma | Chao Peng (CGG); Gopal Mohapatra | Wisley Martins | Gregory Duncan (Hess) | Steve Checkles (Formerly Hess)
©2019 SEG

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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Improving images under complex salt with ocean bottom node data

SEG - Society of Exploration Geophysicists, September, 2019
Yuan Yao* | Hongda Ma | Yan Liu | Chao Peng (CGG); Gopal Mohapatra | Gregory Duncan | Wisley Martins (Hess) | Steve Checkles (Formerly Hess)
©2019 SEG

Subsalt imaging at Stampede field in the Gulf of Mexico has remained challenging for decades due to the existence of a large and complex sediment inclusion inside thick tabular salt. Recently, appropriate full-waveform inversion (FWI) algorithms have been developed for automatic salt model updating (Shen et al., 2017; Zhang et al., 2018). By applying this technology to newly acquired ocean bottom node (OBN) data with good low-frequency content and ultra-long offsets, we are able to invert both the shape and velocity of this complex sediment inclusion at Stampede and provide significant improvement to the subsalt image. A good starting model for FWI is still needed in this workflow, but detailed interpretation efforts are not necessary. Moreover, we expect further improved subsalt imaging if data with even longer offsets becomes available.

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Adaptive quadratic Wasserstein full-waveform inversion

SEG - Society of Exploration Geophysicists, September, 2019
Diancheng Wang and Ping Wang | CGG
©2019 SEG

we present a FWI scheme based on the quadratic Wasserstein metric, with adaptive normalization and integral wavefield. We show that this scheme has better convexity than traditional metrics, and therefore can mitigate cycle-skipping issues, while being insensitive to amplitude effects. We demonstrate the effectiveness of our approach with a streamer data set in an area of complex salt geometry. In addition, we show that this approach can work naturally on reflection data, without the extra procedure of scale separation by either decomposition or demigration. With these learnings, we believe that the fundamentals of FWI research are starting to converge.

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Updating salt model using FWI on WAZ data in the Perdido area: benefits and challenges

SEG - Society of Exploration Geophysicists, September, 2019
Ravi Kumar | Huifeng Zhu | Vivek Vandrasi | Don Dobesh (CGG); Alfredo Vazquez (Pemex)
©2019 SEG

In this case study, we discuss the impact of using a not-so-appropriate Wide Azimuth Data (WAZ) data which lacks good usable low frequencies and long offset full azimuth coverage for FWI based model building. We use Time-Lag FWI (TLFWI) demonstrated to be an appropriate algorithm by Zhigang et al 2018. We observe significant uplift in the shallow velocity model and salt overhang definitions at most locations. In this abstract, we discuss the limitations of using WAZ data for FWI and the additional effort to obtain better starting models to overcome the lack of sufficient diving energy and lack of low-frequency signal (<3Hz). We also briefly discuss the uncertainties in the inverted models esp. beyond the penetration depth of transmission waves.

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

SEG - Society of Exploration Geophysicists, September, 2019
Florian Jouno | Adriano Martinez | Denis Ferreira | Daniela Donno | and Adel Khalil (CGG)
©2019 SEG

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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