Salt interpretation for depth imaging ‐ where geology is working in the geophysical world

Zhang, Quincy; Chang, Itze; Li, Li

doi:10.1190/1.3255627

Cited by 4 publications

(1 citation statement)

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“…Headway has been made in developing methods for integrating migration and interpretation tools for a faster turnaround as well as establishing a feedback process whereby migration results can be efficiently used to update the model for a better final image (Mosher et al, 2007;Foss et al, 2008;Ahmed et al, 2012;Chopra and Marfurt 2012;O'Briain et al, 2013). Li et al (2009) and Zhang et al (2009) show how changing salt geometry based on regional geologic knowledge of the area can lead to a better subsalt image. Ritter (2010) explains the importance of identifying geobodies that may have significantly different velocities than surrounding sediments, such as overpressured shale and carbonate carapaces, based on subsalt images and common image gathers.…”

Section: Introductionmentioning

confidence: 99%

Hybrid target-oriented salt interpretation in the Gulf of Mexico

Maranganti¹,

Agnihotri²

2014

Interpretation

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The top-down method for salt interpretation starts with the search for salt-sediment interfaces, which begins at the shallowest depths and progressively moves deeper. This search is typically conducted on intermediate seismic products such as sediment flood, salt flood volumes, overhang sediment flood, and overhang salt flood depth migrations. However, with this approach, poorly imaged subsalt areas become known only after spending considerable time interpreting intermediate salt features. We evaluated a new method for streamlining this traditional approach to salt-model building wherein a reference salt geometry was obtained earlier in the salt interpretation process. Having a reference seismic volume helped to identify poorly imaged subsalt targets much sooner. A hybrid of model-based and data-based interpretations was then performed for the poorly imaged subsalt areas, whereby interpretation was completed by proposing geologically viable models that were continuously verified by their impact on geophysical (seismic) data. In poorly imaged areas, we looked bottom-up, rather than top-down, to establish a salt geometry that best fit the geologic model as well as the geophysical data. Our hybrid target-oriented approach is useful for not only reducing the interpretation cycle time but also for improving images beneath the salt.

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Section: Introductionmentioning

confidence: 99%

Hybrid target-oriented salt interpretation in the Gulf of Mexico

Maranganti¹,

Agnihotri²

2014

Interpretation

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Variance-based model interpolation for improved full-waveform inversion in the presence of salt bodies

et al. 2018

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When present in the subsurface salt bodies impact the complexity of wave-equation-based seismic imaging techniques, such as least-squares reverse-time migration, and full-waveform inversion (FWI). Typically, the Born approximation used in every iteration of least-squaresbased inversions is incapable of handling the sharp, high-contrast boundaries of salt bodies.We develop a variance-based method for reconstruction of velocity models to resolve the imaging and inversion issues caused by salt bodies. Our main idea lies in retrieving useful information from independent updates corresponding to FWI at di↵erent frequencies. After several FWI iterations we compare the model updates by considering the variance distribution between them to identify locations most prone to cycle skipping. We interpolate velocities from the surrounding environment into these high-variance areas. This approach allows the model to gradually improve from identifying easily resolvable areas and extrapolating the model updates from those to the areas that are di cult to resolve at early FWI iterations. In numerical tests, our method demonstrates the ability to obtain convergent FWI results at higher frequencies.Downloaded 08/06/18 to 109.171.137.210. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/ de Hoop (2013), who used a total variation norm to invert a salt-a↵ected subsurface using FWI. Regularized inversion methods could be computationally expensive when multiple runs on a fine mesh are needed and these methods often require prior assumptions about the subsurface structure (Kadu et al., 2016).The second approach involves image or gradient manipulations with single iteration updates of FWI. Processed gradients can lead to shorter paths toward the global minimum without being trapped in the local minima (Alkhalifah, 2015b. Image-guided inversion (Ma et al., 2012), gradient optimization (Wu and Alkhalifah, 2016) or gradient conditioning through scattering angle-based filters (Alkhalifah, 2015a;Kazei et al., 2016) can serve the same purpose.The objective of FWI is to minimize both amplitude and phase di↵erences between observed and modeled seismic data. For successful inversion the Born approximation requires the initial velocity model to deliver mismatches in travel times less than half the period (Beydoun and Tarantola, 1988). Larger errors cause cycle-skipping artifacts on the wavepaths specific to each source-receiver pair. These artifacts in the image domain can be identified as repeated contrast velocity anomalies that, in turn, lead the inversion to convergence toward a local minimum on an objective function (Virieux and Operto, 2009).Inversion of mono-frequency data allows features to be resolved at a specific scale. The inversion results from di↵erent mono-frequencies therefore do not match exactly.When mono-frequency data are modeled, the phase of a harmonic plane wave at a given point on a wavepath depends on the frequency and travel time. Hence, at di↵erent frequencies, the phases of ...

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