9 results on '"Operto, Stéphane"'
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2. High-resolution speed of sound estimation from ultrasound waves using extended full wave inversion
- Author
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Oraevsky, Alexander A., Wang, Lihong V., Ranjbaran, Seyed Mohsen, Aghamiry, Hossein S., Gholami, Ali, Operto, Stéphane, and Avanaki, Kamran
- Published
- 2022
- Full Text
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3. Toward a robust workflow for deep crustal imaging by FWI of OBS data: The eastern Nankai Trough revisited
- Author
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Górszczyk, Andrzej, Operto, Stéphane, and Malinowski, Michał
- Abstract
Crustal‐scale imaging by the full‐waveform inversion (FWI) of long‐offset seismic data is inherently difficult because the large number of wavelengths propagating through the crust makes the inversion prone to cycle skipping. Therefore, efficient crustal‐scale FWI requires an accurate starting model and a stable workflow minimizing the nonlinearity of the inversion. Here we attempt to reprocess a challenging 2‐D ocean‐bottom seismometer (OBS) data set from the eastern Nankai Trough. The starting model is built by first‐arrival traveltime tomography (FAT), which is FWI assisted for tracking cycle skipping. We iteratively refine the picked traveltimes and then reiterate the FAT until the traveltime residuals remain below the cycle‐skipping limit. Subsequently, we apply Laplace‐Fourier FWI, in which progressive relaxation of time damping is nested within frequency continuation to hierarchically inject more data into the inversion. These two multiscale levels are complemented by a layer‐stripping approach implemented through offset continuation. The reliability of the FWI velocity model is assessed by means of source wavelet estimation, synthetic seismogram modeling, ray tracing modeling, dynamic warping, and checkerboard tests. Although the viscoacoustic approximation is used for wave modeling, the synthetic seismograms reproduce most of the complexity of the data with a high traveltime accuracy. The revised FWI scheme produces a high‐resolution velocity model of the entire crust that can be jointly interpreted with migrated images derived from multichannel seismic data. This study opens a new perspective on the design of OBS crustal‐scale experiments amenable to FWI; however, a further assessment of the optimal OBS spacing is required for reliable FWI. New robust workflow minimizing nonlinearity of the crustal‐scale FWIExtensive quality control for validation of the starting and final FWI modelNew geological insight into structure of the eastern Nankai Trough
- Published
- 2017
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4. Travel time inversion from ground level to gallery: protocol for the characterization of P‐wave seismic signature in a fractured‐porous Urgonian platform at hectometric scale
- Author
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Maufroy, Emeline, Gaffet, Stéphane, Operto, Stéphane, Guglielmi, Yves, and Boyer, Daniel
- Abstract
A tomographic P‐wave velocity model is inferred from a ground level‐to‐gallery vertical 500 m × 800 m seismic experiment conducted at the inter‐Disciplinary Underground Science and Technology Laboratory (LSBB, France). No initial knowledge of the velocity structure of the surrounding fractured‐porous carbonates was previously available. Ninety‐four shots at the surface were recorded by a line of 189 seismometers on the steep slope of the topographic surface and by a line of 150 geophones in an 800 m‐long, 250‐500 m‐depth gallery. The P‐wave velocities inferred from first‐arrival travel time inversion display a relatively large set of values ranging from 4000 to 6000 m/s. Such Vpvariations correlate well with the 5 to 20% porosity variations between the main geological units that consist of two sedimentary facies affected by a complex cemented fault zone. Taking advantage of the known geology of the site, this study explores the influence of the acquisition geometry impacted by the topography and of the near‐surface weathered zone onto the shallow Vptomography resolution ability. Considering the mesoscopic scale of the targeted medium, reliable imaging of hectometric geological bodies with 10% contrasts in porosities can be achieved only with the simultaneous association of (i) a high density of sources and receivers in the monitoring array geometry, and (ii) the equal consideration of surface‐to‐gallery and surface‐to‐surface first‐arrival travel times, as an essential constraint to correctly image the underlying structures.
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- 2014
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5. Which parameterization is suitable for acoustic vertical transverse isotropic full waveform inversion? Part 2: Synthetic and real data case studies from Valhall.
- Author
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Gholami, Yaser, Brossier, Romain, Operto, Stéphane, Prieux, Vincent, Ribodetti, Alessandra, and Virieux, Jean
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WAVES (Physics) ,WAVELENGTHS ,AZIMUTH ,ACOUSTIC emission ,PARAMETER estimation - Abstract
It is necessary to account for anisotropy in full waveform inversion (FWI) of wide-azimuth and wide-aperture seismic data in most geologic environments, for correct depth positioning of reflectors, and for reliable estimations of wave speeds as a function of the direction of propagation. In this framework, choosing a suitable anisotropic subsurface parameterization is a central issue in monoparameter and multiparameter FWI. This is because this parameterization defines the influence of each physical parameter class on the data as a function of the scattering angle, and hence the resolution of the parameter reconstruction, and on the potential trade-off between different parameter classes. We apply monoparameter and multiparameter frequency-domain acoustic vertical transverse isotropic FWI to synthetic and real wide-aperture data, representative of the Valhall oil field. We first show that reliable monoparameter FWI can be performed to build a high-resolution velocity model (for the vertical, the horizontal, or normal move-out velocity), provided that the background models of two Thomsen parameters describe the long wavelengths of the subsurface sufficiently accurately. Alternatively, we show the feasibility of the joint reconstruction of two wave speeds (e.g., the vertical and horizontal wave speeds) with limited trade-off effects, while Thomsen parameter ,5 is kept fixed during the inversion. The influence of the wave speeds on the data for a limited range of scattering angles when combined each other can, however, significantly hamper the resolution with which the two wave speeds are imaged. These conclusions inferred from the application to the mat data are fully consistent with those inferred from the theoretical parameterization analysis of acoustic vertical transverse isotropic FWI performed in the companion report. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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6. Which parameterization is suitable for acoustic vertical transverse isotropic full waveform inversion? Part 1: Sensitivity and trade-off analysis.
- Author
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Gholami, Yaser, Brossier, Romain, Operto, Stéphane, Ribodetti, Alessandra, and Virieux, Jean
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ANISOTROPY ,AZIMUTH ,SEISMIC wave studies ,ACOUSTIC emission ,MATHEMATICAL models - Abstract
In most geologic environments, accounting for anisotropy is necessary to perform acoustic full waveform inversion (FWI) of wide-azimuth and wide-aperture seismic data because of the potential dependence of wave speeds on the direction of the wave propagation. In the framework of multiparameter FWI, the subsurface parameterization controls the influence of the different parameter classes on the modeled seismic data as a function of the scattering angle and hence the resolution with which the parameters can be reconstructed and the potential trade-off between different parameters. We have evaluated a numerical procedure based on computation of the scattering patterns of the different parameters to assess the sensitivity of the seismic data to different parameterizations of vertical transverse isotropic media in the acoustic approximation. Among the different categories we have tested, a monoparametric FWI was found for imaging one wave speed with a broad wavenumber content, keeping the Thomsen parameters fixed, which have a small influence on the data relative to the wave speed. This raises the question of the initial information required in the background models of the Thomsen parameters to perform reliable monoparameter FWI. Alternatively, simultaneously inverting the horizontal and vertical wave speeds introduces limited trade-off effects because these wave speeds have significant influence on the data for distinct Panges of scattering angles, while the influence of the Thomsen parameter δ remains weak. With such parameterization, the short-to-intermediate wavelengths of the vertical velocity are updated from the short-to-intermediate scattering angles, while the long-to-intermediate wavelengths of the horizontal velocity are updated from the wide-to-intermediate scattering angles. We concluded that the choice of the subsurface parameterization can be driven by the acquisition geometry, which controls the scattering-angle coverage and hence the resolving power of FWI, and by the accuracy of the available initial FWI models. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
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7. Shallow-structure characterization by 2D elastic full-waveform inversion.
- Author
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Romdhane, Anouar, Grandjean, Gilles, Brossier, Romain, Rejiba, Fayçal, Operto, Stéphane, and Virieux, Jean
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CATHODE ray oscillographs ,INVERSION (Geophysics) ,ALGORITHMS ,EARTHFLOWS ,GALERKIN methods - Abstract
Assessing the effectiveness of elastic full-waveform-inversion (FWI) algorithms when applied to shallow 2D structures in the presence of a complex topography is critically important. By using FWI, we overcome inherent limitations of conventional seismic methods used for near-surface prospecting (acoustic tomography and multichannel spectral analysis of surface waves). The elastic forward problem, formulated in the frequency domain, is based on a mixed finite-element P0PI discontinuous Galerkin method to ensure accurate modeling of complex topography effects at a reasonable computing cost. The inversion problem uses an FWI algorithm to minimize the misfit between observed and calculated data. Based on results from a numerical experiment performed on a realistic landslide model inspired from the morphostructure of the Super-Sauze earthflow, we analyzed the effect of using a hierarchical preconditioning strategy, based on a simultaneous multifrequency inversion of damped data, to mitigate the strong nonlinearities coming from the surface waves. This strategy is a key point in alleviating the strong near-surface effects and avoiding convergence toward a local minimum. Using a limited-memory quasi-Newton method improved the convergence level. These findings are analogous to recent applications on large-scale domains, although limited source-receiver offset ranges, low-frequency content of the source, and domination of surface waves on the signal led to some difficulties. Regarding the impact of data decimation on the inversion results, we have learned that an inversion restricted to the vertical data component can be successful without significant loss in terms of parameter imagery resolution. In our investigations of the effect of increased source spacing, we found that a sampling of 4 m (less than three times the theoretical maximum of one half-wavelength) led to severe aliasing. [ABSTRACT FROM AUTHOR]
- Published
- 2011
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8. Imaging and inversion -- Introduction.
- Author
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Buske, Stefan, Lecomte, Isabelle, Nemeth, Tamas, Operto, Stéphane, and Sallares, Valenti
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WAVE equation ,GEOPHYSICS - Abstract
The article discusses various reports in this issue, including one about the prestack depth migration approach and another on a new wave equation for P-waves.
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- 2009
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9. Deep structure of the northern Kerguelen Plateau and hotspot-related activity
- Author
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Charvis, Philippe, Recq, Maurice, Operto, Stéphane, and Brefort, Daniel
- Abstract
Seismic refraction profiles were carried out in 1983 and 1987 throughout the Kerguelen Isles (southern Indian Ocean, Terres Australes & Antarctiques Franqaises, TAAF) and thereafter at sea on the Kerguelen‐Heard Plateau during the MD66/KeOBS cruise in 1991. These profiles substantiate the existence of oceanic‐type crust beneath the Kerguelen‐Heard Plateau stretching from 46°s to 55°S, including the archipelago. Seismic velocities within both structures are in the range of those encountered in ‘standard’ oceanic crust. However, the Kerguelen Isles and the Kerguelen‐Heard Plateau differ strikingly in their velocity‐depth structure. Unlike the Kerguelen Isles, the thickening of the crust below the Kerguelen‐Heard Plateau is caused by a 17 km thick layer 3. Velocities of 7.4 km s−1 or so within the transition to mantle zone below the Kerguelen Isles are ascribed to the lower crust intruded and/or underplated by upper mantle material. The crust‐mantle boundary below the Kerguelen‐Heard Plateau is abrupt and devoid of any underplated material. The difference in structure between the northern edge of the Kerguelen Plateau (including the archipelago) and the Kerguelen‐Heard Plateau may be related to variability of the time‐dependent hotspot activity. The Kerguelen‐Heard Plateau was emplaced during the Cretaceous time (110 Ma) when the volcanic output rate of the Kerguelen Plateau and the Ninetyeast Ridge was high (as well as high potential temperature). The northernmost Kerguelen Plateau and the archipelago were emplaced during Tertiary time (40–45 Ma), as the volcanic output rate reduced. Furthermore, intraplate volcanism continued in the Kerguelen archipelago for at least 40 Ma. The isostatic compensation of the Kerguelen Isles and the Kerguelen‐Heard Plateau is achieved by low‐density mantle material, as shown by refraction and geoid studies. The velocity‐depth structure below the Kerguelen Isles is similar to that found below intraplate oceanic islands such as Hawaii. Despite the differences in age, the crust below Iceland (0Ma) and the Kerguelen Plateau (100–120 Ma) are strikingly akin. The similarity between the Kerguelen‐Heard Plateau and Iceland, then, strongly supports a similar origin for both structures, the Kerguelen‐Heard Plateau being a fossil equivalent of present‐day Iceland. Crustal thickening beneath the Kerguelen‐Heard Plateau, which may result from an Iceland‐type setting (i.e. an active spreading centre over a hotspot), is mostly produced by thickening of layer 3, layer 2 representing 25 per cent only of the thickness of the igneous crust. The Kerguelen Isles, despite the initial volcanism near the active Southeast Indian Ridge, behave as a midplate volcanic island and are definitely not representative of the whole Kerguelen Plateau structure.
- Published
- 1995
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