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1
Carrion, Philip, Jesse Costa, Jose E. Ferrer Pinheiro et Michael Schoenberg. « Cross‐borehole tomography in anisotropic media ». GEOPHYSICS 57, no 9 (septembre 1992) : 1194–98. http://dx.doi.org/10.1190/1.1443333.
Texte intégralRésumé :
Anisotropy has significant effect on traveltime cross‐borehole tomography. Even relatively weak anisotropy cannot be ignored if accurate velocity estimates are desired, since isotropic traveltime tomography treats anisotropy as inhomogeneity. Traveltime data in our examples were synthetically generated by a ray‐tracing code for anisotropic media, and the computed quasi‐P‐wave traveltimes were subsequently inverted using the “dual tomography” technique (Carrion, 1991). The results of the tomographic inversion show typical artifacts due to the anisotropy, and that accurate imaging is impossible without taking the anisotropy into account.
2
Koren, Zvi, Igor Ravve, Gladys Gonzalez et Dan Kosloff. « Anisotropic local tomography ». GEOPHYSICS 73, no 5 (septembre 2008) : VE75—VE92. http://dx.doi.org/10.1190/1.2953979.
Texte intégralRésumé :
Local tomography is interactive, ray-based, residual-interval-parameter analysis for updating background anisotropic velocity parameters. The method operates directly on image gathers generated by anisotropic curved-ray Kirchhoff time migration. A locally 1D, spatially varying, vertical transversely isotropic model is assumed. The background anisotropy parameters are the instantaneous (interval) vertical compression velocity [Formula: see text] and the two Thomsen anisotropy parameters, [Formula: see text] and [Formula: see text]. The interval velocity [Formula: see text] is updated from short-offset reflection events, and [Formula: see text] is updated from available long-offset data. The medium parameters are updated from the top down both vertically and by layers, one parameter at a time. The picked residual-anisotropy parameters correspond to the residual-moveout (RMO) curves that best fit the migrated reflection events. The method is based on splitting the contribution to the computed RMO at a given point into two parts: from overburden residual parameters and from the actual picked residual parameter. This approach allows for direct residual-interval-parameter analysis to be applied in the same way we perform the commonly used residual-effective-parameter analysis. The local tomography enables a controlled interactive estimation of the long-wavelength anisotropy parameters. The reliable anisotropy parameters estimated by the local approach are used as a background (guiding) model for a global tomography. This makes it possible to successfully apply a global constrained inversion that is performed simultaneously for all parameters of all output intervals using detailed RMO information.
3
Pratt, R. G., W. J. McGaughey et C. H. Chapman. « Anisotropic velocity tomography : A case study in a near‐surface rock mass ». GEOPHYSICS 58, no 12 (décembre 1993) : 1748–63. http://dx.doi.org/10.1190/1.1443389.
Texte intégralRésumé :
Cross‐borehole data were acquired in the surface crown pillar of a massive sulfide ore mine. The data consist of five, two‐dimensional (2-D), cross‐borehole panels, each with approximately 900 source‐receiver pairs. The panels were located within the crown pillar at either side of and within a major subvertical fault zone that intersects the orebody. An initial analysis of the data indicates that the bedrock containing the orebody is seismically anisotropic. A rigorous analysis of the traveltimes using anisotropic velocity tomography confirms the initial assessment that anisotropy exists within the crown pillar rock mass. Anisotropic velocity tomography is the generalization of tomographic methods to anisotropic media. As in any geophysical problem, the data are insufficient to completely resolve the distributions of the rock properties at all scale lengths; we use external constraints on the roughness of the final solution to ensure an algebraically well‐posed problem. Plots of the data residuals (the “traveltime surfaces”) are an essential tool in determining an optimal level of constraint. Of equal importance are plots of the relationship between the solution roughness and the rms level of the residuals. The final results of anisotropic velocity tomography are a set of images (tomograms) of the velocity and selected anisotropy parameters for the five panels. Our images do not contain the distortions typically exhibited when using isotropic tomography in anisotropic media. The velocity tomograms clearly show the geometry of the overburden contact at the top of the bedrock. The anisotropy tomograms show a decrease in anisotropy with depth on two of the panels. They also show a decrease in anisotropy with proximity to the fault zone. These features of the seismic velocity anisotropy are consistent with observations of fracture orientation and distribution. The results of the crosshole data interpretation contribute to the overall site investigation and provide a reliable interrogation of the bulk properties of the rock mass.
4
Zhou, Chaoguang, Junru Jiao, Sonny Lin, John Sherwood et Sverre Brandsberg-Dahl. « Multiparameter joint tomography for TTI model building ». GEOPHYSICS 76, no 5 (septembre 2011) : WB183—WB190. http://dx.doi.org/10.1190/geo2010-0395.1.
Texte intégralRésumé :
Model building for tilted transversely isotropic media has commonly been performed by a single parameter tomography that updates the velocity in the symmetry direction, while the orientation of the symmetry axis and Thomsen parameters [Formula: see text] and [Formula: see text] are typically estimated from the migration stack and well data. Unfortunately, well data are often not available. In addition, when they are available, their lateral sampling is typically very sparse and their vertical sampling usually spans only a limited range of depths. In order to obtain spatially varying anisotropic models, with or without well data, we developed a multiparameter joint tomographic approach that simultaneously inverts for the velocity in the symmetry axis direction, [Formula: see text] and [Formula: see text]. We derived a set of reflection tomography equations for slowness in the symmetry axis direction and Thomsen parameters [Formula: see text] and [Formula: see text]. In order to address the nonuniqueness of the tomography, we developed a regularization strategy that uses an independent regularization operator and regularization factor for each individual anisotropy parameter. Synthetic tests found that ambiguity exists between the anisotropy parameters and that velocity has a better resolution than [Formula: see text] and [Formula: see text]. They also confirmed that joint tomography provides a better data fit than single parameter tomography. The field example was used to test a way to incorporate the sonic data in the model building process and limit the tomographic updates on certain anisotropy parameters by adjusting the regularization.
5
Hadden, Shaun, R. Gerhard Pratt et Brendan Smithyman. « Anisotropic full-waveform inversion of crosshole seismic data : A vertical symmetry axis field data application ». GEOPHYSICS 84, no 1 (1 janvier 2019) : B15—B32. http://dx.doi.org/10.1190/geo2017-0790.1.
Texte intégralRésumé :
Anisotropic waveform tomography (AWT) uses anisotropic traveltime tomography followed by anisotropic full-waveform inversion (FWI). Such an approach is required for FWI in cases in which the geology is likely to exhibit anisotropy. An important anisotropy class is that of transverse isotropy (TI), and the special case of TI media with a vertical symmetry axis (VTI) media is often used to represent elasticity in undeformed sedimentary layering. We have developed an approach for AWT that uses an acoustic approximation to simulate waves in VTI media, and we apply this approach to crosshole data. In our approach, the best-fitting models of seismic velocity and Thomsen VTI anisotropy parameters are initially obtained using anisotropic traveltime tomography, and they are then used as the starting models for VTI FWI within the acoustic approximation. One common problem with the acoustic approach to TI media is the generation of late-arriving (spurious) S-waves as a by-product of the equation system. We used a Laplace-Fourier approach that effectively damps the spurious S-waves to suppress artifacts that might otherwise corrupt the final inversion results. The results of applying AWT to synthetic data illustrate the trade-offs in resolution between the two parameter classes of velocity and anisotropy, and they also verify anisotropic traveltime tomography as a valid method for generating starting models for FWI. The synthetic study further indicates the importance of smoothing the anisotropy parameters before proceeding to FWI inversions of the velocity parameter. The AWT technique is applied to real crosshole field gathers from a sedimentary environment in Western Canada, and the results are compared with the results from a simpler (elliptical) anisotropy model. The transversely isotropic approach yields an FWI image of the vertical velocity that (1) exhibits a superior resolution and (2) better predicts the field data than does the elliptical approach.
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Michelena, Reinaldo J. « Singular value decomposition for cross‐well tomography ». GEOPHYSICS 58, no 11 (novembre 1993) : 1655–61. http://dx.doi.org/10.1190/1.1443381.
Texte intégralRésumé :
I perform singular value decomposition (SVD) on the matrices that result in tomographic velocity estimation from cross‐well traveltimes in isotropic and anisotropic media. The slowness model is parameterized in four ways: One‐dimensional (1-D) isotropic, 1-D anisotropic, two‐dimensional (2-D) isotropic, and 2-D anisotropic. The singular value distribution is different for the different parameterizations. One‐dimensional isotropic models can be resolved well but the resolution of the data is poor. One‐dimensional anisotropic models can also be resolved well except for some variations in the vertical component of the slowness that are not sensitive to the data. In 2-D isotropic models, “pure” lateral variations are not sensitive to the data, and when anisotropy is introduced, the result is that the horizontal and vertical component of the slowness cannot be estimated with the same spatial resolution because the null space is mostly related to horizontal and high frequency variations in the vertical component of the slowness. Since the distribution of singular values varies depending on the parametrization used, the effect of conventional regularization procedures in the final solution may also vary. When the model is isotropic, regularization translates into smoothness, and when the model is anisotropic regularization not only smooths but may also alter the anisotropy in the solution.
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Gao, Zirui, Manuel Guizar-Sicairos, Viviane Lutz-Bueno, Aileen Schröter, Marianne Liebi, Markus Rudin et Marios Georgiadis. « High-speed tensor tomography : iterative reconstruction tensor tomography (IRTT) algorithm ». Acta Crystallographica Section A Foundations and Advances 75, no 2 (6 février 2019) : 223–38. http://dx.doi.org/10.1107/s2053273318017394.
Texte intégralRésumé :
The recent advent of tensor tomography techniques has enabled tomographic investigations of the 3D nanostructure organization of biological and material science samples. These techniques extended the concept of conventional X-ray tomography by reconstructing not only a scalar value such as the attenuation coefficient per voxel, but also a set of parameters that capture the local anisotropy of nanostructures within every voxel of the sample. Tensor tomography data sets are intrinsically large as each pixel of a conventional X-ray projection is substituted by a scattering pattern, and projections have to be recorded at different sample angular orientations with several tilts of the rotation axis with respect to the X-ray propagation direction. Currently available reconstruction approaches for such large data sets are computationally expensive. Here, a novel, fast reconstruction algorithm, named iterative reconstruction tensor tomography (IRTT), is presented to simplify and accelerate tensor tomography reconstructions. IRTT is based on a second-rank tensor model to describe the anisotropy of the nanostructure in every voxel and on an iterative error backpropagation reconstruction algorithm to achieve high convergence speed. The feasibility and accuracy of IRTT are demonstrated by reconstructing the nanostructure anisotropy of three samples: a carbon fiber knot, a human bone trabecula specimen and a fixed mouse brain. Results and reconstruction speed were compared with those obtained by the small-angle scattering tensor tomography (SASTT) reconstruction method introduced by Liebiet al.[Nature(2015),527, 349–352]. The principal orientation of the nanostructure within each voxel revealed a high level of agreement between the two methods. Yet, for identical data sets and computer hardware used, IRTT was shown to be more than an order of magnitude faster. IRTT was found to yield robust results, it does not require prior knowledge of the sample for initializing parameters, and can be used in cases where simple anisotropy metrics are sufficient,i.e.the tensor approximation adequately captures the level of anisotropy and the dominant orientation within a voxel. In addition, by greatly accelerating the reconstruction, IRTT is particularly suitable for handling large tomographic data sets of samples with internal structure or as a real-time analysis tool during the experiment for online feedback during data acquisition. Alternatively, the IRTT results might be used as an initial guess for models capturing a higher complexity of structural anisotropy such as spherical harmonics based SASTT in Liebiet al.(2015), improving both overall convergence speed and robustness of the reconstruction.
8
Leinss, S., H. Löwe, M. Proksch, J. Lemmetyinen, A. Wiesmann et I. Hajnsek. « Anisotropy of seasonal snow measured by polarimetric phase differences in radar time series ». Cryosphere Discussions 9, no 6 (5 novembre 2015) : 6061–123. http://dx.doi.org/10.5194/tcd-9-6061-2015.
Texte intégralRésumé :
Abstract. Snow settles under the force of gravity and recrystallizes by vertical temperature gradients. Both effects are assumed to form oriented ice crystals which induce an anisotropy in mechanical, thermal, and dielectric properties of the snow pack. On microscopic scales, the anisotropy could be hitherto determined only from stereology or computer tomography of samples taken from snow pits. In this paper we present an alternative method and show how the anisotropy of a natural snow pack can be observed contact- and destruction-free with polarimetric radar measurements. The copolar phase differences (CPD) of polarized microwaves transmitted through dry snow were analyzed for four winter seasons (2009–2013) from the SnowScat Instrument, installed at a test site near the town of Sodankylä, Finnland. An electrodynamic model was established based on anisotropic optics and on Maxwell–Garnett-type mixing formulas to provide a link between the structural anisotropy and the measured CPD. The anisotropy values derived from the CPD were compared with in-situ anisotropy measurements obtained by computer tomography. In addition, we show that the CPD measurements obtained from SnowScat show the same temporal evolution as space-borne CPD measurements from the satellite TerraSAR-X. The presented dataset provides a valuable basis for the future development of snow models capable of including the anisotropic structure of snow.
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Pratt, R. Gerhard, et Mark S. Sams. « Reconciliation of crosshole seismic velocities with well information in a layered sedimentary environment ». GEOPHYSICS 61, no 2 (mars 1996) : 549–60. http://dx.doi.org/10.1190/1.1443981.
Texte intégralRésumé :
In sedimentary environments, horizontal fine layering can cause significant complications when analyzing and comparing seismic data with different frequencies and propagation directions. At the Whitchester test site, three boreholes penetrate upper Carboniferous cyclical sediments, involving interbedded carbonates, sandstones, and mudstones, with seismic velocities ranging from less than 3.0 km/s in the mudstones to over 4.5 km/s in the carbonates. Initial crosshole results from two boreholes approximately 200 m deep and 75 m apart showed a poor correlation with the borehole logs recorded in a third, intermediate borehole. Furthermore, the tomographic images were inconsistent with the expected geology. The objective of this paper is to reconcile the crosshole seismic data with the borehole log information. The borehole information must be upscaled to match the resolution of the crosshole experiment. However, upscaling techniques based on Backus averaging lead to the prediction of significant anisotropy (of the order of 20% in places) with a vertical symmetry axis. This anisotropy is a result of a combination of fine layering and the intrinsic mineral anisotropy measured on the core samples, although it is the layer‐induced anisotropy that dominates at the crosshole frequencies of 400 Hz. This prediction was tested by including anisotropy parameters into the analysis of the crosshole data, using anisotropic velocity tomography. In the central part of the final anisotropic velocity tomograms, where the ray coverage is adequate, these crosshole results are consistent with the anisotropy predictions. Once the anisotropy was properly accounted for, the tomographic images are consistent with the layered nature of the geology at the site, and they show the location and throw of a known fault in the section. We conclude that, at this site and at crosshole frequencies, seismic layer‐induced anisotropy plays a significant role and must be accounted for when processing and interpreting crosshole seismic data.
10
Han, S.-M., et J.-Y. Rho. « Dependence of broadband ultrasound attenuation on the elastic anisotropy of trabecular bone ». Proceedings of the Institution of Mechanical Engineers, Part H : Journal of Engineering in Medicine 212, no 3 (1 mars 1998) : 223–26. http://dx.doi.org/10.1243/0954411981534006.
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The effect of trabecular elastic anisotropy on broadband ultrasound attenuation (BUA) and bone mineral density (BMD) was investigated with human and bovine cubic cancellous bones. Ultrasonic parameters describing trabecular anisotropy were found from the three orthogonal ultrasound velocities. BMD was measured using quantitative computed tomography. Three elastic anisotropy ratios were compared to BUA in all three directions and to BMD. The combined effect of anisotropic characteristics and BMD was also correlated with BUA. The results showed that the anisotropy ratios were significantly related to BUA (p<0.05). There was, however, no correlation between BMD and the elastic anisotropy ratios. The combination of BMD and the anisotropy produced a significantly enhanced relationship with BUA.
11
Leinss, Silvan, Henning Löwe, Martin Proksch, Juha Lemmetyinen, Andreas Wiesmann et Irena Hajnsek. « Anisotropy of seasonal snow measured by polarimetric phase differences in radar time series ». Cryosphere 10, no 4 (17 août 2016) : 1771–97. http://dx.doi.org/10.5194/tc-10-1771-2016.
Texte intégralRésumé :
Abstract. The snow microstructure, i.e., the spatial distribution of ice and pores, generally shows an anisotropy which is driven by gravity and temperature gradients and commonly determined from stereology or computer tomography. This structural anisotropy induces anisotropic mechanical, thermal, and dielectric properties. We present a method based on radio-wave birefringence to determine the depth-averaged, dielectric anisotropy of seasonal snow with radar instruments from space, air, or ground. For known snow depth and density, the birefringence allows determination of the dielectric anisotropy by measuring the copolar phase difference (CPD) between linearly polarized microwaves propagating obliquely through the snowpack. The dielectric and structural anisotropy are linked by Maxwell–Garnett-type mixing formulas. The anisotropy evolution of a natural snowpack in Northern Finland was observed over four winters (2009–2013) with the ground-based radar instrument "SnowScat". The radar measurements indicate horizontal structures for fresh snow and vertical structures in old snow which is confirmed by computer tomographic in situ measurements. The temporal evolution of the CPD agreed in ground-based data compared to space-borne measurements from the satellite TerraSAR-X. The presented dataset provides a valuable basis for the development of new snow metamorphism models which include the anisotropy of the snow microstructure.
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Zhang, Chao, Xiangzhuang Kong, Xian Wang, Yanxia Du et Guangming Xiao. « A Predicting Model for the Effective Thermal Conductivity of Anisotropic Open-Cell Foam ». Energies 15, no 16 (22 août 2022) : 6091. http://dx.doi.org/10.3390/en15166091.
Texte intégralRésumé :
The structural anisotropy of open-cell foam leads to the anisotropy of effective thermal conductivity (ETC). To quantitatively analyze the effect of structural anisotropy on the anisotropy of ETC, a new predicting model for the ETC of anisotropic open-cell foam was proposed based on an anisotropy tetrakaidecahedron cell (ATC). Feret diameters in three orthogonal directions obtained by morphological analysis of real foam structures were used to characterize the anisotropy of ATC. To validate our proposed anisotropic model, the ETCs of real foam structures in three orthogonal directions predicted by it were compared with the numerical results, for which the structures of numerical models are reconstructed by X-ray computed tomography (X-CT). Using the present anisotropic model, the influences of the thermal conductivity ratio (TCR) and porosity of the foams on the anisotropic ratios of ETCs are also investigated. Results show that there is good consistency between the ETCs obtained by the anisotropic model and the numerical method. The maximum relative errors between them are 2.84% and 13.57% when TCRs are 10 and 100, respectively. The present anisotropic model can not only predict the ETCs in different orthogonal directions but also quantitatively predict the anisotropy of ETC. The anisotropies of the ETCs decrease with porosity because the proportion of the foam skeleton decreases. However, the anisotropies of ETCs increase with TCR, and there exist asymptotic values in anisotropic ratios of ETCs as TCR approaches infinity and they are equal to the relative Feret diameters in different orthogonal directions.
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Gosselin, Jeremy M., Pascal Audet, Andrew J. Schaeffer, Fiona A. Darbyshire et Clément Estève. « Azimuthal anisotropy in Bayesian surface wave tomography : application to northern Cascadia and Haida Gwaii, British Columbia ». Geophysical Journal International 224, no 3 (20 novembre 2020) : 1724–41. http://dx.doi.org/10.1093/gji/ggaa561.
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SUMMARY Surface wave tomography is a valuable tool for constraining azimuthal anisotropy at regional scales. However, sparse and uneven coverage of dispersion measurements make meaningful uncertainty estimation challenging, especially when applying subjective model regularization. This paper considers azimuthal anisotropy constrained by measurements of surface wave dispersion data within a Bayesian trans-dimensional (trans-d) tomographic inversion. A recently proposed alternative model parametrization for trans-d inversion is implemented in order to produce more realistic models than previous studies considering trans-d surface wave tomography. The reversible-jump Markov chain Monte Carlo sampling technique is used to numerically estimate the posterior probability density of the model parameters. Isotropic and azimuthally anisotropic components of surface wave group velocity maps (and their associated uncertainties) are estimated while avoiding model regularization and allowing model complexity to be determined by the data information content. Furthermore, data errors are treated as unknown, and solved for within the inversion. The inversion method is applied to measurements of surface wave dispersion from regional earthquakes recorded over northern Cascadia and Haida Gwaii, a region of complex active tectonics but highly heterogeneous station coverage. Results for isotropic group velocity are consistent with previous studies that considered the southern part of the study region over Cascadia. Azimuthal anisotropic fast-axis directions are generally margin-parallel between Vancouver Island and Haida Gwaii, with a small change in direction and magnitude along the margin which may be attributed to the changing tectonic regime (from subduction to transform tectonics). Estimated errors on the dispersion data (solved for within the inversion) reveal a correlation between surface wave period and the dependence of data errors on travel path length. This paper demonstrates the value of considering azimuthal anisotropy within Bayesian tomographic inversions. Furthermore, this work provides structural context for future studies of tectonic structure and dynamics of northern Cascadia and Haida Gwaii, with the aim of improving our understanding of seismic and tsunami hazards.
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VanderBeek, Brandon P., et Manuele Faccenda. « Imaging upper mantle anisotropy with teleseismic P-wave delays : insights from tomographic reconstructions of subduction simulations ». Geophysical Journal International 225, no 3 (1 mars 2021) : 2097–119. http://dx.doi.org/10.1093/gji/ggab081.
Texte intégralRésumé :
SUMMARY Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodelled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities. Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy. However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as synthetic testing has been restricted to the recovery of simplified block-like structures using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm). Here, we present a modified parametrization for imaging arbitrarily oriented hexagonal anisotropy and test the method by reconstructing geodynamic simulations of subduction. Our inversion approach allows for isotropic starting models and includes approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Synthetic seismic data are created by propagating teleseismic waves through an elastically anisotropic subduction zone model created via petrologic-thermomechanical modelling. Delay times across a synthetic seismic array are measured using conventional cross-correlation techniques. We find that our imaging algorithm is capable of resolving large-scale features in subduction zone anisotropic structure (e.g. toroidal flow pattern and dipping fabrics associated with the descending slab). Allowing for arbitrarily oriented anisotropy also results in a more accurate reconstruction of isotropic slab structure. In comparison, models created assuming isotropy or only azimuthal anisotropy contain significant isotropic and anisotropic imaging artefacts that may lead to spurious interpretations. We conclude that teleseismic P-wave traveltimes are a useful observable for probing the 3-D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies particularly in subduction settings.
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Puro, A. �. « Reconstructive tomography with weak optical anisotropy ». Journal of Applied Mechanics and Technical Physics 32, no 2 (1991) : 252–55. http://dx.doi.org/10.1007/bf00858045.
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Shalaginov, Aleksandr, Nina Nevedrova, Aidisa Sanchaa, Ilya Shaparenko et Petr Ponomarev. « ELECTRICAL ANISOTROPY ACCORDING TO DC METHODS IN THE BYSTROVKA FIELD AREA (SHORE RESERVOIR IN THE NOVOSIBIRSK REGION) ». Interexpo GEO-Siberia 2, no 2 (2019) : 158–64. http://dx.doi.org/10.33764/2618-981x-2019-2-2-158-164.
Texte intégralRésumé :
The article presents the results of the study of electrical anisotropy by two methods of DC (vertical electrical sounding and electrical tomography) in the area of the Bystrovka field area, on the shore of the Novosibirsk reservoir. Taking into account a priori well data, the parameters of the geoelectric model and the anisotropic characteristics of the section are determined. On the site of the study, according to the data of two methods, electrical anisotropy observed in the reference geoelectric horizon, represented by shale. In addition, the main direction of crack propagation is determined.
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Woodward, Marta Jo, Dave Nichols, Olga Zdraveva, Phil Whitfield et Tony Johns. « A decade of tomography ». GEOPHYSICS 73, no 5 (septembre 2008) : VE5—VE11. http://dx.doi.org/10.1190/1.2969907.
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Over the past 10 years, ray-based postmigration grid tomography has become the standard model-building tool for seismic depth imaging. While the basics of the method have remained unchanged since the late 1990s, the problems it solves have changed dramatically. This evolution has been driven by exploration demands and enabled by computer power. There are three main areas of change. First, standard model resolution has increased from a few thousand meters to a few hundred meters. This order of magnitude improvement may be attributed to both high-quality, complex residual-moveout data picked as densely as [Formula: see text] to [Formula: see text] vertically and horizontally, and to a strategy of working down from long-wavelength to short-wavelength solutions. Second, more and more seismic data sets are being acquired along multiple azimuths, for improved illumination and multiple suppression. High-resolution velocity tomography must solve for all azimuths simultaneously, to prevent short-wavelength velocity heterogeneity from being mistaken for azimuthal anisotropy. Third, there has been a shift from predominantly isotropic to predominantly anisotropic models, both VTI and TTI. With four-component data, anisotropic grid tomography can be used to build models that tie PZ and PS images in depth.
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Rao, Ying, Yanghua Wang, Shumin Chen et Jianmin Wang. « Crosshole seismic tomography with cross-firing geometry ». GEOPHYSICS 81, no 4 (juillet 2016) : R139—R146. http://dx.doi.org/10.1190/geo2015-0677.1.
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We have developed a case study of crosshole seismic tomography with a cross-firing geometry in which seismic sources were placed in two vertical boreholes alternatingly and receiver arrays were placed in another vertical borehole. There are two crosshole seismic data sets in a conventional sense. These two data sets are used jointly in seismic tomography. Because the local sediment is dominated by periodic, flat, thin layers, there is seismic anisotropy with different velocities in the vertical and horizontal directions. The vertical transverse isotropy anisotropic effect is taken into account in inversion processing, which consists of three stages in sequence. First, isotropic traveltime tomography is used for estimating the maximum horizontal velocity. Then, anisotropic traveltime tomography is used to invert for the anisotropic parameter, which is the normalized difference between the maximum horizontal velocity and the maximum vertical velocity. Finally, anisotropic waveform tomography is implemented to refine the maximum horizontal velocity. The cross-firing acquisition geometry significantly improves the ray coverage and results in a relatively even distribution of the ray density in the study area between two boreholes. Consequently, joint inversion of two crosshole seismic data sets improves the resolution and increases the reliability of the velocity model reconstructed by tomography.
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Pandey, Biswajit. « Tomography of stellar halos : what does anisotropy in a stellar halo tell us ? » Journal of Cosmology and Astroparticle Physics 2022, no 10 (1 octobre 2022) : 058. http://dx.doi.org/10.1088/1475-7516/2022/10/058.
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Abstract The stellar halo of the Milky Way is known to have a highly lumpy structure due to the presence of tidal debris and streams accreted from the satellite galaxies. The abundance and distribution of these substructures can provide a wealth of information on the assembly history of the Milky Way. We use some information-theoretic measures to study the anisotropy in a set of Milky Way-sized stellar halos from the Bullock & Johnston suite of simulations that uses a hybrid approach coupling semi-analytic and N-body techniques. Our analysis shows that the whole-sky anisotropy in each stellar halo increases with the distance from its centre and eventually plateaus out beyond a certain radius. All the stellar halos have a very smooth structure within a radius of ∼ 50 kpc and a highly anisotropic structure in the outskirts. At a given radius, the anisotropies at a fixed polar or azimuthal angle have two distinct components: (i) an approximately isotropic component and (ii) a component with large density fluctuations on small spatial scales. We remove the contributions of the substructures and any non-spherical shape of the halo by randomizing the polar and azimuthal coordinates of the stellar particles while keeping their radial distances fixed. We observe that the fluctuating part of the anisotropy is completely eliminated, and the approximately uniform component of the anisotropy is significantly reduced after the sphericalization. A comparison between the original halos and their sphericalized versions reveals that the approximately uniform part of the anisotropy originates from the discreteness noise and the non-spherical shape of the halo whereas the substructures contribute to the fluctuating part. We show that such distinction between the anisotropies has the potential to constrain the shape of the stellar halo and its substructures.
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Yang, Eomzi, Tae Sup Yun, Kwang Yeom Kim, Seong Woo Moon et Yong-Seok Seo. « Estimation of the Structural and Geomechanical Anisotropy in Fault Gouges Using 3D Micro-Computed Tomography (μ-CT) ». Sensors 20, no 17 (20 août 2020) : 4706. http://dx.doi.org/10.3390/s20174706.
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Fault gouges play an important role in the shear deformation of fault zones, by causing weakness and frictional instability in structures. Previous studies have investigated the evolution of shear deformation of fault zones by observing experiments using remolded and synthetic gouge specimens at a micro-scale. However, how the spatial configuration of the rock constituents accounts for the 3D anisotropy of intact structures of fault gouges, particularly at the core-scale, is not well understood. We obtained 3D μ-CT images of directionally cored gouge specimens and performed statistical analysis to quantify the major orientation of the internal structures. Direct shear tests were conducted to investigate the relationship between the distribution of the internal structures and geomechanical behavior. The results show that the undisturbed fault gouge has a clear anisotropy parallel to the fault plane even at the core-scale. Moreover, the direct shear test results show that the frictional resistance of a fault gouge has anisotropy related to the fault plane. The simple, yet robust method proposed in this study confirms that the core-scale structural anisotropy is correlated to the anisotropic shear resistance.
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David, Marcelo, Omer Amran, Ron Simhi et Franco Simini. « Time-Domain Electrical Impedance Tomography by Numerical Analysis of the Step Response ». Journal of Physics : Conference Series 2008, no 1 (1 août 2021) : 012019. http://dx.doi.org/10.1088/1742-6596/2008/1/012019.
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Abstract This work describes the theoretical basis of an electrical impedance tomography imaging system based on numerical analysis of the step response. Its novelty relies on the use of time domain for rendering the tomographic images. Following the injection of a Heaviside-step current through two electrodes, the voltage-response is measured on all couple of electrodes according to the neighbouring strategy; this process is repeated on every pair of consecutive electrodes. Based on the measurements, a tomographic image is reconstructed using the Gauss-Newton-Raphson algorithm. We tested the technique by simulating two representative circuits: one symmetrical pseudo-isotropic and one pseudo-anisotropic in AC, while both pseudo-isotropic at DC. The time-domain reconstructed images show the second network’s pseudo-anisotropy while allowing the system to show its tendency to pseudo-isotropy when the time elapses towards DC-steady-state. This novel technique for reconstructing electrical impedance tomographic images may shed new light on sensing slight differences in tissues while being fast and low-cost.
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Chen, Liang, Qiang Xiao, Wei Liang, Jingxian Hong et Xingjiang Zou. « A time-of-flight revising approach to improve the image quality of Lamb wave tomography for the detection of defects in composite panels ». Science and Engineering of Composite Materials 25, no 3 (25 avril 2018) : 587–92. http://dx.doi.org/10.1515/secm-2015-0399.
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Abstract Lamb wave tomography can be used to evaluate structural integrity. The time-of-flight (TOF) data are usually recorded as input to the reconstruction algorithm. For composite materials, TOF estimation is complicated due to their anisotropy. To reduce the effects of anisotropy on image reconstruction, the TOF data of flawed plates are revised according to baseline data obtained from an unflawed plate. Tomographic images are reconstructed using the original and revised TOF data, respectively. Results show that images reconstructed using the revised TOF data have better visual quality and that TOF data revision can substantially reduce the artifacts resulting from anisotropy in defect detection of composite materials.
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Kästle, E. D., I. Molinari, L. Boschi et E. Kissling. « Azimuthal anisotropy from eikonal tomography : example from ambient-noise measurements in the AlpArray network ». Geophysical Journal International 229, no 1 (1 novembre 2021) : 151–70. http://dx.doi.org/10.1093/gji/ggab453.
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SUMMARY Ambient-noise records from the AlpArray network are used to measure Rayleigh wave phase velocities between more than 150 000 station pairs. From these, azimuthally anisotropic phase-velocity maps are obtained by applying the eikonal tomography method. Several synthetic tests are shown to study the bias in the Ψ2 anisotropy. There are two main groups of bias, the first one caused by interference between refracted/reflected waves and the appearance of secondary wave fronts that affect the phase traveltime measurements. This bias can be reduced if the amplitude field can be estimated correctly. Another source of error is related to the incomplete reconstruction of the traveltime field that is only sparsely sampled due to the receiver locations. Both types of bias scale with the magnitude of the velocity heterogeneities. Most affected by the spurious Ψ2 anisotropy are areas inside and at the border of low-velocity zones. In the isotropic velocity distribution, most of the bias cancels out if the azimuthal coverage is good. Despite the lack of resolution in many parts of the surveyed area, we identify a number of anisotropic structures that are robust: in the central Alps, we find a layered anisotropic structure, arc-parallel at mid-crustal depths and arc-perpendicular in the lower crust. In contrast, in the eastern Alps, the pattern is more consistently E–W oriented which we relate to the eastward extrusion. The northern Alpine forleand exhibits a preferential anisotropic orientation that is similar to SKS observations in the lowermost crust and uppermost mantle.
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Selvakumaran, Lakshmi, et Gilles Lubineau. « Validation of Micro-Meso Electrical Relations for Laminates with Varying Anisotropy ». Applied Mechanics and Materials 784 (août 2015) : 435–42. http://dx.doi.org/10.4028/www.scientific.net/amm.784.435.
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For electrical impedance tomography (EIT) to be useful in monitoring transverse cracks in composites, it is imperative to establish the relation between conductivity and cracking density. Micro to meso scale homogenization has been developed for classical carbon fiber reinforced polymer (CFRP) laminate which provides such a relationship. However, we have shown in previous studies that the detectability of transverse cracks in such CFRP, which are characterized by very anisotropic electrical properties, is poor. Then, it is better to lower the electrical anisotropy, which can be achieved by various technologies including doping the polymeric resin by conductive nanoparticles. However, the validity of mesoscale homogenization for laminates with such low anisotropy has not been tested before. Here, we show that the mesoscale damage indicator is intrinsic for composites with varying anisotropy.
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Vasco, Don W., John E. Peterson et Ki Ha Lee. « Ground‐penetrating radar velocity tomography in heterogeneous and anisotropic media ». GEOPHYSICS 62, no 6 (novembre 1997) : 1758–73. http://dx.doi.org/10.1190/1.1444276.
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A ray series solution for Maxwell's equations provides an efficient numerical technique for calculating wavefronts and raypaths associated with electromagnetic waves in anisotropic media. Using this methodology and assuming weak anisotropy, we show that a perturbation of the anisotropic structure may be related linearly to a variation in the traveltime of an electromagnetic wave. Thus, it is possible to infer lateral variations in the dielectric permittivity and magnetic permeability matrices. The perturbation approach is used to analyze a series of crosswell ground‐penetrating radar surveys conducted at the Idaho National Engineering Laboratory. Several important geological features are imaged, including a rubble zone at the interface between two basalt flows. Linear low‐velocity anomalies are imaged clearly and are continuous across well pairs.
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Liu, Yongsheng, et Ping Tong. « Eikonal equation-based P-wave seismic azimuthal anisotropy tomography of the crustal structure beneath northern California ». Geophysical Journal International 226, no 1 (13 mars 2021) : 287–301. http://dx.doi.org/10.1093/gji/ggab103.
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SUMMARY Delineating spatial variations of seismic anisotropy in the crust is of great importance for the understanding of structural heterogeneities, regional stress regime and ongoing crustal dynamics. In this study, we present a 3-D anisotropic P-wave velocity model of the crust beneath northern California by using the eikonal equation-based seismic azimuthal anisotropy tomography method. The velocity heterogeneities under different geological units are well resolved. The thickness of the low-velocity sediment at the Great Valley Sequence is estimated to be about 10 km. The high-velocity anomaly underlying Great Valley probably indicates the existence of ophiolite bodies. Strong velocity contrasts are revealed across the Hayward Fault (2–9 km) and San Andreas Fault (2–12 km). In the upper crust (2–9 km), the fast velocity directions (FVDs) are generally fault-parallel in the northern Coast Range, which may be caused by geological structure; while the FVDs are mainly NE–SW in Great Valley and the northern Sierra Nevada possibly due to the regional maximum horizontal compressive stress. In contrast, seismic anisotropy in the mid-lower crust (12–22 km) may be attributed to the alignment of mica schists. The anisotropy contrast across the San Andreas Fault may imply different mechanisms of crustal deformation on the two sides of the fault. Both the strong velocity contrasts and the high angle (∼45° or above) between the FVDs and the strikes of faults suggest that the faults are mechanically weak in the San Francisco bay area (2–6 km). This study suggests that the eikonal equation-based seismic azimuthal anisotropy tomography is a valuable tool to investigate crustal heterogeneities and tectonic deformation.
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Mastio, Nicolas, Pierre Thore, Marianne Conin et Guillaume Caumon. « Determination of a stress-dependent rock-physics model using anisotropic time-lapse tomographic inversion ». GEOPHYSICS 85, no 4 (10 juin 2020) : C141—C152. http://dx.doi.org/10.1190/geo2019-0526.1.
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In the petroleum industry, time-lapse (4D) studies are commonly used for reservoir monitoring, but they are also useful to perform risk assessment for potential overburden deformations (e.g., well shearing, cap-rock integrity). Although complex anisotropic velocity changes are predicted in the overburden by geomechanical studies, conventional time-lapse inversion workflows only deal with vertical velocity changes. To retrieve the geomechanically induced anisotropy, we have adopted a reflection traveltime tomography method coupled with a time-shift estimation algorithm of prestack data of the baseline and monitor simultaneously. For the 2D approach, we parameterize the anisotropy using five coefficients, enough to cover any type of anisotropy. Before applying the workflow to a real data set, we first study a synthetic data set based on the real data set and include velocity variations between baseline and monitor found in the literature (vertical P-wave velocity decrease in the cap rock and isotropic P-wave velocity change in the reservoir). On the synthetics, we measure the angular ray coverage necessary to retrieve the target anisotropy and observe that the retrieved anisotropies depend on the offset range. Based on a synthetic experiment, we believe that the acquisition of the real case study is suitable for performing tomographic inversion. The anisotropic velocity changes obtained on three inlines separated by 375 m are consistent and show a strong positive anomaly in the cap rock along the 45° direction (the [Formula: see text] parameter in Thomsen notation), whereas the vertical velocity change is surprisingly almost negligible. We adopt a rock-physics explanation compatible with these observations and geologic considerations: a reactivation of water-filled subvertical cracks.
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Peng, Chengbin (Chuck), et Jun Tang. « Automatic early arrival traveltime tomography and its applications ». GEOPHYSICS 82, no 2 (1 mars 2017) : U1—U11. http://dx.doi.org/10.1190/geo2016-0303.1.
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We have developed a method of macrovelocity inversion that does not require explicit picking of either common-image point gathers or first breaks. The method uses head waves, diving waves, and wide-angle reflections in seismic data (collectively early arrival energies) for accurate estimation of velocity and anisotropy parameters. In this method, seismic data are first decomposed into Gaussian packets. Packets associated with early arrival energies are selected and used as input to a tomography solver. The outputs of the solver are velocity and Thomsen’s anisotropy parameters, or any of their combinations. Using information contained in the packets, we can correctly model the early arrival energies (first breaks and/or other refractions). The workflow is fully automatic and can be used in a batch processing environment with minimum human intervention. We have tested the method on synthetic and field data sets. In one synthetic test, we were able to reduce traveltime residuals of diving waves from 400 to 5 ms and recover anisotropic model parameters that are sensitive to early arrival traveltimes. In another synthetic test, we were able to recover a large shallow low-velocity anomaly with a very simple starting velocity model. The first field data set was for a shallow marine seismic data project. We were able to obtain a better shallow velocity model using our method than when using a legacy approach. In the second field data test, we applied our method on a deepwater data set from a dual-coil acquisition, with full-azimuth and long-offset coverage. Our method can correctly model early arrival energies recorded at long offsets and use them in the iterative inversion such that better estimation of velocities and anisotropy parameters in shallow sediments can be achieved. We have tested different starting models for the inversion. We are able to get very similar results, suggesting that our method is not sensitive to the accuracy of a starting model.
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Palmer, Derecke. « The measurement of weak anisotropy with the generalized reciprocal method ». GEOPHYSICS 65, no 5 (septembre 2000) : 1583–91. http://dx.doi.org/10.1190/1.1444846.
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Anisotropy parameters can be determined from seismic refraction data using the generalized reciprocal method (GRM) for a layer in which the velocity can be described with the Crampin approximation for transverse isotropy. The parameters are the standard anisotropy factor, which is the horizontal velocity divided by the vertical velocity, and a second poorly determined parameter which, for weak anisotropy, is approximated by a linear relationship with the anisotropy factor. Although only one anisotropy parameter is effectively determined, the second parameter is essential to ensure that the anisotropy does not degenerate to the elliptical condition which is indeterminate using the approach described in this paper. The anisotropy factor is taken as the value for which the phase velocity at the critical angle given by the Crampin equation is equal to the average velocity computed with the optimum XY value obtained from a GRM analysis of the refraction data. The anisotropy parameters can be used to improve the estimate of the refractor velocity, which can exhibit marked dip effects when the overlying layer is anisotropic. In a model study, depths computed with the phase velocity at the critical angle are within 3% of the true values, whereas those calculated with the horizontal phase velocity (which assumes isotropy) are greater than the true depths by about 25%. Anisotropy illustrates the pitfalls of model‐based inversion strategies, which seek agreement between the travetime data and the computed response of the model. With anisotropic layers, the traveltime data provide the seismic velocity in the overlying layer in the horizontal direction, whereas the seismic velocity near the critical angle is required for depth computations. If anisotropy is applicable, then the GRM using the methods described in this paper is able to provide a good starting model for other approaches, such as refraction tomography.
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Wang, Xiaoxiang, et Ilya Tsvankin. « Ray-based gridded tomography for tilted transversely isotropic media ». GEOPHYSICS 78, no 1 (1 janvier 2013) : C11—C23. http://dx.doi.org/10.1190/geo2012-0066.1.
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Reflection tomography in the migrated domain can help reconstruct heterogeneous, anisotropic velocity fields needed for accurate depth imaging of complex geologic structures. The presence of anisotropy, however, increases the uncertainty in velocity analysis and typically requires a priori constraints on the model parameters. Here, we develop a 2D P-wave tomographic algorithm for heterogeneous transversely isotropic media with a tilted symmetry axis (TTI) and investigate the conditions necessary for stable estimation of the symmetry-direction velocity [Formula: see text] and the anisotropy parameters [Formula: see text] and [Formula: see text]. The model is divided into rectangular cells, and the parameters [Formula: see text], [Formula: see text], [Formula: see text], and the tilt [Formula: see text] of the symmetry axis are defined at the grid points. To increase the stability of the inversion, the symmetry axis is set orthogonal to the imaged reflectors, with the tilt interpolated inside each layer. The iterative migration velocity analysis involves efficient linearized parameter updating designed to minimize the residual moveout in image gathers for all available reflection events. The moveout equation in the depth-migrated domain includes a nonhyperbolic term that describes long-offset data, which are particularly sensitive to [Formula: see text]. Synthetic tests for models with a “quasi-factorized” TTI syncline (i.e., [Formula: see text] and [Formula: see text] are constant inside the anisotropic layer) and a TTI thrust sheet demonstrate that stable parameter estimation requires either strong smoothness constraints or additional information from walkaway VSP (vertical seismic profiling) traveltimes. If the model is quasi-factorized with a linear spatial variation of [Formula: see text], it may be possible to obtain the interval TTI parameters just from long-spread reflection data.
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Barber, Quinn M., et Roger J. Zemp. « Ultrasound Scattering Anisotropy Visualization With Ultrasound Tomography ». IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 64, no 2 (février 2017) : 335–39. http://dx.doi.org/10.1109/tuffc.2016.2622898.
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Rao, Ying, et Yanghua Wang. « Crosshole seismic tomography including the anisotropy effect ». Journal of Geophysics and Engineering 8, no 2 (28 avril 2011) : 316–21. http://dx.doi.org/10.1088/1742-2132/8/2/016.
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Perlin, Lourenço Panosso, Roberto Caldas de Andrade Pinto et Ângela do Valle. « Ultrasonic tomography in wood with anisotropy consideration ». Construction and Building Materials 229 (décembre 2019) : 116958. http://dx.doi.org/10.1016/j.conbuildmat.2019.116958.
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Maurer, Hansruedi, Sandy Isabelle Schubert, Fritz Bächle, Sebastian Clauss, Daniel Gsell, Jürg Dual et Peter Niemz. « A simple anisotropy correction procedure for acoustic wood tomography ». Holzforschung 60, no 5 (1 août 2006) : 567–73. http://dx.doi.org/10.1515/hf.2006.094.
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Abstract Anisotropy of acoustic propagation velocities is a ubiquitous feature of wood. This needs to be considered for successful application of travel time tomography, an increasingly popular technique for non-destructive testing of living trees. We have developed a simple correction scheme that removes first-order anisotropy effects. The corrected travel-time data can be inverted with isotropic inversion codes that are commercially available. Using a numerical experiment, we demonstrate the consequences of ignoring anisotropy effects and outline the performance of our correction scheme. The new technique has been applied to two spruce samples. Subsequent inspection of the samples revealed a good match with the tomograms.
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Lines, Larry, Henry Tan, Sven Treitel, John Beck, Richard Chambers, John Eager, Charles Savage et al. « Integrated reservoir characterization : Beyond tomography ». GEOPHYSICS 60, no 2 (mars 1995) : 354–64. http://dx.doi.org/10.1190/1.1443771.
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In 1992, there was a collaborative effort in reservoir geophysics involving Amoco, Conoco, Schlumberger, and Stanford University in an attempt to delineate variations in reservoir properties of the Grayburg unit in a West Texas [Formula: see text] pilot at North Cowden Field. Our objective was to go beyond traveltime tomography in characterizing reservoir heterogeneity and flow anisotropy. This effort involved a comprehensive set of measurements to do traveltime tomography, to image reflectors, to analyze channel waves for reservoir continuity, to study shear‐wave splitting for borehole stress‐pattern estimation, and to do seismic anisotropy analysis. All these studies were combined with 3-D surface seismic data and with sonic log interpretation. The results are to be validated in the future with cores and engineering data by history matching of primary, water, and [Formula: see text] injection performance. The implementation of these procedures should provide critical information on reservoir heterogeneities and preferential flow direction. Geophysical methods generally indicated a continuous reservoir zone between wells.
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Cagáň, Jan, Jaroslav Pelant, Martin Kyncl, Martin Kadlec et Lenka Michalcová. « Damage detection in carbon fiber–reinforced polymer composite via electrical resistance tomography with Gaussian anisotropic regularization ». Structural Health Monitoring 18, no 5-6 (29 décembre 2018) : 1698–710. http://dx.doi.org/10.1177/1475921718820013.
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Electrical resistance tomography is a method for sensing the spatial distribution of electrical conductivity. Therefore, this type of tomography is suitable for sensing damages, which affect electrical conductivity. The utilization of resistance tomography for the structural health monitoring of carbon fiber–reinforced polymer composites is questionable owing to its low spatial resolution and the strong anisotropy of carbon fiber–reinforced polymer composites. This article deals with the employment of resistance tomography with regularization based on a Gaussian anisotropic smoothing filter for the detection of cuts. The advantages of the filter are shown through the image reconstruction of rectangular composite specimens with three different laminate stacking sequences. The cuts are implemented by a milled groove. Visual comparison of the images shows a substantial improvement in the shape reconstruction ability. In addition to visual comparison, the image reconstructions are assessed in terms of the reconstruction error and cross-correlation.
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Williamson, Paul R. « On resolution and uniqueness in anisotropic crosshole traveltime tomography ». GEOPHYSICS 63, no 4 (juillet 1998) : 1184–89. http://dx.doi.org/10.1190/1.1444418.
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The inclusion of anisotropy in P-wave traveltime tomography has been undertaken by several authors and in all cases some loss of resolution and uniqueness compared to the isotropic problem was observed. The origin of this problem is analysed for straight‐ray tomography using the Radon transform and the projection slice theorem. This analysis shows that the separation of the anisotropy from the isotropic velocity field can only be guaranteed for the dc component. Resolution at higher spatial frequencies depends upon the spatial support of the object, that is the separation of the holes in crosshole work, and the range of ray angles available. Specific calculations in the crosshole case suggest that even in media known to be horizontally stratified, it will probably be difficult to estimate spatially varying elliptical anisotropy at wavelengths less than a few times smaller than the hole separation, and Thomsen’s parameter, delta at wavelengths less than the hole separation itself. These results broadly agree with the empirical observations in the literature.
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Ushenko, V. A., A. Yu Sdobnov, W. D. Mishalov, A. V. Dubolazov, O. V. Olar, V. T. Bachinskyi, A. G. Ushenko, Yu A. Ushenko, O. Ya Wanchuliak et I. Meglinski. « Biomedical applications of Jones-matrix tomography to polycrystalline films of biological fluids ». Journal of Innovative Optical Health Sciences 12, no 06 (novembre 2019) : 1950017. http://dx.doi.org/10.1142/s1793545819500172.
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Algorithms for reconstruction of linear and circular birefringence-dichroism of optically thin anisotropic biological layers are presented. The technique of Jones-matrix tomography of polycrystalline films of biological fluids of various human organs has been developed and experimentally tested. The coordinate distributions of phase and amplitude anisotropy of bile films and synovial fluid taken from the knee joint are determined and statistically analyzed. Criteria (statistical moments of 3rd and 4th orders) of differential diagnostics of early stages of cholelithiasis and septic arthritis of the knee joint with excellent balanced accuracy were determined. Data on the diagnostic efficiency of the Jones-matrix tomography method for polycrystalline plasma (liver disease), urine (albuminuria) and cytological smears (cervical cancer) are presented.
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Hamanaka, Senji, Chisato Nonomura, Thanh Binh Nguyen Thi et Atsushi Yokoyama. « Correlation between fiber orientation distribution and mechanical anisotropy in glass-fiber-reinforced composite materials ». Journal of Polymer Engineering 39, no 7 (26 juillet 2019) : 653–60. http://dx.doi.org/10.1515/polyeng-2018-0371.
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Abstract This study investigates the correlation between the fiber orientation distribution along the thickness and mechanical anisotropy in injection-molded products using a thermoplastic resin reinforced by short fibers. To this end, polyamide-6 samples containing 15, 30, 50, and 65 wt% of short fiberglass were compounded, and flat plates with side gates were injection-molded. The fiber orientation distribution near the center of the plates was observed via X-ray computed tomography and that along the thickness was quantified via a fiber orientation tensor. Coupon test pieces were cut from the plates along the machine and transverse directions, and a three-point bending test was performed. Mechanical anisotropy was evaluated from the ratio of the flexural modulus in each direction. Evaluation results of the fiber orientation distribution and mechanical anisotropy were compared. As a result of the above investigation, a clear correlation was found between the fiber orientation distribution and mechanical anisotropy when the glass fiber content was 15–50 wt%. In the anisotropic expression under the condition of high glass fiber content (65 wt%), contributions of parameters other than the fiber orientation distribution became evident.
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Liu, Yongsheng, Iman Suardi, Xueyuan Huang, Shaolin Liu et Ping Tong. « Seismic velocity and anisotropy tomography of southern Sumatra ». Physics of the Earth and Planetary Interiors 316 (juillet 2021) : 106722. http://dx.doi.org/10.1016/j.pepi.2021.106722.
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Kim, Jung-Ho, Seong-Jun Cho, Myeong-Jong Yi et Motoyuki Sato. « Application of anisotropy borehole radar tomography in Korea ». Near Surface Geophysics 4, no 1 (1 avril 2005) : 13–18. http://dx.doi.org/10.3997/1873-0604.2005027.
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Zhao, Dapeng, Sheng Yu et Xin Liu. « Seismic anisotropy tomography : New insight into subduction dynamics ». Gondwana Research 33 (mai 2016) : 24–43. http://dx.doi.org/10.1016/j.gr.2015.05.008.
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Lin, Yu-Pin, Li Zhao et Shu-Huei Hung. « Full-wave multiscale anisotropy tomography in Southern California ». Geophysical Research Letters 41, no 24 (23 décembre 2014) : 8809–17. http://dx.doi.org/10.1002/2014gl061855.
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Grazzi, Francesco, Carlo Cialdai, Marco Manetti, Mirko Massi, Maria Pia Morigi, Matteo Bettuzzi, Rosa Brancaccio et al. « A multi-technique tomography-based approach for non-invasive characterization of additive manufacturing components in view of vacuum/UHV applications : preliminary results ». Rendiconti Lincei. Scienze Fisiche e Naturali 32, no 3 (12 mai 2021) : 463–77. http://dx.doi.org/10.1007/s12210-021-00994-2.
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AbstractIn this paper, we have studied an additively manufactured metallic component, intended for ultra-high vacuum application, the exit-snout of the MACHINA transportable proton accelerator beam-line. Metal additive manufacturing components can exhibit heterogeneous and anisotropic microstructures. Two non-destructive imaging techniques, X-ray computed tomography and Neutron Tomography, were employed to examine its microstructure. They unveiled the presence of porosity and channels, the size and composition of grains and intergranular precipitates, and the general behavior of the spatial distribution of the solidification lines. While X-ray computed tomography evidenced qualitative details about the surface roughness and internal defects, neutron tomography showed excellent ability in imaging the spatial density distribution within the component. The anisotropy of the density was attributed to the material building orientation during the 3D printing process. Density variations suggest the possibility of defect pathways, which could affect high vacuum performances. In addition, these results highlight the importance of considering building orientation in the design for additive manufacturing for UHV applications. Graphical Abstract
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Kulenkampff, J., M. Gründig, A. Zakhnini, R. Gerasch et J. Lippmann-Pipke. « Process tomography of diffusion, using PET, to evaluate anisotropy and heterogeneity ». Clay Minerals 50, no 3 (août 2015) : 369–75. http://dx.doi.org/10.1180/claymin.2015.050.3.09.
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AbstractAnisotropy and compositional and structural heterogeneity in clays are causes of considerable deviations from homogeneous diffusion, in particular in terms of direction-dependent transport rates and preferred transport zones. Conventional diffusion experiments, in which the sample is treated as a homogeneous black box in a concentration gradient, are interminable and insensitive to spatial effects. In contrast, tomographic imaging methods are capable of both reducing the amount of observation time required and revealing space-dependent features of the diffusion process.In the present study, positron-emission-tomography (PET) was applied as the most sensitive quantitative spatiotemporal tomographic modality for direct observation of positron-emitting radiotracers in opaque media at reasonable resolution (1 mm) on a laboratory scale (100 mm).Geoscientific applications of PET, or GeoPET, have revealed anisotropic and heterogeneous effects in diffusion experiments that have been conducted on Opalinus clay samples of different sizes, as well as on other rock types. Applying the Comsol Optimization Module to 2D-image sections of the PET tomograms, effective parameter values were derived, thereby quantifying the anisotropic diffusion.
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Andriampenomanana, Fenitra, Andrew A. Nyblade, Michael E. Wysession, Raymond J. Durrheim, Frederik Tilmann, Guilhem Barruol, Gérard Rambolamanana et Tsiriandrimanana Rakotondraibe. « Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography ». Geophysical Journal International 224, no 1 (24 septembre 2020) : 290–305. http://dx.doi.org/10.1093/gji/ggaa458.
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SUMMARY The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn-tomography study in which Pn traveltimes are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ±0.30 km s–1 about a mean of 8.10 km s–1. Low-Pn-velocity zones (<8.00 km s–1) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100–300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E–W belt in central Madagascar. This belt is located at the edge of a broader low S-velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High-Pn-velocity zones (>8.20 km s–1) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region.
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Ayadi, Walid, Lucien Laiarinandrasana et Kacem Saï. « Anisotropic (Continuum Damage Mechanics)-based multi-mechanism model for semi-crystalline polymer ». International Journal of Damage Mechanics 27, no 3 (1 décembre 2016) : 357–86. http://dx.doi.org/10.1177/1056789516679494.
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In this work, the anisotropic damage of semi-crystalline polymers is investigated. The model, developed within a thermodynamic framework, includes the following features: (i) the degree of crystallinity; (ii) the hydrostatic pressure effect; and (iii) the damage anisotropy. The adopted tensorial damage variable is based on the Continuum Damage Mechanics approach under the energy equivalence assumption. For the quantification of the anisotropy, a parameter called “shape factor” is defined as the ratio between the void mean diameter and the void mean height. This parameter is linked to the main axial and the main radial damage components. Experimental data taken from the recent literature using the tomography technique were selected to assess the model capability. Finite element simulations of notched round bar specimens subjected to tensile test stopped at three key loading stages are systematically compared with experimental data. The proposed model was able not only to accurately simulate the macroscopic response of the material, but more interestingly, to reproduce the spatial distribution of the shape factor. This demonstrates the anisotropy effects of the material under study induced at different stages of the deformation.
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Zhu, Hejun, et Jeroen Tromp. « Mapping Tectonic Deformation in the Crust and Upper Mantle Beneath Europe and the North Atlantic Ocean ». Science 341, no 6148 (8 août 2013) : 871–75. http://dx.doi.org/10.1126/science.1241335.
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We constructed a three-dimensional azimuthally anisotropic model of Europe and the North Atlantic Ocean based on adjoint seismic tomography. Several features are well correlated with historical tectonic events in this region, such as extension along the North Atlantic Ridge, trench retreat in the Mediterranean, and counterclockwise rotation of the Anatolian Plate. Beneath northeastern Europe, the direction of the fast anisotropic axis follows trends of ancient rift systems older than 350 million years, suggesting “frozen-in” anisotropy related to the formation of the craton. Local anisotropic strength profiles identify the brittle-ductile transitions in lithospheric strength. In continental regions, these profiles also identify the lower crust, characterized by ductile flow. The observed anisotropic fabric is generally consistent with the current surface strain rate measured by geodetic surveys.
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Magali, J. K., T. Bodin, N. Hedjazian, H. Samuel et S. Atkins. « Geodynamic tomography : constraining upper-mantle deformation patterns from Bayesian inversion of surface waves ». Geophysical Journal International 224, no 3 (3 décembre 2020) : 2077–99. http://dx.doi.org/10.1093/gji/ggaa577.
Texte intégralRésumé :
SUMMARY In the Earth’s upper mantle, seismic anisotropy mainly originates from the crystallographic preferred orientation (CPO) of olivine due to mantle deformation. Large-scale observation of anisotropy in surface wave tomography models provides unique constraints on present-day mantle flow. However, surface waves are not sensitive to the 21 coefficients of the elastic tensor, and therefore the complete anisotropic tensor cannot be resolved independently at every location. This large number of parameters may be reduced by imposing spatial smoothness and symmetry constraints to the elastic tensor. In this work, we propose to regularize the tomographic problem by using constraints from geodynamic modelling to reduce the number of model parameters. Instead of inverting for seismic velocities, we parametrize our inverse problem directly in terms of physical quantities governing mantle flow: a temperature field, and a temperature-dependent viscosity. The forward problem consists of three steps: (1) calculation of mantle flow induced by thermal anomalies, (2) calculation of the induced CPO and elastic properties using a micromechanical model, and (3) computation of azimuthally varying surface wave dispersion curves. We demonstrate how a fully nonlinear Bayesian inversion of surface wave dispersion curves can retrieve the temperature and viscosity fields, without having to explicitly parametrize the elastic tensor. Here, we consider simple flow models generated by spherical temperature anomalies. The results show that incorporating geodynamic constraints in surface wave inversion help to retrieve patterns of mantle deformation. The solution to our inversion problem is an ensemble of models (i.e. thermal structures) representing a posterior probability, therefore providing uncertainties for each model parameter.
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Bakulin, Andrey, Marta Woodward, Dave Nichols, Konstantin Osypov et Olga Zdraveva. « Building tilted transversely isotropic depth models using localized anisotropic tomography with well information ». GEOPHYSICS 75, no 4 (juillet 2010) : D27—D36. http://dx.doi.org/10.1190/1.3453416.
Texte intégralRésumé :
Tilted transverse isotropy (TTI) is increasingly recognized as a more geologically plausible description of anisotropy in sedimentary formations than vertical transverse isotropy (VTI). Although model-building approaches for VTI media are well understood, similar approaches for TTI media are in their infancy, even when the symmetry-axis direction is assumed known. We describe a tomographic approach that builds localized anisotropic models by jointly inverting surface-seismic and well data. We present a synthetic data example of anisotropic tomography applied to a layered TTI model with a symmetry-axis tilt of 45 degrees. We demonstrate three scenarios for constraining the solution. In the first scenario, velocity along the symmetry axis is known and tomography inverts for Thomsen’s [Formula: see text] and [Formula: see text] parame-ters. In the second scenario, tomography inverts for [Formula: see text], [Formula: see text], and velocity, using surface-seismic data and vertical check-shot traveltimes. In contrast to the VTI case, both these inversions are nonunique. To combat nonuniqueness, in the third scenario, we supplement check-shot and seismic data with the [Formula: see text] profile from an offset well. This allows recovery of the correct profiles for velocity along the symmetry axis and [Formula: see text]. We conclude that TTI is more ambiguous than VTI for model building. Additional well data or rock-physics assumptions may be required to constrain the tomography and arrive at geologically plausible TTI models. Furthermore, we demonstrate that VTI models with atypical Thomsen parameters can also fit the same joint seismic and check-shot data set. In this case, although imaging with VTI models can focus the TTI data and match vertical event depths, it leads to substantial lateral mispositioning of the reflections.