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Articles de revues sur le sujet « Global tomography »

Auteur : Grafiati
Publié le 10 mars 2023

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1

Wenhao Chen, Wenhao Chen, Yudan Wang Yudan Wang, Huiqiang Liu Huiqiang Liu, Biao Deng Biao Deng, Yushuang Yang Yushuang Yang et Tiqiao Xiao Tiqiao Xiao. « Pseudo-global tomography for local micro-computed tomography with high-brightness synchrotron X-rays ». Chinese Optics Letters 12, no 2 (2014) : 023401–23404. http://dx.doi.org/10.3788/col201412.023401.

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Haned, A., E. Stutzmann, M. Schimmel, S. Kiselev, A. Davaille et A. Yelles-Chaouche. « Global tomography using seismic hum ». Geophysical Journal International 204, no 2 (30 décembre 2015) : 1222–36. http://dx.doi.org/10.1093/gji/ggv516.

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Ritzwoller, Michael H., Nikolai M. Shapiro, Mikhail P. Barmin et Anatoli L. Levshin. « Global surface wave diffraction tomography ». Journal of Geophysical Research : Solid Earth 107, B12 (décembre 2002) : ESE 4–1—ESE 4–13. http://dx.doi.org/10.1029/2002jb001777.

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Root, B. C. « Comparing global tomography-derived and gravity-based upper mantle density models ». Geophysical Journal International 221, no 3 (24 février 2020) : 1542–54. http://dx.doi.org/10.1093/gji/ggaa091.

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SUMMARY Current seismic tomography models show a complex environment underneath the crust, corroborated by high-precision satellite gravity observations. Both data sets are used to independently explore the density structure of the upper mantle. However, combining these two data sets proves to be challenging. The gravity-data has an inherent insensitivity in the radial direction and seismic tomography has a heterogeneous data acquisition, resulting in smoothed tomography models with de-correlation between different models for the mid-to-small wavelength features. Therefore, this study aims to assess and quantify the effect of regularization on a seismic tomography model by exploiting the high lateral sensitivity of gravity data. Seismic tomography models, SL2013sv, SAVANI, SMEAN2 and S40RTS are compared to a gravity-based density model of the upper mantle. In order to obtain similar density solutions compared to the seismic-derived models, the gravity-based model needs to be smoothed with a Gaussian filter. Different smoothening characteristics are observed for the variety of seismic tomography models, relating to the regularization approach in the inversions. Various S40RTS models with similar seismic data but different regularization settings show that the smoothening effect is stronger with increasing regularization. The type of regularization has a dominant effect on the final tomography solution. To reduce the effect of regularization on the tomography models, an enhancement procedure is proposed. This enhancement should be performed within the spectral domain of the actual resolution of the seismic tomography model. The enhanced seismic tomography models show improved spatial correlation with each other and with the gravity-based model. The variation of the density anomalies have similar peak-to-peak magnitudes and clear correlation to geological structures. The resolvement of the spectral misalignment between tomographic models and gravity-based solutions is the first step in the improvement of multidata inversion studies of the upper mantle and benefit from the advantages in both data sets.
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Lei, Wenjie, Youyi Ruan, Ebru Bozdağ, Daniel Peter, Matthieu Lefebvre, Dimitri Komatitsch, Jeroen Tromp, Judith Hill, Norbert Podhorszki et David Pugmire. « Global adjoint tomography—model GLAD-M25 ». Geophysical Journal International 223, no 1 (21 mai 2020) : 1–21. http://dx.doi.org/10.1093/gji/ggaa253.

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SUMMARY Building on global adjoint tomography model GLAD-M15, we present transversely isotropic global model GLAD-M25, which is the result of 10 quasi-Newton tomographic iterations with an earthquake database consisting of 1480 events in the magnitude range 5.5 ≤ Mw ≤ 7.2, an almost sixfold increase over the first-generation model. We calculated fully 3-D synthetic seismograms with a shortest period of 17 s based on a GPU-accelerated spectral-element wave propagation solver which accommodates effects due to 3-D anelastic crust and mantle structure, topography and bathymetry, the ocean load, ellipticity, rotation and self-gravitation. We used an adjoint-state method to calculate Fréchet derivatives in 3-D anelastic Earth models facilitated by a parsimonious storage algorithm. The simulations were performed on the Cray XK7 ‘Titan’ and the IBM Power 9 ‘Summit’ at the Oak Ridge Leadership Computing Facility. We quantitatively evaluated GLAD-M25 by assessing misfit reductions and traveltime anomaly histograms in 12 measurement categories. We performed similar assessments for a held-out data set consisting of 360 earthquakes, with results comparable to the actual inversion. We highlight the new model for a variety of plumes and subduction zones.
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Bozdağ, Ebru, Daniel Peter, Matthieu Lefebvre, Dimitri Komatitsch, Jeroen Tromp, Judith Hill, Norbert Podhorszki et David Pugmire. « Global adjoint tomography : first-generation model ». Geophysical Journal International 207, no 3 (22 septembre 2016) : 1739–66. http://dx.doi.org/10.1093/gji/ggw356.

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Dziewonski, Adam M. « Global seismic tomography of the mantle ». Reviews of Geophysics 33 (1995) : 419. http://dx.doi.org/10.1029/95rg00738.

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Wang, Yuchen, Nan Ding, Yu Zhang, Long Li, Xiaoyan Yang et Qingzhi Zhao. « A New Approach of the Global Navigation Satellite System Tomography for Any Size of GNSS Network ». Remote Sensing 12, no 4 (13 février 2020) : 617. http://dx.doi.org/10.3390/rs12040617.

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Global Navigation Satellite System (GNSS) tomography is a popular method for measuring and modelling water vapor in the troposphere. Presently, most studies use a cuboid-shaped tomographic region in their modelling, which represents the modelling region for all measurement epochs. This region is defined by the distribution of the GNSS signals skywards from a network of ground based GNSS stations for all epochs of measurements. However, in reality at each epoch the shape of the GNSS tomographic region is more likely to be an inverted cone. Unfortunately, this fixed conic tomographic region does not properly reflect the fact that the GNSS signal changes quickly over time. Therefore a dynamic or adaptive tomographic region is better suited. In this study, a new approach that adjusts the GNSS tomographic model to adapt the size of the GNSS network is proposed, which referred to as The High Flexibility GNSS Tomography (HFGT). Test data from different numbers of the GNSS stations are used and the results from HFGT are compared against that of radiosonde data (RS) to assess the accuracy of the HFGT approach. The results showed that the new approach is feasible for different numbers of the GNSS stations when a sufficient and uniformed distribution of GNSS signals is used. This is a novel approach for GNSS tomography.
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Cyril Patrick Masalu, Desiderius. « Global Mid-Ocean Ridges Mantle Tomography Profiles ». Earth Sciences 4, no 2 (2015) : 80. http://dx.doi.org/10.11648/j.earth.20150402.13.

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Zaroli, Christophe. « Global seismic tomography using Backus–Gilbert inversion ». Geophysical Journal International 207, no 2 (23 août 2016) : 876–88. http://dx.doi.org/10.1093/gji/ggw315.

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Patella, Domenico. « Self‐potential global tomography including topographic effects ». Geophysical Prospecting 45, no 5 (septembre 1997) : 843–63. http://dx.doi.org/10.1046/j.1365-2478.1997.570296.x.

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Patella, D. « Self-potential global tomography including topographic effects ». Geophysical Prospecting 46, no 1 (janvier 1998) : 103. http://dx.doi.org/10.1046/j.1365-2478.1998.830318.x.

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Nishida, K., J. P. Montagner et H. Kawakatsu. « Global Surface Wave Tomography Using Seismic Hum ». Science 326, no 5949 (1 octobre 2009) : 112. http://dx.doi.org/10.1126/science.1176389.

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Ruan, Youyi, Wenjie Lei, Ryan Modrak, Rıdvan Örsvuran, Ebru Bozdağ et Jeroen Tromp. « Balancing unevenly distributed data in seismic tomography : a global adjoint tomography example ». Geophysical Journal International 219, no 2 (1 août 2019) : 1225–36. http://dx.doi.org/10.1093/gji/ggz356.

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SUMMARY The uneven distribution of earthquakes and stations in seismic tomography leads to slower convergence of nonlinear inversions and spatial bias in inversion results. Including dense regional arrays, such as USArray or Hi-Net, in global tomography causes severe convergence and spatial bias problems, against which conventional pre-conditioning schemes are ineffective. To save computational cost and reduce model bias, we propose a new strategy based on a geographical weighting of sources and receivers. Unlike approaches based on ray density or the Voronoi tessellation, this method scales to large full-waveform inversion problems and avoids instabilities at the edges of dense receiver or source clusters. We validate our strategy using a 2-D global waveform inversion test and show that the new weighting scheme leads to a nearly twofold reduction in model error and much faster convergence relative to a conventionally pre-conditioned inversion. We implement this geographical weighting strategy for global adjoint tomography.
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Wen, Debao, Dengkui Mei et Yanan Du. « Imaging the Three-Dimensional Ionospheric Structure with a Blob Basis Functional Ionospheric Tomography Model ». Sensors 20, no 8 (12 avril 2020) : 2182. http://dx.doi.org/10.3390/s20082182.

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A new ionospheric tomography model is presented in this work. In the new model, the traditional voxel basis function is replaced by the blob basis function. Due to the overlapping nature of their rotational symmetric basis functions, the new model introduces certain weighting from nearby tomographic spherical blobs. To confirm the feasibility of the new tomography model, a numerical simulation scheme is devised, and the simulation demonstrates that the reconstructed quality of the blob basis tomographic model is higher than that of the voxel basis tomographic model. Meanwhile, the variable blob radius is adopted in order to improve the efficiency of the new model. Finally, the new ionospheric tomography model is applied to reconstruct the temporal-spatial distribution of ionospheric electron density using actual global navigation satellite system observations. The comparisons between the tomographic profiles and those obtained from ionosonde data further demonstrate the reliability and the superiority of the new ionospheric tomography model.
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Trampert, J. « Global seismic tomography : the inverse problem and beyond ». Inverse Problems 14, no 3 (1 juin 1998) : 371–85. http://dx.doi.org/10.1088/0266-5611/14/3/002.

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van der Hilst, R. D., S. Widiyantoro et E. R. Engdahl. « Evidence for deep mantle circulation from global tomography ». Nature 386, no 6625 (avril 1997) : 578–84. http://dx.doi.org/10.1038/386578a0.

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Zhou, Ying, F. A. Dahlen, Guust Nolet et Gabi Laske. « Finite-frequency effects in global surface-wave tomography ». Geophysical Journal International 163, no 3 (décembre 2005) : 1087–111. http://dx.doi.org/10.1111/j.1365-246x.2005.02780.x.

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Boschi, L., J. P. Ampuero, D. Peter, P. M. Mai, G. Soldati et D. Giardini. « Petascale computing and resolution in global seismic tomography ». Physics of the Earth and Planetary Interiors 163, no 1-4 (août 2007) : 245–50. http://dx.doi.org/10.1016/j.pepi.2007.02.011.

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Romanowicz, Barbara. « Global seismic tomography : Present status and future perspectives ». Journal of the Acoustical Society of America 117, no 4 (avril 2005) : 2431. http://dx.doi.org/10.1121/1.4786695.

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Liu, Lou, Zhang, Huang, Zhou et Zhang. « On the Study of Influences of Different Factors on the Rapid Tropospheric Tomography ». Remote Sensing 11, no 13 (28 juin 2019) : 1545. http://dx.doi.org/10.3390/rs11131545.

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A rapid tropospheric tomography system was developed by using algebraicreconstruction technique. Influences of different factors on the tomographic results, including theground meteorological data, the multi-Global Navigation Satellite System (GNSS) observations, theground station distribution and the tomographic horizontal resolution, were systematicallyinvestigated. In order to exclude the impacts from discrepancies of water vapor informationbetween input observations and references on the tomographic results, the latest reanalysisproducts, ERA5, which were taken as references for result evaluations, were used to simulate slantwet delay (SWD) observations at GNSS stations. Besides, the slant delays derived from GNSSprocessing were also used to evaluate the reliability of simulated observations. Tomography resultsshow that the input both SWD and ground meteorological data could improve the tomographicresults where SWD mainly improve the results at middle layers (500 to 5000m, namely 2 to 16 layer)and ground meteorological data could improve the humidity fields at bottom layers further (0 to500m, namely 0 to 2 layer). Compared to the usage of Global Positioning System (GPS) only SWD,the inclusion of multi-GNSS SWD does not significantly improve the tomographic results at alllayers due to the almost unchanged dispersion of puncture points of GNSS signals. However,increases in the ground GNSS stations can benefit the tomography, with improvements of morethan 10% at bottom and middle layers. Higher tomographic horizontal resolution can furtherslightly improve the tomographic results (about 3-6% from 0.5 to 0.25 degrees), which, however,will also increase the computational burden at the same time.
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Coghill, Robert C., Christine N. Sang, Karen Faith Berman, Gary J. Bennett et Michael J. Iadarola. « Global Cerebral Blood Flow Decreases during Pain ». Journal of Cerebral Blood Flow & ; Metabolism 18, no 2 (février 1998) : 141–47. http://dx.doi.org/10.1097/00004647-199802000-00003.

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Positron emission tomography studies have identified a common set of brain regions activated by pain. No studies, however, have quantitatively examined pain-induced CBF changes. To better characterize CBF during pain, 14 subjects received positron emission tomography scans during rest, during capsaicin-evoked pain (250 μg, intradermal injection), and during innocuous vibration. Using the H215O intravenous bolus method with arterial blood sampling, global CBF changes were assessed quantitatively. Painful stimulation produced a 22.8% decrease in global CBF from resting levels ( P < 0.0005). This decrease was not accounted for by arterial PCO2 or heart rate changes. Although the exact mechanism remains to be determined, this pain-induced global decrease represents a previously unidentified response of CBF.
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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.

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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.
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Peter, D., C. Tape, L. Boschi et J. H. Woodhouse. « Surface wave tomography : global membrane waves and adjoint methods ». Geophysical Journal International 171, no 3 (15 septembre 2007) : 1098–117. http://dx.doi.org/10.1111/j.1365-246x.2007.03554.x.

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Valentine, Andrew P., et John H. Woodhouse. « Approaches to automated data selection for global seismic tomography ». Geophysical Journal International 182, no 2 (11 mai 2010) : 1001–12. http://dx.doi.org/10.1111/j.1365-246x.2010.04658.x.

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Gudmundsson, Olafur, et Robert W. Clayton. « A 2-D synthetic study of global traveltime tomography ». Geophysical Journal International 106, no 1 (juillet 1991) : 53–65. http://dx.doi.org/10.1111/j.1365-246x.1991.tb04600.x.

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Montagner, Jean-Paul, et Toshiro Tanimoto. « Global upper mantle tomography of seismic velocities and anisotropies ». Journal of Geophysical Research : Solid Earth 96, B12 (10 novembre 1991) : 20337–51. http://dx.doi.org/10.1029/91jb01890.

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Spetzler, Jesper, et Jeannot Trampert. « Implementing spectral leakage corrections in global surface wave tomography ». Geophysical Journal International 155, no 2 (novembre 2003) : 532–38. http://dx.doi.org/10.1046/j.1365-246x.2003.02079.x.

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Jeejeebhoy, Khursheed. « Subjective Global Assessment versus Sarcopenia Detected by Computed Tomography ». Journal of Parenteral and Enteral Nutrition 39, no 3 (23 février 2015) : 271. http://dx.doi.org/10.1177/0148607114562887.

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Stark, Cyril Jakob. « Global Completability with Applications to Self-Consistent Quantum Tomography ». Communications in Mathematical Physics 348, no 1 (22 septembre 2016) : 1–25. http://dx.doi.org/10.1007/s00220-016-2760-2.

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Wen, Debao, Dengkui Mei et Yanan Du. « Adaptive Smoothness Constraint Ionospheric Tomography Algorithm ». Sensors 20, no 8 (23 avril 2020) : 2404. http://dx.doi.org/10.3390/s20082404.

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Ionospheric tomography reconstruction based on global navigation satellite system observations is usually an ill-posed problem. To resolve it, an adaptive smoothness constraint ionospheric tomography algorithm is proposed in this work. The new algorithm performs an adaptive adjustment for the constrained weight coefficients of the tomography system. The computational efficiency and the reconstructed quality of ionospheric imaging are improved by using the new algorithm. A numerical simulation experiment was conducted in order to validate the feasibility and superiority of the algorithm. The statistical results of the reconstructed errors and the comparisons of ionospheric profiles confirmed the superiority of the new algorithm. Finally, the new algorithm was successfully applied to reconstruct three-dimensional ionospheric images under geomagnetic quiet and geomagnetic disturbance conditions over Hunan province. The tomographic results are reasonable and consistent with the general behavior of the ionosphere. The positive and negative phase storm effects are found during geomagnetic storm occurrence.
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Heublein, Marion, Patrick Erik Bradley et Stefan Hinz. « Observing geometry effects on a Global Navigation Satellite System (GNSS)-based water vapor tomography solved by least squares and by compressive sensing ». Annales Geophysicae 38, no 1 (6 février 2020) : 179–89. http://dx.doi.org/10.5194/angeo-38-179-2020.

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Abstract. In this work, the effect of the observing geometry on the tomographic reconstruction quality of both a regularized least squares (LSQ) approach and a compressive sensing (CS) approach for water vapor tomography is compared based on synthetic Global Navigation Satellite System (GNSS) slant wet delay (SWD) estimates. In this context, the term “observing geometry” mainly refers to the number of GNSS sites situated within a specific study area subdivided into a certain number of volumetric pixels (voxels) and to the number of signal directions available at each GNSS site. The novelties of this research are (1) the comparison of the observing geometry's effects on the tomographic reconstruction accuracy when using LSQ or CS for the solution of the tomographic system and (2) the investigation of the effect of the signal directions' variability on the tomographic reconstruction. The tomographic reconstruction is performed based on synthetic SWD data sets generated, for many samples of various observing geometry settings, based on wet refractivity information from the Weather Research and Forecasting (WRF) model. The validation of the achieved results focuses on a comparison of the refractivity estimates with the input WRF refractivities. The results show that the recommendation of Champollion et al. (2004) to discretize the analyzed study area into voxels with horizontal sizes comparable to the mean GNSS intersite distance represents a good rule of thumb for both LSQ- and CS-based tomography solutions. In addition, this research shows that CS needs a variety of at least 15 signal directions per site in order to estimate the refractivity field more accurately and more precisely than LSQ. Therefore, the use of CS is particularly recommended for water vapor tomography applications for which a high number of multi-GNSS SWD estimates are available.
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Yao, Yibin, Chen Liu, Chaoqian Xu, Yu Tan et Mingshan Fang. « A Refined Tomographic Window for GNSS-Derived Water Vapor Tomography ». Remote Sensing 12, no 18 (15 septembre 2020) : 2999. http://dx.doi.org/10.3390/rs12182999.

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Global navigation satellite system (GNSS) tomography can effectively sense the three-dimensional structure of tropospheric water vapor (WV) using the GNSS observations. Numerous studies have utilized a tomographic window to include more epochs of observations, which significantly increases the number of valid signals. However, considering the tomography grid limits, a massive number of valid signals inevitably exhibits linear dependence. This dependence makes it impossible to improve the rank score of the tomography coefficient matrix by blindly introducing a large number of valid rays. Furthermore, excessive valid signals may lead to a high condition number in the coefficient matrix (ill-condition problem), which causes unstable results using the GNSS-WV tomography. Considering these problems, we proposed an improved tomographic approach, which applies a refined tomographic window. It differs from the general tomographic window in that the window is refined to traverse the valid signals available 15 min before and after the target epoch while retaining only the linearly independent parts (characteristic signal). Compared to the conventional method, the proposed method can filter the characteristic signal, which increases the rank score of the coefficient matrix and improves the stability of the tomography model. In this paper, we used GNSS observations from the Hong Kong Satellite Positioning Reference Station Network (SatRef) to validate the performance of the proposed method over the day-of-year (DOY) periods of 130–132, 2019 and 146–148, 2019. The numerical results showed that, by using a refined tomographic window, the proposed method obtained superior WV products in comparison with that of the conventional method.
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Ritsema, Jeroen, et Vedran Lekić. « Heterogeneity of Seismic Wave Velocity in Earth's Mantle ». Annual Review of Earth and Planetary Sciences 48, no 1 (30 mai 2020) : 377–401. http://dx.doi.org/10.1146/annurev-earth-082119-065909.

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Seismology provides important constraints on the structure and dynamics of the deep mantle. Computational and methodological advances in the past two decades improved tomographic imaging of the mantle and revealed the fine-scale structure of plumes ascending from the core-mantle boundary region and slabs of oceanic lithosphere sinking into the lower mantle. We discuss the modeling aspects of global tomography including theoretical approximations, data selection, and model fidelity and resolution. Using spectral, principal component, and cluster analyses, we highlight the robust patterns of seismic heterogeneity, which inform us of flow in the mantle, the history of plate motions, and potential compositionally distinct reservoirs. In closing, we emphasize that data mining of vast collections of seismic waveforms and new data from distributed acoustic sensing, autonomous hydrophones, ocean-bottom seismometers, and correlation-based techniques will boost the development of the next generation of global models of density, seismic velocity, and attenuation. ▪ Seismic tomography reveals the 100-km to 1,000-km scale variation of seismic velocity heterogeneity in the mantle. ▪ Tomographic images are the most important geophysical constraints on mantle circulation and evolution.
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Galyavich, A. S., et A. Yu Rafikov. « Evaluation of the right ventricular ejection fraction according to multislice computed tomography in patients with pulmonary embolism ». Kazan medical journal 96, no 6 (15 décembre 2015) : 901–5. http://dx.doi.org/10.17750/kmj2015-901.

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Aim. Evaluation of the global systolic function of the right ventricle according to multislice computed tomography in patients with pulmonary embolism. Methods. 37 people aged 31 to 75 years (20 women and 17 men, mean age 55±12 years) were examined. The study group included 15 patients without clinical or instrumental signs of congenital heart disease and myocardial infarction of the left and right ventricles, with signs of pulmonary embolism according to multislice computed tomography. The control group included 22 patients. Tomographic analysis of end-diastolic volume, ejection fraction of the left and right ventricles was performed during noninvasive multislice computed tomography - coronary angiography, angiopulmonography. The study was conducted on a 64-helical computed tomography Aquillon 64 (Toshiba, Japan). Results. Analyzing group medians in patients with pulmonary embolism there was a decrease of the right ventricular ejection fraction and end-diastolic volume of the left ventricle (р
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Herscovitch, Peter, Alexander P. Auchus, Mokhtar Gado, David Chi et Marcus E. Raichle. « Correction of Positron Emission Tomography Data for Cerebral Atrophy ». Journal of Cerebral Blood Flow & ; Metabolism 6, no 1 (février 1986) : 120–24. http://dx.doi.org/10.1038/jcbfm.1986.14.

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Because positron emission tomography (PET) provides measurements per unit volume of intracranial contents, these measurements may be affected by the inclusion of metabolically inactive CSF spaces in the volume in which they are made. Thus, PET measurements of CBF and metabolism may be artifactually lowered in normal aging and dementia, which are both associated with significant brain atrophy. We describe a method to correct global PET data, averaged over several tomographic slices, for cerebral atrophy by using measurements of CSF space volume obtained with quantitative x-ray computed tomography. The importance of making such a correction is demonstrated using PET measurements of CBF and oxygen metabolism obtained in normal young, normal elderly, and demented subjects.
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Ding, Nan, Xiangrong Yan, Shubi Zhang, Suqin Wu, Xiaoming Wang, Yu Zhang, Yuchen Wang et al. « Node-Based Optimization of GNSS Tomography with a Minimum Bounding Box Algorithm ». Remote Sensing 12, no 17 (25 août 2020) : 2744. http://dx.doi.org/10.3390/rs12172744.

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Global Navigation Satellite Systems (GNSS) tomography plays an important role in the monitoring and tracking of the tropospheric water vapor. In this study, a new approach for improving the node-based GNSS tomography is proposed, which makes a trade-off between the real observed region and the complexity of the discretization of the tomographic region. To obtain dynamically the approximate observed region, the convex hull algorithm and minimum bounding box algorithm are used at each tomographic epoch. This new approach can dynamically define the tomographic model for all types of study areas based on the GNSS data. The performance of the new approach is tested by comparing it against the common node-based GNSS tomographic approach. Test data in May 2015 are obtained from the Hong Kong GNSS network to build the tomographic models and the radiosonde data as a reference are used for validating the quality of the new approach. The experimental results show that the root-mean-square errors of the new approach, in most cases, have a 38 percent improvement and the values of standard deviation reduce to over 43 percent compared with the common approach. The results indicate that the new approach is applicable to the node-based GNSS tomography.
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Zhao, Qingzhi, Yibin Yao et Wanqiang Yao. « Troposphere Water Vapour Tomography : A Horizontal Parameterised Approach ». Remote Sensing 10, no 8 (7 août 2018) : 1241. http://dx.doi.org/10.3390/rs10081241.

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Global Navigation Satellite System (GNSS) troposphere tomography has become one of the most cost-effective means to obtain three-dimensional (3-d) image of the tropospheric water vapour field. Traditional methods divide the tomography area into a number of 3-d voxels and assume that the water vapour density at any voxel is a constant during the given period. However, such behaviour breaks the spatial continuity of water vapour density in a horizontal direction and the number of unknown parameters needing to be estimated is very large. This is the focus of the paper, which tries to reconstruct the water vapor field using the tomographic technique without imposing empirical horizontal and vertical constraints. The proposed approach introduces the layered functional model in each layer vertically and only an a priori constraint is imposed for the water vapor information at the location of the radiosonde station. The elevation angle mask of 30° is determined according to the distribution of intersections between the satellite rays and different layers, which avoids the impact of ray bending and the error in slant water vapor (SWV) at low elevation angles on the tomographic result. Additionally, an optimal weighting strategy is applied to the established tomographic model to obtain a reasonable result. The tomographic experiment is performed using Global Positioning System (GPS) data of 12 receivers derived from the Satellite Positioning Reference Station Network (SatRef) in Hong Kong. The quality of the established tomographic model is validated under different weather conditions and compared with the conventional tomography method using 31-day data, respectively. The numerical result shows that the proposed method is applicable and superior to the traditional one. Comparisons of integrated water vapour (IWV) of the proposed method with that derived from radiosonde and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim data show that the root mean square (RMS)/Bias of their differences are 3.2/−0.8 mm and 3.3/−1.7 mm, respectively, while the values of traditional method are 5.1/−3.9 mm and 6.3/−5.9 mm, respectively. Furthermore, the water vapour density profiles are also compared with radiosonde and ECMWF data, and the values of RMS/Bias error for the proposed method are 0.88/0.06 g/m3 and 0.92/−0.08 g/m3, respectively, while the values of the traditional method are 1.33/0.38 g/m3 and 1.59/0.40 g/m3, respectively.
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39

Goncalves, Doglas S., Marcia A. Gomes-Ruggiero et Carlile Lavor. « Global convergence of diluted iterations in maximum-likelihood quantum tomography ». Quantum Information and Computation 14, no 11&12 (septembre 2014) : 966–80. http://dx.doi.org/10.26421/qic14.11-12-5.

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In this paper we address convergence issues of the Diluted $R \rho R$ algorithm \cite{rehacek2007}, used to obtain the maximum likelihood estimate for the density matrix in quantum state tomography. We give a new interpretation to the diluted $R \rho R$ iterations that allows us to prove the global convergence under weaker assumptions. Thus, we propose a new algorithm which is globally convergent and suitable for practical implementation.
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de Viron, Olivier, Michel Van Camp, Alexia Grabkowiak et Ana M. G. Ferreira. « Comparing global seismic tomography models using varimax principal component analysis ». Solid Earth 12, no 7 (19 juillet 2021) : 1601–34. http://dx.doi.org/10.5194/se-12-1601-2021.

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Abstract. Global seismic tomography has greatly progressed in the past decades, with many global Earth models being produced by different research groups. Objective, statistical methods are crucial for the quantitative interpretation of the large amount of information encapsulated by the models and for unbiased model comparisons. Here we propose using a rotated version of principal component analysis (PCA) to compress the information in order to ease the geological interpretation and model comparison. The method generates between 7 and 15 principal components (PCs) for each of the seven tested global tomography models, capturing more than 97 % of the total variance of the model. Each PC consists of a vertical profile, with which a horizontal pattern is associated by projection. The depth profiles and the horizontal patterns enable examining the key characteristics of the main components of the models. Most of the information in the models is associated with a few features: large low-shear-velocity provinces (LLSVPs) in the lowermost mantle, subduction signals and low-velocity anomalies likely associated with mantle plumes in the upper and lower mantle, and ridges and cratons in the uppermost mantle. Importantly, all models highlight several independent components in the lower mantle that make between 36 % and 69 % of the total variance, depending on the model, which suggests that the lower mantle is more complex than traditionally assumed. Overall, we find that varimax PCA is a useful additional tool for the quantitative comparison and interpretation of tomography models.
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Morozov, A. K., et D. C. Webb. « A sound projector for acoustic tomography and global ocean monitoring ». IEEE Journal of Oceanic Engineering 28, no 2 (avril 2003) : 174–85. http://dx.doi.org/10.1109/joe.2003.811888.

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Bijwaard, Harmen, Wim Spakman et E. Robert Engdahl. « Closing the gap between regional and global travel time tomography ». Journal of Geophysical Research : Solid Earth 103, B12 (10 décembre 1998) : 30055–78. http://dx.doi.org/10.1029/98jb02467.

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Hosseini, Kasra, et Karin Sigloch. « Multifrequency measurements of core-diffractedPwaves (Pdiff) for global waveform tomography ». Geophysical Journal International 203, no 1 (28 août 2015) : 506–21. http://dx.doi.org/10.1093/gji/ggv298.

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Liu, Kui, et Ying Zhou. « Global Rayleigh wave phase-velocity maps from finite-frequency tomography ». Geophysical Journal International 205, no 1 (7 février 2016) : 51–66. http://dx.doi.org/10.1093/gji/ggv555.

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Lu, Chang, et Stephen P. Grand. « The effect of subducting slabs in global shear wave tomography ». Geophysical Journal International 205, no 2 (5 mars 2016) : 1074–85. http://dx.doi.org/10.1093/gji/ggw072.

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Levshin, Anatoli L., Michael P. Barmin, Michael H. Ritzwoller et Jeannot Trampert. « Minor-arc and major-arc global surface wave diffraction tomography ». Physics of the Earth and Planetary Interiors 149, no 3-4 (avril 2005) : 205–23. http://dx.doi.org/10.1016/j.pepi.2004.10.006.

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Malischewsky, Peter. « Seismic tomography. With applications in global seismology and exploration geophysics ». Tectonophysics 172, no 3-4 (février 1990) : 369–70. http://dx.doi.org/10.1016/0040-1951(90)90043-8.

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Zhao, Dapeng, Yoshihiro Yamamoto et Takahiro Yanada. « Global mantle heterogeneity and its influence on teleseismic regional tomography ». Gondwana Research 23, no 2 (mars 2013) : 595–616. http://dx.doi.org/10.1016/j.gr.2012.08.004.

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Behounkova, M., H. Cizkova et C. Matyska. « Resolution tests of global geodynamic models by travel-time tomography ». Studia Geophysica et Geodaetica 49, no 3 (juillet 2005) : 343–63. http://dx.doi.org/10.1007/s11200-005-0014-4.

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Agaltsov, A. D. « A global uniqueness result for acoustic tomography of moving fluid ». Bulletin des Sciences Mathématiques 139, no 8 (décembre 2015) : 937–42. http://dx.doi.org/10.1016/j.bulsci.2015.06.003.

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