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1
Stoffa, P. L., J. T. Fokkema, R. M. de Luna Freire e W. P. Kessinger. "Split‐step Fourier migration". GEOPHYSICS 55, n. 4 (aprile 1990): 410–21. http://dx.doi.org/10.1190/1.1442850.
Testo completoAbstract (sommario):
The split‐step Fourier method is developed and applied to migrating stacked seismic data in two and three dimensions. This migration method, which is implemented in both the frequency‐wavenumber and frequency‐space domains, takes into account laterally varying velocity by defining a reference slowness (reciprocal of velocity) as the mean slowness in the migration interval and a perturbation term that is spatially varying. The mean slowness defines a reference vertical wavenumber which is used in the frequency‐wavenumber domain to downward continue the data across a depth interval as in constant‐velocity phase‐shift migration. The perturbation term is used to define a “source” contribution that is taken into account by the application of a second phase shift in the frequency‐space domain. Since the method does not include the effects of second and higher order spatial derivatives of the slowness field, the method theoretically is accurate only when there are no rapid lateral slowness variations combined with steep angles of propagation. However, synthetic and real examples indicate that good results are obtained for realistic geologic structures.
2
Larner, Ken, e Craig Beasley. "Cascaded migrations: Improving the accuracy of finite‐difference migration". GEOPHYSICS 52, n. 5 (maggio 1987): 618–43. http://dx.doi.org/10.1190/1.1442331.
Testo completoAbstract (sommario):
The accuracy of time migrations done with finite‐difference schemes deteriorates with increasing reflector dip. Some properties of migration in general, and of finite‐difference approaches in particular, suggest a way of improving the accuracy of finite‐difference schemes for migrating steep dips. First, although data will be undermigrated when too low a velocity is used in migration, a correctly migrated result can be obtained by migrating again, this time with the previously undermigrated result as input. In fact, a sequence of undermigrations will yield the correct result as long as the sum of the squares of the migration velocities used in the different migration stages equals the square of the correct migration velocity. A second property is that the apparent spatial dip of a reflector perceived by the migration process is a function of not only the time dip of the unmigrated reflection, but also the velocity used in the migration. In a sequence of low‐velocity migrations, the apparent spatial dip perceived at each migration stage can be considerably less than the true dip. Thus, because finite‐difference migration is accurate for small spatial dips, the cascaded migrations yield a more accurate result than that of single‐stage migration. Also, because each migration stage is done with low velocity, the depth step can be large; hence, the computational effort need not be. The accuracy of the method is not compromised (in fact, it improves) in media in which velocity increases with depth. Moreover, the cascaded approach suffers no more than other methods of time migration where velocity varies mildly in the lateral direction. In applications of the method to stacked data from the Gulf of Mexico, reflections from near‐vertical flanks of salt domes were migrated with accuracy comparable to that achieved by frequency‐wavenumber domain migration.
3
Ashraf, Syed Ziauarrahman. "Data Migration Strategies: A Step-by-Step Guide". Journal of Artificial Intelligence, Machine Learning and Data Science 1, n. 1 (20 gennaio 2024): 1593–97. https://doi.org/10.51219/jaimld/syed-ziaurrahman-ashraf/357.
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Cooke, Dennis, Andrej Bóna e Benn Hansen. "Simultaneous time imaging, velocity estimation, and multiple suppression using local event slopes". GEOPHYSICS 74, n. 6 (novembre 2009): WCA65—WCA73. http://dx.doi.org/10.1190/1.3242751.
Testo completoAbstract (sommario):
Starting with the double-square-root equation we derive expressions for a velocity-independent prestack time migration and for the associated migration velocity. We then use that velocity to identify multiples and suppress them as part of the imaging step. To describe our algorithm, workflow, and products, we use the terms velocity-independent and oriented. While velocity-independent imaging does not require an input migration velocity, it does require input [Formula: see text]-values (also called local event slopes) measured in both the shot and receiver domains. There are many possible methods of calculating these required input [Formula: see text]-values, perhaps the simplest is to compute the ratio of instantaneous spatial frequency to instantaneous temporal frequency. Using a synthetic data set rich in multiples, we test the oriented algorithm and generate migrated prestack gathers, the oriented migration velocity field, and stacked migrations. We use oriented migration velocities for prestack multiple suppression. Without this multiple suppression step, the velocity-independent migration is inferior to a conventional Kirchhoff migration because the oriented migration will flatten primaries and multiples alike in the common image domain. With this multiple suppression step, the velocity-independent are very similar to a Kirchhoff migration generated using the known migration velocity of this test data set.
5
Badov, A. D. "New Step in Migration Study". Izvestiya Rossiiskoi Akademii Nauk. Seriya Geograficheskaya., n. 3 (21 luglio 2015): 118. http://dx.doi.org/10.15356/0373-2444-2012-3-118-119.
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Lee, Hong‐Yuan, e Shie‐Tsung Hwang. "Migration of Backward‐Facing Step". Journal of Hydraulic Engineering 120, n. 6 (giugno 1994): 693–705. http://dx.doi.org/10.1061/(asce)0733-9429(1994)120:6(693).
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Arntsen, Børge, Constantin Gerea e Tage Røsten. "Imaging salt bodies using explicit migration operators offshore Norway". GEOPHYSICS 74, n. 2 (marzo 2009): S25—S32. http://dx.doi.org/10.1190/1.3063660.
Testo completoAbstract (sommario):
We have tested the performance of 3D shot-profile depth migration using explicit migration operators on a real 3D marine data set. The data were acquired offshore Norway in an area with a complex subsurface containing large salt bodies. We compared shot-profile migration using explicit migration operators with conventional Kirchhoff migration, split-step Fourier migration, and common-azimuth by generalized screen propagator (GSP) migration in terms of quality and computational cost. Image quality produced by the explicit migration operator approach is slightly better than with split-step Fourier migration and clearly better than in common-azimuth by GSP and Kirchhoff migrations. The main differences are fewer artifacts and better-suppressed noise within the salt bodies. Kirchhoff migration shows considerable artifacts (migration smiles) within and close to the salt bodies, which are not present in images produced by the other three wave-equation methods. Expressions for computational cost were developed for all four migration algorithms in terms of frequency content and acquisition parameters. For comparable frequency content, migration cost using explicit operators is four times the cost of the split-step Fourier method, up to 260 times the cost of common-azimuth by GSP migration, and 25 times the cost of Kirchhoff migration. Our results show that in terms of image quality, shot-profile migration using explicit migration operators is well suited for imaging in areas with complex geology and significant velocity changes. However, computational cost of the method is high and makes it less attractive in terms of efficiency.
8
Popovici, Alexander Mihai. "Prestack migration by split‐step DSR". GEOPHYSICS 61, n. 5 (settembre 1996): 1412–16. http://dx.doi.org/10.1190/1.1444065.
Testo completoAbstract (sommario):
The double‐square‐root (DSR) prestack migration equation, though defined for depth variable velocity, can be used to image media with strong velocity variations using a phase‐shift plus interpolation (PSPI) or split‐step correction. The split‐step method is based on applying a phase‐shift correction to the extrapolated wavefield—a correction that attempts to compensate for the lateral velocity variations. I show how to extend DSR prestack migration to lateral velocity media and exemplify the method by applying the new algorithm to the Marmousi data set. The split‐step DSR migration is very fast and offers excellent imaging results.
9
Wenzel, F. "Frequency‐wavenumber migration in laterally heterogeneous media". GEOPHYSICS 56, n. 10 (ottobre 1991): 1671–73. http://dx.doi.org/10.1190/1.1442977.
Testo completoAbstract (sommario):
In a recent paper, Stoffa et al. (1990) presented a split‐step Fourier migration method. It takes laterally varying migration velocities into account by splitting the reference slowness into an average slowness, constant within the migration interval, and a perturbation term which accounts for the lateral variations. The method works accurately only if the lateral slowness variations are smooth and angles of propagation are near‐vertical. The restriction to steep propagation angles stems from the fact that the wavefield is accurately propagated with respect to the reference slowness but a five‐degree approximation is used to accommodate the lateral variations. This note presents a theory that avoids the five‐degree assumption and thus generalizes the split‐step algorithm.
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Rickman, J. M., S. R. Phillpot, D. Wolf, D. L. Woodraska e S. Yip. "On the mechanism of grain-boundary migration in metals: A molecular dynamics study". Journal of Materials Research 6, n. 11 (novembre 1991): 2291–304. http://dx.doi.org/10.1557/jmr.1991.2291.
Testo completoAbstract (sommario):
The migration of a (100) θ = 43.6°(Σ29) twist grain boundary is observed during the course of a molecular-dynamics simulation. The atomic-level details of the migration are investigated by determining the time dependence of the planar structure factor, a function of the planar interparticle bond angles, and the location of the center of a mass of planes near the grain boundary. It is found that a migration step consists of local bond rearrangements which, when the simulation cell is made large enough, produce domain-like structures in the migrating plane. Although no overall sliding is observed during migration, a local sliding of the planes near the migrating grain boundary accompanies the migration process. It is suggested that a three-dimensional cloud of thermally produced Frenkel-like point defects near the boundary accompanies, and facilitates, its migration.
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Zhang, Linbin, James W. Rector, G. Michael Hoversten e Sergey Fomel. "Split-step complex Padé-Fourier depth migration". Geophysical Journal International 171, n. 3 (10 ottobre 2007): 1308–13. http://dx.doi.org/10.1111/j.1365-246x.2007.03610.x.
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Ristow, Dietrich, e Thomas Rühl. "Fourier finite‐difference migration". GEOPHYSICS 59, n. 12 (dicembre 1994): 1882–93. http://dx.doi.org/10.1190/1.1443575.
Testo completoAbstract (sommario):
Many existing migration schemes cannot simultaneously handle the two most important problems of migration: imaging of steep dips and imaging in media with arbitrary velocity variations in all directions. For example, phase‐shift (ω, k) migration is accurate for nearly all dips but it is limited to very simple velocity functions. On the other hand, finite‐difference schemes based on one‐way wave equations consider arbitrary velocity functions but they attenuate steeply dipping events. We propose a new hybrid migration method, named “Fourier finite‐difference migration,” wherein the downward‐continuation operator is split into two downward‐continuation operators: one operator is a phase‐shift operator for a chosen constant background velocity, and the other operator is an optimized finite‐difference operator for the varying component of the velocity function. If there is no variation of velocity, then only a phase‐shift operator will be applied automatically. On the other hand, if there is a strong variation of velocity, then the phase‐shift component is suppressed and the optimized finite‐difference operator will be fully applied. The cascaded application of phase‐shift and finite‐difference operators shows a better maximum dip‐angle behavior than the split‐step Fourier migration operator. Depending on the macro velocity model, the Fourier finite‐difference migration even shows an improved performance compared to conventional finite‐difference migration with one downward‐continuation step. Finite‐difference migration with two downward‐continuation steps is required to reach the same migration performance, but this is achieved with about 20 percent higher computation costs. The new cascaded operator of the Fourier finite‐difference migration can be applied to arbitrary velocity functions and allows an accurate migration of steeply dipping reflectors in a complex macro velocity model. The dip limitation of the cascaded operator depends on the variation of the velocity field and, hence, is velocity‐adaptive.
13
Lin, Luchun, Baoping Shi e Ping An. "Multiwavelet prestack Kirchhoff migration". GEOPHYSICS 81, n. 3 (maggio 2016): S79—S85. http://dx.doi.org/10.1190/geo2015-0140.1.
Testo completoAbstract (sommario):
The problems of stretching and aliasing usually occur in prestack Kirchhoff migration. A multiwavelet-based approach is proposed to provide an alternative approach to resolve these problems. Two steps are involved in the multiwavelet-based algorithm: The first step is the decomposition of the seismic traces with a series of wavelets of different dominant frequencies. This step is based on the principle of basis pursuit and is enhanced for more accurate and sparse decomposition by adding an adaptive subdictionary that is associated with the minima and maxima of a seismic trace. The second step is wavelet migration based on the Kirchhoff formulation using a novel approach. The outer iteration is the wavelets of the input seismic traces, which ensures that an input wavelet is only used once in the entire migration process. This enables output of multiple migrated images of different wavelets of dominant frequency ranges, elimination of noisy images, and composition of images for specific interpretation and reservoir characterization purposes with very little extra computational cost. The stretching and aliasing problems are naturally resolved because interpolation on the seismic trace is not required in the migration. We tested the algorithm using a synthetic model with a dipping layer and a 2D real seismic section. We compared our results with results obtained by conventional Kirchhoff migration.
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Hillmann, Felicitas, e Ching Lin Pang. "Migration-led Regeneration:". Cosmopolitan Civil Societies: An Interdisciplinary Journal 12, n. 1 (26 giugno 2020): i—xii. http://dx.doi.org/10.5130/ccs.v12.i1.7297.
Testo completoAbstract (sommario):
It is timely to reopen the discussion on inequalities in connection with migration-related processes. Our special issue might be a first step in shedding more light on this issue which had all but vanished at the discourse level but that has not ceased to exist. It presents an international and interdisciplinary selection of scholars that are concerned with questions of urban transformation, diversity and inequality.
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Sena, Armando R., Paul L. Stoffa e Mrinal K. Sen. "Split-step Fourier migration of GPR data in lossy media". GEOPHYSICS 71, n. 4 (luglio 2006): K77—K91. http://dx.doi.org/10.1190/1.2217157.
Testo completoAbstract (sommario):
Migration of ground-penetrating radar (GPR) data traditionally has been implemented assuming uniform, nonconductive, and nondispersive media. However, in many real applications, the effects of heterogeneities, conductivity, or dispersion can be important, so it is necessary to consider these effects to image the data correctly. We implement the split-step Fourier technique for migration of GPR data in 2D media transverse-electric (TE) or transverse-magnetic (TM) propagation modes and demonstrate how it takes into account, naturally and efficiently, the effects of dispersion, attenuation, and heterogeneities. Compensation for attenuation during migration implies applying a gain that can make the numerical algorithm unstable. We introduce a homogeneous plane-wave approximation that gives greater stability to the migration technique, allowing a stable and satisfactory migration of the GPR data up to depths equivalent to three times the skin depth computed at the dominant frequency of the radar signal. Multiple slowness references of split-step migration for lossy media are introduced and compared with the single-reference slowness technique. Finally, we propose two limited gain modifications of the migration algorithm and study their usefulness when highly lossy zones are in the media.
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Wu, Wenjia, Junzhou Luo, Kai Dong, Ming Yang e Zhen Ling. "Energy-Efficient User Association with Congestion Avoidance and Migration Constraint in Green WLANs". Wireless Communications and Mobile Computing 2018 (28 giugno 2018): 1–10. http://dx.doi.org/10.1155/2018/9596141.
Testo completoAbstract (sommario):
Green wireless local area networks (WLANs) have captured the interests of academia and industry recently, because they save energy by scheduling an access point (AP) on/off according to traffic demands. However, it is very challenging to determine user association in a green WLAN while simultaneously considering several other factors, such as avoiding AP congestion and user migration constraints. Here, we study the energy-efficient user association with congestion avoidance and migration constraint (EACM). First, we formulate the EACM problem as an integer linear programming (ILP) model, to minimize APs’ overall energy consumption within a time interval while satisfying the following constraints: traffic demand, AP utilization threshold, and maximum number of demand node (DN) migrations allowed. Then, we propose an efficient migration-constrained user reassociation algorithm, consisting of two steps. The first step removes k AP-DN associations to eliminate AP congestion and turn off as many idle APs as possible. The second step reassociates these k DNs according to an energy efficiency strategy. Finally, we perform simulation experiments that validate our algorithm’s effectiveness and efficiency.
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Christophe, Barbara. "Migration in German Textbooks". Journal of Educational Media, Memory, and Society 1, n. 1 (1 marzo 2009): 190–202. http://dx.doi.org/10.3167/jemms.2009.010112.
Testo completoAbstract (sommario):
This article raises the question of how German textbooks should deal with issues of migration as one of the main challenges in a globalizing age. In order to prepare the ground for a well-founded answer it follows a twofold agenda. In a rst step, previous attempts at analyzing textbook representations of migration are critically scrutinized and read against the background of current debates on methodological approaches to textbook research. In a second step, anthropological research on the structure of public German discourses on migration is cited as a key to developing a truly multiperspectival mode of representing it. Ultimately, the article demonstrates that education alone cannot be given the responsibility of clarifying questions that politics have failed to articulate and that pupils must be taught to participate competently in the discourse on migration policy. They should be familiarized with the various positions advocated in the political sphere, and simultaneously equipped with the necessary tools for critical re ection.
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MENDEZ-OTERO, ROSALIA, e MARCELO F. SANTIAGO. "Functional role of a glycolipid in directional movements of neurons". Anais da Academia Brasileira de Ciências 73, n. 2 (giugno 2001): 221–29. http://dx.doi.org/10.1590/s0001-37652001000200007.
Testo completoAbstract (sommario):
Migration of neurons from their site of origin to their final destination is a critical and universal step in the formation of the complex structure of the nervous system. The migratory process is thought to be governed in part by genetically and epigenetically defined sequences of signals which are interpreted by migrating cells. The molecular mechanisms that underlie neuronal migration have been the subject of intense investigation. As in other developmental processes, many molecules must participate in neuronal migration. Some molecules, such as cell adhesion molecules and motor proteins, may contribute to discrete steps in the migration act; others, like extracellular signaling molecules, may regulate the activation and/or termination of the migration program. In this article we review findings from our group that demonstrate the functional role(s) of a specific glycolipid in neuronal migration and neurite outgrowth in the developing and adult nervous system.
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Friedl, Peter, e Katarina Wolf. "Proteolytic interstitial cell migration: a five-step process". Cancer and Metastasis Reviews 28, n. 1-2 (21 gennaio 2009): 129–35. http://dx.doi.org/10.1007/s10555-008-9174-3.
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Zhang, Yu, e Guanquan Zhang. "One-step extrapolation method for reverse time migration". GEOPHYSICS 74, n. 4 (luglio 2009): A29—A33. http://dx.doi.org/10.1190/1.3123476.
Testo completoAbstract (sommario):
We have proposed a new method, a one-step extrapolation algorithm, to solve the acoustic wave equation. By introducing a square-root operator, the two-way wave equation can be formulated as a first-order partial differential equation in time, which is similar to the one-way wave equation. To solve the new wave equation, we used a stable explicit extrapolation method in the time direction and handled lateral velocity variations in the space and wavenumber domains. Unlike the conventional explicit finite-difference schemes, the new method does not suffer from numerical instability or numerical dispersion problems. It can be used to design cost-effective and high-quality reverse time migration or modeling code.
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Zhao, Jingxia, Shulun Zhang, Changlong Wang e Yi Ni. "Adaptive spatial-division split-step fourier migration method". Applied Geophysics 2, n. 2 (giugno 2005): 75–79. http://dx.doi.org/10.1007/s11770-005-0035-3.
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Deng, Zijun, Weiwei Wang, Xun Xu, Nan Ma e Andreas Lendlein. "Modulation of Mesenchymal Stem Cell Migration using Programmable Polymer Sheet Actuators". MRS Advances 5, n. 46-47 (2020): 2381–90. http://dx.doi.org/10.1557/adv.2020.235.
Testo completoAbstract (sommario):
AbstractRecruitment of mesenchymal stem cells (MSCs) to damaged tissue is a crucial step to modulate tissue regeneration. Here, the migration of human adipose-derived stem cells (hADSCs) responding to thermal and mechanical stimuli was investigated using programmable shape-memory polymer actuator (SMPA) sheets. Changing the temperature repetitively between 10 and 37 °C, the SMPA sheets are capable of reversibly changing between two different pre-defined shapes like an artificial muscle. Compared to non-actuating sheets, the cells cultured on the programmed actuating sheets presented a higher migration velocity (0.32 ± 0.1 vs. 0.57 ± 0.2 μm/min). These results could motivate the next scientific steps, for example, to investigate the MSCs pre-loaded in organoids towards their migration potential.
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Whitcombe, David N. "Fast model building using demigration and single‐step ray migration". GEOPHYSICS 59, n. 3 (marzo 1994): 439–49. http://dx.doi.org/10.1190/1.1443606.
Testo completoAbstract (sommario):
This work provides explorationists with simple procedures to perform depth conversion more accurately than can be achieved with simple vertical layer cake depth conversion. The use of image rays, which are inadequate in structurally complex areas, is avoided. Migrated time interpretations are still used and are "demigrated" using the Kirchhoff time migration equations. This backs out the effect of the time migration prior to a ray depth migration and enables the lateral shifts between the time migrated image and a depth migrated image to be quantified. These shifts can be separated into a mismigration component and a refraction component. The relative size of the components define whether time or depth migration is required and may be used to justify a remigration of the seismic image. Furthermore, the tedious layer by layer approach to ray depth migration may be avoided by using the velocity depth model from the vertical layer cake depth conversion of the time‐migrated data for ray depth migration of the unmigrated data for all horizons in a single step. A satisfactory result is usually achieved without the need to iterate. These methods are illustrated with both a synthetic example and a real 3-D data set from the Norwegian North Sea.
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Sörensson, Erika, e Hildur Kalman. "Care and concern in the research process: meeting ethical and epistemological challenges through multiple engagements and dialogue with research subjects". Qualitative Research 18, n. 6 (30 novembre 2017): 706–21. http://dx.doi.org/10.1177/1468794117743466.
Testo completoAbstract (sommario):
By addressing a case of data collection strategies applied in research on Thai migration to the Swedish wild-berry industry, this article argues for how a feminist approach based on care and concern for research subjects both safeguards ethical concerns and promotes good knowing. The data collection procedures were designed in a step-by-step manner, including the research subjects as much as possible at different times and in different ways in an attempt to create preconditions for a more inclusive production of knowledge. In-depth interviews, participant observation, photo documentation and group interviews were used, which facilitated the possibility to understand the content and meanings of wild-berry picking from the workers’ points of view. Through prolonged contact, including repeated encounters and dialogue with research subjects, in-depth knowledge was produced concerning Thai migrations to Sweden, as migration was set in relation to the migrants’ life courses and living conditions.
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Yilmaz, Ö. "Interpretive evaluation of migrated data". Exploration Geophysics 20, n. 2 (1989): 17. http://dx.doi.org/10.1071/eg989017.
Testo completoAbstract (sommario):
In practice, migration of seismic data requires decision making with regard to: (1) Different migration strategies ? 2-D/3-D, post-stack/prestack, and time/depth migrations; (2) different migration algorithms for a given strategy ? integral, finite-difference and frequency-wavenumber methods; (3) different parameters for a given algorithm ? aperture width, depth-step size, stretch factor; (4) the input data ? profile length, noise content, spatial aliasing and boundary effects; (5) and finally, migration velocities ? the weak link between the seismic method and the subsurface geology that the former tries to image.The seismic interpreter, whose main role is to infer subsurface geology from the migrated data, normally should not be burdened with the decisions concerning the above factors. Fortunately, migration results often are self-evident; a feature considered geologically implausible on a migrated section can be associated with one or more of the above factors. Based on large number of field data cases, I will discuss each of these factors and provide some generally applicable guidelines for migration that an interpreter can invoke in practice.
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Sun, Junzhe, Sergey Fomel e Lexing Ying. "Low-rank one-step wave extrapolation for reverse time migration". GEOPHYSICS 81, n. 1 (1 gennaio 2016): S39—S54. http://dx.doi.org/10.1190/geo2015-0183.1.
Testo completoAbstract (sommario):
Reverse time migration (RTM) relies on accurate wave extrapolation engines to image complex subsurface structures. To construct such operators with high efficiency and numerical stability, we have developed a one-step wave extrapolation approach using complex-valued low-rank decomposition to approximate the mixed-domain space-wavenumber wave extrapolation symbol. The low-rank one-step method involves a complex-valued phase function, which is more flexible than a real-valued phase function of two-step schemes, and thus it is capable of modeling a wider variety of dispersion relations. Two novel designs of the phase function leads to the desired properties in wave extrapolation. First, for wave propagation in inhomogeneous media, including a velocity gradient term assures a more accurate phase behavior, particularly when the velocity variations are large. Second, an absorbing boundary condition, which is propagation-direction-dependent, can be incorporated into the phase function as an anisotropic attenuation term. This term allows waves to travel parallel to the boundary without absorption, thus reducing artificial reflections at wide incident angles. Using numerical experiments, we revealed the stability improvement of a one-step scheme in comparison with two-step schemes. We observed the low-rank one-step operator to be remarkably stable and capable of propagating waves using large time step sizes, even beyond the Nyquist limit. The stability property can help to minimize the computational cost of seismic modeling or RTM. The low-rank one-step wave extrapolation also handles anisotropic wave propagation accurately and efficiently. When applied to RTM in anisotropic media, the proposed method generated high-quality images.
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Popova, Ekaterina. "Mother Russia, Step-Mother Hungary: New Home for Russian Migrants". Multidiszciplináris kihívások, sokszínű válaszok, n. 2 (31 dicembre 2021): 28–46. http://dx.doi.org/10.33565/mksv.2021.02.02.
Testo completoAbstract (sommario):
The issue of migration is not new, but clearly requires renewed attention, especially when it comes to the national interest and the identification and integration of future migrants. This paper describes some of the problems of modern Russian emigration and offers an analysis, in the context of modern realities, as to why Russians choose to emigrate to Hungary.
This essay explores the real causes of “push” and “pull” factors and the ability or readiness of Russians who live in Hungary to be a part of the society. Additionally, considering the globalisation process, it examines circular and return migration as a new ‘hybrid’ form of migration in which the final destination cannot be assured.
The study uses the systematic methodology of grounded theory and interview and is a part of the dissertation. This research can be useful for either migration researchers or policymakers.
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Stoufflet, Julie, e Isabelle Caillé. "The Primary Cilium and Neuronal Migration". Cells 11, n. 21 (26 ottobre 2022): 3384. http://dx.doi.org/10.3390/cells11213384.
Testo completoAbstract (sommario):
The primary cilium (PC) is a microtubule-based tiny sensory organelle emanating from the centrosome and protruding from the surface of most eukaryotic cells, including neurons. The extremely severe phenotypes of ciliopathies have suggested their paramount importance for multiple developmental events, including brain formation. Neuronal migration is an essential step of neural development, with all neurons traveling from their site of birth to their site of integration. Neurons perform a unique type of cellular migration called cyclic saltatory migration, where their soma periodically jumps along with the stereotyped movement of their centrosome. We will review here how the role of the PC on cell motility was first described in non-neuronal cells as a guide pointing to the direction of migration. We will see then how these findings are extended to neuronal migration. In neurons, the PC appears to regulate the rhythm of cyclic saltatory neuronal migration in multiple systems. Finally, we will review recent findings starting to elucidate how extracellular cues sensed by the PC could be intracellularly transduced to regulate the machinery of neuronal migration. The PC of migrating neurons was unexpectedly discovered to display a rhythmic extracellular emergence during each cycle of migration, with this transient exposure to the external environment associated with periodic transduction of cyclic adenosine monophosphate (cAMP) signaling at the centrosome. The PC in migrating neurons thus uniquely appears as a beat maker, regulating the tempo of cyclic saltatory migration.
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Ryazantsev, Sergey V., Irina N. Molodikova e Olga D. Vorobeva. "BETWEEN THE EURASIAN AND EUROPEAN SUBSYSTEMS: MIGRATION AND MIGRATION POLICY IN THE CIS AND BALTIC COUNTRIES IN THE 1990s—2020s". Baltic Region 14, n. 2 (2022): 115–43. http://dx.doi.org/10.5922/2079-8555-2022-2-8.
Testo completoAbstract (sommario):
The article analyses migration from border countries (the so-called overlapping area) of two migration subsystems — Eurasian (centred in the Russian Federation) and European (the European Union) from 1991 to 2021 (before the recent events in Ukraine). A step-by-step analysis of the migration situation in the countries of the former USSR — Belarus, Latvia, Lithuania, Moldova, Ukraine and Estonia was conducted. The article examines bilateral and multilateral migration processes, analyses the main factors influencing their development and explores migration policy measures and their impact on the regulation of migration processes in the countries of the overlapping area. These countries, located between the two centres of major migration subsystems in Eurasia (Eurasian and European, or, in other words, between the Russian Federation and the core of the EU), are subject to their strong influence and ‘competitive gravitation’. The strength of this gravitation depends not only on pull and push factors but also on the attractiveness and non-attractiveness of the migration policies prevailing in these migration subsystems at a given point in time.
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Motealleh, Andisheh, e Nermin S. Kehr. "Directed vertical cell migration via bifunctionalized nanomaterials in 3D step-gradient nanocomposite hydrogels". Biomaterials Science 8, n. 20 (2020): 5628–37. http://dx.doi.org/10.1039/d0bm01133a.
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Kim, Y. C., R. Gonzalez e J. R. Berryhill. "Recursive wavenumber‐frequency migration". GEOPHYSICS 54, n. 3 (marzo 1989): 319–29. http://dx.doi.org/10.1190/1.1442657.
Testo completoAbstract (sommario):
There are many approaches for migrating seismic data using velocities varying only with depth. These methods are capable of accommodating quasi‐continuous vertical velocity variation at the expense of a considerably larger amount of computation than with the Stolt method, which assumes a constant velocity for the subsurface of the earth. However, the errors involved in estimating migration velocities from seismic data are often too large to justify such a large amount of computational effort. Furthermore, because there is a resolution limit in velocity estimation, a time‐depth curve based on the velocity estimates may be represented by a series of line segments that typically are much larger than the migration step size. For a time‐depth curve segmented in this way, we may successively apply the fast Stolt method interleaved with phase shift for downward continuation. This approach, recursive wavenumber‐frequency (k-f ) migration, retains the speed of the Stolt method and produces from seismic data a subsurface image as good as that from the phase‐shift method. The recursive k-f method is a powerful tool, particularly for the migration of 3-D data.
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Berkhout, A. J., e D. J. Verschuur. "Seismic imaging beyond depth migration". GEOPHYSICS 66, n. 6 (novembre 2001): 1895–912. http://dx.doi.org/10.1190/1.1487132.
Testo completoAbstract (sommario):
If seismic imaging is formulated in terms of two focusing steps—focusing in emission and focusing in detection (or vice versa)—the output of the first focusing step yields a new type of seismic gather, the common‐focus‐point (CFP) gather, which is available for data analysis and information extraction. One important consequence of this novel option is that the involved focusing operators can be updated without updating the underlying velocity model. Introducing the concept of “dynamic focusing,” it is proposed to verify the validity of focusing operators by comparing the “gather of focus‐point responses” with the “gather of focusing operators.” Compared with velocity‐driven time and depth migration, operator‐driven CFP migration can be considered as the most general approach to seismic imaging: it does not require a velocity model, and it automatically takes into account unknown complex propagation effects such as conversion, anisotropy, and dispersion. In addition, in CFP migration, the second focusing step can be extended to produce both angle‐averaged reflection information and angle‐dependent reflection information. The CFP approach to seismic migration allows new solutions in the situation of complex near‐surface layers, subsalt targets, multicomponent processing, and time lapse analysis.
33
Lim, Chinten J., Kristin H. Kain, Eugene Tkachenko, Lawrence E. Goldfinger, Edgar Gutierrez, Michael D. Allen, Alex Groisman, Jin Zhang e Mark H. Ginsberg. "Integrin-mediated Protein Kinase A Activation at the Leading Edge of Migrating Cells". Molecular Biology of the Cell 19, n. 11 (novembre 2008): 4930–41. http://dx.doi.org/10.1091/mbc.e08-06-0564.
Testo completoAbstract (sommario):
cAMP-dependent protein kinase A (PKA) is important in processes requiring localized cell protrusion, such as cell migration and axonal path finding. Here, we used a membrane-targeted PKA biosensor to reveal activation of PKA at the leading edge of migrating cells. Previous studies show that PKA activity promotes protrusion and efficient cell migration. In live migrating cells, membrane-associated PKA activity was highest at the leading edge and required ligation of integrins such as α4β1 or α5β1 and an intact actin cytoskeleton. α4 integrins are type I PKA-specific A-kinase anchoring proteins, and we now find that type I PKA is important for localization of α4β1 integrin-mediated PKA activation at the leading edge. Accumulation of 3′ phosphorylated phosphoinositides [PtdIns(3,4,5)P3] products of phosphatidylinositol 3-kinase (PI3-kinase) is an early event in establishing the directionality of migration; however, polarized PKA activation did not require PI3-kinase activity. Conversely, inhibition of PKA blocked accumulation of a PtdIns(3,4,5)P3-binding protein, the AKT-pleckstrin homology (PH) domain, at the leading edge; hence, PKA is involved in maintaining cell polarity during migration. In sum, we have visualized compartment-specific PKA activation in migrating cells and used it to reveal that adhesion-mediated localized activation of PKA is an early step in directional cell migration.
34
Bourgeois, A., B. F. Jiang e P. Lailly. "Linearized inversion: a significant step beyond pre-stack migration". Geophysical Journal International 99, n. 2 (novembre 1989): 435–45. http://dx.doi.org/10.1111/j.1365-246x.1989.tb01700.x.
Testo completo
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Liu, Qiancheng, e Daniel Peter. "One-step data-domain least-squares reverse time migration". GEOPHYSICS 83, n. 4 (1 luglio 2018): R361—R368. http://dx.doi.org/10.1190/geo2017-0622.1.
Testo completoAbstract (sommario):
Least-squares reverse time migration (LSRTM) is an iterative inversion algorithm for estimating the broadband-wavenumber reflectivity model. Although it produces superior results compared with conventional reverse time migration (RTM), LSRTM is computationally expensive. We have developed a one-step LSRTM method by considering the demigrated and observed data to design a deblurring preconditioner in the data domain using the Wiener filter. For the Wiener filtering, we further use a stabilized division algorithm via the Taylor expansion. The preconditioned observed data are then remigrated to obtain a deblurred image. The total cost of this method is about two RTMs. Through synthetic and real data experiments, we see that one-step LSRTM is able to enhance image resolution and balance source illumination at low computational costs.
36
Lee, Delman, Iain M. Mason e Geoffrey M. Jackson. "Split‐step Fourier shot‐record migration with deconvolution imaging". GEOPHYSICS 56, n. 11 (novembre 1991): 1786–93. http://dx.doi.org/10.1190/1.1442991.
Testo completoAbstract (sommario):
The split‐step Fourier method is used to prestack migrate two synthetic borehole‐to‐surface shot gathers. Model structures in the zone of specular illumination beneath the shot are reconstructed by using the split‐step Fourier method both to back‐propagate the recorded wavefield and to forward propagate the source wavelet. The overburden is vertically and laterally inhomogeneous. Each depth interval is treated as a homogeneous strip with the mean velocity plus an inhomogeneity correction term. The inhomogeneity correction term is split and spatially multiplied with each spectral component of the wavefield on its entry to and upon its exit from each strip. Propagation through each strip is effected by multiplication in the spatial frequency domain. The split‐step Fourier method offers a valuable alternative to finite‐difference migration for machines with limited memory. Three imaging methods are compared for two signal‐to‐noise ratios. They are: image extraction by traveltime, crosscorrelation with source wavelet, and deconvolution with source wavelet. At high signal‐to‐noise level, the image formed by deconvolution offers better spatial resolution than images formed by crosscorrelation with the source wavelet or by extraction using traveltime. If the signal‐to‐noise level is low, traveltime imaging deteriorates rapidly, while deconvolution images degrade towards those created by crosscorrelation imaging.
37
Freyburg, Tina. "The Janus Face of EU Migration Governance: Impairing Democratic Governance at Home Improving It Abroad?" European Foreign Affairs Review 17, Special Issue (1 aprile 2012): 125–42. http://dx.doi.org/10.54648/eerr2012018.
Testo completoAbstract (sommario):
In the field of European Union (EU) migration policies, a large number of legal, administrative, and institutional tightening measures have been established in the name of domestic security since the early 1990s. These measures have been criticized because they allegedly lower the democratic quality of EU migration governance. Yet, the EU seeks to export its rules and regulations beyond its borders to neighbouring third states through its association policies. This article argues that what may be seen as a step back in terms of democratic governance inside the EU may present a step forward in its authoritarian Southern neighbourhood. It explores whether the rules of EU migration can constitute a step toward democratization of migration governance in neighbouring countries, where provisions of transparent, participatory, or accountable decision-making cannot be taken for granted. Taking the example of the Moroccan 2003 law on migration, the paper demonstrates that the EU was fairly successful in inducing the adoption of legislation in line with provisions of democratic governance. The contribution concludes with a discussion of the prospect of implementation of these provisions, in particular, against the background of the EU's extended external competences under the Lisbon Treaty.
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Trung, Dang Hoai, Nguyen Van Giang, Nguyen Thanh Van, Nguyen Van Thuan e Vo Minh Triet. "The application of split step fourier migration to interpreting GPR data in Vietnam". Tạp chí Khoa học và Công nghệ biển 17, n. 4B (15 dicembre 2017): 161–66. http://dx.doi.org/10.15625/1859-3097/17/4b/13004.
Testo completoAbstract (sommario):
Migration methods play an essential role in processing ground penetrating radar data. For estimating electromagnetic propagation velocity distribution, the finite - difference migration is used because of its reliable performance with high noise conditions. To optimize this migration algorithm, we propose using energy diagram as a criterion of looking for the correct velocity. If the velocity varies laterally and vertically, split step Fourier migration is used for creating a true image of subsurface structures. We applied these steps to real data in Vietnam. The results verified on field data show that migrated images with calculated velocity from energy diagram have the best quality.
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Ohkado, A., P. Douglas, D. Zenner e L. Kawatsu. "Pre-migration TB screening—the first step is always the hardest". International Journal of Tuberculosis and Lung Disease 24, n. 12 (1 dicembre 2020): 1261–64. http://dx.doi.org/10.5588/ijtld.20.0102.
Testo completoAbstract (sommario):
As the proportion of foreign-born persons among TB notifications continues to rise, Japan is preparing to introduce pre-migration TB screening for those coming from selected countries, who are intending to stay for more than 90 days. It has announced that the programme will commence in 2020. In this review, the authors examine the experiences from two countries which already have years of experience in operating pre-migration TB screening, namely the United Kingdom and Australia. The authors point out that both countries have developed strong health information system not only to collect and analyse screening results, but also to use the data to effectively monitor and evaluate the screening programme itself. The critical role which health information system plays within pre-migration screening is often overlooked. Here we argue that Japan, as with any other countries planning to introduce pre-migration screening for TB, must also plan for data management.
40
Gray, Samuel H., John Etgen, Joe Dellinger e Dan Whitmore. "Seismic migration problems and solutions". GEOPHYSICS 66, n. 5 (settembre 2001): 1622–40. http://dx.doi.org/10.1190/1.1487107.
Testo completoAbstract (sommario):
Historically, seismic migration has been the practice (science, technology, and craft) of collapsing diffraction events on unmigrated records to points, thereby moving (“migrating”) reflection events to their proper locations, creating a true image of structures within the earth. Over the years, the scope of migration has broadened. What began as a structural imaging tool is evolving into a tool for velocity estimation and attribute analysis, making detailed use of the amplitude and phase information in the migrated image. With its expanded scope, migration has moved from the final step of the seismic acquisition and processing flow to a more central one, with links to both the processes preceding and following it. In this paper, we describe the mechanics of migration (the algorithms) as well as some of the problems related to it, such as algorithmic accuracy and efficiency, and velocity estimation. We also describe its relationship with other processes, such as seismic modeling. Our approach is tutorial; we avoid presenting the finest details of either the migration algorithms themselves or the problems to which migration is applied. Rather, we focus on presenting the problems themselves, in the hope that most geophysicists will be able to gain an appreciation of where this imaging method fits in the larger problem of searching for hydrocarbons.
41
Moskal, Marta, e Naomi Tyrrell. "Family migration decision-making, step-migration and separation: children's experiences in European migrant worker families". Children's Geographies 14, n. 4 (8 dicembre 2015): 453–67. http://dx.doi.org/10.1080/14733285.2015.1116683.
Testo completo
42
Hyun, Young-Min, Ronen Sumagin, Pranita Sarangi, Michael Overstreet, Christina Baker, Deborah Fowell, Ingrid Sarelius e Minsoo Kim. "A novel final step in leukocyte extravasation (102.19)". Journal of Immunology 186, n. 1_Supplement (1 aprile 2011): 102.19. http://dx.doi.org/10.4049/jimmunol.186.supp.102.19.
Testo completoAbstract (sommario):
Abstract The real-time dynamics of cell surface adhesion receptor reflect leukocyte migration and trafficking in lymphoid and non-lymphoid tissues, providing valuable insight into the evolving events of the immune response. Despite notable advances in studying leukocyte migration in simplified systems in vitro, live imaging of subcellular adhesion complexes expressed at physiological density in intact tissues has been challenging. To this end we generated a knock-in mouse with CD18 fused to CFP. To enable imaging of CD18 dynamics during transendothelial migration of T lymphocytes, monocytes, and neutrophils, we used multiphoton-intravital microscopy and enhanced three dimensional detection of the fluorescent CD18 fusion protein. These in vivo analyses and parallel experiments in vitro show that extravasating leukocytes become extremely elongated before completing transmigration across the endothelium and subsequent retraction of the stretched uropod from the basolateral side of the endothelial cells takes longer than 20 min, occasionally leaving CD18-enriched microparticle at the basement membrane. Experiments with knock-out mice and blocking antibodies reveal that the uropod elongation is mediated by LFA-1. Therefore we propose that uropod elongation/delayed tail retraction is a novel final, rate-limiting step in the leukocyte extravasation cascade, which may be important for precise regulation of leukocyte recruitment and maintaining endothelial barrier function during inflammation.
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Kaplan, Sam T., Partha S. Routh e Mauricio D. Sacchi. "Derivation of forward and adjoint operators for least-squares shot-profile split-step migration". GEOPHYSICS 75, n. 6 (novembre 2010): S225—S235. http://dx.doi.org/10.1190/1.3506146.
Testo completoAbstract (sommario):
The forward and adjoint operators for shot-profile least-squares migration are derived. The forward operator is demigration, and the adjoint operator is migration. The demigration operator is derived from the Born approximation. The process begins with a Green’s function that allows for a laterally varying migration velocity model using the split-step approximation. Next, the earth is divided into horizontal layers, and within each layer the migration velocity model is made to be constant with respect to depth. For a given layer, (1) the source-side wavefield is propagated down to its top using the background wavefield. This gives a background wavefield incident at the layer’s upper boundary. (2) The layer’s contribution to the scattered wavefield is computed using the Born approximation to the scattered wavefield and the background wavefield. (3) Next, its scattered wavefield is propagated back up to the measurement surface using, again, the background wavefield. The measured wavefield is approximated by the sum of scattered wavefields from each layer. In the derivation of the measured wavefield, the shot-profile migration geometry is used. For each shot, the resulting wavefield modeling operator takes the form of a Fredholm integral equation of the first kind, and this is used to write down its adjoint, the shot-profile migration operator. This forward/adjoint pair is used for shot-profile least-squares migration. Shot-profile least-squares migration is illustrated with two synthetic examples. The first uses data collected over a four-layer acoustic model, and the second uses data from the Sigsbee 2a model.
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Dipankar, Pankaj, Puneet Kumar, Shiba Prasad Dash e Pranita P. Sarangi. "Functional and Therapeutic Relevance of Rho GTPases in Innate Immune Cell Migration and Function during Inflammation: An In Silico Perspective". Mediators of Inflammation 2021 (13 febbraio 2021): 1–10. http://dx.doi.org/10.1155/2021/6655412.
Testo completoAbstract (sommario):
Systematic regulation of leukocyte migration to the site of infection is a vital step during immunological responses. Improper migration and localization of immune cells could be associated with disease pathology as seen in systemic inflammation. Rho GTPases act as molecular switches during inflammatory cell migration by cycling between Rho-GDP (inactive) to Rho-GTP (active) forms and play an essential role in the precise regulation of actin cytoskeletal dynamics as well as other immunological functions of leukocytes. Available reports suggest that the dysregulation of Rho GTPase signaling is associated with various inflammatory diseases ranging from mild to life-threatening conditions. Therefore, it is crucial to understand the step-by-step activation and inactivation of GTPases and the functioning of different Guanine Nucleotide Exchange Factors (GEFs) and GTPase-Activating Proteins (GAPs) that regulate the conversion of GDP to GTP and GTP to GDP exchange reactions, respectively. Here, we describe the molecular organization and activation of various domains of crucial elements associated with the activation of Rho GTPases using solved PDB structures. We will also present the latest evidence available on the relevance of Rho GTPases in the migration and function of innate immune cells during inflammation. This knowledge will help scientists design promising drug candidates against the Rho-GTPase-centric regulatory molecules regulating inflammatory cell migration.
45
Verwiebe, Roland, Laura Wiesböck e Roland Teitzer. "New forms of intra-European migration, labour market dynamics and social inequality in Europe". Migration Letters 11, n. 2 (4 maggio 2014): 125–36. http://dx.doi.org/10.33182/ml.v11i2.234.
Testo completoAbstract (sommario):
This article deals mainly with new forms of Intra-European migration, processes of integration and inequality, and the dynamics of emerging transnational labour markets in Europe. We discuss these issues against the background of fundamental changes which have been taking place on the European continent over the past two decades. Drawing on available comparative European data, we examine, in a first step, whether the changes in intra-European migration patterns have been accompanied by a differentiation of the causes of migration. In a second step, we discuss the extent to which new forms of transnational labour markets have been emerging within Europe and their effects on systems of social stratification.
46
Oda, T., M. Katori, K. Hatanaka e Y. Nagai. "Inhibition of neutrophil migration by a selective inhibitor of matrix metalloproteinase: analysis by intravital microscopy". Mediators of Inflammation 4, n. 2 (1995): 133–37. http://dx.doi.org/10.1155/s0962935195000238.
Testo completoAbstract (sommario):
Observation of the microcirculation of the hamster cheek pouch by intravital microscopy revealed five steps of neutrophil migration from the venules after topical application of leukotriene B4to the microvasculature: rolling along the venular wall (Step 1), adhesion to it (Step 2), disappearance from the vascular lumen (Step 3), presence between the endothelial cells and the subendothelial basement membrane (Step 4) and passage through the basement membrane (Step 5). The present study was performed to examine whether a metalloproteinase inhibitor inhibits neutrophil migration at any of the above five steps. Chymostatin and leupeptin did not inhibit any of these five steps. In contrast, FN-439, a selective inhibitor of matrix metalloproteinase, reduced the number of neutrophils in the perivascular space without affecting Steps 1 to 3. It was concluded that neutrophils may use metalloproteinase (collagenase/gelatinase) to penetrate the subendothelial basement membrane.
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Kühl, Henning, Maurico D. Sacchi e Jürgen Fertig. "The Hartley transform in seismic imaging". GEOPHYSICS 66, n. 4 (luglio 2001): 1251–57. http://dx.doi.org/10.1190/1.1487072.
Testo completoAbstract (sommario):
Phase‐shift migration techniques that attempt to account for lateral velocity variations make substantial use of the fast Fourier transform (FFT). Generally, the Hermitian symmetry of the complex‐valued Fourier transform causes computational redundancies in terms of the number of operations and memory requirements. In practice a combination of the FFT with the well‐known real‐to‐complex Fourier transform is often used to avoid such complications. As an alternative means to the Fourier transform, we introduce the inherently real‐valued, non‐symmetric Hartley transform into phase‐shift migration techniques. By this we automatically avoid the Hermitian symmetry resulting in an optimized algorithm that is comparable in efficiency to algorithms based on the real‐to‐complex FFT. We derive the phase‐shift operator in the Hartley domain for migration in two and three dimensions and formulate phase shift plus interpolation, split‐step migration, and split‐step double‐square‐root prestack migration in terms of the Hartley transform as examples. We test the Hartley phase‐shift operator for poststack and prestack migration using the SEG/EAGE salt model and the Marmousi data set, respectively.
48
Xue, Zhiguang, Yangkang Chen, Sergey Fomel e Junzhe Sun. "Seismic imaging of incomplete data and simultaneous-source data using least-squares reverse time migration with shaping regularization". GEOPHYSICS 81, n. 1 (1 gennaio 2016): S11—S20. http://dx.doi.org/10.1190/geo2014-0524.1.
Testo completoAbstract (sommario):
Simultaneous-source acquisition improves the efficiency of the seismic data acquisition process. However, direct imaging of simultaneous-source data may introduce crosstalk artifacts in the final image. Likewise, direct imaging of incomplete data avoids the step of data reconstruction, but it can suffer from migration artifacts. We have proposed to incorporate shaping regularization into least-squares reverse time migration (LSRTM) and use it for suppressing interference noise caused by simultaneous-source data or migration artifacts caused by incomplete data. To implement LSRTM, we have applied lowrank one-step reverse time migration and its adjoint iteratively in the conjugate-gradient algorithm to minimize the data misfit. A shaping operator imposing structure constraints on the estimated model was applied at each iteration. We constructed the shaping operator as a structure-enhancing filtering to attenuate migration artifacts and crosstalk noise while preserving structural information. We have carried out numerical tests on synthetic models in which the proposed method exhibited a fast convergence rate and was effective in attenuating migration artifacts and crosstalk noise.
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Zhang, Jianfeng, e Linong Liu. "Optimum split-step Fourier 3D depth migration: Developments and practical aspects". GEOPHYSICS 72, n. 3 (maggio 2007): S167—S175. http://dx.doi.org/10.1190/1.2715658.
Testo completoAbstract (sommario):
We present an efficient scheme for depth extrapolation of wide-angle 3D wavefields in laterally heterogeneous media. The scheme improves the so-called optimum split-step Fourier method by introducing a frequency-independent cascaded operator with spatially varying coefficients. The developments improve the approximation of the optimum split-step Fourier cascaded operator to the exact phase-shift operator of a varying velocity in the presence of strong lateral velocity variations, and they naturally lead to frequency-dependent varying-step depth extrapolations that reduce computational cost significantly. The resulting scheme can be implemented alternatively in spatial and wavenumber domains using fast Fourier transforms (FFTs). The accuracy of the first-order approximate algorithm is similar to that of the second-order optimum split-step Fourier method in modeling wide-angle propagation through strong, laterally varying media. Similar to the optimum split-step Fourier method, the scheme is superior to methods such as the generalized screen and Fourier finite difference. We demonstrate the scheme’s accuracy by comparing it with 3D two-way finite-difference modeling. Comparisons with the 3D prestack Kirchhoff depth migration of a real 3D data set demonstrate the practical application of the proposed method.
50
Rosner, Margit, e Markus Hengstschläger. "Three-Dimensional Migration of Human Amniotic Fluid Stem Cells Involves Mesenchymal and Amoeboid Modes and is Regulated by mTORC1". Stem Cells 39, n. 12 (4 agosto 2021): 1718–32. http://dx.doi.org/10.1002/stem.3441.
Testo completoAbstract (sommario):
Abstract Three-dimensional (3D) cell migration is an integral part of many physiologic processes. Although being well studied in the context of adult tissue homeostasis and cancer development, remarkably little is known about the invasive behavior of human stem cells. Using two different kinds of invasion assays, this study aimed at investigating and characterizing the 3D migratory capacity of human amniotic fluid stem cells (hAFSCs), a well-established fetal stem cell type. Eight hAFSC lines were found to harbor pronounced potential to penetrate basement membrane (BM)-like matrices. Morphological examination and inhibitor approaches revealed that 3D migration of hAFSCs involves both the matrix metalloprotease-dependent mesenchymal, elongated mode and the Rho-associated protein kinase-dependent amoeboid, round mode. Moreover, hAFSCs could be shown to harbor transendothelial migration capacity and to exhibit a motility-associated marker expression pattern. Finally, the potential to cross extracellular matrix was found to be induced by mTORC1-activating growth factors and reduced by blocking mTORC1 activity. Taken together, this report provides the first demonstration that human stem cells exhibit mTORC1-dependent invasive capacity and can concurrently make use of mesenchymal and amoeboid 3D cell migration modes, which represents an important step toward the full biological characterization of fetal human stem cells with relevance to both developmental research and stem cell-based therapy.