1,720,952 research outputs found
Reduction of computing time for seismic applications based on the Helmholtz equation by Graphics Processing Units
No full textThe oil and gas industry makes use of computational intensive algorithms to provide an image of the subsurface. The image is obtained by sending wave energy into the subsurface and recording the signal required for a seismic wave to reflect back to the surface from the Earth interfaces that may have different physical properties. A seismic wave is usually generated by shots of known frequencies, placed close to the surface on land or close to the water surface in the sea. Returning waves are usually recorded in time by hydrophones in a marine environment or by geophones during land acquisition. The goal of seismic imaging is to transform the seismograms to a spatial image of the subsurface. Migration algorithms produce an image of the subsurface given the seismic data measured at the surface. In this thesis we focus on solving the Helmholtz equation which represents the wave propagation in the frequency domain. We can easily convert from the time domain to the frequency domain and vice-versa using the Fourier transformation. A discretization with second-order finite differences gives a sparse linear system of equations that needs to be solved for each frequency. Two- as well as three-dimensional problems are considered. Krylov subspace methods such as Bi-CGSTAB and IDR(s) have been chosen as solvers. Since the convergence of the Krylov subspace solvers deteriorates with an increasing wave number, a shifted Laplacian multigrid preconditioner is used to improve the convergence. Here, we extend the matrix-dependent multigrid method to solve complex-valued matrices in three dimensions. As the smoother, we have considered parallelizable methods such as weighted Jacobi (?-Jacobi), multi-colored Gauss-Seidel and damped multi-colored Gauss-Seidel (?-GS). The implementation of the preconditioned solver on a CPU (Central Processing Unit) is compared to an implementation on the GPU (Graphics Processing Units or graph- ics card) using CUDA (Compute Unified Device Architecture). The results show that in two dimensions the preconditioned Bi-CGSTAB method on the GPU as well as the pre- conditioned IDR(s) method on a single GPU are about 30 times faster than on a single- threaded CPU. To achieve double precision accuracy on the GPU we have used the iterative refinement in Chapter 2. However, problems of realistic size are too large to fit in the memory of one GPU. One solution for this is to use multiple GPUs. A currently widely used architecture consists of a multi-core computer connected to one or at most two GPUs. Moreover, those GPUs can have different characteristics and memory sizes. A setup with four or more identical GPUs is rather uncommon, but it would be ideal from a memory point of view. It would imply that the maximum memory is four times more than on a single GPU. How- ever GPUs are connected to a PCI bus and in some cases two GPUs share the same PCI bus, which creates data transfer limitations. To summarize, using multi-GPUs increases the total memory size but data transfer problems appear. Therefore, in Chapter 3 we consider different multi-GPU approaches and understand how data transfer affects the performance of a Krylov subspace solver with shifted Laplace multigrid preconditioner for the three-dimensional Helmholtz equation using CUDA (Compute Unified Device Architecture). Two multi-GPU approaches are considered: data parallelism and split of the algorithm. Their implementations on a multi-GPU architecture are compared to a multi-threaded CPU and single GPU implementation. The results show that the data parallel implementation suffers from communication between GPUs and the CPU, but is still a number of times faster compared to many-cores. The split of the algorithm across GPUs limits communication and delivers speedups comparable to a single GPU implementation. As a geophysical application which requires an efficient numerical method we con- sider 3-D reverse time migration with the constant-density acoustic wave equation in Chapter 4. The idea of migration in the time domain is to calculate the forward wave- field by injecting the source wavelet. Secondly, we compute the wavefield backward in time by injecting the recorded signal at the receiver locations. Subsequently, we cross- correlate the forward and backward wavefields at given timesteps. An explicit finite- difference scheme in the time domain is a common choice. However, it requires a significant amount of disk space to store the forward wavefields. The advantage of migration with a frequency domain solver is that it does not require large amounts of disk space to store the snapshots. However, a disadvantage is the memory usage of the solver. As GPUs have generally much less memory available than CPUs, this impacts the size of the problem significantly. The frequency-domain approach simplifies the correlation of the source and receiver wavefields, but requires the solution of a large sparse linear system of equations. The question is whether migration in the frequency domain can compete with a time-domain implementation when both are performed on a parallel architecture. Both methods are naturally parallel over shots, but the frequency-domain method is also parallel over frequencies. If we have a sufficiently large number of compute nodes, we can compute the result for each frequency in parallel and the required time is dominated by the number of iterations for the highest frequency. Here, GPUs are used as accelerators and not as independent compute nodes. We optimize the throughput of the latter with dynamic load balancing, asynchronous I/O and compression of snapshots. Since the frequency- domain solver employs a matrix-dependent prolongation, the coarse grid operators required more storage than available on GPUs for problems of realistic sizes. An alternative to the depth migration is least-squares migration (LSM). LSM was introduced as a bridge between full waveform inversion and migration. Like migration, LSM does not attempt to retrieve the background velocity model, however, like full wave- form inversion the modeled data should fit the observations. In Chapter 5 an efficient LSM algorithm is presented using several enhancements. Firstly, a frequency decimation approach is introduced that makes use of the redundant information present in the data. It leads to a speedup of LSM, whereas the impact on accuracy is kept minimal. Secondly, to store the sparse discretization and matrix-dependent prolongation matrices efficiently, a new matrix storage format VCRS (Very Compressed Row Storage) is presented. This format is capable of handling lossless compression. It does not only reduce the size of the stored matrix by a certain factor but also increases the efficiency of the matrix-vector computations. The study shows that the effect of lossless and lossy compression with a proper choice of the compression parameters are positive. Thirdly, we accelerate the LSM engine by GPUs. A GPU is used as an accelerator, where the data is partially transferred to a GPU to execute a set of operations, or as a replacement, where the complete data is stored in the GPU memory. We demonstrate that using GPU as a replacement leads to higher speedups and allows us to solve larger problem sizes. Summarizing the effects of each improvement, the resulting speedup can be at least an order of magnitude compared to the original LSM method.Delft Institute of Applied MathematicsElectrical Engineering, Mathematics and Computer Scienc
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Closing the performance gap between an iterative frequency-domain solver and an explicit time-domain scheme for 3D migration on parallel architectures
Full text linkThree-dimensional reverse-time migration with the constant-density acoustic wave equation requires an efficient numerical scheme for the computation of wavefields. An explicit finite-difference scheme in the time domain is a common choice. However, it requires a significant amount of disk space for the imaging condition. The frequency-domain approach simplifies the correlation of the source and receiver wavefields, but requires the solution of a large sparse linear system of equations. For the latter, we use an iterative Krylov solver based on a shifted Laplace multigrid preconditioner with matrix-dependent prolongation. The question is whether migration in the frequency domain can compete with a time-domain implementation when both are performed on a parallel architecture. Both methods are naturally parallel over shots, but the frequency-domain method is also parallel over frequencies. If we have a sufficiently large number of compute nodes, we can compute the result for each frequency in parallel and the required time is dominated by the number of iterations for the highest frequency. As a parallel architecture, we consider a commodity hardware cluster that consists of multicore central processing units (CPUs), each of them connected to two graphics processing units (GPUs). Here, GPUs are used as accelerators and not as an independent compute node. The parallel implementation of the 3D migration in frequency domain is compared to a time-domain implementation. We optimize the throughput of the latter with dynamic load balancing, asynchronous I/O, and compression of snapshots. Because the frequency-domain solver uses matrix-dependent prolongation, the coarse-grid operators require more storage than available on GPUs for problems of realistic size. Due to data transfer, there is no significant speedup using GPU-accelerators. Therefore, we consider an implementation on CPUs only. Nevertheless, with the parallelization over shots and frequencies, this approach could compete with the time-domain implementation on multiple GPUs.Delft Institute of Applied MathematicsElectrical Engineering, Mathematics and Computer Scienc
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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