1,721,169 research outputs found
Two-grid full-waveform Rayleigh-wave inversion via a genetic algorithm — Part 2: Application to two actual data sets
Get PDFWe present the application of our two-grid genetic-algorithm Rayleigh-wave full-waveform inversion (FWI), which has been described in a companion paper, to two actual data sets acquired in Luni (Italy) and Grenoble (France), respectively. As our technique employs 2D elastic finite difference modeling for solving the forward problem, the observed data were 3D to 2D corrected prior to the inversion. To limit the computing time, both the inversions were focused on predicting low-resolution, smooth models by using quite coarse inversion grids. The wavelets for the FWI were estimated directly from the observed data via the Wiener method. In the Luni case, due to the strong dispersion effects on the data, to strengthen the inversion both envelopes and waveforms were considered in the objective function and an offset-marching strategy was applied. Though no a priori information was exploited, both the outcomes of the Luni and Grenoble data inversion were fair. The predicted Luni Vs model shows a strong velocity increase from about 3 to 6 m and velocity inversions have been detected at around 2 m and 9 m depth. Analyzing the dispersion spectra, it results that the predicted Luni data reasonably reproduced the waveforms related to the fundamental mode and, likely, a small part of those related to the 1st higher mode. Concerning the Grenoble example, the predicted Vs model coincides reasonably well with the long-wavelength structures presented in the Vs profiles obtained from nearby boreholes. The data reconstruction is generally satisfactory and when mismatches occur between the predicted and observed traces, the phase differences are always within half-periods. The fair inversion outcomes suggest that both the predicted Luni and Grenoble model would likely be adequate initial models for local FWI, which could further increase the resolution and the details of the estimated Vs models
Two-grid full-waveform Rayleigh-wave inversion via a genetic algorithm — Part 1: Method and synthetic examples
Get PDFWhen reliable a priori information is not available, it is difficult to correctly predict near-surface shear wave velocity models from Rayleigh waves through existing techniques, especially in the case of complex geology. To tackle this issue, we propose a new method, two-grid genetic-algorithm Rayleigh-wave full-waveform inversion (FWI).
Adopting a two-grid parameterization of the model, the genetic algorithm inverts for unknown velocities and densities at the nodes of a coarse grid, while forward modeling is performed on a fine grid to avoid numerical dispersion. A bilinear interpolation brings the coarse grid results into the fine grid models. The coarse inversion grid allows for a significant reduction of the computing time required by the genetic algorithm to converge. The coarser the grid, the less the unknowns, the less the required computing time, at the expense of the model resolution.
To further increase efficiency, our inversion code can perform the optimization employing a frequency-marching strategy and/or an offset-marching strategy, can make use of different kind of objective function and it allows for parallel computing.
We illustrate the effect of the proposed inversion method using three synthetic examples with rather complex near-surface models. Though no a priori information was used in all the three tests, the long-wavelength structures of the reference models were fairly predicted, and satisfactory matches between “observed” and predicted data were achieved.
The fair predictions of the reference models suggest that the final models estimated by our genetic-algorithm FWI, which we call macro-models, would be suitable input to gradient-based Rayleigh-wave FWI for further refinement.
Other issues related to the practical use of the method are presented in a companion paper that shows the applications of the method to field data
CAI, XING-ZHEN
No full text一、研究目的：固氮性藍綠藻為唯一可在有氧條件下，利用光能固定空氣中二氧化碳
和氮氣，合成細胞內有機物的生物；尤其是固氮作用為一種對氧氣敏感的反應，因此
我們希望能瞭解固氮性藍綠藻固氮酵素活性的特性及固氮性藍綠藻對不同氮源的需求
，並進而探求其有關氮的代謝途徑。
二、研究方法：
(1) 藍綠藻的培養：本實驗以台灣水稻田所分離純化的藍綠細菌、念珠藻Anabaena
CH3 為材料。(2) 葉綠素與蛋白質的測定。(3) 生長速率的測定。(4) 異型細胞頻率
的測定。(5) 固氮酵素活性的測定。(6) 培養液中硝酸鹽濃度的測定。(7) 培養液中
銨離子濃度的測定。
三、研究結果：
1.不同氮源對念珠藻Anabaena CH3生長的影響：
短時間（24小時）及長時間（16天）培養時，不同氮源對生長、固氮活性比及異型細
胞頻率的影響。
2.念珠藻Anabaena CH3的氮源吸收特性：
各以硝酸鹽及銨鹽為單獨氮源時的吸收曲線的比較及混合以上兩種氮源時的吸收曲線
比較，發現念珠藻Anabaena CH3在有氮氣存在下時會先吸收銨鹽再吸收硝酸鹽。而且
知硝酸鹽及銨鹽的吸收為主動運輸
Rayleigh Waves Modelling Complexities in the Perspective of Full Waveform Inversion of Surface Waves - Synthetic Examples
No full textFull waveform inversion (FWI) has recently been considered for extracting detailed near surface information through the surface waves inversion. But the need of long computing time and the risk to get trapped into local minima make FWI cumbersome. Both these shortcomings are strongly dependent on forward modeling. In fact, forward modeling must yield very accurate seismograms and be computationally feasible. In this study we use an elastic finite difference modeling and discuss means to address, at least partially, these two conflicting requirements. In particular, we illustrate the possibility of attenuating the computing time problem by implementing the convolutional perfectly matched layers (CPML), and by performing the 3D to 2D correction on the observed data, thus allowing for using simple 2D forward modeling. It turns out that CPML has the ability to accelerate the computations compared with the standard tapering method, while maintaining the same efficiency in attenuating the unwanted artifacts from the model boundaries, and that 3D to 2D correction gives satisfactory results. Both of them result very useful in the perspective of surface waves FWI applications, also in view of the fact that other features, such as irregular topography, which cannot be neglected, require additional computing efforts
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Comparison between sparse Radon transforms towards an MPI implementation.
No full textIn exploration geophysics the linear Radon transform projects seismic data from
time-offset (t-x) domain into intercept time-ray parameter (τ-p) domain while the parabolic Radon
transform maps t-x domain seismic data into intercept time-curvature (τ-q) domain.
Seismic reflections can be approximated as hyperbolae when the dips are gentle and the spread is
small (Yilmaz, 2001). Ideal linear Radon transform maps a hyperbolic seismic event in an ellipse
with the center at (0,0) and the vertical and horizontal radii equal to the intercept time τ and the
reciprocal of the root mean square velocity 1/v, respectively. Normal moveout (NMO) correction
approximates hyperbolic seismic events as parabolas that become points when an ideal parabolic
Radon transform is applied to the seismic gather.
In Radon transform domain Random noise is mapped destructively while seismic reflections are
projected constructively and separately. This makes Radon transform methods useful in enhancing
signal to noise ratio, doing trace interpolation and attenuating multiples in seismic data processing,
especially for marine seismic data. In this work we focus our attention mainly on the parabolic
Radon transform.
Two main problems that arise in the Radon transform computation are i) the artifacts generated
in the transform domain and ii) the long computing time.
The artifacts originate from two sources. One is related with a too coarse transform domain
sampling rate or a too wide sampling range, the other is related with the limited offset range in t-x
domain. Different sampling criterions can be used to deal with the limits of the sampling rate and
range (Turner, 1990; Schonewille and Duijindam, 2001). A common statement of the sampling
criterions is that a higher frequency coincides with a more stringent limit of sampling rate and
range. So we can relieve these requirements by doing a filtering before Radon transform. But we
should only filter out the frequencies between the maximum useful frequency of the signal and the
Nyquist frequency to avoid degrading the resolution of the seismic data.
The limited offset range in t-x domain makes NMO corrected seismic events be projected into
butterfly structures instead of points in τ-q domain. This fact causes the inverse parabolic Radon
transform to be unable to reconstruct the original data faithfully and leads to the difficulty in doing
multiple attenuation with the Radon transform method. To deal with this difficulty different authors
developed various sparse Radon transform algorithms to compress the butterfly structures. Sacchi
and Ulrych (1995) proposed the frequency domain sparse Radon transform (FDS-RT). Trad et al.
(2003) presented the time-frequency domain sparse Radon transform (TFDS-RT). Lu (2013)
formulated the iterative shrinkage sparse Radon transform (ISS-RT). We developed our code
according to different standard and sparse Radon transform algorithms and compared their
performances in the reconstruction of the original data and in the demultiple processing. We also
compared our results with the outcomes of the ProMAX® software for what concerns the ability of
multiple attenuation.
To deal with the long computing time problem, we used Open MPI to parallelize the Radon
transform algorithms. Open MPI is a Message Passing Interface (MPI) library project. MPI allows
users to simultaneously use multiple processors to perform a calculation
Variations on the Author
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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
- …
