1,549 research outputs found
Defining the 3D geometry of thin shale units in the Sleipner reservoir using seismic attributes
Acknowledgments The seismic interpretation and image processing was carried out in the SeisLab facility at the University of Aberdeen (sponsored by BG BP and Chevron). Seismic imaging analysis was performed using GeoTeric (ffA), and analysis of seismic amplitudes was performed in Petrel 2015 (Schlumberger). We would like to thank the NDDC (RG11766-10) for funding this research and Statoil for the release of the Sleipner field seismic dataset utilized in this research paper and also Anne-Kari Furre and her colleagues for their assistance. We also thank the editor, Alejandro Escalona and the two anonymous reviewers for their constructive and in depth comments that improved the paper.Peer reviewedPostprin
Ultraviolet and soft X--ray photon--photon elastic scattering in an electron gas
We have considered the processes which lead to elastic scattering between two
far ultraviolet or X--ray photons while they propagate inside a solid, modeled
as a simple electron gas. The new ingredient, with respect to the standard
theory of photon--photon scattering in vacuum, is the presence of low--energy,
nonrelativistic electron--hole excitations. Owing to the existence of
two--photon vertices, the scattering processes in the metal are predominantly
of second order, as opposed to fourth order for the vacuum case. The main
processes in second order are dominated by exchange of virtual plasmons between
the two photons. For two photons of similar energy , this gives
rise to a cross section rising like up to maximum of around
~cm, and then decreasing like . The maximal cross
section is found for the photon wavevector , the Fermi surface
size, which typically means a photon energy in the keV range.
Possible experiments aimed at checking the existence of these rare but
seemingly measurable elastic photon--photon scattering processes are discussed,
using in particular intense synchrotron sources.Comment: 33 pages, TeX, Version 3.1, S.I.S.S.A. preprint 35/93/C
Bayesian nonparametric graphical models for time-varying parameters VAR
Over the last decade, big data have poured into econometrics, demanding new
statistical methods for analysing high-dimensional data and complex non-linear
relationships. A common approach for addressing dimensionality issues relies on
the use of static graphical structures for extracting the most significant
dependence interrelationships between the variables of interest. Recently,
Bayesian nonparametric techniques have become popular for modelling complex
phenomena in a flexible and efficient manner, but only few attempts have been
made in econometrics. In this paper, we provide an innovative Bayesian
nonparametric (BNP) time-varying graphical framework for making inference in
high-dimensional time series. We include a Bayesian nonparametric dependent
prior specification on the matrix of coefficients and the covariance matrix by
mean of a Time-Series DPP as in Nieto-Barajas et al. (2012). Following Billio
et al. (2019), our hierarchical prior overcomes over-parametrization and
over-fitting issues by clustering the vector autoregressive (VAR) coefficients
into groups and by shrinking the coefficients of each group toward a common
location. Our BNP timevarying VAR model is based on a spike-and-slab
construction coupled with dependent Dirichlet Process prior (DPP) and allows
to: (i) infer time-varying Granger causality networks from time series; (ii)
flexibly model and cluster non-zero time-varying coefficients; (iii)
accommodate for potential non-linearities. In order to assess the performance
of the model, we study the merits of our approach by considering a well-known
macroeconomic dataset. Moreover, we check the robustness of the method by
comparing two alternative specifications, with Dirac and diffuse spike prior
distributions
Bayesian Markov-Switching Tensor Regression for Time-Varying Networks
Modeling time series of multilayer network data is challenging due to the peculiar characteristics of real-world networks, such as sparsity and abrupt structural changes. Moreover, the impact of external factors on the network edges is highly heterogeneous due to edge- and time-specific effects. Capturing all these features results in a very high-dimensional inference problem. A novel tensor-on-tensor regression model is proposed, which integrates zero-inflated logistic regression to deal with the sparsity, and Markov-switching coefficients to account for structural changes. A tensor representation and decomposition of the regression coefficients are used to tackle the high-dimensionality and account for the heterogeneous impact of the covariate tensor across the response variables. The inference is performed following a Bayesian approach, and an efficient Gibbs sampler is developed for posterior approximation. Our methodology applied to financial and email networks detects different connectivity regimes and uncovers the role of covariates in the edge-formation process, which are relevant in risk and resource management. Code is available on GitHub. Supplementary materials for this article are available online
Bayesian SAR model with stochastic volatility and multiple time-varying weights
A novel spatial autoregressive model for panel data is introduced, which
incorporates multilayer networks and accounts for time-varying relationships.
Moreover, the proposed approach allows the structural variance to evolve
smoothly over time and enables the analysis of shock propagation in terms of
time-varying spillover effects. The framework is applied to analyse the
dynamics of international relationships among the G7 economies and their impact
on stock market returns and volatilities. The findings underscore the
substantial impact of cooperative interactions and highlight discernible
disparities in network exposure across G7 nations, along with nuanced patterns
in direct and indirect spillover effects
Static and Dynamic BART for Rank-Order Data
Ranking lists are often provided at regular time intervals by one or multiple
rankers in a range of applications, including sports, marketing, and politics.
Most popular methods for rank-order data postulate a linear specification for
the latent scores, which determine the observed ranks, and ignore the temporal
dependence of the ranking lists. To address these issues, novel nonparametric
static (ROBART) and autoregressive (ARROBART) models are introduced, with
latent scores defined as nonlinear Bayesian additive regression tree functions
of covariates. To make inferences in the dynamic ARROBART model, closed-form
filtering, predictive, and smoothing distributions for the latent time-varying
scores are derived. These results are applied in a Gibbs sampler with data
augmentation for posterior inference. The proposed methods are shown to
outperform existing competitors in simulation studies, and the advantages of
the dynamic model are demonstrated by forecasts of weekly pollster rankings of
NCAA football teams.Comment: The Supplementary Material is available upon request to the author
Quantifying human performance in chess
From sports to science, the recent availability of large-scale data has
allowed to gain insights on the drivers of human innovation and success in a
variety of domains. Here we quantify human performance in the popular game of
chess by leveraging a very large dataset comprising of over 120 million games
between almost 1 million players. We find that individuals encounter hot
streaks of repeated success, longer for beginners than for expert players, and
even longer cold streaks of unsatisfying performance. Skilled players can be
distinguished from the others based on their gaming behaviour. Differences
appear from the very first moves of the game, with experts tending to
specialize and repeat the same openings while beginners explore and diversify
more. However, experts experience a broader response repertoire, and display a
deeper understanding of different variations within the same line. Over time,
the opening diversity of a player tends to decrease, hinting at the development
of individual playing styles. Nevertheless, we find that players are often not
able to recognize their most successful openings. Overall, our work contributes
to quantifying human performance in competitive settings, providing a first
large-scale quantitative analysis of individual careers in chess, helping
unveil the determinants separating elite from beginner performance.Comment: 8 pages, 5 figure
A Spatiotemporal Gamma Shot Noise Cox Process
A new discrete-time shot noise Cox process for spatiotemporal data is
proposed. The random intensity is driven by a dependent sequence of latent
gamma random measures. Some properties of the latent process are derived, such
as an autoregressive representation and the Laplace functional. Moreover, these
results are used to derive the moment, predictive, and pair correlation
measures of the proposed shot noise Cox process. The model is flexible but
still tractable and allows for capturing persistence, global trends, and latent
spatial and temporal factors. A Bayesian inference approach is adopted, and an
efficient Markov Chain Monte Carlo procedure based on conditional Sequential
Monte Carlo is proposed. An application to georeferenced wildfire data
illustrates the properties of the model and inference
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