12,871 research outputs found
Multi-scale uncertainty quantification in geostatistical seismic inversion
Geostatistical seismic inversion is commonly used to infer the spatial
distribution of the subsurface petro-elastic properties by perturbing the model
parameter space through iterative stochastic sequential
simulations/co-simulations. The spatial uncertainty of the inferred
petro-elastic properties is represented with the updated a posteriori variance
from an ensemble of the simulated realizations. Within this setting, the
large-scale geological (metaparameters) used to generate the petro-elastic
realizations, such as the spatial correlation model and the global a priori
distribution of the properties of interest, are assumed to be known and
stationary for the entire inversion domain. This assumption leads to
underestimation of the uncertainty associated with the inverted models. We
propose a practical framework to quantify uncertainty of the large-scale
geological parameters in seismic inversion. The framework couples
geostatistical seismic inversion with a stochastic adaptive sampling and
Bayesian inference of the metaparameters to provide a more accurate and
realistic prediction of uncertainty not restricted by heavy assumptions on
large-scale geological parameters. The proposed framework is illustrated with
both synthetic and real case studies. The results show the ability retrieve
more reliable acoustic impedance models with a more adequate uncertainty spread
when compared with conventional geostatistical seismic inversion techniques.
The proposed approach separately account for geological uncertainty at
large-scale (metaparameters) and local scale (trace-by-trace inversion)
Power spectrum multipoles on the curved sky: an application to the 6-degree Field Galaxy Survey
The peculiar velocities of galaxies cause their redshift-space clustering to
depend on the angle to the line-of-sight, providing a key test of gravitational
physics on cosmological scales. These effects may be described using a
multipole expansion of the clustering measurements. Focussing on Fourier-space
statistics, we present a new analysis of the effect of the survey window
function, and the variation of the line-of-sight across a survey, on the
modelling of power spectrum multipoles. We determine the joint covariance of
the Fourier-space multipoles in a Gaussian approximation, and indicate how
these techniques may be extended to studies of overlapping galaxy populations
via multipole cross-power spectra. We apply our methodology to one of the
widest-area galaxy redshift surveys currently available, the 6-degree Field
Galaxy Survey, deducing a normalized growth rate f*sigma_8(z=0.06) = 0.38 +/-
0.12 in the low-redshift Universe, in agreement with previous analyses of this
dataset using different techniques. Our framework should be useful for
processing future wide-angle galaxy redshift surveys.Comment: 17 pages, 7 figures, version accepted by MNRA
A hybrid approach for predicting the distribution of vibro-acoustic energy in complex built-up structures
Finding the distribution of vibro-acoustic energy in complex built-up
structures in the mid-to-high frequency regime is a difficult task. In
particular, structures with large variation of local wavelengths and/or
characteristic scales pose a challenge referred to as the mid-frequency
problem. Standard numerical methods such as the finite element method (FEM)
scale with the local wavelength and quickly become too large even for modern
computer architectures. High frequency techniques, such as statistical energy
analysis (SEA), often miss important information such as dominant resonance
behaviour due to stiff or small scale parts of the structure. Hybrid methods
circumvent this problem by coupling FEM/BEM and SEA models in a given built-up
structure. In the approach adopted here, the whole system is split into a
number of subsystems which are treated by either FEM or SEA depending on the
local wavelength. Subsystems with relative long wavelengths are modelled using
FEM. Making a diffuse field assumption for the wave fields in the short wave
length components, the coupling between subsystems can be reduced to a weighted
random field correlation function. The approach presented results in an
SEA-like set of linear equations which can be solved for the mean energies in
the short wavelength subsystems
Asteroseismology of Solar-Type and Red-Giant Stars
We are entering a golden era for stellar physics driven by satellite and
telescope observations of unprecedented quality and scope. New insights on
stellar evolution and stellar interiors physics are being made possible by
asteroseismology, the study of stars by the observation of natural, resonant
oscillations. Asteroseismology is proving to be particularly significant for
the study of solar-type and red-giant stars. These stars show rich spectra of
solar-like oscillations, which are excited and intrinsically damped by
turbulence in the outermost layers of the convective envelopes. In this review
we discuss the current state of the field, with a particular emphasis on recent
advances provided by the Kepler and CoRoT space missions and the wider
significance to astronomy of the results from asteroseismology, such as stellar
populations studies and exoplanet studies.Comment: The following paper will appear in the 2013 volume of Annual Reviews
of Astronomy and Astrophysics (88 pages, 7 figures; references updated;
further corrections to typos during galley-proof review
Phoneme and sentence-level ensembles for speech recognition
We address the question of whether and how boosting and bagging can be used for speech recognition. In order to do this, we compare two different boosting schemes, one at the phoneme level and one at the utterance level, with a phoneme-level bagging scheme. We control for many parameters and other choices, such as the state inference scheme used. In an unbiased experiment, we clearly show that the gain of boosting methods compared to a single hidden Markov model is in all cases only marginal, while bagging significantly outperforms all other methods. We thus conclude that bagging methods, which have so far been overlooked in favour of boosting, should be examined more closely as a potentially useful ensemble learning technique for speech recognition
A Subband-Based SVM Front-End for Robust ASR
This work proposes a novel support vector machine (SVM) based robust
automatic speech recognition (ASR) front-end that operates on an ensemble of
the subband components of high-dimensional acoustic waveforms. The key issues
of selecting the appropriate SVM kernels for classification in frequency
subbands and the combination of individual subband classifiers using ensemble
methods are addressed. The proposed front-end is compared with state-of-the-art
ASR front-ends in terms of robustness to additive noise and linear filtering.
Experiments performed on the TIMIT phoneme classification task demonstrate the
benefits of the proposed subband based SVM front-end: it outperforms the
standard cepstral front-end in the presence of noise and linear filtering for
signal-to-noise ratio (SNR) below 12-dB. A combination of the proposed
front-end with a conventional front-end such as MFCC yields further
improvements over the individual front ends across the full range of noise
levels
- …