449 research outputs found
Fast probabilistic petrophysical mapping of reservoirs from 3D seismic data
A fast probabilistic inversion method for 3D petrophysical property prediction from inverted prestack seismic data has been developed and tested on a real data set. The inversion objective is to estimate the joint probability density function (PDF) of model vectors consisting of porosity, clay content, and water saturation components at each point in the reservoir, from data vectors with compressional- and shear-wave-impedance components that are obtained from the inversion of seismic data. The proposed inversion method is based on mixture density network (MDN), which is trained by a given set of training samples, and provides an estimate of the joint posterior PDFâs of the model parameters for any given data point. This method is much more time and memory efficient than conventional nonlinear inversion methods. The training data set is constructed using nonlinear petrophysical forward relations and includes different sources of uncertainty in the inverse problem such as variations in effective pressure, bulk modulus and density of hydrocarbon, and random noise in recorded data. Results showed that the standard deviations of all model parameters were reduced after inversion, which shows that the inversion process provides information about all parameters. The reduction of uncertainty in water saturation was smaller than that for porosity or clay content; nevertheless the maximum of the a posteriori (MAP) of model PDF clearly showed the boundary between brine saturated and oil saturated rocks at wellbores. The MAP estimates of different model parameters show the lateral and vertical continuity of these boundaries. Errors in the MAP estimate of different model parameters can be reduced using more accurate petrophysical forward relations. This fast, probabilistic, nonlinear inversion method can be applied to invert large seismic cubes for petrophysical parameters on a standard desktop computer. </jats:p
The rise and demise of the Glanville fritillary on the Isle of Wight
The Glanville fritillary is one of Britainâs rarest breeding butterflies, and is predominantly restricted to the south coast of the Isle of Wight. Populations have been monitored annually at a high proportion of known sites by counting the number of larval âwebsâ during spring since 1996. In this paper, we present population time series for eight core sites. Populations have been observed to fluctuate considerably over the last 18 years, with a high degree of synchrony between sites. Recently, numbers of larval webs have shown a severe decline, with simultaneous extinctions occurring across many former strongholds. We combine our web count data with counts of adult butterflies from five sites of the UK Butterfly Monitoring Scheme. Together, these data suggest that the Glanville fritillary is threatened by extinction on the Isle of Wight, and that the total area used for breeding is likely no more than a few km2. The results flag up the need for a national census of remaining populations and further research to understand causes of decline, so that a conservation recovery plan can be developed
Modified spontaneous symmetry breaking pattern by brane-bulk interaction terms
We show how translational invariance can be broken by the vacuum that drives
the spontaneous symmetry breaking of extra-dimensional extensions of the
Standard Model, when delta-like interactions between brane and bulk scalar
fields are present. We explicitly build some examples of vacuum configurations,
which induce the spontaneous symmetry breaking, and have non trivial profile in
the extra coordinate.Comment: 13 pages, two figure
Strong tree level unitarity violations in the extra dimensional Standard Model with scalars in the bulk
We show how the tree level unitarity violations of compactified extra
dimensional extensions of the Standard Model become much stronger when the
scalar sector is included in the bulk. This effect occurs when the couplings
are not suppressed for larger Kaluza-Klein levels, and could have relevant
consequences for the phenomenology of the next generation of colliders. We also
introduce a simple and generic formalism to obtain unitarity bounds for finite
energies, taking into account coupled channels including the towers of
Kaluza-Klein excitations.Comment: Version to appear in Phys. Rev. D Typos corrected and remarks added
to clarify figure
Pseudo Goldstones at Future Colliders from the Extended Bess Model
We consider the production of the lightest pseudo-Goldstone bosons at future
colliders through the vector resonances predicted by the extended BESS model,
which consists of an effective lagrangian parametrization with dynamical
symmetry breaking, describing scalar, vector and axial-vector bound states in a
rather general framework. We find that the detection of pseudo-Goldstone pairs
at LHC requires a careful evaluation of backgrounds. For e+e- collisions in the
TeV range the backgrounds can be easily reduced and the detection of
pseudo-Goldstone pairs is generally easier.Comment: 17 pages and 12 figures (included as a uuencoded tar file), LaTeX
(style article), UGVA-DPT 1994/03-84
The effect of two-temperature post-shock accretion flow on the linear polarization pulse in magnetic cataclysmic variables
The temperatures of electrons and ions in the post-shock accretion region of
a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass
flow rates or for sufficiently weak magnetic fields. At lower mass flow rates
or in stronger magnetic fields, efficient cyclotron cooling will cool the
electrons faster than the electrons can cool the ions and a two-temperature
flow will result. Here we investigate the differences in polarized radiation
expected from mCV post-shock accretion columns modeled with one- and
two-temperature hydrodynamics. In an mCV model with one accretion region, a
magnetic field >~30 MG and a specific mass flow rate of ~0.5 g/cm/cm/s, along
with a relatively generic geometric orientation of the system, we find that in
the ultraviolet either a single linear polarization pulse per binary orbit or
two pulses per binary orbit can be expected, depending on the accretion column
hydrodynamic structure (one- or two-temperature) modeled. Under conditions
where the physical flow is two-temperature, one pulse per orbit is predicted
from a single accretion region where a one-temperature model predicts two
pulses. The intensity light curves show similar pulse behavior but there is
very little difference between the circular polarization predictions of one-
and two-temperature models. Such discrepancies indicate that it is important to
model some aspect of two-temperature flow in indirect imaging procedures, like
Stokes imaging, especially at the edges of extended accretion regions, were the
specific mass flow is low, and especially for ultraviolet data.Comment: Accepted for publication in Astrophysics & Space Scienc
Analytic properties of the Landau gauge gluon and quark propagators
We explore the analytic structure of the gluon and quark propagators of
Landau gauge QCD from numerical solutions of the coupled system of renormalized
Dyson--Schwinger equations and from fits to lattice data. We find sizable
negative norm contributions in the transverse gluon propagator indicating the
absence of the transverse gluon from the physical spectrum. A simple analytic
structure for the gluon propagator is proposed. For the quark propagator we
find evidence for a mass-like singularity on the real timelike momentum axis,
with a mass of 350 to 500 MeV. Within the employed Green's functions approach
we identify a crucial term in the quark-gluon vertex that leads to a positive
definite Schwinger function for the quark propagator.Comment: 42 pages, 16 figures, revtex; version to be published in Phys Rev
Degenerate BESS Model: The possibility of a low energy strong electroweak sector
We discuss possible symmetries of effective theories describing spinless and
spin 1 bosons, mainly to concentrate on an intriguing phenomenological
possibility: that of a hardly noticeable strong electroweak sector at
relatively low energies. Specifically, a model with both vector and axial
vector strong interacting bosons may possess a discrete symmetry imposing
degeneracy of the two sets of bosons (degenerate BESS model). In such a case
its effects at low energies become almost invisible and the model easily passes
all low energy precision tests. The reason lies essentially in the fact that
the model automatically satisfies decoupling, contrary to models with only
vectors. For large mass of the degenerate spin one bosons the model becomes
identical at the classical level to the standard model taken in the limit of
infinite Higgs mass. For these reasons we have thought it worthwhile to fully
develop the model, together with its possible generalizations, and to study the
expected phenomenology. For instance, just because of its invisibility at low
energy, it is conceivable that degenerate BESS has low mass spin one states and
gives quite visible signals at existing or forthcoming accelerators.Comment: 37 pages, LaTeX, 14 figures (uuencoded
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sunâs centre, equal to half of Mercuryâs perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
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