751 research outputs found
Science requirements and the design of cabled ocean observatories
The ocean sciences are beginning a new phase in which scientists will enter the ocean environment and adaptively
observe the Earth-Ocean system through remote control of sensors and sensor platforms. This new ocean
science paradigm will be implemented using innovative facilities called ocean observatories which provide unprecedented
levels of power and communication to access and manipulate real-time sensor networks deployed
within many different environments in the ocean basins. Most of the principal design drivers for ocean observatories
differ from those for commercial submarine telecommunications systems. First, ocean observatories require
data to be input and output at one or more seafloor nodes rather than at a few land terminuses. Second,
ocean observatories must distribute a lot of power to the seafloor at variable and fluctuating rates. Third, the
seafloor infrastructure for an ocean observatory inherently requires that the wet plant be expandable and reconfigurable.
Finally, because the wet communications and power infrastructure is comparatively complex, ocean
observatory infrastructure must be designed for low life cycle cost rather than zero maintenance. The origin of
these differences may be understood by taking a systems engineering approach to ocean observatory design
through examining the requirements derived from science and then going through the process of iterative refinement
to yield conceptual and physical designs. This is illustrated using the NEPTUNE regional cabled observatory
power and data communications sub-systems
Localization of thermal packets and metastable states in Sinai model
We consider the Sinai model describing a particle diffusing in a 1D random
force field. As shown by Golosov, this model exhibits a strong localization
phenomenon for the thermal packet: the disorder average of the thermal
distribution of the relative distance y=x-m(t), with respect to the
(disorder-dependent) most probable position m(t), converges in the limit of
infinite time towards a distribution P(y). In this paper, we revisit this
question of the localization of the thermal packet. We first generalize the
result of Golosov by computing explicitly the joint asymptotic distribution of
relative position y=x(t)-m(t) and relative energy u=U(x(t))-U(m(t)) for the
thermal packet. Next, we compute in the infinite-time limit the localization
parameters Y_k, representing the disorder-averaged probabilities that k
particles of the thermal packet are at the same place, and the correlation
function C(l) representing the disorder-averaged probability that two particles
of the thermal packet are at a distance l from each other. We moreover prove
that our results for Y_k and C(l) exactly coincide with the thermodynamic limit
of the analog quantities computed for independent particles at equilibrium in a
finite sample of length L. Finally, we discuss the properties of the
finite-time metastable states that are responsible for the localization
phenomenon and compare with the general theory of metastable states in glassy
systems, in particular as a test of the Edwards conjecture.Comment: 17 page
Finite time and asymptotic behaviour of the maximal excursion of a random walk
We evaluate the limit distribution of the maximal excursion of a random walk
in any dimension for homogeneous environments and for self-similar supports
under the assumption of spherical symmetry. This distribution is obtained in
closed form and is an approximation of the exact distribution comparable to
that obtained by real space renormalization methods. Then we focus on the early
time behaviour of this quantity. The instantaneous diffusion exponent
exhibits a systematic overshooting of the long time exponent. Exact results are
obtained in one dimension up to third order in . In two dimensions,
on a regular lattice and on the Sierpi\'nski gasket we find numerically that
the analytic scaling holds.Comment: 9 pages, 4 figures, accepted J. Phys.
Pliocene Te Aute limestones, New Zealand: Expanding concepts for cool-water shelf carbonates
Acceptance of a spectrum of warm- through cold-water shallow-marine carbonate facies has become of fundamental importance for correctly interpreting the origin and significance of all ancient platform limestones. Among other attributes, properties that have become a hallmark for characterising many Cenozoic non-tropical occurrences include: (1) the presence of common bryozoan and epifaunal bivalve skeletons; (2) a calcite-dominated mineralogy; (3) relatively thin deposits exhibiting low rates of sediment accumulation; (4) an overall destructive early diagenetic regime; and (5) that major porosity destruction and lithification occur mainly in response to chemical compaction of calcitic skeletons during moderate to deep burial. The Pliocene Te Aute limestones are non-tropical skeletal carbonates formed at paleolatitudes near 40-42°S under the influence of commonly strong tidal flows along the margins of an actively deforming and differentially uplifting forearc basin seaway, immediately inboard of the convergent Pacific-Australian plate boundary off eastern North Island, New Zealand. This dynamic depositional and tectonic setting strongly influenced both the style and subsequent diagenetic evolution of the limestones. Some of the Te Aute limestones exhibit the above kinds of "normal" non-tropical characteristics, but others do not. For example, many are barnacle and/or bivalve dominated, and several include attributes that at least superficially resemble properties of certain tropical carbonates. In this regard, a number of the limestones are infaunal bivalve rich and dominated by an aragonite over a calcite primary mineralogy, with consequently relatively high diagenetic potential. Individual limestone units are also often rather thick (e.g., up to 50-300 m), with accumulation rates from 0.2 to 0.5 m/ka, and locally as high as 1 m/ka. Moreover, there can be a remarkable array of diagenetic features in the limestones, involving grain alteration and/or cementation to widely varying extents within any, or some combination of, the marine phreatic, burial, and meteoric diagenetic environments, including locally widespread development of meteoric cement sourced from aragonite dissolution. The message is that non-tropical shelf carbonates include a more diverse array of geological settings, of skeletal and mineralogical facies, and of diagenetic features than current sedimentary models mainly advocate. While several attributes positively distinguish tropical from non-tropical limestones, continued detailed documentation of the wide spectrum of shallow-marine carbonate deposits formed outside tropical regions remains an important challenge in carbonate sedimentology
Einstein-Weyl structures corresponding to diagonal K\"ahler Bianchi IX metrics
We analyse in a systematic way the four dimensionnal Einstein-Weyl spaces
equipped with a diagonal K\"ahler Bianchi IX metric. In particular, we show
that the subclass of Einstein-Weyl structures with a constant conformal scalar
curvature is the one with a conformally scalar flat - but not necessarily
scalar flat - metric ; we exhibit its 3-parameter distance and Weyl one-form.
This extends previous analysis of Pedersen, Swann and Madsen , limited to the
scalar flat, antiself-dual case. We also check that, in agreement with a
theorem of Derdzinski, the most general conformally Einstein metric in the
family of biaxial K\"ahler Bianchi IX metrics is an extremal metric of Calabi,
conformal to Carter's metric, thanks to Chave and Valent's results.Comment: 15 pages, Latex file, minor modifications, to be published in Class.
Quant. Gra
Concerted changes in tropical forest structure and dynamics: evidence from 50 South American long-term plots
Several widespread changes in the ecology of old-growth tropical forests have recently been documented for the late twentieth century, in particular an increase in stem turnover (pan-tropical), and an increase in above-ground biomass (neotropical). Whether these changes are synchronous and whether changes in growth are also occurring is not known. We analysed stand-level changes within 50 long-term. monitoring plots from across South America spanning 1971-2002. We show that: (i) basal area (BA: sum of the cross-sectional areas of all trees in a plot) increased significantly over time (by 0.10 +/- 0.04 m(2) ha(-1) yr(-1), mean +/- 95% CI); as did both (ii) stand-level BA growth rates (sum of the increments of BA of surviving trees and BA of new trees that recruited into a plot); and (iii) stand-level BA mortality rates (sum of the cross-sectional areas of all trees that died in a plot). Similar patterns were observed on a per-stem basis: (i) stem density (number of stems per hectare; 1 hectare is 10(4) m(2)) increased significantly over time (0.94 +/- 0.63 stems ha(-1) yr(-1)); as did both (ii) stem recruitment rates; and (iii) stem mortality rates. In relative terms, the pools of BA and stem density increased by 0.38 +/- 0.15% and 0.18 +/- 0.12% yr(-1), respectively. The fluxes into and out of these pools-stand-level BA growth, stand-level BA mortality, stem recruitment and stem mortality rates-increased, in relative terms, by an order of magnitude more. The gain terms (BA growth, stem recruitment) consistently exceeded the loss terms (BA loss, stem mortality) throughout the period, suggesting that whatever process is driving these changes was already acting before the plot network was established. Large long-term increases in stand-level BA growth and simultaneous increases in stand BA and stem density imply a continent-wide increase in resource availability which is increasing net primary productivity and altering forest dynamics. Continent-wide changes in incoming solar radiation, and increases in atmospheric concentrations of CO2 and air temperatures may have increased resource supply over recent decades, thus causing accelerated growth and increased dynamism across the world's largest tract of tropical forest
Slepian functions and their use in signal estimation and spectral analysis
It is a well-known fact that mathematical functions that are timelimited (or
spacelimited) cannot be simultaneously bandlimited (in frequency). Yet the
finite precision of measurement and computation unavoidably bandlimits our
observation and modeling scientific data, and we often only have access to, or
are only interested in, a study area that is temporally or spatially bounded.
In the geosciences we may be interested in spectrally modeling a time series
defined only on a certain interval, or we may want to characterize a specific
geographical area observed using an effectively bandlimited measurement device.
It is clear that analyzing and representing scientific data of this kind will
be facilitated if a basis of functions can be found that are "spatiospectrally"
concentrated, i.e. "localized" in both domains at the same time. Here, we give
a theoretical overview of one particular approach to this "concentration"
problem, as originally proposed for time series by Slepian and coworkers, in
the 1960s. We show how this framework leads to practical algorithms and
statistically performant methods for the analysis of signals and their power
spectra in one and two dimensions, and on the surface of a sphere.Comment: Submitted to the Handbook of Geomathematics, edited by Willi Freeden,
Zuhair M. Nashed and Thomas Sonar, and to be published by Springer Verla
Theoretical biology - Comparing models of species abundance
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62867/1/nature04826.pd
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