1,010 research outputs found
Baseline studies and evaluation of the physical, chemical, and biological characteristics of nearshore dredge spoil disposal, Pearl Harbor, Hawaii: Part A: Baseline studies, investigation and selection of a suitable dredge spoil site; final report
Results of the geology, physical oceanography, water chemistry and biological baseline investigations of the proposed dredge spoil disposal site off Pearl Harbor, Honolulu, Hawaii, have yielded no evidence to suggest that dumping of dredge spoil will create significant adverse effects on the environment. Results obtained during these investigations have shown that the proposed site is suitable as a permanent dredge spoil disposal site. Further confirmation of this conclusion will be obtained through the proposed monitoring activities conducted during actual disposal operations (phase B). The general circulation and current patterns in the disposal area lead to the further recommendation that actual dumping take place in the southeast corner of the disposal area, moving from east to west.Pacific Division Naval Facilities Engineering Command, Contract Number N62742-76, C-0050
On the physics of frequency domain controlled source electromagnetics in shallow water, 2: transverse anisotropy
Author Posting. © The Authors, 2017. This article is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 211 (2017): 1046–1061, doi:10.1093/gji/ggx360.In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. It is the purpose of this paper to evaluate the physics of CSEM for an ocean whose electrical thickness is comparable to or much thinner than that of the overburden using the in-line configuration through examination of the elliptically-polarized seafloor electric field, the time-averaged energy flow depicted by the real part of the complex Poynting vector, energy dissipation through Joule heating and the Fréchet derivatives of the seafloor field with respect to the sub-seafloor conductivity that is assumed to be transversely anisotropic, with a vertical-to-horizontal resistivity ratio of 3:1. For an ocean whose electrical thickness is comparable to that of the overburden, the seafloor electromagnetic response for a model containing a resistive reservoir layer has a greater amplitude and reduced phase as a function of offset compared to that for a halfspace, or a stronger and faster response, and displays little to no evidence for the air interaction. For an ocean whose electrical thickness is much smaller than that of the overburden, the electric field displays a greater amplitude and reduced phase at small offsets, shifting to a stronger amplitude and increased phase at intermediate offsets, and a weaker amplitude and enhanced phase at long offsets, or a stronger and faster response that first changes to stronger and slower, and then transitions to weaker and slower. By comparison to the isotropic case with the same horizontal conductivity, transverse anisotropy stretches the Poynting vector and the electric field response from a thin resistive layer to much longer offsets. These phenomena can be understood by visualizing the energy flow throughout the structure caused by the competing influences of the dipole source and guided energy flow in the reservoir layer, and the air interaction caused by coupling of the entire sub-seafloor resistivity structure with the sea surface. The Fréchet derivatives are dominated by preferential sensitivity to the vertical conductivity in the reservoir layer and overburden at short offsets. The horizontal conductivity Fréchet derivatives are weaker than to comparable to the vertical derivatives at long offsets in the substrate. This means that the sensitivity to the horizontal conductivity is present in the shallow parts of the subsurface. In the presence of transverse anisotropy, it is necessary to go to higher frequencies to sense the horizontal conductivity in the overburden as compared to an isotropic model with the same horizontal conductivity. These observations in part explain the success of shallow towed CSEM using only measurements of the in-line component of the electric field.This work was supported at WHOI by an Independent Research and Development award, and by the Walter A. and Hope Noyes Smith Chair for Excellence in Oceanography
On the physics of frequency-domain controlled source electromagnetics in shallow water. 1: isotropic conductivity
Author Posting. © The Authors, 2016. This article is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 208 (2017): 1026-1042, doi:10.1093/gji/ggw435.In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. It is the purpose of this paper to evaluate the physics of CSEM for an ocean whose electrical thickness is comparable to or much thinner than that of the overburden using the in-line configuration through examination of the elliptically polarized seafloor electric field, the time-averaged energy flow depicted by the real part of the complex Poynting vector, energy dissipation through Joule heating and the Fréchet derivatives of the seafloor field with respect to the subseafloor conductivity that is assumed to be isotropic. The deep water (ocean layer electrically much thicker than the overburden) seafloor EM response for a model containing a resistive reservoir layer has a greater amplitude and reduced phase as a function of offset compared to that for a half-space, or a stronger and faster response. For an ocean whose electrical thickness is comparable to or much smaller than that of the overburden, the electric field displays a greater amplitude and reduced phase at small offsets, shifting to a stronger amplitude and increased phase at intermediate offsets and a weaker amplitude and enhanced phase at long offsets, or a stronger and faster response that first changes to stronger and slower, and then transitions to weaker and slower. These transitions can be understood by visualizing the energy flow throughout the structure caused by the competing influences of the dipole source and guided energy flow in the reservoir layer, and the air interaction caused by coupling of the entire subseafloor resistivity structure with the sea surface. A stronger and faster response occurs when guided energy flow is dominant, while a weaker and slower response occurs when the air interaction is dominant. However, at intermediate offsets for some models, the air interaction can partially or fully reverse the direction of energy flux in the reservoir layer toward rather than away from the source, resulting in a stronger and slower response. The Fréchet derivatives are dominated by preferential sensitivity to the reservoir layer conductivity for all water depths except at high frequencies, but also display a shift with offset from the galvanic to the inductive mode in the underburden and overburden due to the interplay of guided energy flow and the air interaction. This means that the sensitivity to the horizontal conductivity is almost as strong as to the vertical component in the shallow parts of the subsurface, and in fact is stronger than the vertical sensitivity deeper down. However, the sensitivity to horizontal conductivity is still weak compared to the vertical component within thin resistive regions. The horizontal sensitivity is gradually decreased when the water becomes deep. These observations in part explain the success of shallow towed CSEM using only measurements of the in-line component of the electric field
FIREBALL: Instrument pointing and aspect reconstruction
The Faint Intergalactic Redshifted Emission Balloon (FIREBALL) had its first scientific flight in June 2009. The instrument is a 1 meter class balloon-borne telescope equipped with a vacuum-ultraviolet integral field spectrograph intended to detect emission from the inter-galactic medium at redshifts 0.3 < z < 1.0. The scientific goals and the challenging environment place strict constraints on the pointing and tracking systems of the gondola. In this manuscript we briefly review our pointing requirements, discuss the methods and solutions used to meet those requirements, and present the aspect reconstruction results from the first successful scientific flight
Contact spheres and hyperk\"ahler geometry
A taut contact sphere on a 3-manifold is a linear 2-sphere of contact forms,
all defining the same volume form. In the present paper we completely determine
the moduli of taut contact spheres on compact left-quotients of SU(2) (the only
closed manifolds admitting such structures). We also show that the moduli space
of taut contact spheres embeds into the moduli space of taut contact circles.
This moduli problem leads to a new viewpoint on the Gibbons-Hawking ansatz in
hyperkahler geometry. The classification of taut contact spheres on closed
3-manifolds includes the known classification of 3-Sasakian 3-manifolds, but
the local Riemannian geometry of contact spheres is much richer. We construct
two examples of taut contact spheres on open subsets of 3-space with nontrivial
local geometry; one from the Helmholtz equation on the 2-sphere, and one from
the Gibbons-Hawking ansatz. We address the Bernstein problem whether such
examples can give rise to complete metrics.Comment: 29 pages, v2: Large parts have been rewritten; previous Section 6 has
been removed; new Section 5.2 on the Gibbons-Hawking ansatz; new Sections 6
and
Spatial effects on species persistence and implications for biodiversity
Natural ecosystems are characterized by striking diversity of form and
functions and yet exhibit deep symmetries emerging across scales of space, time
and organizational complexity. Species-area relationships and species-abundance
distributions are examples of emerging patterns irrespective of the details of
the underlying ecosystem functions. Here we present empirical and theoretical
evidence for a new macroecological pattern related to the distributions of
local species persistence times, defined as the timespans between local
colonizations and extinctions in a given geographic region. Empirical
distributions pertaining to two different taxa, breeding birds and herbaceous
plants, analyzed in a new framework that accounts for the finiteness of the
observational period, exhibit power-law scaling limited by a cut-off determined
by the rate of emergence of new species. In spite of the differences between
taxa and spatial scales of analysis, the scaling exponents are statistically
indistinguishable from each other and significantly different from those
predicted by existing models. We theoretically investigate how the scaling
features depend on the structure of the spatial interaction network and show
that the empirical scaling exponents are reproduced once a two-dimensional
isotropic texture is used, regardless of the details of the ecological
interactions. The framework developed here also allows to link the cut-off
timescale with the spatial scale of analysis, and the persistence-time
distribution to the species-area relationship. We conclude that the inherent
coherence obtained between spatial and temporal macroecological patterns points
at a seemingly general feature of the dynamical evolution of ecosystems.Comment: 5 pages, 5 figures. Supplementary materials avaliable on
http://www.pnas.org/content/108/11/434
Deficiências de macronutrientes na sota (Glycine max L., Merril, var. IAC-2)
Sintomas de deficiência de macronutrientes foram induzidos na soja, var. IAC-2. Foi verificado o efeito da omissão de N, R, K, Ca, Mg e S no crescimento, produção e composição mineral das folhas.Soybean plants, var. IAC-2 were grown in nutrient solution in presence and absence of macronutrients whose symptoms of deficiency were induced. The main conclusions are as follws : 1) The symptoms of deficiency herein obtained follow the general pattern as found in other plants; 2) growth and yield were drastically reduced by absence of N and Ca from the substrate; 3) omission of on element caused a decrease in its content in the leaves down to deficiency levels, except in the case of S
Anomalous diffusion, Localization, Aging and Sub-aging effects in trap models at very low temperature
We study in details the dynamics of the one dimensional symmetric trap model,
via a real-space renormalization procedure which becomes exact in the limit of
zero temperature. In this limit, the diffusion front in each sample consists in
two delta peaks, which are completely out of equilibrium with each other. The
statistics of the positions and weights of these delta peaks over the samples
allows to obtain explicit results for all observables in the limit .
We first compute disorder averages of one-time observables, such as the
diffusion front, the thermal width, the localization parameters, the
two-particle correlation function, and the generating function of thermal
cumulants of the position. We then study aging and sub-aging effects : our
approach reproduces very simply the two different aging exponents and yields
explicit forms for scaling functions of the various two-time correlations. We
also extend the RSRG method to include systematic corrections to the previous
zero temperature procedure via a series expansion in . We then consider the
generalized trap model with parameter and obtain that the
large scale effective model at low temperature does not depend on in
any dimension, so that the only observables sensitive to are those
that measure the `local persistence', such as the probability to remain exactly
in the same trap during a time interval. Finally, we extend our approach at a
scaling level for the trap model in and obtain the two relevant time
scales for aging properties.Comment: 33 pages, 3 eps figure
Height-diameter allometry of tropical forest trees
Tropical tree height-diameter (H:D) relationships may vary by forest type and region making large-scale estimates of above-ground biomass subject to bias if they ignore these differences in stem allometry. We have therefore developed a new global tropical forest database consisting of 39 955 concurrent H and D measurements encompassing 283 sites in 22 tropical countries. Utilising this database, our objectives were:
1. to determine if H:D relationships differ by geographic region and forest type (wet to dry forests, including zones of tension where forest and savanna overlap).
2. to ascertain if the H:D relationship is modulated by climate and/or forest structural characteristics (e.g. stand-level basal area, A).
3. to develop H:D allometric equations and evaluate biases to reduce error in future local-to-global estimates of tropical forest biomass.
Annual precipitation coefficient of variation (PV), dry season length (SD), and mean annual air temperature (TA) emerged as key drivers of variation in H:D relationships at the pantropical and region scales. Vegetation structure also played a role with trees in forests of a high A being, on average, taller at any given D. After the effects of environment and forest structure are taken into account, two main regional groups can be identified. Forests in Asia, Africa and the Guyana Shield all have, on average, similar H:D relationships, but with trees in the forests of much of the Amazon Basin and tropical Australia typically being shorter at any given D than their counterparts elsewhere. The region-environment-structure model with the lowest Akaike\u27s information criterion and lowest deviation estimated stand-level H across all plots to within amedian −2.7 to 0.9% of the true value. Some of the plot-to-plot variability in H:D relationships not accounted for by this model could be attributed to variations in soil physical conditions. Other things being equal, trees tend to be more slender in the absence of soil physical constraints, especially at smaller D. Pantropical and continental-level models provided less robust estimates of H, especially when the roles of climate and stand structure in modulating H:D allometry were not simultaneously taken into account
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