3,871 research outputs found
Negative refraction in natural ferromagnetic metals
It is generally believed that Veselago's criterion for negative refraction
cannot be fulfilled in natural materials. However, considering imaginary parts
of the permittivity ({\epsilon}) and permeability ({\mu}) and for metals at not
too high frequencies the general condition for negative refraction becomes
extremely simple: Re({\mu}) Re(n) < 0. Here we demonstrate
experimentally that in such natural metals as pure Co and FeCo alloy the
negative values of the refractive index are achieved close to the frequency of
the ferromagnetic resonance. Large values of the negative refraction can be
obtained at room temperature and they can easily be tuned in moderate magnetic
fields
Permafrost, landscape and ecosystem responses to late Quaternary warm stages in Northeast Siberia
Permafrost, landscape and ecosystem responses to late Quaternary warm stages in
Northeast Siberia
S. Wetterich1, F. Kienast2, L. Schirrmeister1, M. Fritz1, A. Andreev3, P. Tarasov4
1Alfred Wegener Institute for Polar and Marine Research, Department of Periglacial Research,
Potsdam, Germany; 2Senckenberg Research Institute and Natural History Museum, Research
Station for Quaternary Palaeontology, Weimar, Germany; 3Institute of Geology and Mineralogy,
University of Cologne, Germany; 4Institute of Geological Sciences, Free University Berlin,
Germany
Perennially frozen ground is widely distributed in Arctic lowlands and beyond. Permafrost
responds sensitive to changes in climate conditions. Climate-driven dynamics of landscape,
sedimentation and ecology in periglacial regions are frequently recorded in permafrost deposits.
The study of late Quaternary permafrost can therefore reveal past glacial-interglacial and stadialinterstadial
environmental dynamics. One of the most striking processes under warming climate
conditions is the extensive thawing of permafrost (thermokarst) and subsequent surface
subsidence. Thermokarst basins promote the development of lakes, whose sedimentological and
paleontological records give insights into past interglacial and interstadial (warm).
In this paper we present results of qualitative and quantitative reconstructions of climate and
environmental conditions for the last Interglacial (MIS 5e, Kazantsevo; ca. 130 to 115 ka ago), the
lateglacial Allerød Interstadial (ca. 13 to 11 uncal. ka BP), and the early Holocene (ca. 10.5 to 8
uncal. ka BP). The study was performed in course of the IPY project #15 ‘Past Permafrost’ with
permafrost deposits exposed at the coasts of the Dmitry Laptev Strait (East Siberian Sea, East
Siberia). The reconstruction based on fossil-rich findings of plants (pollen, macro-remains) and
invertebrates (beetles, chironomids, ostracods gastropods).
Interglacial vegetation dynamics are reflected in the pollen records by changes from early
interglacial grass-sedge-tundra to shrub-tundra during the interglacial thermal optimum followed
by grass-sedge-tundra vegetation at the end of the Kazantsevo warm period. Terrestrial beetle
and plant remains prove the former existence of open forest tundra with Dahurian larch, grey
alder and boreal shrubs interspersed with patches of steppes and meadows during the
interglacial thermal optimum. Mean temperature reconstructions of the warmest month (MTWA,
TJuly) for the interglacial thermal optimum are based on quantitative chironomid transfer functions
revealed a TJuly of 12.9 ± 0.9 °C. The TJuly reconstructed by plant macrofossils amounts to 13.2 ±
0.5 °C, and the pollen-based TJuly reaches 14.3 ± 3.3 °C. Low net precipitation is reflected by
steppe plants and beetles. The temperature reconstruction based on three independent
approaches. Nethertheless, all methods consistently indicate an interglacial TJuly about 10 °C
higher than today, which is interpreted as a result of a combination of increased insolation and
higher climatic continentality during the last Interglacial.
Grass-sedge dominated tundra vegetation occurred during the lateglacial to Holocene transition
which was replaced by shrub tundra during the early Holocene. The presence of Salix and Betula
pollen reflects temperatures about 4 °C higher than present between 12 to 11 uncal. ka BP,
during the Allerød Interstadial, but shrubs disappeared in the following Younger Dryas stadial,
reflecting a climate deterioration. Alnus fruticosa, Betula nana, Poaceae and Cyperaceae
dominate early Holocene pollen spectra. Pollen-based reconstructions point to TJuly 4 °C warmer
than present. Shrubs gradually disappeared from coastal areas after 7.6 uncal. ka BP when
vegetation cover became similar to modern wet tundra.
Thermokarst acted as response to warming conditions on landscape scale in permafrost regions.
Concurrent changes in relief, hydrology and ecosystems are obvious and detectable by analyses
of the paleontological record preserved in thermokarst deposits
Rapid resistivity imaging for marine CSEM surveys with two transmitter polarizations: An application to the North Alex mud volcano
To image the internal resistivity structure of the North Alex mud volcano offshore Egypt, the marine electromagnetics group at the Helmholtz Centre for Ocean Research Kiel (GEOMAR) developed and conducted a novel transient marine controlled-source electromagnetic experiment. The system, which was specifically developed to image the mud volcano, is also generally suitable for surveys of other small seafloor targets, such as gas-hydrate reservoirs, fluid-flow features, and submarine massive-sulfide deposits. An electric bipole antenna is set down by a remotely operated vehicle on the seafloor sequentially in two perpendicular polarizations at each transmission station. Two orthogonal horizontal electric field components are recorded on the seabed by an array of independently deployed nodal receivers (RXs). With two transmitter polarizations, the unique acquisition geometry of the system provides a very rich data set. However, for this geometric setup, conventional marine electromagnetic interpretation schemes (such as normalized magnitude variation with offset plots) have been difficult to implement. We have developed a simple imaging technique, which can be used for a first-step mapping of seafloor apparent resistivity with the GEOMAR system. Images can be produced in just a few minutes on a regular laptop computer, and the robustness of the approach was demonstrated using two synthetic data sets from simple seafloor models. The method was then applied to the real data acquired at the North Alex mud volcano in 2008. Results found increased apparent sediment resistivities of up to 4 Omega m near the center of the mud volcano occurring at source-RX offsets greater than 500 m, which mapped to apparent depths of greater than 150 m. This may be caused by large quantities of free gas or freshwater in the sediment pore space
Permafrost sequences on Kurungnakh Island, Lena Delta (NE Siberia, Russia) as key site of the late Quaternary environmental history of West Beringia
On electric fields produced by inductive sources on the seafloor
The transient electromagnetic (TEM) method has recently been proposed as a tool for mineral exploration on the seafloor. Similar to airborne TEM surveys conducted on land, marine TEM systems can use a concentric or coincident wire-loop transmitter and receiver towed behind a ship. Such towed-loop TEM surveys can be further augmented by placing additional stationary receivers on the seafloor throughout the survey area. We examine the electric fields measured by remote receivers from an inductive source transmitter within a 1D layered earth model. At sea, it is conceivable to deploy either a horizontal transmitter (such as the analogous standard airborne configuration) or a more exotic vertical transmitter. Therefore, we study and compare the sensitivity of the vertical and horizontal towed-loop systems with a variety of seafloor conductivity structures. Our results indicate that the horizontal loop system is more sensitive to the thickness of a buried conductive layer and would be advantageous over the vertical loop system in characterizing the size of a shallowly buried mineralized zone. The vertical loop system is more sensitive to a resistive layer than the horizontal loop system. The vertical electric field produced by the vertical loop transmitter is sensitive to greater depths than the horizontal fields, and measuring the vertical field at the receivers would therefore be advantageous. We also conducted a novel test of a towed horizontal loop system with remote dipole receivers in a marine setting. The system was tested at the Palinuro volcanic complex in the Tyrrhenian Sea, a site of known massive sulfide mineralization. Preliminary results are consistent with shallowly buried material in the seafloor of conductivities >1  S/m
The impact of patient-dependent risk factors on morbidity and mortality following gastric surgery for malignancies
Gastric cancer remains a leading cause of mortality worldwide. The treatment for gastric cancer is multimodal, in which gastrectomy remains the only curative approach. However, gastric resection is often associated with increased morbidity and mortality rates, depending on several factors. These factors can be attributed to the patient as comorbidities or effects of the disease upon him and, on the other hand, there are risk factors independent of the patient, such as aspects of the tumor (type, staging, location), experience of the surgical and anesthetic team, logistics of the hospital, yield of adjuvant therapies etc. We recognize the fact that patient-related risk factors are often overlooked and not taken into consideration prior to surgery, thus becoming a source of morbidity and mortality. These factors are more susceptible to modulating in order to better select candidates for gastric resection and thus create a better outcome. Therefore, identifying and modulating patient-related risk factors is paramount in order to decrease the incidence of morbidity and mortality following gastric resections
Claim Reserving via Inverse Probability Weighting: A Micro-Level Chain-Ladder Method
Claim reserving is primarily accomplished using macro-level models, with the
Chain-Ladder method being the most widely adopted method. These methods are
usually constructed heuristically and rely on oversimplified data assumptions,
neglecting the heterogeneity of policyholders, and frequently leading to modest
reserve predictions. In contrast, micro-level reserving leverages on stochastic
modeling with granular information for improved predictions, but usually comes
at the cost of more complex models that are unattractive to practitioners. In
this paper, we introduce a simple macro-level type approach that can
incorporate granular information from the individual level. To do so, we imply
a novel framework in which we view the claim reserving problem as a population
sampling problem and propose a reserve estimator based on inverse probability
weighting techniques, with weights driven by policyholders' attributes. The
framework provides a statistically sound method for aggregate claim reserving
in a frequency and severity distribution-free fashion, while also incorporating
the capability to utilize granular information via a regression-type framework.
The resulting reserve estimator has the attractiveness of resembling the
Chain-Ladder claim development principle, but applied at the individual claim
level, so it is easy to interpret and more appealing to practitioners
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