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101.
Elisa Cenci Matteo PizzolonNicole Chimento Carlotta Mazzoldi 《Estuarine, Coastal and Shelf Science》2011
Deployed for diverse purposes, artificial substrata can, theoretically, act as fish attractors or as fish producers. We tested these hypotheses by comparing fish assemblages on two artificial hard substrata, the Ca’ Roman and Sottomarina jetties, before (1994 and 2003) and after (2005 and 2006) the deployment of a new breakwater in the area. Our results support the fish production hypothesis. A remarkable increase in richness and abundance of sedentary species was observed on the extant substrata with the presence of the new barrier. More mobile species showed less definite trends. The net result has been a total increase in abundance and richness in the area. While fish assemblage showed important temporal variation in composition before deployment (Ca’ Roman, comparison between 1994 and 2003), clear trends of increasing abundance were concurrent with the presence of the new barrier. This increase in abundance, particularly of sedentary species, may be attributed to enhanced connectivity and/or an improvement in larval retention due to the changes in water circulation connected with the new breakwater, or to an increase in juveniles and/or adults promoted by changes in environmental conditions. Indeed, changes in benthic cover were recorded, with an increase of uncovered rock and the appearance of previously unrecorded algae after breakwater deployment. 相似文献
102.
James S. Bolton Martin G. Haehnelt Matteo Viel Robert F. Carswell 《Monthly notices of the Royal Astronomical Society》2006,366(4):1378-1390
The low-density hydrogen and helium in the intergalactic medium (IGM) probed by quasi-stellar object (QSO) absorption lines is sensitive to the amplitude and spectral shape of the metagalactic ultraviolet (UV) background. We use realistic H i and He ii Lyα forest spectra, constructed from state-of-the-art hydrodynamical simulations of a Λ cold dark matter (ΛCDM) universe to confirm the reliability of using line profile fitting techniques to infer the ratio of the metagalactic H i and He ii ionization rates. We further show that the large spatial variations and the anticorrelation with H i absorber density observed in the ratio of the measured He ii to H i column densities can be explained in a model where the H i ionization rate is dominated by the combined UV emission from young star-forming galaxies and QSOs and the He ii ionization rate is dominated by emission from QSOs only. In such a model the large fluctuations in the column density ratio are due to the small number of QSOs expected to contribute at any given point to the He ii ionization rate. A significant contribution to UV emission at the He ii photoelectric edge from hot gas in galaxies and galaxy groups would decrease the expected fluctuations in the column density ratio. Consequently, this model appears difficult to reconcile with the large increase in He ii opacity fluctuations towards higher redshift. Our results further strengthen previous suggestions that observed He ii Lyα forest spectra at z ∼ 2–3.5 probe the tail end of the reionization of He ii by QSOs. 相似文献
103.
Sebastian Heinz Andrea Merloni Tiziana Di Matteo Rashid Sunyaev 《Astrophysics and Space Science》2005,300(1-3):15-21
The scale invariance model (Heinz, S. and Sunyaev, R.A.: 2003, MNRAS 343, L59) can be used to derive robust scaling relations between the radio luminosity from accreting black holes and the black
hole mass and accretion rate. These relations agree well with the recently found “fundamental plane” of black hole activity
(Merloni, A., Heinz, S. and Di Matteo, T.: 2003, MNRAS 345, 1057). This relation provides a new, powerful tool for the comparison of jets from black holes of different masses and accretion
rates. The regression coefficients of this relation contain information about the nature of the X-ray emission mechanism driving
the correlation. We argue that X-ray synchrotron emission from the base of the jets is unlikely to be the dominant contribution
to the X-ray spectrum in most of the sources. 相似文献
104.
105.
106.
John A. Regan Martin G. Haehnelt Matteo Viel 《Monthly notices of the Royal Astronomical Society》2007,374(1):196-205
We compare simulations of the Lyman α forest performed with two different hydrodynamical codes, gadget-2 and enzo . A comparison of the dark matter power spectrum for simulations run with identical initial conditions show differences of 1–3 per cent at the scales relevant for quantitative studies of the Lyman α forest. This allows a meaningful comparison of the effect of the different implementations of the hydrodynamic part of the two codes. Using the same cooling and heating algorithm in both codes, the differences in the temperature and the density probability distribution function are of the order of 10 per cent. The differences are comparable to the effects of box size and resolution on these statistics. When self-converged results for each code are taken into account, the differences in the flux power spectrum – the statistics most widely used for estimating the matter power spectrum and cosmological parameters from Lyman α forest data – are about 5 per cent. This is again comparable to the effects of box size and resolution. Numerical uncertainties due to a particular implementation of solving the hydrodynamic or gravitational equations appear therefore to contribute only moderately to the error budget in estimates of the flux power spectrum from numerical simulations. We further find that the differences in the flux power spectrum for enzo simulations run with and without adaptive mesh refinement are also of the order of 5 per cent or smaller. The latter require 10 times less CPU time making the CPU time requirement similar to that of a version of gadget-2 that is optimized for Lyman α forest simulations. 相似文献
107.
108.
Paolo Padovani Eric Perlman Hermine Landt Paolo Giommi Matteo Perri 《Astrophysics and Space Science》2004,294(1-2):71-78
We present a new population of radio quasars whose X-ray band, unlike previously known sources which have (flat) inverse Compton
radiation, is characterized by (steep) synchrotron emission, with a broad-band spectral energy distribution similar to that
of BL Lacs with high energy synchrotron peaks. We discuss how this new class was discovered, the class properties, and the
implications of its existence for our understanding of jets and active galactic nuclei in general. 相似文献
109.
Sebastiano Foti Stefano Parolai Dario Albarello Matteo Picozzi 《Surveys in Geophysics》2011,32(6):777-825
Surface-wave dispersion analysis is widely used in geophysics to infer a shear wave velocity model of the subsoil for a wide
variety of applications. A shear-wave velocity model is obtained from the solution of an inverse problem based on the surface
wave dispersive propagation in vertically heterogeneous media. The analysis can be based either on active source measurements
or on seismic noise recordings. This paper discusses the most typical choices for collection and interpretation of experimental
data, providing a state of the art on the different steps involved in surface wave surveys. In particular, the different strategies
for processing experimental data and to solve the inverse problem are presented, along with their advantages and disadvantages.
Also, some issues related to the characteristics of passive surface wave data and their use in H/V spectral ratio technique
are discussed as additional information to be used independently or in conjunction with dispersion analysis. Finally, some
recommendations for the use of surface wave methods are presented, while also outlining future trends in the research of this
topic. 相似文献
110.
Matteo Picozzi Claus Milkereit Kevin Fleming Eser Çakti Jochen Zschau 《Journal of Seismology》2011,15(4):557-578
A seismic antenna approach based on the generalized zero-lag cross-correlation method for rapid earthquake localization is
proposed. This method is intended to be applied primarily for early warning, whenever the epicentre-to-target distances guarantee
enough lead-time, rapid response purposes, and for those circumstances when a seismogenic area is not directly accessible
with seismic stations or/and a network of instruments is concentrated within the area to be warned. The procedure we propose
aims to provide useful information for magnitude determination and shake-maps generation. Indeed, it relies only on the first
P-wave triggered arrivals from seismic stations, and is designed to work in real-time for the localization of events occurring
outside of the network, that is, under conditions that might be detrimental to standard localization approaches. The procedure
can by summarized by a few preliminary pre-seismic and real-time co-seismic steps. In the pre-seismic time-frame, for the
cases where a large and dense network exists, waiting for all stations to trigger could dramatically reduce the available
lead-time for the warning. Therefore, in such cases, the network could profitably be divided into sub-arrays, while also taking
advantage of available earthquake recordings or simulated data sets. During the co-seismic time-frame, the main operations
are: (1) individual on-site triggering by the P-wave of the seismic stations (e.g. by a STA/LTA algorithm); (2) real-time
communication of key parameters (e.g. P-wave arrival time, and signal quality) to a main centre by SMS/WLAN; (3) setup of
a pseudo data set, composed by a Gaussian function centred at the P-time, and with a bell width that can be set up proportional
to the trigger signal-to-noise ratio (SNR); (4) calculation of a coherency map for the sub-array with triggered stations (preliminary
sub-array location); and (5) stacking of coherency maps from the different sub-arrays (final location). By the stack of coherency
maps estimated by the different sub-arrays in the last step of the procedure, the epicentral area’s location may be better
constrained. This innovative approach for rapid localization was applied to both synthetic data, and real observations of
two small earthquakes that occurred in the Marmara Sea, Turkey, which were recorded by the Istanbul Earthquake Rapid Response
System. 相似文献