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811.
We present a finite difference code intended for computing linear, adiabatic, non radial pulsations of spherical stars. This
code is based on a slight modification of the general Newton-Raphson technique in order to handle the relaxation of the eigenvalue(square
of the eigenfrequency) of the modes and their corresponding eigenfunctions. This code has been tested computing the pulsation
spectra of polytropic spheres finding a good agreement with previous work. Then, we have coupled this code to our evolutionary
code and applied it to the computation of the pulsation spectrum of a low mass, pure-helium white dwarf of 0.3 M
⊙ for a wide range of effective temperatures. In making this calculation we have taken an evolutionary time step short enough
such that eigenmodes corresponding to a given model are used as initial approximation to those of the next one. Specifically,
we have computed periods, period spacing, eigenfunctions, weight functions, kinetic energies and variational periods for a
wide range of modes. To our notice this is the first effort in studying the pulsation properties of helium white dwarfs. The
solution we have found working with these realistic white dwarf models are in good accord with the predictions of the asymptotic
theory of Tassoul (1980) for high order modes. This indicates that the code presented here is able to work adequately also
with realistic stellar models.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
812.
Diatom and chrysophyte cyst-based reconstructions of the dates of spring and autumn lake-mixing enabled us to estimate spring
(STanom) and autumn (ATanom) temperature anomalies as well as ice-cover of the last ca. 4,000 years in a lake sediment core (Oberer Landschitzsee, 2,076
m a.s.l.) from the southern slopes of the Austrian Central Alps. The two independently inferred temperature anomalies were
significantly correlated. On average, spring and autumn temperatures were lower during the two millennia B.C than during 0–1,300
A.D. Marked spring and autumn temperature minima occurred at about 1,300 and 600 B.C. At about 1,300 A.D, STanom declined again. Spring-temperature anomalies during Roman and Medieval times equaled or slightly exceeded the modern values
and paralleled tree-line and glacier fluctuations. The de-coupling of autumn and spring climates, which began during the Medieval
period, might indicate changes in major circulation modes. It was assumed that the North-Atlantic influence, triggering winter-rain
climate in the Northern Mediterranean, became weaker during Medieval times, resulting in a trend towards warmer autumns and
overall more continental climate conditions in the study area. Four pulses of land use, inferred from indicator pollen, occurred
during (1) the Early to Late Bronze, (2) the transition from Late Bronze to Early Iron Age (Hallstatt), (3) Late Iron Age
(La Tène, Celtic time) to Roman times, and (4) during high to late Medieval times. Climate seemed to be an important, though
complex, trigger of Alpine land use. 相似文献
813.
Origin and geodynamic significance of Upper Cretaceous lamprophyres from the Villány Mts (S Hungary)
Summary In the Villány Mts of southern Hungary, ocelli-bearing porphyritic lamprophyre dykes and sills of Upper Cretaceous age occur
sporadically, intruding Mesozoic carbonate rocks. They at places contain metasomatised mantle xenoliths and quartz xenocrysts
of crustal origin. They are moderately fractionated with significant LILE and LREE enrichments and a notable Nb–Ta negative
anomaly. Trace elements indicate that they formed in an intraplate environment by very low degree partial melting of a metasomatised
garnet lherzolite mantle source that was enriched by earlier subduction. Based on petrography, geochemistry and age constraints,
they differ from other Mesozoic basic rocks of the Tisza block (Mecsek Mts and Slavonian basalts); however, they show a significant
geochemical similarity to the Upper Cretaceous lamprophyre dyke swarm from NE Transdanubia (northwestern Hungary) situated
on the Alcapa microplate. Thus we suggest that lamprophyres from the Villány Mts and NE Transdanubia could have originated
from the same or similar enriched asthenospheric mantle sources. 相似文献
814.
Arnoldo Valle-Levinson Nandita Sarkar Rosario Sanay Doris Soto Jorge León 《Estuaries and Coasts》2007,30(1):113-126
Underway current velocity profiles were combined with temperature and salinity profiles at fixed stations to describe tidal
and subtidal flow patterns in the middle of the northernmost Chilean fjord, Estuario Reloncaví. This is the first study involving
current velocity measurements in this fjord. Reloncaví fjord is 55 km long, 2 km wide, and on average is 170 m deep. Measurements
concentrated around a marked bend of the coastline, where an 8-km along-fjord transect was sampled during a semidiurnal tidal
cycle in March 2002 and a 2-km cross-fjord transect was occupied, also during a semidiurnal cycle, in May 2004. The fjord
hydrography showed a relatively thin (<5 m deep), continuously stratified, buoyant layer with stratification values >4 kg
m−3 per meter of depth. Below this thin layer, the water was relatively homogeneous. Semidiurnal tidal currents had low amplitudes
(<10 cm s−1) that allowed the persistence of a surface front throughout the tidal cycle. The front oscillated with a period of ca. 2.5
h and showed excursions of 2 km. The front oscillations could have been produced by a lateral seiche that corresponds to the
natural period of oscillation across the fjord. This front could have also caused large (2 h) phase lags in the semidiurnal
tidal currents, from one end of the transect to the other, within the buoyant layer. Tidal phases were relatively uniform
underneath this buoyant layer. Subtidal flows showed a 3-layer pattern consisting of a surface layer (8 m thick, of 5 cm s−1 surface outflow), an intermediate layer (70 m thick, of 3 cm s−1 net inflow), and a bottom layer (below 80 m depth, of 3 cm s−1 net outflow). The surface outflow and, to a certain extent, the inflow layer were related to the buoyant water interacting
with the ambient oceanic water. The inflowing layer and the bottom outflow were attributed to nonlinear effects associated
with a tidal wave that reflects at the fjord's head. The weak subtidal currents followed the morphology of the bend and caused
downwelling on the inside and upwelling on the outside part of the bend. 相似文献
815.
Anomalous Emissions of SO2 During the Recent Eruption of Santa Ana Volcano, El Salvador, Central America 总被引:1,自引:0,他引:1
Rodolfo Olmos José Barrancos Claudia Rivera Francisco Barahona Dina L. López Benancio Henriquez Agustín Hernández Efrain Benitez Pedro A. Hernández Nemesio M. Pérez Bo Galle 《Pure and Applied Geophysics》2007,164(12):2489-2506
Santa Ana volcano in western El Salvador, Central America, had a phreatic eruption at 8:05 am (local time) on October 1, 2005,
101 years after its last eruption. However, during the last one hundred years this volcano has presented periods of quiet
degassing with fumarolic activity and an acidic lake within its crater. This paper presents results of frequent measurements
of SO2 degassing using the MiniDOAS (Differential Optical Absorption Spectroscopy) system and a comparison with the volcanic seismicity
prior to the eruption, during, and after the eruption. Vehicle measurements of SO2 flux were taken every hour during the first nine days of the eruption and daily after that. The period of time reported here
is from August to December, 2005. Three periods of degassing are distinguished: pre-eruptive, eruptive, and post-eruptive
periods. The intense activity at Santa Ana volcano started in July 2005. During the pre-eruptive period up to 4306 and 5154
ton/day of SO2 flux were recorded on October 24 and September 9, 2005, respectively. These values were of the same order of magnitude as
the recorded values just after the October 1st eruption (2925 ton/day at 10:01 am). Hourly measurements of SO2 flux taken during the first nine days after the main eruptive event indicate that explosions are preceded by an increase
in SO2 flux and that this parameter reaches a peak after the explosion took place. This behavior suggests that increasing accumulation
of exsolved magmatic gases occurs within the magmatic chamber before the explosions, increasing the pressure until the point
of explosion. A correlation between SO2 fluxes and RSAM (Real Time Seismic Amplitude Measurements) is observed during the complete sampling period. Periodic fluctuations
in the SO2 and RSAM values during the entire study period are observed. One possible mechanism explaining these fluctuations it that
convective circulation within the magmatic chamber can bring fresh magma periodically to shallow levels, allowing increasing
degasification and then decreasing degasification as the batch of magma lowers its gas content, becomes denser, and sinks
to give space to a new magma pulse. These results illustrate that the measurements of SO2 flux can provide important warning signals for incoming explosive activity in active volcanoes. 相似文献
816.
817.
In the upwelling area along the Namibian coast of SW-Africa sulphur discolorations were investigated to study the impact of hydrogen sulphide on the ecosystem using satellite imagery. The formation of colloidal sulphur in the upper water layer results from the oxidation of hydrogen sulphide. The occurrence of sulphur plumes as well as their temporal and spatial development was investigated in relation to the driving meteorological and oceanographic conditions. Because of the sporadic occurrence of sulphur events and the limited number of ship-borne investigations in that area remote sensing of ocean colour is the only method to follow these phenomena continuously and synoptically. In the past the sulphur plumes were studied by true colour images derived from ocean colour satellite data like sea-viewing wide field of view sensor or moderate resolution imaging spectroradiometer and identified by their typical milky turquoise discoloration. 相似文献
818.
A finite-volume numerical model is employed to investigate the adaptation of the atmospheric boundary layer to a change in
the underlying surface roughness, such as that existing in the transition from land to the free surface of a water body. Numerical
results are validated by comparison with neutral stratification atmospheric data and compared with the internal boundary-layer
(IBL) heights computed using a number of existing empirical formulae. The numerical analysis allows an extension of the fetch
range in which the existing formulae, calibrated only by comparison with short fetch data, may be applied. An argument is
offered that the spatial variability of the water surface roughness should be also taken into account for the IBL development
over the surface of a water body. 相似文献
819.
Mass budgets of the Lambert, Mellor and Fisher Glaciers and basal fluxes beneath their flowbands on Amery Ice Shelf 总被引:4,自引:0,他引:4
Kenneth C. JEZEK Beata M. CSATHó Ute C. HERZFELD Katy L. FARNESS Philippe HUYBRECHTS 《中国科学D辑(英文版)》2007,50(11):1693-1706
We used in situ measurements and remote-sensing data sets to evaluate the mass budgets of the Lambert, Mellor and Fisher Glaciers and the basal melting and freezing rates beneath their flowbands on the Amery Ice Shelf. Our findings show the Lambert and Mellor Glaciers upstream of the ANARE Lambert Glacier Basin (LGB) traverse may have positive imbalances of 3.9±2.1 Gt a-1 and 2.1±2.4 Gt a-1, respectively, while the Fisher Glacier is approximately in balance. The upstream region as a whole has a positive imbalance of 5.9±4.9 Gt a-1. The three same glaciers downstream of the ANARE LGB traverse line are in negative imbalance, where the whole downstream region has a negative imbalance of -8.5±5.8 Gt a-1. Overall the mass budgets of the Lambert, Mellor, and Fisher Glaciers are close to bal-ance, and the collective three-glacier system is also nearly in balance with a mass budget of -2.6±6.5 Gt a-1. The significant positive imbalances for the interior basin upstream of the ice-movement stations established in the early 1970s (GL line) reported previously are possibly due to an overestimate of the total accumulation and an underestimate of the ice flux through the GL line. The mean melting rate is -23.0±3.5 m ice a-1 near the southern grounding line, which decreases rapidly downstream, and transitions to refreezing at around 300 km from the southern extremity of the Amery Ice Shelf. Freezing rates along the flowbands are around 0.5±0.1 to 1.5±0.2 m ice a-1. The per-centage of ice lost from the interior by basal melting beneath the flowbands is about 80%±5%. The total basal melting and refreezing beneath the three flowbands is 50.3±7.5 Gt ice a-1 and 7.0±1.1 Gt ice a-1, respectively. We find a much larger total basal melting and net melting than the results for the whole Amery Ice Shelf derived from previous modeling and oceanographic measurements. 相似文献
820.
The canadian school of hydrogeology: history and legacy 总被引:1,自引:0,他引:1
Tóth J 《Ground water》2005,43(4):640-644