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61.
Greene Brian R. Kral Stephan T. Chilson Phillip B. Reuder Joachim 《Boundary-Layer Meteorology》2022,183(3):321-353
Boundary-Layer Meteorology - We explore the potential of a new method for the estimation of profiles of turbulence statistics in the stable boundary layer (SBL). By applying gradient-based scaling... 相似文献
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Bastian Joachim Emmanuel Gardés Rainer Abart Wilhelm Heinrich 《Contributions to Mineralogy and Petrology》2011,161(3):389-399
Growth rates of monomineralic, polycrystalline åkermanite (Ca2MgSi2O7) rims produced by solid-state reactions between monticellite (CaMgSiO4) and wollastonite (CaSiO3) single crystals were determined at 0.5 GPa dry argon pressure, 1,000–1,200°C and 5 min to 60 h, using an internally heated pressure vessel. Inert Pt-markers, initially placed at the monticellite–wollastonite interface, indicate symmetrical growth into both directions. This and mass balance considerations demonstrate that rim growth is controlled by transport of MgO. At 1,200°C and run durations between 5 min and 60 h, rim growth follows a parabolic rate law with rim widths ranging from 0.4 to 16.3 μm indicating diffusion-controlled rim growth. The effective bulk diffusion coefficient \( D_{\text{eff,MgO}}^{\text{Ak}} \) is calculated to 10?15.8±0.1 m2 s?1. Between 1,000°C and 1,200°C, the effective bulk diffusion coefficient follows an Arrhenius law with E a = 204 ± 18 kJ/mol and D 0 = 10?8.6±1.6 m2 s?1. Åkermanite grains display a palisade texture with elongation perpendicular to the reaction interface. At 1,200°C, average grain widths measured normal to elongation, increase with the square root of time and range from 0.4 to 5.4 μm leading to a successive decrease in the grain boundary area fraction, which, however, does not affect \( D_{\text{eff,MgO}}^{\text{Ak}} \) to a detectible extent. This implies that grain boundary diffusion only accounts for a minor fraction of the overall chemical mass transfer, and rim growth is essentially controlled by volume diffusion. This is corroborated by the agreement between our estimates of the effective MgO bulk diffusion coefficient and experimentally determined volume diffusion data for Mg and O in åkermanite from the literature. There is sharp contrast to the MgO–SiO2 binary system, where grain boundary diffusion controls rim growth. 相似文献
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66.
Increasing levels and biomagnification of persistent organic pollutants (POPs) in Antarctic biota 总被引:3,自引:0,他引:3
Representatives of the Antarctic food web (krill, cephalopod, fish, penguin, seal) of the area around Elephant Island and from the Weddell Sea were analysed for the most recalcitrant organochlorine compounds. Due to sorption of the compounds to sinking particles and accumulation in sediments, two benthic fish species (Gobionotothen gibberifrons, Chaenocephalus aceratus) feeding on benthos invertebrates and fish reflected significantly increasing concentrations within a decade (1987-1996), while a benthopelagic species (Champsocephalus gunnari) feeding on krill did not. In the pelagic food chain, lipid normalised concentrations of all compounds increased from Antarctic krill to fish proving that biomagnification of highly lipophilic pollutants (log octanol-water partition coefficient>5) occurs in water-breathing animals. As top predators Weddell and southern elephant seals (Leptonychotes weddellii, Mirounga leonina) biomagnified the persistent organic pollutants relative to krill 30-160 fold with the exception of hexachlorobenzene, the levels of which were lower than in fish indicating its intense specific elimination. 相似文献
67.
The determination of seismic amplitude amplification is a fundamental contribution to seismic hazard assessment. While often
only high-frequency amplitude variations (>1 Hz) are taken into account, we analyse broadband waveforms from 0.14 to 8.6 Hz
using a temporary network of 32 stations in and around the earthquake-prone city of Bucharest. Spectral amplitudes are calculated
with an adaptive multiple-taper approach. Across our network (aperture 25 km × 25 km), we find a systematic northwest/southeast-oriented
structural influence on teleseismic P-wave amplitudes from 0.14 to 0.86 Hz that can be explained by constructive interference
in the dipping Cenozoic sedimentary layers. For higher frequencies (1.4–8.75 Hz), more local site effects prevail and can
be correlated partly among neighbouring stations. The transition between systematic and localised amplitude variations occurs
at about 1 Hz. 相似文献
68.
Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average
depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx.
2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the
bay. The hydro- and thermodynamical structures of Hervey Bay and their variability are presented here for the first time,
using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed
with the Coupled Hydrodynamical Ecological Model for Regional Shelf Seas (COHERENS). Due to the high tidal range (>3.5 m),
the bay is considered as a vertically well-mixed system, and therefore, only horizontal fronts are likely. Recent field measurements,
but also the numerical simulations, indicate characteristic features of an inverse/hypersaline estuary with low salinity (35.5 psu)
in the open ocean and peak values (>39.0 psu) in the head water of the bay. The model further predicts a nearly persistent
mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore). The investigation further shows
that air temperature, wind direction and tidal regime are mainly responsible for the stability of the inverse circulation
and the strength of the salinity gradient across the bay. Due to an ongoing drying trend, the occurrence of severe droughts
at the central east coast of Australia and, therefore, a reduction in freshwater supply, the salinity flux out of the bay
has increased, and the inverse circulation has also strengthened. 相似文献
69.
Three sympathetic flares were observed with the Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station of Beijing Astronomical Observatory on 29 August, and 1 September 1990. Each set of sympathetic flares had three ribbons. Two ribbons appeared in active region NOAA 6233 and one ribbon occurred in NOAA 6240 embedded in a single polarity area. Photospheric vector magnetograms were simultaneously obtained from both regions as well. We use a new numerical technique to reconstruct the chromospheric and coronal magnetic fields by making use of the observed vector magnetic fields in the photosphere as boundary conditions. Magnetic field loops linking both regions were identified from the reconstructed 3-D fields. The analysis of chromospheric filtergrams and reconstructed 3-D magnetic fields indicates that interaction between a sheared lower loop in the active region NOAA 6233 and a higher loop linking the two regions resulted in sympathetic flares. The analysis of the time delay between flare ribbons in NOAA 6233 and 6240 indicates that heat conduction along the higher loop from the primary energy release site is responsible for the sympathetic flaring in NOAA 6240. The events reported in this paper represent only one alternative as the cause of sympathetic flaring in which energy transport along coronal interconnecting loops plays the major role, and no in-situ energy release is required. 相似文献
70.
Small-scale elastic heterogeneities (<5 km) are found in the upper lithosphere underneath the Gräfenberg array, southeast Germany. The results are based on the analysis of broadband recordings of 17 intermediate-depth (201–272 km) events from the Hindu Kush region. The wavefront of the first P arrival and the following 40 s coda are separated into coherent and incoherent (scattered) parts in the frequency range from 0.05 to 5 Hz. The frequency-dependent intensities of the mean and fluctuation wavefields are used to describe the scattering characteristics of the lithosphere underneath the receivers. It is possible to discriminate a weak-fluctuation regime of the wavefield in the frequency range below approximately 1.5–2.5 Hz and a strong-fluctuation regime starting at 2.0–2.5 Hz and continuing to higher frequencies. In order to explain the observed wavefield fluctuations, an approach with seismic scattering at random media-type structures is proposed. The preferred model contains heterogeneities with 3–7 per cent perturbations in seismic velocity and correlation lengths of 0.6–4.8 km in the crust. This is compatible with models from active seismic experiments. Scattering in the lithospheric mantle is not required, but cannot be excluded at weak velocity contrasts (<3 per cent). 相似文献