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841.
Scheelite-mineralized microtonalite sheets occur on the SE margin of the end-Caledonian Leinster Granite in SE Ireland. Scheelite,
polymetallic sulphides and minor cassiterite occur in veins in the microtonalites, disseminated throughout the greisened microtonalite
sheets and in the adjacent wallrocks. Two major mineralized vein types occur in the microtonalite sheets: (1) Scheelite ± arsenopyrite ± pyrrhotite
occur in quartz-fluorite veins, generally without a muscovite selvage; (2) Sphalerite ± chalcopyrite ± pyrite ± galena ± cassiterite ± stannite
occur in quartz + fluorite veins with a coarse muscovite selvage and are often intergrown with the muscovite. Quartz-hosted
fluid inclusions were examined from representative samples of both vein types using petrographic, microthermometric and laser
Raman spectroscopic techniques. Three distinct types of fluid inclusions have been recognized. Primary, vapour rich Type 1
inclusions in quartz from the scheelite-mineralized veins are of H2O-CO2-CH4-N2 ± H2S ± NaCl composition and formed between 360–530 °C. Primary and secondary, liquid-rich Type 2 fluid inclusions in the base
metal sulphide-mineralized veins are of H2O-CH4-N2 ± H2S-NaCl composition and formed between 340–480 °C. They also occur as pseudosecondary and secondary inclusions in scheelite-mineralized
veins. Late dilute, low temperature H2O-NaCl + KCl fluid inclusions may be related to late-Caledonian convection of meteoric waters around the cooling Leinster
Granite batholith.
Received: 4 September 1996 / Accepted: 23 May 1997 相似文献
842.
843.
A nitrogen isotope study of soil water and groundwater in southern Indiana, USA, in 1991–1992 demonstrated considerable variations
in nitrate degradation processes compared to an earlier investigation in 1986–1987. Although N-fertilizers were applied in
May 1991, the δ15N values in soil water decreased in February 1992, indicating its delayed release into the system after substantial rainfall.
The δ15N values of groundwater decreased from +12.3‰ in November 1991 to +11.3‰ in February 1992, and to +7.5‰ in March 1992. The
increased residence time of nitrate in the soil resulted in increased denitrification, ammonia volatilization and plant uptake,
and reduced threat to the groundwater quality. The 1986–1987 study in the area reported that excessive rainfall during the
summer rapidly transported the nitrate to deeper horizons and drastically reduced volatilization and microbial reduction of
nitrate, thus increasing the immediate threat to the groundwater quality in the area. The present study demonstrated that
nitrogen isotopic signatures can be used to determine the effects of local soil type, rainfall, and land-use practices on
the fate of nitrate in the subsurface.
Received: 18 February 1997 · Accepted: 17 June 1997 相似文献
844.
The 1900–1700 Ma Waterberg Group in the main Waterberg fault-bounded basin consists of dominantly coarse siliciclastic red
beds with minor volcanic rocks. The sedimentary rocks were deposited mainly by alluvial fans, fluvial braidplains and transgressive
shallow marine environments, with lesser lacustrine and aeolian settings. Uplifted, largely granitic source areas were located
along the Thabazimbi-Murchison lineament (TML) fault system in the south, and along the Palala shear zone in the northeast.
Palaeoplacer titanomagnetite-ilmenite-zircon heavy mineral deposits, best developed in the Cleremont Formation in the centre
of the basin, reflect initial fluvial reworking and subsequent littoral marine concentration. Coarse alluvial cassiterite
placer deposits are found in the Gatkop area in the southwest of the basin, and appear to have been derived from stanniferous
Bushveld Complex lithologies south of the TML. Hydrothermal zinc and U-Cu mineralisation in the Alma lithologies in the same
area appears to be related to the TML fault system. Small manganese deposits and anomalous tungsten values occur in the south
of the basin, where they are again closely spatially associated with the TML. Copper-barium mineralisation is found associated
with dolerite dykes, and in stratigraphically controlled, inferred syngenetic settings. The most interesting of these apparently
syngenetic occurrences is found within green coloured reduced mudrocks and inferred volcanic rocks, at an unconformity developed
within the overall red bed sequence of the Waterberg Group, adjacent to the TML in the southwest of the basin. The most important
potential mineralisation in the main Waterberg basin thus encompasses shoreline placer Ti and the possibility of substantial
sediment-hosted copper deposits.
Received: 31 May 1996 / Accepted: 17 February 1997 相似文献
845.
H2O activities in supercritical fluids in the system KCl-H2O-(MgO) were measured at pressures of 1, 2, 4, 7, 10 and 15 kbar by numerous reversals of vapor compositions in equilibrium
with brucite and periclase. Measurements spanned the range 550–900 °C. A change of state of solute KCl occurs as pressures
increase above 2 kbar, by which H2O activity becomes very low and, at pressures of 4 kbar and above, nearly coincident with the square of the mole fraction
(x
H2O). The effect undoubtedly results primarily from ionic dissociation as H2O density (ρH2O) approaches 1 gm/cm3, and is more pronounced than in the NaCl-H2O system at the same P-T-X conditions. Six values of solute KCl activity were yielded by terminal points of the isobaric brucite-periclase T-x
H2O curves where sylvite saturation occurs. The H2O mole fraction of the isobaric invariant assemblage brucite-periclase-sylvite-fluid is near 0.52 at all pressures, and the
corresponding temperatures span only 100 °C between 1 and 15 kbar. This remarkable convergence of the isobaric equilibrium
curves reflects the great influence of pressure on lowering of both KCl and H2O activities. The H2O and KCl activities can be expressed by the formulas: a
H2O = γH2O[x
H2O+(1 + (1 + α)x
KCl)], and a
KCL = γKCl[(1 + α)x
KCl/(x
H2O +(1 + α)x
KCl)](1 + α), where α is a degree of dissociation parameter which increases from zero at the lowest pressures to near one at high pressures
and the γ's are activity coefficients based on an empirical regular solution parameter W: ln γi = (1 − xi)2W. Least squares fitting of our H2O and KCl activity data evaluates the parameters: α = exp(4.166 −2.709/ρH2O) − 212.1P/T, and W = (−589.6 − 23.10P) /T, with ρH2O in gm/cm3, P in kbar and T in K. The standard deviation from the measured activities is only ± 0.014. The equations define isobaric liquidus curves,
which are in perfect agreement with previous DTA liquidus measurements at 0.5–2 kbar, but which depart progressively from
their extrapolation to higher pressures because of the pressure-induced dissociation effect. The great similarity of the NaCl-H2O and KCl-H2O systems suggests that H2O activities in the ternary NaCl-KCl-H2O system can be described with reasonable accuracy by assuming proportionality between the binary systems. This assumption
was verified by a few reconnaissance measurements at 10 kbar of the brucite-periclase equilibrium with a Na/(Na + K) ratio
of 0.5 and of the saturation temperature for Na/(Na + K) of 0.35 and 0.50. At that pressure the brucite-periclase curves reach
a lowest x
H2O of 0.45 and a temperature of 587 °C before salt saturation occurs, values considerably lower than in either binary. This
double-salt eutectic effect may have a significant application to natural polyionic hypersaline solutions in the deep crust
and upper mantle in that higher solute concentrations and very low H2O activities may be realized in complex solutions before salt saturation occurs. Concentrated salt solutions seem, from this
standpoint, and also because of high mechanical mobility and alkali-exchanging potential, feasible as metasomatic fluids for
a variety of deep-crust and upper mantle processes.
Received: 9 August 1996 / Accepted: 15 November 1996 相似文献
846.
The seismic ground motion of a test area in the eastern district of Naples is computed with a hybrid technique based on the mode summation and the finite difference methods. This technique allows us the realistic modelling of source and propagation effects, including local soil conditions. In the modelling, we consider the 1980 Irpinia earthquake, a good example of strong shaking for the area of Naples, which is located about 90 km from the epicenter.The detailed geological setting is reconstructed from a large number of drillings. The sub-soil is mainly formed by alluvial (ash, stratified sand and peat) and pyroclastic materials overlying a pyroclastic rock (yellow neapolitan tuff), representing the neapolitan bedrock. The detailed information available on mechanical properties of the sub-soil and its geometry warrants the application of the sophisticated hybrid technique.As expected, the sedimentary cover causes an increase of the signal's amplitudes and duration. If thin peat layers are present, the amplification effects are reduced, and the peak ground accelerations are similar to those observed for the bedrock model. This can be explained by the backscattering of wave energy at such layers, that tend to seismically decouple the upper from the lower part of the structure.For SH-waves, the influence of the variations of the S-wave velocities on the spectral amplification is studied, by considering locally measured velocities and values determined from near-by down-hole measurements. The comparison between the computed spectral amplifications confirms the key role of an accurate determination of the seismic velocities of the different layers.The comparison performed between a realistic 2-D seismic response and a standard 1-D response, based on the vertical propagation of waves in a plane layered structure, shows considerable difference, from which it is evident that serious caution must be taken in the modelling of expected ground motion at a specific site. 相似文献
847.
848.
—The clustering structure of the Vesuvian earthquakes occurring is investigated by means of statistical tools: the inter-event time distribution, the running mean and the multifractal analysis. The first cannot clearly distinguish between a Poissonian process and a clustered one due to the difficulties of clearly distinguishing between an exponential distribution and a power law one. The running mean test reveals the clustering of the earthquakes, but looses information about the structure of the distribution at global scales. The multifractal approach can enlighten the clustering at small scales, while the global behaviour remains Poissonian. Subsequently the clustering of the events is interpreted in terms of diffusive processes of the stress in the earth crust. 相似文献
849.
Study of sporadic-E clouds by backscatter radar 总被引:1,自引:0,他引:1
It is shown that swept-frequency backscatter ionograms covering a range of azimuths can be used to study the dynamics of sporadic-E clouds. A simple technique based on analytic ray tracing can be used to simulate the observed narrow traces associated with Es patches. This enables the location and extent of the sporadic-E clouds to be determined. The motion of clouds can then be determined from a time sequence of records. In order to demonstrate the method, results are presented from an initial study of 5 days of backscatter ionograms from the Jindalee Stage B data base obtained during March-April 1990. Usually 2–3 clouds were observed each day, mainly during the evening and up to midnight. The clouds lasted from 1–4 h and extended between 30°–80° in azimuth and 150–800 km in range. The clouds were mostly stationary or drifted generally westward with velocities of up to 80 ms–1. Only one cloud was observed moving eastward. 相似文献
850.
Effects of convection instability due to incompatibility between ocean dynamics and surface forcings
The study demonstrates that an incompatibility between a surface temperature climatology and a given ocean model, into which the climatology is assimilated via Haney restoration, can cause model ocean climate drift and interdecadal oscillations when the ocean is switched to a weaker restoration. This is made using an idealized Atlantic Ocean model driven by thermal and wind forcing only. Initially, the temperature climatology is forcefully assimilated into the model, and an implied heat flux field is diagnosed. During this stage any incompatibility is suppressed. The restoring boundary condition is then switched to a new forcing consisting of a part of the diagnosed flux and a part of the restoring forcing in such a way that at the moment of the switching the heat flux is identical to that prior to the switching. Under this new forcing condition, the incompatibility becomes manifest, causing changes in convection patterns, and producing drift and interdecadal oscillations. The mechanisms are described. 相似文献