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651.
David Banks 《Hydrogeology Journal》1992,1(4):5-19
Several theoretical and semi-empirical studies indicate a near linear relationship between the specific capacity (F) of boreholes in bedrock aquifers and apparent transmissivity (T) of the form T=F/!, where !=c. 0.9. For several boreholes in Hvaler and Trondheim, Norway, the specific capacity of individual fractures has been determined by plotting QA (the yield of water from the aquifer during pumping or recovery) against s (drawdown). Individual apparent fracture transmissivities can be calculated, as can average apparent hydraulic conductivities for individual borehole sections. The method presented is easily performed, requires modest amounts of data, is not mathematically difficult, and is believed to yield at least an order-of-magnitude estimate of transmissivity. Given that hydraulic conductivity in bedrock aquifers can range over seven or eight orders of magnitude, even such a coarse estimate can be of significant use. This is verified by comparison of hydraulic conductivities derived from testing of boreholes in Hvaler with conductivities deduced from water leakage into a nearby subsea tunnel in the same lithology. Résumé: Plusieurs études théoriques et semi-empiriques font envisager un relation pratiquement linéaire entre la capacité d'emmagasinement spécifique (F) de forages dans des aquifères de socle et la transmissivité apparente (T), de la forme T = F/a, où a = 0,9 c. Pour plusieurs forages de Hvaler et de Trondheim (Norvège), la capacité spécifique de fractures individuelles a été déterminée à partir de la relation Q (débit d'alimentation de l'aquifère au cours du pompage ou de la remontée) en fonction de s (rabattement). On a pu calculer les transmissivités individuelles apparentes des fractures et, quand c'était possible, les conductivités hydrauliques apparentes moyennes de portion de forages. La méthode proposée est facile à mettre en oeuvre; elle demande peu de données, n'est pas difficile mathématiquement et parait donner au moins un ordre de grandeur de la transmissivité. Etant donné que la conductivité hydraulique dans les aquifères de socle peut couvrir 7 ou 8 ordres de grandeur, une estimation, même grossière comme coll-ci, peut être significative. Ceci est vérifié en effectuant la comparasion entre les conductivités hydrauliques, obtenues en testant les forages de Hvaler, et celles déduites de l'étude de fuites dans un tunnel sous-marin voisin, dans les mêmes formations géologiques. 相似文献
652.
Internally consistent gahnitic spinel-cordierite-garnet equilibria in the FMASHZn system: geothermobarometry and applications 总被引:8,自引:0,他引:8
Geoffrey T. Nichols Ron F. Berry David H. Green 《Contributions to Mineralogy and Petrology》1992,111(3):362-377
The equilibrium (Mg, Fe, Zn)3Al2Si3O12+2Al2SiO5=3(Mg, Fe, Zn)Al2O4+5SiO2 garnet + sillimanite/kyanitc = spinel + quartz was calibrated in the piston-cylinder apparatus between 11 and 30 kbar, and over the temperature range of 950 to 1200°C. Three experimental mixes of Mg no. [100*MgO/(MgO+FeO)] 40, 47 and 60, in the FeO –MgO–Al2O3–SiO2–ZnO (FMASZn) system were used under low oxygen fugacities and anhydrous conditions. We derive a ternary Fe–Mg–Zn symmetric mixing model for aluminous spinels in equilibrium with garnet, to quantify the increase in gahnitic end-member of spinel with increasing pressure and descreasing temperature. Further experiments in the spinel-cordieritequartz-sillimanite field were combined with garnet-cordierite data from the literature to produce a consistent set of equations describing the exchange reactions in FMASHZn relevant to quartz-sillimanite bearing rocks at granulite facies conditions. As spinel is an important mineral participating in many rocks of aluminous composition at granulite-facies conditions, and as zinc contributes to an enlargement of spinel's stability field towards higher pressures and lower temperatures, the thermobarometric calibrations presented here will be most significant in delineating the prograde and retrograde trajectory of P-T paths. 相似文献
653.
654.
David Alexander 《Environmental Geology》1992,20(3):165-179
Conceptual equations are presented for the net benefits, total risk, and total vulnerability associated with hazard zone occupation. It is shown that, as they are polycausal phenomena, landslides require a more sophisticated approach to this form of analysis than that employed for most other forms of hazard.Three examples of landsliding are examined. At Cuyocuyo, in the Peruvian Cordillera Oriental, slope instability is virtually inevitable, but human intervention may be disturbing some very fragile natural equilibria. At Calciano, in southern Italy, deforestation has resulted in a mudflow disaster, although man-made factors are not the only causes at work. Finally, in the Valle dell'Orco (also in southern Italy) demographic and agrarian change have aided the partial metamorphosis of the catchment into a large-scale mudflow complex (although the longer-term causes are natural, tectonic ones). These examples illustrate parts of the continuum that extends from dominant natural to dominant anthropogenic causes.Human intervention has played a key role in stimulating the natural antecedents of landslides occurring in the study areas. It is concluded that risk perception has not been wide enough to promote adequate risk mitigation, in part because of a failure properly to conceive of landslides as polycausal phenomena, in which man-made causes effectively cannot—and should not—be separated from natural ones. 相似文献
655.
656.
657.
An estimate of late Pleistocene geomagnetic intensity variation from Sulu Sea sediments 总被引:2,自引:0,他引:2
David A. Schneider 《Earth and Planetary Science Letters》1993,120(3-4):301-310
Sediments from Site 769 of the Ocean Drilling Program's Leg 124 provide a record of geomagnetic intensity variation over the past 110 ky. Using continous shipboard measurements exclusively, I estimate the variation in the geomagnetic field strength by employing low-field magnetic susceptibility as a normalization parameter for the measured remanence intensity. By calibrating the resultant relative paleointensity record against previously available Holocene age estimates of absolute paleointensity, I derive an estimate of virtual dipole moment since 110 ka. The record obtained from these Sulu Sea sediments is strikingly like that previously obtained from sediments of a similar age in the Mediterranean Basin with distinct intervals of low intensity near 15, 20, 40 and 65–70 ka. The Sulu sediments also indicate a low-intensity feature near 108 ka. Important differences in paleointensity estimates obtained from these different regions for the interval between 30 and 20 ka suggest that a relatively large non-dipolar component of the geomagnetic field might have been present at that time. 相似文献
658.
David Kadko 《Earth and Planetary Science Letters》1993,120(3-4):361-374
Profiles of 210Pb over the Endeavour and North Cleft Segments of the Juan de Fuca Ridge are used to model a time scale for the scavenging, by hydrothermal plumes, of reactive elements in seawater. The hydrothermal plumes above these ridge segments are sites of intense scavenging removal of 210Pb. At Endeavour, the total 210Pb activities within the plume are as low as 8 dpm/100 l and dissolved activities are as low as 3 dpm/100 l. At the North Cleft, which is characterized by higher particulate Fe concentrations, the total 210Pb activities are 4.5 dpm/100 l, the dissolved activities are 1–2 dpm/100 l and the 210Pb activities are deficient with respect to the activity of the 210Po daughter. These are perhaps the lowest 210Pb activities ever measured in the deep sea. The large gradient of 210Pb between the plume and surrounding deep water suggests that scavenging is focused into the plumes through horizontal transport. The implication, therefore, is that this process might impact the ocean on a scale larger than that local to the ridge crest. By coupling published measurements of particle flux from Endeavour with 210Pb activities on particles trapped at that site, the total volume of seawater stripped of 210Pb per year for that site was calculated to be 7.4 × 1012 l/y. Globally, the extrapolated volume flux of seawater stripped of reactive constituents is 5.7 × 1015 l/y, such that the entire ocean is processed in this manner in 2.4×105 y. The geochemical cycle of elements with ocean residence times much shorter than this (e.g., Pb and Th) will not be greatly affected by hydrothermal scavenging. On the other hand, this process holds significance for the geochemistry of other elements scavenged by hydrothermal plumes, such as P and V, whose ocean residence times are > 104 y. 相似文献
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