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991.
992.
993.
Ettore Perozzi Lorenzo Casalino Guido Colasurdo Alessandro Rossi Giovanni B. Valsecchi 《Celestial Mechanics and Dynamical Astronomy》2002,83(1-4):49-62
The authors here propose a mission scenario, aimed at close exploration of a Near-Earth Asteroid, exploiting low-V resonant trajectories. These trajectories allow repeated fly-bys of a chosen target. A selection procedure, which has been used to find some interesting mission opportunities in the 2005-2015 time frame, is described and the corresponding detailed mission profiles are derived, using an indirect optimization method. 相似文献
994.
The nearshore region exhibits many striking morphological patterns with a variety of spatial and temporal scales. The formation of these rhythmic features has been initially ascribed, depending on the pattern in question, to spatial structures in the flow or geological constrains. These forcing templates have been hypothesized to provide the spatial structure that becomes imprinted on the shoreline or seabed morphology. More recently, new explanations for rhythmic patterns have involved interactions between fluid flow and sediment transport that create morphological feedbacks and lead to pattern self-organization. While forcing-template models do not explicitly treat transport of the sediment that makes up the pattern, self-organization models focus on the strong couplings within flow/sediment systems, and on interactions between emergent structures. We illustrate the sweeping shift from template explanations to self-organization by discussing four nearshore patterns: beach cusps, surfzone crescentic sandbars, inner-shelf sorted bedforms, and large-scale cuspate shorelines. Models involving self-organization show that local interactions between flow and sediment transport can collectively give rise to patterns with large-scale coherence and that the driving feedbacks can be associated with either topographical or grain-size composition instabilities. An approach based on self-organization also allows researchers to establish limitations in the predictability of the occurrence of rhythmic patterns and characteristics as well as to study mechanisms leading to the observed variability of pattern or lack of regularity. 相似文献
995.
Giovanni Zanchetta Alessandra Borghini Anthony E. Fallick Francesco P. Bonadonna Gabriello Leone 《Journal of Paleolimnology》2007,38(2):227-239
Ten meters of lacustrine deposits retrieved from Lake Pergusa (Sicily, southern Italy) were investigated through stable isotope
composition (carbon and oxygen) of authigenic carbonate (calcareous muds) and freshwater shells. The core chronology was established
through three AMS dates, and by correlation with a previously dated nearby core. Stable isotope data show that the lake water
evolution was mainly dominated by evaporation. Between ca. 20 and 28 ka the recovered sediments have very high δ18O values, likely corresponding to very dry climatic conditions. The observed rapid oscillations in the δ18O of the recovered sediments during this period also suggest important climatic fluctuations. More humid conditions dominated
during the Holocene period, with the wettest interval occurring between ca. 9000 and 3000 years BP. Late Holocene sediments
represent a substantial return to drier conditions. The available pollen data from a nearby core substantially confirm this
general climatic trend during the Holocene. The positive correlation between δ13C of the calcareous muds and carbonate content suggests that biological activity played a key role in the carbon isotope evolution
of dissolved inorganic carbon. However, a clear climatic signal is not evident from the δ13C record. 相似文献
996.
The Supramonte limestone complex, of Jurassic–Cretaceous age, lies within the municipal borders of Urzulei, Oliena and Orgosolo
(north-central Sardinia). For the most part, the Supramonte groundwater drains towards the outcrops in the northernmost part
of the massif. A minor, almost negligible quantity of water drains towards a series of outcrops along the edge of the carbonate
structure, with numerous subaerial and submarine springs. The groundwater in the entire system represents one of the most
important water resources in Sardinia, especially for drinking purposes. Taking into consideration the development of karst
cavities, permanent groundwater reserves were cautiously estimated to be at least 144 Mm3. Intrinsic vulnerability to pollution has been assessed by means of the EPIK method. 相似文献
997.
John Douglas Hideo Aochi Peter Suhadolc Giovanni Costa 《Bulletin of Earthquake Engineering》2007,5(1):17-26
In this short article, the possible reduction in the standard deviation of empirical ground motion estimation equations through
the modelling of the effect of crustal structure is assessed through the use of ground-motion simulations. Simulations are
computed for different source-to-site distances, focal depths, focal mechanisms and for crustal models of the Pyrenees, the
western Alps and the upper Rhine Graben. Through the method of equivalent hypocentral distance introduced by Douglas et al.
[(2004) Bull Earthquake Eng 2(1): 75–99] to model the effect of crustal structure in empirical equations, the scatter associated
with such equations derived using these simulated data could be reduced to zero if real-to-equivalent hypocentral distance
mapping functions were derived for every combination of mechanism, depth and crustal structure present in the simulated dataset.
This is, obviously, impractical. The relative importance of each parameter in affecting the decay of ground motions is assessed
here. It is found that variation in focal depth is generally more important than the effect of crustal structure when deriving
the real-to-equivalent hypocentral distance mapping functions. In addition, mechanism and magnitude do not have an important
impact on the decay rate. 相似文献
998.
999.
Maura Bussolotto Antonio Benedicto Chiara Invernizzi Luca Micarelli Valrie Plagnes Giovanni Deiana 《Journal of Structural Geology》2007,29(12):2017-2037
The Gubbio fault is an active normal fault defined by an important morphological scarp and normal fault focal mechanism solutions. This fault truncates the inherited Miocene Gubbio anticline and juxtaposes Mesozoic limestones in the footwall against Quaternary lacustrine deposits in the hanging wall. The offset is more than 2000 m of geological throw accumulated during a poly-phased history, as suggested by previous works, and has generated a complex zone of carbonate-rich fault-related structures. We report the results of a multidisciplinary study that integrates detailed outcrop and petrographic analysis of two well-exposed areas along the Gubbio fault zone, geochemical analysis (fluid inclusions, stable isotopes, and trace elements) of calcite-sealed fault-related structures and fault rocks, and biostratigraphic controls. Our aims are: (i) the characterization of the deformation features and their spatial–temporal relationships, and (ii) the determination of the P/T conditions and the fluid behaviour during deformation to achieve a better understanding of fluid–rock interaction in fault zones.We show that few of the observed structures can be attributed to an inherited shortening phase while the most abundant structures and fault rocks are related to extensional tectonics. The outcropping extensional patterns formed at depths less than 2.5–3 km, in a confined fluid system isolated from meteoric water, and the fault structures are the response to a small amount of cumulated displacement, 12–19% of the total geological throw. 相似文献
1000.
Marco Mancini Elisabetta D'Anastasio Mario Barbieri Paolo Marco De Martini 《Quaternary Research》2007,67(3):487-501
The eastern border of the Middle Valley of the Tiber River is characterized by several Plio-Pleistocene paleoshorelines, which extend for about 100 km along the western margin of the Central Apennines (Italy). We studied these paleoshorelines by the means of geological and paleontological analyses and new 87Sr/86Sr isotope analyses. The youngest and uppermost paleoshorelines have been detected and mapped through detailed geologic and stratigraphic surveys, which led to the recognition of nearshore deposits, cliff breccias, alignments of Lithophaga borings, fossil abrasion notches and wave-cut platforms. The altitude of these paleoshorelines decreases almost regularly in the NNW–SSE direction from 480 to 220 m a.s.l. Measurements of the 87Sr/86Sr isotope ratio have been conducted on corals and mollusks collected from sediments outcropping close to the paleoshorelines. The isotopic dating results indicate numerical values that range between 0.70907 and 0.70910 all over the 100-km outcrop. These results, together with biostratigraphic data, constrain the age of the youngest paleoshorelines to 1.65–1.50 Ma. These paleoshorelines are thus considered almost isochronous, giving an estimated uplift rate of 0.34–0.17 ± 0.03 mm/a moving from NNW to SSE. Shape, length and continuity of the 100-km-long observed movements indicate that the studied paleoshorelines are an important marker of the Quaternary uplift of the Central Apennines. 相似文献