全文获取类型
收费全文 | 1066篇 |
免费 | 27篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 32篇 |
大气科学 | 78篇 |
地球物理 | 196篇 |
地质学 | 302篇 |
海洋学 | 120篇 |
天文学 | 279篇 |
自然地理 | 87篇 |
出版年
2022年 | 5篇 |
2021年 | 10篇 |
2020年 | 11篇 |
2019年 | 14篇 |
2018年 | 9篇 |
2017年 | 15篇 |
2016年 | 24篇 |
2015年 | 20篇 |
2014年 | 26篇 |
2013年 | 54篇 |
2012年 | 24篇 |
2011年 | 61篇 |
2010年 | 31篇 |
2009年 | 47篇 |
2008年 | 43篇 |
2007年 | 52篇 |
2006年 | 51篇 |
2005年 | 37篇 |
2004年 | 45篇 |
2003年 | 41篇 |
2002年 | 46篇 |
2001年 | 40篇 |
2000年 | 30篇 |
1999年 | 25篇 |
1998年 | 26篇 |
1997年 | 15篇 |
1996年 | 18篇 |
1995年 | 16篇 |
1994年 | 17篇 |
1993年 | 14篇 |
1992年 | 9篇 |
1991年 | 16篇 |
1990年 | 12篇 |
1989年 | 8篇 |
1988年 | 13篇 |
1987年 | 11篇 |
1986年 | 6篇 |
1985年 | 21篇 |
1984年 | 18篇 |
1983年 | 11篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1979年 | 15篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1971年 | 5篇 |
1970年 | 4篇 |
排序方式: 共有1094条查询结果,搜索用时 328 毫秒
891.
Despite the exposures of Precambrian and Paleozoic rocks and the accretionary tectonic history of the northern Pacific (northeastern Asia, Alaska, and Kamchatka), it is likely that a considerable portion of the lower crust of the continental margins is much younger and was generated by Cretaceous postaccretion magmatic events. Data on xenoliths suggest that Late Cretaceous and Paleocene mafic intrusions and cumulates of calc-alkaline magmas may become more important with increasing depth. This conclusion is based on the petrological and geochronological investigation of lower-middle crustal xenoliths borne by mantlederived alkali basalt lavas and U-Pb dating of zircon cores from the igneous rocks of the region. We studied deep mafic xenoliths of granulites and gabbroids (accounting for <2% of the general xenolith population) from the Late Neogene alkali basalt lavas of the Enmelen and Viliga volcanic fields (Russia) and the Imuruk volcanic field in the Seward Peninsula, St. Lawrence Island, and Nunivak Island (Alaska). Depleted MORB-like varieties and relatively enriched in radiogenic isotopes and LREE rocks were distinguished among plagioclase-bearing xenoliths. The most representative collection of Enmelen xenoliths was subdivided into three groups: LREE enriched charnockitoids and mafic melts, pyroxene-plagioclase cumulates with a positive Eu anomaly, and LREE depleted garnet gabbroids. Mineral thermobarometry and calculated seismic velocities (P = 5–12 kbar, T = 740–1100°C, and V p = 7.1 ± 0.3 km/s) suggest that the xenoliths were transported from the lower and middle crust, and the rocks show evidence for their formation through the magmatic fractionation of calc-alkaline magmas and subsequent granulite-facies metamorphism. The U-Pb age of zircon from the xenoliths ranges from the Cretaceous to Paleocene, clustering mainly within 107–56 Ma (147 crystals from 17 samples were dated). The zircon dates were interpreted as reflecting the magmatic and metamorphic stages of the growth and modification of the regional crust. The distribution of the obtained age estimates corresponds to the main magmatic pulses in two largest magmatic belts of the region, Okhotsk-Chukchi and Anadyr-Bristol. The absence of older inherited domains in zircons from both the xenoliths and igneous rocks of the regions is a strong argument in favor of the idea on the injection of juvenile material and underplating of calc-alkaline magmas in the lower crust during that time interval. This conclusion is supported by isotope geochemical data: the Sr, Nd, and Pb isotope ratios of the rocks and xenolith minerals show mantle signatures (87Sr/86Sr = 0.7040–0.70463, 143Nd/144Nd = 0.51252–0.51289, 206Pb/204Pb = 18.32–18.69) corresponding to an OIB source and are in general similar to those of the Cretaceous calc-alkaline basalts and andesites from continental-margin suprasubduction volcanoplutonic belts. Xenoliths from Nunivak Island and Cape Navarin show more depleted (MORB-like) geochemical and isotopic characteristics, which indicates variations in the composition of the lower crust near the southern boundary of the Bering Sea shelf. 相似文献
892.
D. J. Rawlings 《Australian Journal of Earth Sciences》2013,60(5):703-723
The Broadmere Structure is a concealed complex inversion structure that developed in a well preserved part of the Batten Trough in the southern McArthur Basin of northern Australia. The structure formed in response to a number of major tectonic events over a period of at least 600 million years. Evaluation of more than 300 km of seismic data gathered across the structure has provided significant insight into the evolution of the basin architecture which, in turn, has provided a framework for understanding the spatial and temporal distribution of mineralisation. The Palaeoproterozoic McArthur Basin, a polyphase basin with a long and complex evolutionary history, was initiated some time after 1.8 Ga as a sag basin following mantle activity associated with supercontinent assembly. This shallow sag then began to accumulate a relatively mature clastic sedimentary succession with interspersed basic volcanics. Subsequent extension of the weakened crust, beginning at approximately 1730 Ma, led to the development of a series of normal faults and half grabens within zones (e.g. Batten Trough) defined laterally by major strike slip fault complexes. The half grabens were filled initially by volcaniclastic sediments with associated bimodal volcanics that were later followed by mixed carbonate and clastic sediments exceeding 8 km in thickness. A major change in basinal and regional dynamics between 1575 and 1500 Ma (Isan Orogeny) led to the inversion of the thickened succession overlying the half grabens and extensive erosion of earlier depositional units, followed by deposition of a further 3 km of shallow marine clastic sediments. The analysis of the Broadmere Structure has provided a simple framework in which to evaluate prospectivity. The primary northwest striking half grabens formed localised anoxic, sediment starved depocentres in which organic rich sediments that potentially host mineralisation accumulated. The distribution of subsequent ore bodies is a function of the timing of fluid movement and the nature of fluid pathways, as determined by the evolving basin architecture. Fluids moved laterally through the permeable channels into the Batten Trough in two stages. Spatial data show that Pb and Zn occurrences lie close to the bounding faults, while Cu occurrences are distributed across the width of the Batten Trough, supporting a more or less syngenetic origin for the former and a much later, post inversion, epigenetic origin, for the latter. Mineralisation thus occurred in association with inversion structures formed above the primary half grabens that were generally concealed by later sedimentation. Because the half grabens occur at regularly spaced intervals along the Batten Trough there may well be more mineral occurrences similar to the McArthur River deposit. Clearly, these conclusions must be integrated into any exploration strategy. 相似文献
893.
J. McL. Miller D. Phillips C. J. L. Wilson L. J. Dugdale 《Australian Journal of Earth Sciences》2013,60(6):921-940
40Ar/39Ar age data from the boundary between the Delamerian and Lachlan Fold Belts identify the Moornambool Metamorphic Complex as a Cambrian metamorphic belt in the western Stawell Zone of the Palaeozoic Tasmanide System of southeastern Australia. A reworked orogenic zone exists between the Lachlan and Delamerian Fold Belts that contains the eastern section of the Cambrian Delamerian Fold Belt and the western limit of orogenesis associated with the formation of an Ordovician to Silurian accretionary wedge (Lachlan Fold Belt). Delamerian thrusting is craton-verging and occurred at the same time as the final consolidation of Gondwana. 40Ar/39Ar age data indicate rapid cooling of the Moornambool Metamorphic Complex at about 500 Ma at a rate of 20 – 30°C per million years, temporally associated with calc-alkaline volcanism followed by clastic sedimentation. Extension in the overriding plate of a subduction zone is interpreted to have exhumed the metamorphic rocks within the Moornambool Metamorphic Complex. The Delamerian system varies from a high geothermal gradient with syntectonic plutonism in the west to lower geothermal gradients in the east (no syntectonic plutonism). This metamorphic zonation is consistent with a west-dipping subduction zone. Contrary to some previous models involving a reversal in subduction polarity, the Ross and Delamerian systems of Antarctica and Australia are inferred to reflect deformation processes associated with a Cambrian subduction zone that dipped towards the Gondwana supercontinent. Western Lachlan Fold Belt orogenesis occurred about 40 million years after the Delamerian Orogeny and deformed older, colder, and denser oceanic crust, with metamorphism indicative of a low geothermal gradient. This orogenesis closed a marginal ocean basin by west-directed underthrusting of oceanic crust that produced an accretionary wedge with west-dipping faults that verge away from the major craton. The western Lachlan Fold Belt was not associated with arc-related volcanism and plutonism occurred 40 – 60 million years after initial deformation. The revised orogenic boundaries have implications for the location of world-class 440 Ma orogenic gold deposits. The structural complexity of the 440 Ma Stawell gold deposit reflects its location in a reworked part of the Cambrian Delamerian Fold Belt, while the structurally simpler 440 Ma Bendigo deposit is hosted by younger Ordovician turbidites solely deformed by Lachlan orogenesis. 相似文献
894.
The night and day temperature images from advanced spaceborne thermal emission and reflection radiometer (ASTER) remote sensing images are used to identify ephemeral and perennial stream reaches for use in the calibration of an integrated hydrologic model of an ungauged basin. The concept is based on apparent thermal inertia [ATI = (1-albedo)/(day temperature ? night temperature)]. These calculations help both the conceptual model and the calibration for the hydrologic model by indicating where there are thin alluvium and/or shallow groundwater. The study is on the Sevilleta National Wildlife Refuge, a long-term ecological research project that ASTER has included in its regular duty cycle. There are over 360 ASTER scenes in 8 years; however, only 10 night/day pairs suitable for ATI were found. The results correlate to the soil moisture recorded at two locations near the channel (R 2 of 0.88). The relationship between soil moisture and surrounding materials allows for differentiation of the perennial and ephemeral stream reaches. 相似文献
895.
Yan Gao Prashanth Marpu Imgard Niemeyer Daniel Miller Runfola Nicholas M. Giner Thomas Hamill 《国际地球制图》2013,28(3):211-226
Object-based image analysis (OBIA) uses object features (or attributes) that relateto the pixels contained by the image object to assist in image classification. These object features include spectral, shape, texture and context features. With hundreds of available features, the identification of those that can improve separability between classes is critical for OBIA. The Separability and Thresholds (SEaTH) algorithm calculates the SEaTH of object–classes for the given features. The SEaTH algorithm avoids time-consuming trial-and-error practice for seeking important features and thresholds. This article tests the SEaTH algorithm on Landsat-7 Enhanced Thematic Mapper (ETM+) imagery in a heterogeneous landscape with multiple land cover classes. The results suggest SEaTH is a strong alternative to other automated approaches, yielding an agreement of 79% with reference data. In comparison, an object-based nearest neighbour classifier yielded 66% agreement and a pixel-based maximum likelihood classifier yielded 69% agreement. 相似文献
896.
897.
898.
Bart Kuijpers Harvey J. Miller Tijs Neutens 《International journal of geographical information science》2013,27(8):1223-1248
Space-time prisms capture all possible locations of a moving person or object between two known locations and times given the maximum travel velocities in the environment. These known locations or ‘anchor points’ can represent observed locations or mandatory locations because of scheduling constraints. The classic space-time prism as well as more recent analytical and computational versions in planar space and networks assume that these anchor points are perfectly known or fixed. In reality, observations of anchor points can have error, or the scheduling constraints may have some degree of pliability. This article generalizes the concept of anchor points to anchor regions: these are bounded, possibly disconnected, subsets of space-time containing all possible locations for the anchor points, with each location labelled with an anchor probability. We develop two algorithms for calculating network-based space-time prisms based on these probabilistic anchor regions. The first algorithm calculates the envelope of all space-time prisms having an anchor point within a particular anchor region. The second algorithm calculates, for any space-time point, the probability that a space-time prism with given anchor regions contains that particular point. Both algorithms are implemented in Mathematica to visualize travel possibilities in case the anchor points of a space-time prism are uncertain. We also discuss the complexity of the procedures, their use in analysing uncertainty or flexibility in network-based prisms and future research directions. 相似文献
899.
R. Bacon Y. Copin G. Monnet Bryan W. Miller J.R. Allington-Smith M. Bureau C. Marcella Carollo Roger L. Davies Eric Emsellem Harald Kuntschner Reynier F. Peletier E.K. Verolme P. Tim de Zeeuw 《Monthly notices of the Royal Astronomical Society》2001,326(1):23-35
A new integral-field spectrograph, SAURON, is described. It is based on the TIGER principle, and uses a lenslet array. SAURON has a large field of view and high throughput, and allows simultaneous sky subtraction. Its design is optimized for studies of the stellar kinematics, gas kinematics, and line-strength distributions of nearby early-type galaxies. The instrument design and specifications are described, as well as the extensive analysis software which was developed to obtain fully calibrated spectra, and the associated kinematic and line-strength measurements. A companion paper will report on the first results obtained with SAURON on the William Herschel Telescope. 相似文献
900.
The SAURON project – III. Integral-field absorption-line kinematics of 48 elliptical and lenticular galaxies 总被引:1,自引:0,他引:1