首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   326篇
  免费   16篇
  国内免费   1篇
测绘学   14篇
大气科学   24篇
地球物理   53篇
地质学   86篇
海洋学   25篇
天文学   96篇
综合类   1篇
自然地理   44篇
  2021年   1篇
  2020年   4篇
  2019年   4篇
  2018年   7篇
  2017年   10篇
  2016年   7篇
  2015年   12篇
  2014年   11篇
  2013年   26篇
  2012年   7篇
  2011年   17篇
  2010年   13篇
  2009年   19篇
  2008年   14篇
  2007年   10篇
  2006年   9篇
  2005年   12篇
  2004年   13篇
  2003年   23篇
  2002年   9篇
  2001年   14篇
  2000年   14篇
  1999年   15篇
  1998年   7篇
  1997年   12篇
  1996年   2篇
  1995年   3篇
  1994年   4篇
  1993年   2篇
  1992年   4篇
  1991年   3篇
  1990年   2篇
  1988年   4篇
  1986年   3篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1980年   3篇
  1979年   2篇
  1978年   1篇
  1977年   3篇
  1975年   2篇
  1974年   1篇
  1973年   1篇
  1970年   2篇
  1969年   2篇
  1968年   2篇
  1967年   2篇
  1885年   1篇
排序方式: 共有343条查询结果,搜索用时 15 毫秒
91.
92.
— Surface-wave amplitudes from explosion sources show less variation for a given event han body wave amplitudes, so it is natural to expect that yield estimates derived from surface waves will be more accurate than yield estimates derived from body waves. However, yield estimation from surface waves is complicated by the presence of tectonic strain release, which acts like one or more earthquake sources superimposed on top of the explosion. Moment-tensor inversion can be used to remove the tectonic component of the surface waves, however moment-tensor inversion for shallow sources is inherently non-unique so the explosion isotropic moment cannot be determined with the necessary accuracy by this means. Explosions on an island or near a mountain slope can exhibit anomalous surface waves similar to those caused by tectonic strain release. These complications cause yield estimates derived from surface waves to be less accurate than yield estimates from body waves recorded on a well-calibrated network with good coverage. Surface-wave amplitudes can be expressed as a surface-wave magnitude M s , which is defined as the logarithm of the amplitude plus a distance correction, or as a path corrected spectral magnitude, log $M^{\prime}_0$ , which is derived from the surface-wave spectrum. We derive relations for M s vs. yield and log $M^{\prime}_0$ vs. yield for a large data set and estimate the accuracy of these estimates.  相似文献   
93.
When some magnetic field lines connect a Kerr black hole with a disk rotating around it, energy and angular momentum are transferred between them. If the black hole rotates faster than the disk, ca&solm0;GMH>0.36 for a thin Keplerian disk, then energy and angular momentum are extracted from the black hole and transferred to the disk (MH is the mass and aMH is the angular momentum of the black hole). This way, the energy originating in the black hole may be radiated away by the disk. The total amount of energy that can be extracted from the black hole spun down from ca&solm0;GMH=0.998 to ca&solm0;GMH=0.36 by a thin Keplerian disk is approximately 0.15MHc2. This is larger than approximately 0.09MHc2, which can be extracted by the Blandford-Znajek mechanism.  相似文献   
94.
Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. The Mn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonates are found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonate are associated mainly with laminated intervals.Because of the suspected difference in isotopic fractionation between different carbonate minerals, the stable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonate in the sediment. High 18O and low 13C values are associated with abundant Mn-carbonates. Low 18O and high 13C values are associated with only minor concentrations of Mn-carbonates.The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectra using a multiple regression model with backward elimination. The proposed link between the sedimentary archive and local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periods of forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permitting the formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygen to depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.  相似文献   
95.
 A new data set of Etna lava flows erupted since 1868 has been compiled from eight topographic maps of the volcano published at intervals since then. Volumes of 59 flows or groups of flows were measured from topographic difference maps. Most of these volumes are likely to be considerably more accurate than those published previously. We cut the number of flow volumes down to 25 by selecting those examples for which the volume of an individual eruption could be derived with the highest accuracy. This refined data set was searched for high correlations between flow volume and more directly measurable parameters. Only two parameters showed a correlation coefficient of 70% or greater: planimetric flow area A (70%) and duration of the eruption D (79%). If only short duration (<18 days) flows were used, flow length cubed, L3, had a correlation coefficient of 98%. Using combinations of measured parameters, much more significant correlations with volume were found. Dh had a correlation coefficient of 90% (h is the hydrostatic head of magma above the vent), and  , 92% (where W is mean width and E is the degree of topographic enclosure), and a combination of the two , 97%. These latter formulae were used to derive volumes of all eruptions back to 1868 to compare with those from the complete data set. Values determined from the formulae were, on average, lower by 16% (Dh), 7% (, and 19% . Received: 30 November 1998 / Accepted: 20 June 1999  相似文献   
96.
97.
With the objective of investigating the windwind collision phenomenon and supporting contemporaneous X-ray observations, we have organized a large-scale, coordinated optical monitoring campaign of the massive, highly eccentric O9 III+B1 III binary Iota Orionis. Successfully separating the spectra of the components, we refine the orbital elements and confirm the rapid apsidal motion in the system. We also see strong interaction between the components during periastron passage and detect phase-locked variability in the spectrum of the secondary star. However, we find no unambiguous signs of the bow shock crashing on the surface of the secondary, despite the predictions of hydrodynamic simulations. Combining all available photometric data, we find rapid, phase-locked variations and model them numerically, thus restricting the orbital inclination to 50° i 70°.  相似文献   
98.
In this work, we describe a meshless numerical method based on local collocation with RBFs for the solution of the poroelasticity equation. The RBF finite collocation approach forms a series of overlapping nodal stencils, over which an RBF collocation is performed. These local collocation systems enforce the governing PDE operator throughout their interior, with the intersystem communication occurring via the collocation of field variables at the stencil periphery. The method does not rely on a generalised finite differencing approach, whereby the governing partial differential operator is reconstructed at the global level to drive the solution of the PDE. Instead, the PDE governing and boundary operators are enforced directly within the local RBF collocation systems, and the sparse global assembly is formed by reconstructing the value of the field variables at the centrepoint of the local stencils. In this way, the solution of the PDE is driven entirely by the local RBF collocation, and the method more closely resembles the approach of the full‐domain RBF collocation method. By formulating the problem in this fashion, high rates of convergence may be attained without the computational cost and numerical ill‐conditioning issues that are associated with the full‐domain RBF collocation approach. An analytical solution is formulated for a 2D poroelastic fluid injection scenario and is used to verify the proposed implementation of the method. Highly accurate solutions are produced, and convergence rates in excess of sixth order are observed for each field variable (i.e. pressure and displacement) and field‐variable derivative (i.e. pressure gradients and stresses). The stress and displacement fields resulting from the solution of the poroelasticity equation are then used to describe the formation and propagation of microfractures and microfissures, which may form in the presence of large shear strain, in terms of a continuous damage variable which modifies the mechanical and hydraulic properties of the porous medium. The formation of such hydromechanical damage, and the resulting increase in hydraulic conductivity, is investigated for a pressurised injection into sandstone. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
99.
Dissolved organic matter (DOM) quality and quantity is not measured routinely in‐situ limiting our ability to quantify DOM process dynamics. This is problematic given legislative obligations to determine event based variability; however, recent advances in field deployable optical sensing technology provide the opportunity to address this problem. In this paper, we outline a new approach for in‐situ quantification of DOM quantity (Dissolved Organic Carbon: DOC) and a component of quality (Biochemical Oxygen Demand: BOD) using a multi‐wavelength, through‐flow fluorescence sensor. The sensor measured tryptophan‐like (Peak T) and humic‐like (Peak C) fluorescence, alongside water temperature and turbidity. Laboratory derived coefficients were developed to compensate for thermal quenching and turbidity interference (i.e., light attenuation and scattering). Field tests were undertaken on an urban river with ageing wastewater and stormwater infrastructure (Bourn Brook; Birmingham, UK). Sensor output was validated against laboratory determinations of DOC and BOD collected by discrete grab sampling during baseflow and stormflow conditions. Data driven regression models were then compared to laboratory correction methods. A combination of temperature and turbidity compensated Peak T and Peak C was found to be a good predictor of DOC concentration (R2 = 0.92). Conversely, using temperature and turbidity correction coefficients provided low predictive power for BOD (R2 = 0.46 and R2 = 0.51, for Peak C and T, respectively). For this study system, turbidity appeared to be a reasonable proxy for BOD, R2 = 0.86. However, a linear mixed effect model with temperature compensated Peak T and turbidity provided a robust BOD prediction (R2 = 0.95). These findings indicate that with careful initial calibration, multi‐wavelength fluorescence, coupled with turbidity, and temperature provides a feasible proxy for continuous, in‐situ measurement of DOC concentration and BOD. This approach represents a cost effective monitoring solution, particularly when compared to UV – absorbance sensors and DOC analysers, and could be readily adopted for research and industrial applications.  相似文献   
100.
Federico Farina  Gary Stevens 《Lithos》2011,122(3-4):189-200
The broad Sr isotopic variability exhibited by granitoid rocks is commonly interpreted to reflect the mixing of magmas from different sources. However, evidence from granites and migmatites indicates that melting and magma extraction from crustal sources can occur sufficiently rapidly that trace-element and isotopic equilibration between liquid and residual phases is commonly not achieved. Additionally, evidence from unmelted high-grade metamorphic rocks indicates that major reactant minerals in the fluid-absent melting process, principally biotite and plagioclase, do not always attain equilibrium during regional metamorphism. When these two circumstances occur in combination, the melt does not inherit its radiogenic isotopic signature from the bulk source in a simple way. In such situations, the isotopic composition of the melt will be dependent on the isotopic compositions of the reactant phases and the stoichiometry of the melting reaction. This study has used information from experimental studies of fluid absent partial melting in metapelites and metagreywackes to investigate the consequences of Sr isotopic disequilibrium between the reactant minerals for magma composition. The study demonstrates that a range of isotopically distinct magmas can arise from progressive melting of a single source that is able to undergo melting through different reactions as temperature increases. When translated to the typically layered sources constituted by sedimentary and volcano-sedimentary rocks, this process will produce magmas characterized by Sr isotope variability that reflects the differences in melting reaction stoichiometry within the different layers, even with no bulk-rock isotopic variability between layers. This study demonstrates that the Sr isotope variability commonly observed within granitic suites, as well as at the grain and sub-grain scale within individual magmatic bodies, can be primary, reflecting differences in composition between magma batches produced from the progressive melting of a single source.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号