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141.
142.
The logical structure of species richness determinations is outlined in order to explain how their accuracy is affected by nonrandomly distributed populations. The transformation of a species-area curve to a species-range curve is discussed in general terms and conditions required for accurate species richness determinations are given. Applications of the approach to scleractinid coral and pine-tree communities illustrate the method. 相似文献
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Pascal Richet Alan Whittington François Holtz Harald Behrens Susanne Ohlhorst Max Wilke 《Contributions to Mineralogy and Petrology》2000,138(4):337-347
A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial
molar volume of water is 12.0 ± 0.5 cm3/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite
compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H2O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization
or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction
between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent
with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO2–M2O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H2O is about 4 × 10−5 K−1 in silicate glasses.
Received: 30 June 1999 / Accepted: 5 November 1999 相似文献
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Isotope mixing models require individual isotopic tracer content for correct quantification of sediment source contributions 下载免费PDF全文
Hari Ram Upadhayay Samuel Bodé Marco Griepentrog Roshan Man Bajracharya William Blake Wim Cornelis Pascal Boeckx 《水文研究》2018,32(7):981-989
The use of isotopic tracers for sediment source apportionment is gaining interest with recent introduction of compound‐specific stable isotope tracers. The method relies on linear mixing of source isotopic tracers, and deconvolution of a sediment mixture initially quantifies the contribution of sources to the mixture's tracer signature. Therefore, a correction to obtain real sediment source proportions is subsequently required. As far as we are aware, all published studies to date have used total isotopic tracer content or a proxy (e.g., soil carbon content) for this post‐unmixing correction. However, as the relationship between the isotopic tracer mixture and the source mixture is different for each isotopic tracer, post‐unmixing corrections cannot be carried out with one single factor. This contribution presents an isotopic tracer model structure—the concentration‐dependent isotope mixing model (CD‐IMM)—to overcome this limitation. Herein, we aim to clarify why the “conventional” approach to converting isotopic tracer proportions to source proportions using a single factor is wrong. In an initial mathematical assessment, error incurred by not using CD‐IMM (NCD‐IMM) in unmixing two sources with two isotopic tracers showed a complex relation as a function of relative tracer contents. Next, three artificial mixtures with different proportions of three soil sources were prepared and deconvoluted using 13C of fatty acids using CD‐IMM and NCD‐IMM. Using NCD‐IMM affected both accuracy (mean average error increased up to a threefold compared with the CD‐IMM output) and precision (interquartile range was up to 2.5 times larger). Finally, as an illustrative example, the proportional source contribution reported in a published study was recalculated using CD‐IMM. This resulted in changes in estimated source proportions and associated uncertainties. Content of isotopic tracers is seldom reported in published work concerning use of isotopic tracers for sediment source partitioning. The magnitude of errors made by miscalculation in former studies is therefore difficult to assess. With this contribution, we hope the community will acknowledge the limitations of prior approaches and use a CD‐IMM in future studies. 相似文献
147.
The anisotropy of propagation of radio waves used by global navigation satellite systems is investigated using high-resolution
observational data assimilations produced by the European Centre for Medium-range Weather Forecast. The geometry and the refractivity
of the neutral atmosphere are built introducing accurate geodetic heights and continuous formulations of the refractivity
and its gradient. Hence the realistic ellipsoidal shape of the refractivity field above the topography is properly represented.
Atmospheric delays are obtained by ray-tracing through the refractivity field, integrating the eikonal differential system.
Ray-traced delays reveal the anisotropy of the atmosphere. With the aim to preserve the classical mapping function strategy,
mapping functions can evolve to adapt to high-frequency atmospheric fluctuations and to account for the anisotropy of propagation
by fitting at each site and time the zenith delays and the mapping functions coefficients. Adaptive mapping functions (AMF)
are designed with coefficients of the continued fraction form which depend on azimuth. The basic idea is to expand the azimuthal
dependency of the coefficients in Fourier series introducing a multi-scale azimuthal decomposition which slightly changes
the elevation functions with the azimuth. AMF are used to approximate thousands of atmospheric ray-traced delays using a few
tens of coefficients. Generic recursive definitions of the AMF and their partial derivatives lead to observe that the truncation
of the continued fraction form at the third term and the truncation of the azimuthal Fourier series at the fourth term are
sufficient in usual meteorological conditions. Delays’ and elevations’ mapping functions allow to store and to retrieve the
ray-tracing results to solve the parallax problem at the observation level. AMF are suitable to fit the time-variable isotropic
and anisotropic parts of the ray-traced delays at each site at each time step and to provide GPS range corrections at the
measurement level with millimeter accuracy at low elevation. AMF to the azimuthal anisotropy of the neutral atmosphere are
designed to adapt to complex weather conditions by adaptively changing their truncations. 相似文献
148.
Leila Hashemi Beni Sébastien Villeneuve Denyse I. LeBlanc Pascal Delaquis 《Transactions in GIS》2011,15(Z1):95-108
A Geographical Information System (GIS)‐based approach was developed for the identification of vulnerabilities and the measurement of risks associated with contamination of food systems with biological agents. In this research work, a tight integration of ArcGIS with the Arena simulation tool has been implemented. Arena was used to simulate and track contamination in a food distribution network and transmit the time dependent information to GIS. ArcGIS was employed to provide the primary user interface, process network data, and visualize the results. In addition, the GIS, through its powerful capabilities to process spatial data, could allow decision‐ makers to quickly determine the potential impact of a contamination event, at any stage, as a function of both time and geography. Two contamination scenarios along the farm‐to‐fork chain were examined to show the geographic zone and the proportion of the population affected by the contamination. A constraint Voronoi data structure was developed to define influence zones (these were color coded according to a dynamic risk index), to identify those areas that are at greatest immediate risk as time progresses, and to estimate the population affected by these contamination events. This approach thus appears to have general application to many GIS‐based risk assessment problems. 相似文献
149.
研究了空间铷钟和被动型氢钟的地面批量和寿命试验测试结果,以及卫星在轨试验所达到的最新性能结果。基于这些星载钟的试验结果,对全球卫星导航系统的地面时间站的关键设备及其相关算法作了简要描述,并介绍了一种新颖的在轨技术,即从星载原子钟组(ONCLE)直接产生高度稳健的时间频率信号。 相似文献
150.