全文获取类型
收费全文 | 1342篇 |
免费 | 47篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 95篇 |
地球物理 | 334篇 |
地质学 | 584篇 |
海洋学 | 106篇 |
天文学 | 155篇 |
综合类 | 5篇 |
自然地理 | 95篇 |
出版年
2022年 | 4篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 26篇 |
2017年 | 31篇 |
2016年 | 47篇 |
2015年 | 30篇 |
2014年 | 33篇 |
2013年 | 77篇 |
2012年 | 48篇 |
2011年 | 70篇 |
2010年 | 75篇 |
2009年 | 79篇 |
2008年 | 60篇 |
2007年 | 64篇 |
2006年 | 58篇 |
2005年 | 81篇 |
2004年 | 65篇 |
2003年 | 42篇 |
2002年 | 44篇 |
2001年 | 36篇 |
2000年 | 21篇 |
1999年 | 28篇 |
1998年 | 24篇 |
1997年 | 11篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 9篇 |
1992年 | 19篇 |
1991年 | 11篇 |
1990年 | 15篇 |
1989年 | 9篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 17篇 |
1985年 | 16篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 18篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 13篇 |
1976年 | 13篇 |
1975年 | 4篇 |
1974年 | 13篇 |
1973年 | 7篇 |
排序方式: 共有1403条查询结果,搜索用时 46 毫秒
851.
MicrOmega is an ultra miniaturized spectral microscope for in situ analysis of samples. It is composed of 2 microscopes; one with a spatial sampling less or equal to 4 μm, working in 4 colors in the visible range: MicrOmega/VIS, and a NIR hyperspectral microscope working in the spectral range 0.9–4 μm with a spatial sampling of 20 μm per pixel: MicrOmega/IR (described in this paper). MicrOmega/IR illuminates and images samples a few mm in size and acquires the NIR spectrum of each resolved pixel in up to 320 contiguous spectral channels. The goal of this instrument is to analyze in situ the composition of collected samples at almost their grain size scale, in a non-destructive way. With the chosen spectral range and resolution, a wide variety of constituents can be identified: minerals, such as pyroxene and olivine, ferric oxides, hydrated phyllosilicates, sulfates and carbonates and ices and organics. The composition of the various phases within a given sample is a critical record of its formation and evolution. Coupled to the mapping information, it provides unique clues to describe the history of the parent body (planet, satellite and small body). In particular, the capability to identify hydrated grains and to characterize their adjacent phases has a huge potential in the search for possible bio-relics. 相似文献
852.
Martin Jutzi Patrick Michel Willy Benz Derek C. Richardson 《Meteoritics & planetary science》2009,44(12):1877-1887
Abstract— In this paper, we present numerical simulations aimed at reproducing the Baptistina family based on its properties estimated by observations. A previous study by Bottke et al. (2007) indicated that this family is probably at the origin of the K/T impactor, is linked to the CM meteorites and was produced by the disruption of a parent body 170 km in size due to the head‐on impact of a projectile 60 km in size at 3 km s?1. This estimate was based on simulations of fragmentation of non‐porous materials, while the family was assumed to be of C taxonomic type, which is generally interpreted as being formed from a porous body. Using both a model of fragmentation of non‐porous materials, and a model that we developed recently for porous ones, we performed numerical simulations of disruptions aimed at reproducing this family and at analyzing the differences in the outcome between those two models. Our results show that a reasonable match to the estimated size distribution of the real family is produced from the disruption of a porous parent body by the head‐on impact of a projectile 54 km in size at 3 km s?1. Thus, our simulations with a model consistent with the assumed dark type of the family requires a smaller projectile than previously estimated, but the difference remains small enough to not affect the proposed scenario of this family history. We then find that the break‐up of a porous body leads to different outcomes than the disruption of a non‐porous one. The real properties of the Baptistina family still contain large uncertainties, and it remains possible that its formation did not involve the proposed impact conditions. However, the simulations presented here already show some range of outcomes and once the real properties are better constrained, it will be easy to check whether one of them provides a good match. 相似文献
853.
Jean-Claude Leprun Michel Grouzis Herizo Randriambanona 《Comptes Rendus Geoscience》2009,341(7):526-537
This study concerns post-cultural dynamics in the semi-arid South-West of Madagascar (Analabo area, near the Mikea Forest). A synchronic comparison was performed on a set of abandoned fields plots aged from 2 to 30 years and on forest and savanna reference ecosystems, located on cambic arenosols developed from same Quaternary dune sands. The studied parameters concerned mainly a few physical and chemical soil properties (density, permeability, compaction, texture, C, N, K, P content). Important changes occur in the post-cultural succession: an increase of the soil density and compaction, and decrease of the soil upper layer permeability. Consequently, slash and burn cultivation leads to a packing and an induration of the soil surface. Results about edaphic indicators have shown that the physical parameters used better discriminate various stages of evolution than the chemical parameters do. The multivariate analysis of soil indicators shows that vegetative succession over 30 years in a forest ecosystem cleared, burnt, cultivated and left, does not lead to features corresponding to a closed-canopy forest but rather to those of a tree savanna with open, mixed woody-herbaceous vegetation. The primary dense dry deciduous regeneration of the primary forest is very low or nil. In the semi-arid SW Madagascar (Analabo region), post-cultural dynamics conditions consists in a savanna-formation process, controlled by: (1) the intensity and duration of the disturbance (during the cultivation phase); and (2) the more drastic environmental conditions (including both climate and soil). 相似文献
854.
855.
The general subject of this paper is subglacial deformation beneath Breiðamerkurjökull, a surging Icelandic glacier. More specifically it discusses the evolution and the role of fluid pressure on the behaviour of subglacial sediments during deformation. During Little Ice Age maximum, the two outcrops studied, North Jökulsarlon (N-Jk) and Brennhola-Alda (BA), were located at 2550 m and 550 m respectively from the front of the Breiðamerkurjökull. Sedimentological analysis at the forefield of the glacier shows thick, coarse glaciofluvial deposits interbedded with thin, fine-grained shallow lacustrine/swamp deposits, overlain by a deformed till unit at N-Jk. BA outcrop shows fine-grained shallow lacustrine/swamp deposits overlain by a deformed till unit. The sequence of deformation events from one outcrop to the other is similar. First, major thrust planes, which were rooted in shallow lacustrine/swamp deposits developed by glacially induced simple shear. Next, the thrusts were folded, indicating the deformation of hydroplastic sediment assisted by moderate fluid pressure. Then clastic dyke swarms crosscut the sedimentary succession, proving that fluid overpressure caused hydrofracturing associated with fluidisation. Finally, as water escaped from the glacier bed, fluid pressure dropped, and normal faulting occurred in brittle-state subglacial sediments. Fluid-pressure variations are related to glacier dynamics. They control the deformation sequence by modifying subglacial rheological behaviour and the nature of the subglacial tectonism. 相似文献
856.
Experimental evidence is reported for Fe2+ disproportionation in Al-free perovskite (Pv), when submitted to large temperature gradients (i.e., under off-equilibrium
conditions) in a laser heated diamond anvil cell (LHDAC). To enable this effect, the experimental procedure was designed to
produce large radial and axial temperature gradients. In the Pv and ferropericlase (Fp) assemblage synthesized after dissociation
of natural olivine, the three chemical states of iron (i.e., Fe0, Fe2+ and Fe3+) could be evidenced by electron probe microanalysis (EPMA), through variations of oxygen contents attached to the Fe cations.
Despite inherent difficulties for determination of O-contents and Fe3+/ΣFe ratios using EPMA, we recorded significant changes in iron oxidation state across the laser-heated strip. These changes
are correlated with variations in composition for the major elements (Fe, Mg, and Si), which evidences that the Pv/Fp assemblage
experienced large segregation under the strong temperature gradients. Grains of metallic iron were detected in parts of the
laser-heated strip coexisting with a Pv phase with Fe/(Mg + Fe) = 6 at% and most of its iron as Fe3+. This Fe2+-disproportionation reaction involves insertion of Fe3+-defects in the Pv lattice. This Fe3+-bearing Pv phase is presumably unstable and decomposes into a mineral assemblage including magnesioferrite, which is detected
at the border of the laser-heated strip. 相似文献
857.
Reductions of greenhouse gas emissions in Annex I and non-Annex I countries for meeting concentration stabilisation targets 总被引:2,自引:1,他引:1
The IPCC Fourth Assessment Report, Working Group III, summarises in Box 13.7 the required emission reduction ranges in Annex I and non-Annex I countries as a group, to achieve greenhouse gas concentration stabilisation levels between 450 and 650 ppm CO2-eq. The box summarises the results of the IPCC authors’ analysis of the literature on the regional allocation of the emission reductions. The box states that Annex I countries as a group would need to reduce their emissions to below 1990 levels in 2020 by 25% to 40% for 450 ppm, 10% to 30% for 550 ppm and 0% to 25% for 650 ppm CO2-eq, even if emissions in developing countries deviate substantially from baseline for the low concentration target. In this paper, the IPCC authors of Box 13.7 provide background information and analyse whether new information, obtained after completion of the IPCC report, influences these ranges. The authors concluded that there is no argument for updating the ranges in Box 13.7. The allocation studies, which were published after the writing of the IPCC report, show reductions in line with the reduction ranges in the box. From the studies analysed, this paper specifies the “substantial deviation” or “deviation from baseline” in the box: emissions of non-Annex I countries as a group have to be below the baseline roughly between 15% to 30% for 450 ppm CO2-eq, 0% to 20% for 550 ppm CO2-eq and from 10% above to 10% below the baseline for 650 ppm CO2-eq, in 2020. These ranges apply to the whole group of non-Annex I countries and may differ substantially per country. The most important factor influencing these ranges above, for non-Annex I countries, and in the box, for Annex I countries, is new information on higher baseline emissions (e.g. that of Sheehan, Climatic Change, 2008, this issue). Other factors are the assumed global emission level in 2020 and assumptions on land-use change and forestry emissions. The current, slow pace in climate policy and the steady increase in global emissions, make it almost unfeasible to reach relatively low global emission levels in 2020 needed to meet 450 ppm CO2-eq, as was first assumed feasible by some studies, 5 years ago. 相似文献
858.
Micro-scale flood risk analysis based on detailed 2D hydraulic modelling and high resolution geographic data 总被引:2,自引:0,他引:2
Julien Ernst Benjamin J. Dewals Sylvain Detrembleur Pierre Archambeau Sébastien Erpicum Michel Pirotton 《Natural Hazards》2010,55(2):181-209
The paper presents a consistent micro-scale flood risk analysis procedure, relying on detailed 2D inundation modelling as
well as on high resolution topographic and land use database. The flow model is based on the shallow-water equations, solved
by means of a finite volume scheme on multi-block structured grids. Using highly accurate laser altimetry, the simulations
are performed with a typical grid spacing of 2 m, which is fine enough to represent the flow at the scale of individual buildings.
Consequently, the outcomes of hydraulic modelling constitute suitable inputs for the subsequent exposure analysis, performed
at a micro-scale using detailed land use maps and geographic database. Eventually, the procedure incorporates social flood
impact analysis and evaluation of direct economic damage to residential buildings. Besides detailing the characteristics and
performance of the hydraulic model, the paper describes the flow of data within the overall flood risk analysis procedure
and demonstrates its applicability by means of a case study, for which two different flood protection measures were evaluated. 相似文献
859.
Eolian deposits of central and southern Yukon, northwestern Canada, consist of loess mantles, small areas of active dunes, and larger stabilized dune fields. Dune fields in valley settings within the region are situated both within and beyond the limit of the last glaciation. Infrared stimulation luminescence (IRSL) dating in central and southern Yukon reveals that these dune fields stabilized as late as 9–8.5 ka, well after the retreat of Cordilleran glaciers. These findings are comparable to other valley-setting dune fields and loess from central Alaska, which record activity during the period from the Lateglacial to the Holocene Thermal Maximum (HTM), and reduced or altered activity after 9–8 ka. Post-glacial dune activity was most likely related to warm, dry conditions during the HTM, under predominantly shrub-tundra vegetation. Early Holocene stabilization of these dunes probably occurred in response to cooler, moister conditions, and replacement of predominantly tundra by boreal forest cover, dominated by spruce. Stabilization of dune fields in southern Yukon and Alaska most likely represented an extension of the time-transgressive stabilization of dune fields that occurred across northwestern North America with the post-glacial expansion of the boreal forest. 相似文献
860.