全文获取类型
收费全文 | 5732篇 |
免费 | 198篇 |
国内免费 | 71篇 |
专业分类
测绘学 | 169篇 |
大气科学 | 451篇 |
地球物理 | 1317篇 |
地质学 | 2274篇 |
海洋学 | 482篇 |
天文学 | 952篇 |
综合类 | 14篇 |
自然地理 | 342篇 |
出版年
2022年 | 43篇 |
2021年 | 76篇 |
2020年 | 81篇 |
2019年 | 86篇 |
2018年 | 177篇 |
2017年 | 174篇 |
2016年 | 217篇 |
2015年 | 145篇 |
2014年 | 189篇 |
2013年 | 293篇 |
2012年 | 232篇 |
2011年 | 319篇 |
2010年 | 259篇 |
2009年 | 339篇 |
2008年 | 284篇 |
2007年 | 216篇 |
2006年 | 253篇 |
2005年 | 266篇 |
2004年 | 332篇 |
2003年 | 270篇 |
2002年 | 191篇 |
2001年 | 134篇 |
2000年 | 128篇 |
1999年 | 101篇 |
1998年 | 111篇 |
1997年 | 73篇 |
1996年 | 44篇 |
1995年 | 58篇 |
1994年 | 55篇 |
1993年 | 42篇 |
1992年 | 54篇 |
1991年 | 49篇 |
1990年 | 51篇 |
1989年 | 27篇 |
1988年 | 25篇 |
1987年 | 36篇 |
1986年 | 30篇 |
1985年 | 30篇 |
1984年 | 39篇 |
1983年 | 39篇 |
1982年 | 31篇 |
1981年 | 35篇 |
1980年 | 22篇 |
1978年 | 33篇 |
1977年 | 29篇 |
1976年 | 23篇 |
1975年 | 29篇 |
1974年 | 22篇 |
1973年 | 25篇 |
1971年 | 17篇 |
排序方式: 共有6001条查询结果,搜索用时 0 毫秒
911.
We present arguments to show that torsion-coupling constant (which depends on energy asE
–2) can pass through the values of the coupling constants of the other interactions during the evolution of the Universe. An energy-dependent torsion-coupling constant helps in a natural way to understand the ratios of the coupling strengths of the different fundamental interactions. 相似文献
912.
Mariano Moreno‐de las Heras Rubén Díaz‐Sierra José M. Nicolau Miguel A. Zavala 《地球表面变化过程与地形》2011,36(10):1367-1377
The management of reclaimed slopes derived from industrial and civil activities (e.g. surface mining and road construction) requires the development of practical stability analysis approaches that integrate the processes and mechanisms that rule the dynamics of these ubiquitous emerging ecosystems. This work describes a new modelling approach focused on stability analysis of water‐limited reclaimed slopes, where interactive relationships between rill erosion and vegetation regulate ecosystem stability. Our framework reproduces two main groups of possible trends along the temporal evolution of reclaimed slopes: successful trends, characterized by widespread vegetation development and the effective control of rill erosion processes; and gullying trends, characterized by the progressive loss of vegetation and a sharp logistic increase in erosion rates. Furthermore, this analytical approach allows the determination of threshold values for the state variables (i.e. vegetation cover and rill erosion) that drive the system's stability, facilitating the identification of critical situations that require specific human intervention (e.g. revegetation or, in very problematic cases, revegetation combined with rill network destruction) to ensure the long‐term sustainability of the restored ecosystem. The application of our threshold analysis framework in Mediterranean‐dry reclaimed slopes derived from surface coal mining (the Teruel coalfield in central‐eastern Spain) showed a good field‐based performance. Therefore, we believe that this model is a valuable contribution for the management of water‐limited reclaimed systems, including those associated with rill erosion, as it provides a tool for the evaluation of restoration success and can play an important role in decision‐making during ecosystem restoration in severely disturbed landscapes. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
913.
A statistical study of the dependence between various critical fusion temperatures of a certain kind ofcoal and its chemical components is carried out.As well as using classical dependence techniques(multiple,stepwise and PLS regression,principal components,canonical correlation,etc.)together withthe corresponding inference on the parameters of interest,non-parametric regression and bootstrapinference are also performed. 相似文献
914.
Inclement weather creates a chronic hazard for Canadian travellers. Past studies indicate that road collision rates increase during precipitation, although the magnitude of theincrease varies from study to study, partly as a result of variations in weather and driving conditions,but also because of differences in methods. The goal of the current study is to improve ourunderstanding of the links between weather and travel risk in mid-sized Canadian cities by using astandardized method for analyzing data from six cities with different climates: Halifax-Dartmouth,Ottawa, Québec, Hamilton, Waterloo Region, and Regina. The study has four interrelated objectives: (1) Toconduct a sensitivity analysis to determine the extent to which risk estimates vary depending onthe criteria used to define precipitation events and `normal' conditions; (2) To compare therelative risk of collision and injury during precipitation relative to `normal' conditions; (3) Todetermine the extent to which weather-related risks vary fordifferent Canadian cities; and (4) To explore any differences in collision characteristics between events and controls, especially as theyvary from city to city. Results are based on a matched-pair analysis, using six-hour time blocks over afour-year period, 1995 to 1998. Results indicate only modest sensitivity to the criteria used to define precipitation events and `normal' conditions. On average, precipitation is associated with a 75 percentincrease in traffic collisions and a 45 percent increase in related injuries, as compared to `normal'seasonal conditions, but risk levels vary depending on the characteristics of the weather event.Both sensitivity to specific weather conditions and weather-related accident profiles vary from city tocity in ways that are not easily explained. 相似文献
915.
Harm Jan Pierik Jelle I. M. Moree Karianne M. van der Werf Lonneke Roelofs Marcio Boechat Albernaz Antoine Wilbers Bert van der Valk Marieke van Dinter Wim Z. Hoek Tjalling de Haas Maarten G. Kleinhans 《Sedimentology》2023,70(1):179-213
In the transformation from tidal systems to freshwater coastal landscapes, plants act as eco-engineering species that reduce hydrodynamics and trap sediment, but nature and timing of the mechanisms of land creation along estuaries remains unclear. This article focuses on the Old Rhine estuary (The Netherlands) to show the importance of vegetation in coastal landscape evolution, predominantly regarding tidal basin filling and overbank morphology. This estuary hosted the main outflow channel of the river Rhine between ca 6500 to 2000 cal bp , and was constrained by peat during most of its existence. This study reconstructs its geological evolution, by correlating newly integrated geological data and new field records to varying conditions. Numerical modelling was performed to test the inferred mechanisms. It was found that floodbasin vegetation and resulting organic accumulation strongly accelerated back-barrier infill, by minimizing tidal influence. After tidal and wave transport had already sufficiently filled the back-barrier basin, reed rapidly expanded from its edges under brackish conditions, as shown by diatom analysis and datings. Reed growth provided a positive infilling feedback by reducing tidal flow and tidal prism, accelerating basin infilling. New radiocarbon dates show that large-scale crevassing along the Old Rhine River – driven by tidal backwater effect – only started as nutrient-rich river water transformed the floodbasin into an Alder carr in a next phase of estuary evolution. Such less dense vegetation promotes crevassing as sediments are more easily transported into the floodbasin. As river discharge increased and estuary mouth infilling progressed, crevasse activity diminished around 3800 to 3000 cal bp , likely due to a reduced tidal backwater effect. The insights from this data-rich Holocene study showcase the dominant role that vegetation may have in the long-term evolution of coastal wetlands. It provides clues for effective use of vegetation in vulnerable wetland landscapes to steer sedimentation patterns to strategically adapt to rising water levels. 相似文献
916.
917.
918.
Jean de la Paix Mupenzi Lanhai Li Jiwen Ge Achal Varenyam Gabriel Habiyaremye Nzayisenga Theoneste Kamanzi Emmanuel 《地学前缘(英文版)》2011,2(4):599-607
This study has focused on the processes of soil degradation and chemical element concentration
in tea-growing regions of Rwanda, Africa. Soil degradation accelerated by erosion is caused not only
by topography but also by human activities. This soil degradation involves both the physical loss and
reduction in the amount of topsoil associated with nutrient decline. Soil samples were collected from
eleven tropical zones in Rwanda and from variable depth within each collecting site. Of these, Samples
from three locations in each zone were analyzed in the laboratory, with the result that the pH of all soil
samples is shown to be less than 5 (pH < 5) with a general average of 4.4. The elements such as iron (Fe),
copper (Cu), manganese (Mn), and zinc (Zn) are present in high concentration levels. In contrast calcium
(Ca) and sodium (Na) are present at low-level concentrations and carbon (C) was found in minimal concentrations. In addition, elements derived from fertilizers, such as nitrogen (N), phosphorous (P), and
potassium (K) which is also from minerals such as feldspar, are also present in low-level concentrations.
The results indicate that the soil in certain Rwandan tea plantations is acidic and that this level of pH may
help explain, in addition to natural factors, the deficiency of some elements such as Ca, Mg, P and N. The
use of chemical fertilizers, land use system and the location of fields relative to household plots are also
considered to help explain why tea plantation soils are typically degraded. 相似文献
919.
Catherine Pierre Germain Bayon Marie-Madeleine Blanc-Valleron Jean Mascle Stéphanie Dupré 《Geo-Marine Letters》2014,34(2-3):253-267
On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ13C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ13C (up to –5.2‰) and low δ18O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by 13C-rich CO2 and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals. 相似文献
920.
东北地区水文地质环境地质调查未来工作方向探讨 总被引:1,自引:0,他引:1
东北地区资源丰富,在为国家经济建设做出贡献的同时,自身地质环境也在不断变化.在简述东北地区区域水文、环境地质条件等自然地理概况的基础上,讨论了存在的水文地质、环境地质问题.认为未来在东北地区应加强战略性地下水资源的调查评价工作,提高全区研究程度,加强低山丘陵区地下水资源及其环境地质调查评价,在东北西部严重缺水的干旱-半干旱地区开展以解决生活用水、经济建设用水和生态环境建设用水的地下水勘查工作,加强东北中新生代盆地地热资源勘察工作,对地下水动态监测网格应加以补充、完善,加强城市地质调查工作,加强矿山环境地质工作,开展区域地质环境条件调查,建立地质环境监测工作的预报、预警系统. 相似文献