全文获取类型
| 收费全文 | 9781篇 |
| 免费 | 1772篇 |
| 国内免费 | 2560篇 |
专业分类
| 测绘学 | 1024篇 |
| 大气科学 | 1909篇 |
| 地球物理 | 2156篇 |
| 地质学 | 4775篇 |
| 海洋学 | 1651篇 |
| 天文学 | 440篇 |
| 综合类 | 955篇 |
| 自然地理 | 1203篇 |
出版年
| 2024年 | 57篇 |
| 2023年 | 158篇 |
| 2022年 | 468篇 |
| 2021年 | 531篇 |
| 2020年 | 518篇 |
| 2019年 | 537篇 |
| 2018年 | 549篇 |
| 2017年 | 499篇 |
| 2016年 | 547篇 |
| 2015年 | 555篇 |
| 2014年 | 605篇 |
| 2013年 | 672篇 |
| 2012年 | 663篇 |
| 2011年 | 667篇 |
| 2010年 | 615篇 |
| 2009年 | 601篇 |
| 2008年 | 545篇 |
| 2007年 | 528篇 |
| 2006年 | 451篇 |
| 2005年 | 413篇 |
| 2004年 | 342篇 |
| 2003年 | 260篇 |
| 2002年 | 298篇 |
| 2001年 | 307篇 |
| 2000年 | 295篇 |
| 1999年 | 332篇 |
| 1998年 | 270篇 |
| 1997年 | 245篇 |
| 1996年 | 226篇 |
| 1995年 | 201篇 |
| 1994年 | 195篇 |
| 1993年 | 160篇 |
| 1992年 | 155篇 |
| 1991年 | 135篇 |
| 1990年 | 94篇 |
| 1989年 | 84篇 |
| 1988年 | 71篇 |
| 1987年 | 55篇 |
| 1986年 | 34篇 |
| 1985年 | 28篇 |
| 1984年 | 23篇 |
| 1983年 | 18篇 |
| 1982年 | 29篇 |
| 1981年 | 18篇 |
| 1980年 | 8篇 |
| 1979年 | 9篇 |
| 1977年 | 5篇 |
| 1976年 | 8篇 |
| 1958年 | 4篇 |
| 1954年 | 4篇 |
排序方式： 共有10000条查询结果，搜索用时 31 毫秒
191.
192.
A multi-scale study on land use and land cover quality change: The case of the Yellow River Delta in China 总被引：5，自引：0，他引：5
This paper presents a case study of the Yellow River Delta in China, to trace land use and land cover changes during the past
20 years, with an emphasis on land quality changes. Three sets of data are used in this case study: remote sensing data derived
from satellite images; crop yield data from statistics; and soil data collected by the researchers in the field. Our study
reveals that at the regional scale, LUCC has taken place in a positive direction: vegetation cover has been expanding and
crop yields per hectare have been on rise. However, while the overall eco-environment has improved, the improvement is uneven
across the Delta region. At local levels, some areas show signs of increased salinization and declining organic content. Both
natural forces and human activities are responsible for the LUCC, but human activities play a more important role. While some
impacts of human activities are positive, the damages are often long-lasting and irreversible. We also conclude that it is
necessary to use both macro data (such as remote sensing data) and micro data (data collected in the field) to study land
quality change. The former are efficient in examining land quality changes at the regional scale, the latter can serve to
verify ground patterns revealed from macro data and help to identify local variations, so as to get a comprehensive understanding
of LUCC and promote sustainable land use and land management.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
193.
Lin Y. Hu 《Mathematical Geology》2002,34(8):953-963
Gradual deformation is a parameterization method that reduces considerably the unknown parameter space of stochastic models. This method can be used in an iterative optimization procedure for constraining stochastic simulations to data that are complex, nonanalytical functions of the simulated variables. This method is based on the fact that linear combinations of multi-Gaussian random functions remain multi-Gaussian random functions. During the past few years, we developed the gradual deformation method by combining independent realizations. This paper investigates another alternative: the combination of dependent realizations. One of our motivations for combining dependent realizations was to improve the numerical stability of the gradual deformation method. Because of limitations both in the size of simulation grids and in the precision of simulation algorithms, numerical realizations of a stochastic model are never perfectly independent. It was shown that the accumulation of very small dependence between realizations might result in significant structural drift from the initial stochastic model. From the combination of random functions whose covariance and cross-covariance are proportional to each other, we derived a new formulation of the gradual deformation method that can explicitly take into account the numerical dependence between realizations. This new formulation allows us to reduce the structural deterioration during the iterative optimization. The problem of combining dependent realizations also arises when deforming conditional realizations of a stochastic model. As opposed to the combination of independent realizations, combining conditional realizations avoids the additional conditioning step during the optimization process. However, this procedure is limited to global deformations with fixed structural parameters. 相似文献
194.
This study uses factorial and indicator kriging methods with a geographic information system (GIS), to overlay and delineate the variation and pollution sources of soil heavy metals in fields in the north of Changhua County, Taiwan. Accordingly, the spatial variation and pollution probability patterns of soil cadmium, chromium and copper are estimated to support further environmental monitoring, remediation and planning. The overlay maps of industrial plants, irrigation channels and kriging estimates, constitute a GIS. This reveals that the short- and long-range variations and soil pollution by Cd, Cr and Cu strongly corresponded to the locations of irrigation systems and industrial plants in the area of study. The maps also verify most locations of suspected pollution sources from the GIS database. Moreover, the probability spatial patterns estimated by indicator kriging, based on the pollution thresholds of Cd, Cr and Cu, were also highly correlated with irrigation channels and industrial plants. The GIS, with factorial and indicator kriging under different combinations of variations and pollution probabilities of soil Cd, Cr and Cu, provide alternative scenarios for delineating the areas that contain high variation and high pollution probability of these heavy metals, to enable decision makers to identify pollution sources, and to further investigate and remedy soil pollution. 相似文献
195.
Dissolved and particulate trace metals and their partitioning in a hypoxic estuary: The Tanshui Estuary in Northern Taiwan 总被引：1，自引：0，他引：1
The distribution and partitioning of trace metals (Co, Cu, Fe, Mn, Ni, and Zn) between dissolved and particulate phases were studied in the Tanshui Estuary. The upper reach of the estuary is hypoxic and heavily polluted due to domestic and industrial discharges. The concentration ranges of dissolved and leachable particulate trace metals in the Tanshui Estuary were: Co: 0.3–6.1 nM, 1.8–18.6 mg kg−1; Cu: 5–53 nM, 22–500 mg kg−1; Fe: 388–3,364 nM, 1.08–6.67%; Mn: 57–2,914 nM, 209–1,169 mg kg−1; Ni: 7–310 nM, 6–108 mg kg−1; and Zn: 12–176 nM, 62–1,316 mg kg−1; respectively. The dissolved concentrations of the metals were 2–35 times higher than the average values of the world river water. The distributions of dissolved and particulate studied metals, except Mn, in the estuary showed scattering, which could be attributed to the discharges from many industrial wastewater disposal works located in the upper tributaries. The daily input of dissolved metals from the disposal works to the Tanshui Estuary ranged from 0.1–0.4 tons. Dissolved Mn was nearly conservative in the region with salinity higher than 10 psu, while particulate Mn decreased in the region with salinity of 10–15 psu. The concentration increased significantly seawards, corresponding with the distribution of dissolved oxygen. The distribution coefficient (KD) for Mn in the lower estuary was nearly three orders of magnitude higher than in the upper estuary. This phenomenon may be attributed to the diffusion of Mn from the anoxic sediment in the upper estuary and gradual oxidation into particulate Mn in the middle and lower estuary as the estuarine water became more oxygenated. The distribution coefficient for Cu decreased with increasing salinity. The percentages of trace metals bound by suspended particulate matter decreased in the following order: Fe>Zn, Cu>Co>Mn>Ni. 相似文献
196.
197.
The updated study shows that the taphrogenesis of basement of the Fushun Basin is not a kind of instantaneous process. It intensified gradually and went to extreme in the sedimentary stage of the Guchengzi formation, and then, it weakened rapidly and stopped soon afterwards; the depression did not take place after the taphrogenesis. On the contrary, it almost happened simultaneously with the taphrogenesis. The depression went at a high speed from the beginning of the sedimentary period of the Xilutian formation, and then weakened gradually in the sedimentary period of the Gengjiajie formation. The evolution course of the synsedimentary structure of the Fushun Basin can be summarized as the following six stages: slow taphrogenesis and high speed depression to accelerated taphrogenesis and high speed depression to high speed taphrogenesis and high speed depression to retarded taphrogenesis and high speed depression to gradual halt of taphrogenesis and reduced depression to slow depression and gradual halt of depression. The tectonic evolution resulted in the formation of the "lower taphrogenesis and upper depression" structure. The formation of the binary structure might be due to the suspension of taphrogenesis and the change of the regional structure stress field, but the depression kept going. The result of calculation combining the analysis of the synsedimentary structural frame, the back-stripping method of the subsidence history of the basin basement and the simulation of thermo-settlement history indicates that the great sedimentary space required by the "upper depression part" consists of two parts, namely, 40% from compaction of sediments and 60% from slow depression of the basin basement during a long period of time. Gradual halt of the depression in the Fushun Basin may be attributed to the reversal of the lithosphere hot-recession and gravity isostasy adjustment which may be the result of new hot-events in the depths and accompanied invasion of extremely thick diabase sill, thus revealing a new forming mechanism of "fault subsidence at the base and depression on the top" structure. 相似文献
198.
199.
Lower Cretaceous red sedimentary rocks from the depositional basin of East Qilian fold belt have been collected for a paleomagnetic study. Stepwise thermal demagnetization reveals two or three components of magnetization from dark red sandstones. Low-temperature magnetic component is consistent with the present Earth Field direction in geographic coordinates. High-temperature magnetic components are mainly carried by hematite. The mean pole of 19 sites for high-temperature magnetic components after tilt-correction is λ=62.2°N, φ=193.4°E, A95=3.2°, and it passes fold tests at 99% confidence level and reversal tests at 95% confidence level. The paleopole is insignificantly different from that of Halim et al. (1998) from the same sampling area at the 95% confidence level. Compared with paleomagnetic results for North China, South China, and Eurasia, our results suggest that no significant relative latitudinal displacement has taken place between Lanzhou region and these blocks since Cretaceous time. Remarkably, the pole of Lanzhou shows a 20° clockwise rotation with respect to those of North China, South China, and Eurasia. Geological information indicates that the crustal shortening in the western part of Qilian is greater than that in eastern part. In this case, the clockwise rotation of sampling area was related to India/Eurasia collision, and this collision resulted in a left-lateral strike-slip motion of the Altun fault in north Tibetan Plateau after the Cretaceous. 相似文献
200.