全文获取类型
收费全文 | 39897篇 |
免费 | 3296篇 |
国内免费 | 5020篇 |
专业分类
测绘学 | 2684篇 |
大气科学 | 4992篇 |
地球物理 | 8332篇 |
地质学 | 20044篇 |
海洋学 | 3148篇 |
天文学 | 2237篇 |
综合类 | 3600篇 |
自然地理 | 3176篇 |
出版年
2024年 | 148篇 |
2023年 | 428篇 |
2022年 | 944篇 |
2021年 | 1107篇 |
2020年 | 935篇 |
2019年 | 979篇 |
2018年 | 5614篇 |
2017年 | 4769篇 |
2016年 | 3455篇 |
2015年 | 1103篇 |
2014年 | 1172篇 |
2013年 | 1061篇 |
2012年 | 1967篇 |
2011年 | 3682篇 |
2010年 | 2970篇 |
2009年 | 3162篇 |
2008年 | 2656篇 |
2007年 | 3002篇 |
2006年 | 660篇 |
2005年 | 752篇 |
2004年 | 825篇 |
2003年 | 801篇 |
2002年 | 624篇 |
2001年 | 453篇 |
2000年 | 497篇 |
1999年 | 635篇 |
1998年 | 540篇 |
1997年 | 479篇 |
1996年 | 480篇 |
1995年 | 379篇 |
1994年 | 369篇 |
1993年 | 319篇 |
1992年 | 286篇 |
1991年 | 187篇 |
1990年 | 151篇 |
1989年 | 132篇 |
1988年 | 99篇 |
1987年 | 72篇 |
1986年 | 57篇 |
1985年 | 48篇 |
1984年 | 32篇 |
1983年 | 29篇 |
1982年 | 26篇 |
1981年 | 32篇 |
1980年 | 30篇 |
1979年 | 15篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 9篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
This paper introduces the analysis of three dimensional finite element method for the gravity-arch dam building on the complex ground. The foundation rock of dip and strain-slip, three faults, two weak intercalations and etc. arc considered in this calculation model. Water static pressure, gravity of dam, uplift, temperature stress and so on are computed, including three dimensional osmotic and temperature fields in the foundation. The range of calculation model is about 1400×900×600m2 . There are 1845 elements of all kinds, 3280 nodes in the network of finite element. The superlax iterative method is used to solve, and three dimensional perspective and software of after treatment are applied in analyses. The dimension of this computation is large, various results of treating complex problems are reasonable. The important basis has applied to designing unit. The computation is important practical significance for Geheyan Dam which is being built. 相似文献
52.
结合不同的信号调制方式,在满足一定伪距误差的条件下,从所需信号发射功率小和与北斗一代相互干扰小的角度论证了RNSS应用BOC(6,1)和BOC(6,2)两种调制方式的优势。并且对2 483.5~2 500 MHz频段中导航系统与其他通信系统之间的干扰作出估计,论证了GNSS系统与其他无线电业务的兼容性。 相似文献
53.
欧氏聚类算法是多元统计中的一种重要分类方法,可以将其应用于测绘领域中点云数据的分割。本文首先计算点云数据中两点之间的欧氏距离,将距离小于指定阈值作为分为一类的判定准则;然后迭代计算,直至所有的类间距大于指定阈值,完成欧氏聚类分割。具体步骤为:①利用Octree法建立点云数据拓扑组织结构;②对每个点进行k近邻搜索,计算该点与k个邻近点之间的欧氏距离,最小归为一类;③设置一定的阈值,对步骤②迭代计算,直至所有类与类之间的距离大于指定阈值。试验证明,欧氏聚类算法对不同测量技术手段获取的点云数据均具有适用性,可以成功对点云数据进行分割,分割效果良好。 相似文献
54.
Non-Linear Theory and Power-Law Models for Information Integration and Mineral Resources Quantitative Assessments 总被引:7,自引:0,他引:7
Qiuming Cheng 《Mathematical Geosciences》2008,40(5):503-532
Singular physical or chemical processes may result in anomalous amounts of energy release or mass accumulation that, generally,
are confined to narrow intervals in space or time. Singularity is a property of different types of non-linear natural processes
including cloud formation, rainfall, hurricanes, flooding, landslides, earthquakes, wildfires, and mineralization. The end
products of these non-linear processes can be modeled as fractals or multifractals. Hydrothermal processes in the Earth’s
crust can result in ore deposits characterized by high concentrations of metals with fractal or multifractal properties. Here
we show that the non-linear properties of the end products of singular mineralization processes can be applied for prediction
of undiscovered mineral deposits and for quantitative mineral resource assessment, whether for mineral exploration or for
regional, national and global planning for mineral resource utilization. In addition to the general theory and framework for
the non-linear mineral resources assessment, this paper focuses on several power-law models proposed for characterizing non-linear
properties of mineralization and for geoinformation extraction and integration. The theories, methods, and computer system
discussed in this paper were validated using a case study dealing with hydrothermal Au mineral potential in southern Nova
Scotia, Canada. 相似文献
55.
Errors and correction of precipitation measurements in China 总被引:2,自引:0,他引:2
In order to discover the range of various errors in Chinese precipitation measurements and seek a correction method, 30 precipitation evaluation stations were set up countrywide before 1993. All the stations are reference stations in China. To seek a correction method for wind-induced error, a precipitation correction instrument called the "horizontal precipitation gauge" was devised beforehand. Field intercomparison observations regarding 29,000 precipitation events have been conducted using one pit gauge, two elevated operational gauges and one horizontal gauge at the above 30 stations. The range of precipitation measurement errors in China is obtained by analysis of intercomparison measurement results. The distribution of random errors and systematic errors in precipitation measurements are studied in this paper. A correction method, especially for wind-induced errors, is developed. The results prove that a correlation of power function exists between the precipitation amount caught by the horizontal gauge and the absolute difference of observations implemented by the operational gauge and pit gauge. The correlation coefficient is 0.99. For operational observations, precipitation correction can be carried out only by parallel observation with a horizontal precipitation gauge. The precipitation accuracy after correction approaches that of the pit gauge. The correction method developed is simple and feasible. 相似文献
56.
A water quality investigation was carried out in the Deoria district, Ganga plain, to assess the suitability of surface and
groundwaters for domestic, agricultural, and industrial purposes. As much as 50 representative samples from river and groundwater
were collected from various stations to monitor the water chemistry of various ions, comprising Ca2+, Mg2+, Na+, K+, HCO3
−, SO4
2−, NO3
−, Cl−, F−, and trace metals, such as Fe, Cu, Mn, Zn, Cd, and Pb. The results showed that electrical conductance (EC), total dissolved
solids (TDS), HCO3
−, Mg2+, Na+, and total hardness (TH) are above the maximum desirable limit, and apart from Fe and Mn all other trace metals are within
the maximum permissible limit for drinking water. The calculated values for sodium absorption ratio (SAR), salinity, residual
sodium carbonate (RSC), and permeability index (PI) indicate well to permissible use of water for irrigation. High values
of Na%, RSC, and Mg-hazard (MH) at some stations restrict its use for agricultural purpose. Anthropogenic activities affect
the spatial variation of water quality. Economic and social developments of the study area is closely associated with the
characteristics of the hydrological network. 相似文献
57.
Rock slope stability is of great concern along highway routes as stability problems on cut slopes may cause fatal events as well as loss of property. In rock slope engineering, stability evaluations are commonly performed by means of analytical or numerical analyses, principally considering the factor of safety concept. As a matter of fact, the probabilistic assessment of slope stability is progressively getting popularity due to difficulties in assigning the most appropriate values to design parameters in analytical or numerical methods. Additionally, the effect of heterogeneities in rock masses and discontinuities on the analysis results is minimized through the probabilistic concept. In this study, slope stability of high and steep sedimentary rock cut slopes along a state highway in Adilcevaz-Bitlis (Turkey) was evaluated on the basis of probabilistic approach using the Slope Stability Probability Classification (SSPC) system. The probabilistic assessment indicates major slope stability problems because of discontinuity controlled and discontinuity orientation independent mass movements. Almost all studied cut slopes suffer from orientation-independent stability problems with very low stability probabilities. Additionally, the probability of planar and toppling failures is significantly high with respect to the SSPC system. The stability problems along the investigated rock slopes were also verified by field reconnaissance. Remedial measures such as slope re-design and reinforcement at the studied locations should be taken to prevent hazardous events along the highway. On the other hand, the probabilistic approach may be a useful tool during rock slope engineering to overcome numerous uncertainties when probabilistic and analytic results are compared. 相似文献
58.
Wilfred H. Sorrell 《Astrophysics and Space Science》2009,323(2):205-211
Almost all astronomers now believe that the Hubble recession law was directly inferred from astronomical observations. It
turns out that this common belief is completely false. Those models advocating the idea of an expanding universe are ill-founded
on observational grounds. This means that the Hubble recession law is really a working hypothesis. One alternative to the
Hubble recession law is the tired-light hypothesis originally proposed by Zwicky (Proc. Nat. Acad. Sci. 15:773, 1929). This hypothesis leads to a universe that is an eternal cosmos continually evolving without beginning or end. Such a universe
exists in a dynamical state of virial equilibrium. Observational studies of the redshift-magnitude relation for Type Ia supernovae
in distant galaxies might provide the best observational test for a tired-light cosmology. The present study shows that the
model Hubble diagram for a tired-light cosmology gives good agreement with the supernovae data for redshifts in the range
0<z<2. This observational test of a static cosmology shows that the real universe is not necessarily undergoing expansion nor
acceleration.
An erratum to this article can be found at 相似文献
59.
Binh Thai Pham Dieu Tien Bui Indra Prakash Long Hoang Nguyen M. B. Dholakia 《Environmental Earth Sciences》2017,76(10):371
Landslide susceptibility assessment using GIS has been done for part of Uttarakhand region of Himalaya (India) with the objective of comparing the predictive capability of three different machine learning methods, namely sequential minimal optimization-based support vector machines (SMOSVM), vote feature intervals (VFI), and logistic regression (LR) for spatial prediction of landslide occurrence. Out of these three methods, the SMOSVM and VFI are state-of-the-art methods for binary classification problems but have not been applied for landslide prediction, whereas the LR is known as a popular method for landslide susceptibility assessment. In the study, a total of 430 historical landslide polygons and 11 landslide affecting factors such as slope angle, slope aspect, elevation, curvature, lithology, soil, land cover, distance to roads, distance to rivers, distance to lineaments, and rainfall were selected for landslide analysis. For validation and comparison, statistical index-based methods and the receiver operating characteristic curve have been used. Analysis results show that all these models have good performance for landslide spatial prediction but the SMOSVM model has the highest predictive capability, followed by the VFI model, and the LR model, respectively. Thus, SMOSVM is a better model for landslide prediction and can be used for landslide susceptibility mapping of landslide-prone areas. 相似文献
60.