首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   25299篇
  免费   304篇
  国内免费   1113篇
测绘学   1454篇
大气科学   2159篇
地球物理   4707篇
地质学   12023篇
海洋学   1210篇
天文学   1688篇
综合类   2199篇
自然地理   1276篇
  2024年   2篇
  2023年   14篇
  2022年   43篇
  2021年   41篇
  2020年   43篇
  2019年   55篇
  2018年   4812篇
  2017年   4085篇
  2016年   2651篇
  2015年   276篇
  2014年   144篇
  2013年   100篇
  2012年   1037篇
  2011年   2782篇
  2010年   2077篇
  2009年   2371篇
  2008年   1945篇
  2007年   2412篇
  2006年   97篇
  2005年   238篇
  2004年   434篇
  2003年   443篇
  2002年   273篇
  2001年   76篇
  2000年   79篇
  1999年   27篇
  1998年   30篇
  1997年   16篇
  1996年   9篇
  1995年   8篇
  1994年   9篇
  1993年   4篇
  1992年   5篇
  1991年   3篇
  1990年   2篇
  1989年   3篇
  1988年   3篇
  1986年   5篇
  1985年   2篇
  1984年   2篇
  1981年   22篇
  1980年   19篇
  1976年   7篇
  1975年   1篇
  1974年   2篇
  1973年   1篇
  1971年   1篇
  1969年   1篇
  1962年   1篇
  1958年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The Haji-Gak iron deposit of eastern Bamyan Province, eastern Afghanistan, was studied extensively and resource calculations were made in the 1960s by Afghan and Russian geologists. Recalculation of the resource estimates verifies the original estimates for categories A (in-place resources known in detail), B (in-place resources known in moderate detail), and C1 (in-place resources estimated on sparse data), totaling 110.8 Mt, or about 6% of the resources as being supportable for the methods used in the 1960s. C2 (based on a loose exploration grid with little data) resources are based on one ore grade from one drill hole, and P2 (prognosis) resources are based on field observations, field measurements, and an ore grade derived from averaging grades from three better sampled ore bodies. C2 and P2 resources are 1,659.1 Mt or about 94% of the total resources in the deposit. The vast P2 resources have not been drilled or sampled to confirm their extent or quality. The purpose of this article is to independently evaluate the resources of the Haji-Gak iron deposit by using the available geologic and mineral resource information including geologic maps and cross sections, sampling data, and the analog-estimating techniques of the 1960s to determine the size and tenor of the deposit.  相似文献   
132.
133.
This study of fossils (pollen, plant macrofossils, stomata and fish) and sediments (lithostratigraphy and geochemistry) from the Wendel site in North Dakota, USA, emphasizes the importance of considering ground-water hydrology when deciphering paleoclimate signals from lakes in postglacial landscapes. The Wendel site was a paleolake from about 11,500 14C yr BP to 11,100 14C yr BP. Afterwards, the lake-level lowered until it became a prairie marsh by 9,300 14C yr BP and finally, at 8,500 14C yr BP, an ephemeral wetland as it is today. Meanwhile, the vegetation changed from a white spruce parkland (11,500 to 10,500 14C yr BP) to deciduous parkland, followed by grassland at 9,300 14C yr BP. The pattern and timing of these aquatic and terrestrial changes are similar to coeval kettle lake records from adjacent uplands, providing a regional aridity signal. However, two local sources of ground water were identified from the fossil and geochemical data, which mediated atmospheric inputs to the Wendel basin. First, the paleolake received water from the melting of stagnant ice buried under local till for about 900 years after glacier recession. Later, Holocene droughts probably caused the lower-elevation Wendel site to capture the ground water of up-gradient lakes.  相似文献   
134.
The objective of this study is to find the order and coefficients of non-low-phase causal filters for ARMA (auto regressive moving average) filter model, using the Kurtosis minimization criterion. This method is based on the Kurtosis calculation of the treated sample at the input level and its identification at the output of the ARMA model. For this purpose, the order and coefficients of the AR (auto regressive) part are identified using the Yule-Walker algorithm at order two and then extended to order four. To obtain the MA (moving average) part, the AR components are calculated at first from the ARMA filter by deconvolution. Then, spectrally equivalent and minimum phase (SEMP) MA filter is identified using the Durbin algorithm at second and fourth order. Finally, the correct filter is found when the Kurtosis value of the output ARMA filter reconstituted is the closest to the Kurtosis of introduced signal. The proposed method is then tested on simulated processes and applied to real seismic data to perform blind deconvolution and obtain the reflectivity coefficients of subsoil studied.  相似文献   
135.
136.
Wet granular materials are three-dimensionally simulated by the discrete element method with water bridges incorporated between particles. The water bridges are simplified as toroidal shapes, and the matric suction is constantly maintained in the material. A comparison with experimental tests in the literature indicates that the toroidal shape approximation may be one of the best choices with high practicability and decent accuracy. Mechanical behaviours of wet granular materials are studied by triaxial tests. Effects of particle size distributions and void ratios are investigated systematically in this study. The hydraulic limit of the pendular state is also discussed. It gives the capillary cohesion function which is not only determined by the degree of saturation but also positively correlated to relative density and particle size polydispersity and inversely proportional to mean particle size. Furthermore, the capillary strengthening effect is also analysed microscopically in aid of the Stress–Force–Fabric relationship, mainly in fabric anisotropy, coordination number and stress transmission pattern, which revealed the micro-mechanisms of the additional effective stress induced by capillary effect.  相似文献   
137.
Long term competent performance of liner systems is a critical issue in the design and construction of waste repositories due to adverse interactions associated with leachate generated by wastes. This study was conducted to verify the efficacy of fly ash stabilization in enhancing compatibility between lateritic soil and municipal waste leachate. Applications investigated include soil mixtures containing 0, 5, 10, 15, and 20% fly ash compacted at approximately 2% wet of optimum moisture content with modified proctor energy. Baseline hydraulic conductivity was first established at every level of fly ash content by permeating soil mixtures with tap water before permeation with leachate in a compaction mould permeameter using the falling head test method. Results show that the trend in hydraulic conductivity of specimen containing 0% fly ash was characterized by a gradual but erratic decrease which may suggests partial entry of the leachate cations into the double layer. Conversely, specimens containing fly ash showed a general trend consisting of an initial drop in k (up to an order of magnitude) that was followed by slight decrease sustained until k stabilized and later terminated. Above 10% fly ash content, the relatively high values of k observed was not connected with the reactivity of the soil mixtures with leachate, rather it may be attributed to excessive fly ash content that altered their textural and hydraulic properties. The result of this study is potentially significant in the assessment of fly ash as a compatibility enhancing agent which can be admixed in barrier materials that are susceptible to adverse reactions with the liquid to be contained.  相似文献   
138.
Seasonal extreme wave statistics were reproduced by using the 25-km-grid global wave model of WAVEWATCH-III. The results showed that the simulated wave dataset for the present climate (1979-2009) was similar to Climate Forecast System Reanalysis (CFSR) wave data. Statistics such as the root mean squared error (RMSE) and correlation coefficient (CC) over the western North Pacific (WNP) basin were 0.5 m and 0.69 over the analysis domain. The largest trends and standard deviation were around the southern coast of Japan and western edge of the WNP. Linear regression analysis was employed to identify the relationship between the leading principal components (PCs) of significant wave heights (SWHs) in the peak season of July to September and sea surface temperature (SST) anomalies in the equatorial Pacific. The results indicated that the inter-annual variability of SWH can be associated with the El Niño-Southern Oscillation in the peak season. The CC between the first PC of the SWH and anomalies in the Nino 3.4 SST index was also significant at a 99% confidence level. Significant variations in the SWH are affected by tropical cyclones (TCs) caused by increased SST anomalies. The genesis and development of simulated TCs can be important to the variation in SWHs for the WNP in the peak season. Therefore, we can project the variability of SWHs through TC activity based on changes in SST conditions for the equatorial Pacific in the future.  相似文献   
139.
徐健  王驹 《铀矿地质》2006,22(5):295-299,294
岩体质量的评价对于各类大型建筑物特别是地下工程建筑物的安全性具有十分重要的作用。文章基于岩石质量指标(RQD)和岩体块度指数(RBI)的理论,通过对甘肃北山1号钻孔的岩心实测资料证明,岩体块度指数在一定程度上能反映岩体的完整性与相应力学性质的变化特点,同时给出钻孔附近岩体质量分类,进而与岩石质量指标所获得的钻孔附近岩体质量评价分级进行对比。对比结果表明:岩体块度指数在岩石质量评价中能更准确地反映其优越性,具有更为显著的工程意义。  相似文献   
140.
The impact of realistic representation of sea surface temperature (SST) on the numerical simulation of track and intensity of tropical cyclones formed over the north Indian Ocean is studied using the Weather Research and Forecast (WRF) model. We have selected two intense tropical cyclones formed over the Bay of Bengal for studying the SST impact. Two different sets of SSTs were used in this study: one from TRMM Microwave Imager (TMI) satellite and other is the weekly averaged Reynold’s SST analysis from National Center for Environmental Prediction (NCEP). WRF simulations were conducted using the Reynold’s and TMI SST as model boundary condition for the two cyclone cases selected. The TMI SST which has a better temporal and spatial resolution showed sharper gradient when compared to the Reynold’s SST. The use of TMI SST improved the WRF cyclone intensity prediction when compared to that using Reynold’s SST for both the cases studied. The improvements in intensity were mainly due to the improved prediction of surface latent and sensible heat fluxes. The use of TMI SST in place of Reynold’s SST improved cyclone track prediction for Orissa super cyclone but slightly degraded track prediction for cyclone Mala. The present modeling study supports the well established notion that the horizontal SST gradient is one of the major driving forces for the intensification and movement of tropical cyclones over the Indian Ocean.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号