全文获取类型
收费全文 | 1059篇 |
免费 | 204篇 |
国内免费 | 297篇 |
专业分类
测绘学 | 53篇 |
大气科学 | 345篇 |
地球物理 | 250篇 |
地质学 | 507篇 |
海洋学 | 100篇 |
天文学 | 29篇 |
综合类 | 125篇 |
自然地理 | 151篇 |
出版年
2024年 | 7篇 |
2023年 | 12篇 |
2022年 | 62篇 |
2021年 | 53篇 |
2020年 | 54篇 |
2019年 | 77篇 |
2018年 | 71篇 |
2017年 | 78篇 |
2016年 | 86篇 |
2015年 | 58篇 |
2014年 | 78篇 |
2013年 | 47篇 |
2012年 | 61篇 |
2011年 | 65篇 |
2010年 | 53篇 |
2009年 | 57篇 |
2008年 | 37篇 |
2007年 | 54篇 |
2006年 | 44篇 |
2005年 | 50篇 |
2004年 | 33篇 |
2003年 | 27篇 |
2002年 | 28篇 |
2001年 | 22篇 |
2000年 | 32篇 |
1999年 | 72篇 |
1998年 | 45篇 |
1997年 | 35篇 |
1996年 | 31篇 |
1995年 | 23篇 |
1994年 | 23篇 |
1993年 | 21篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有1560条查询结果,搜索用时 187 毫秒
941.
James N. Otoo Norbert H. Maerz Xialing Li Ye Duan 《Rock Mechanics and Rock Engineering》2013,46(3):543-554
Light detection and ranging (LiDAR) scanners are increasingly being used to measure discontinuity orientations on rock cuts to eliminate the bias and hazards of manual measurements which are also time consuming and somewhat subjective. Typically LiDAR data sets (point clouds) are analyzed by sophisticated algorithms that break down when conditions are not ideal, for example when some of the discontinuities are obscured by vegetation, or when significant portions of the rock face are composed of fractured facets, weathering generated surfaces, or anything that should not be identified as a discontinuity for the purposes of slope stability analysis. This paper presents a simple LiDAR point cloud viewer that allows the user to view the point cloud, identify discontinuities, pick three points on the surface (plane) of each discontinuity, and generate discontinuity orientations using the three-point method. Additionally, a test of our 3-D LiDAR viewer for discontinuity orientations on rock cuts in the United States of America and Canada is presented. 相似文献
942.
943.
944.
945.
We use conditional nonlinear optimal perturbation(CNOP)to investigate the optimal precursory disturbances in the ZebiakCane El Nino-Southern Oscillation(ENSO)model.The conditions of the CNOP-type precursors are highly likely to evolve into El Nino events in the Zebiak-Cane model.By exploring the dynamic behaviors of these nonlinear El Nino events caused by the CNOP-type precursors,we find that they,as expected,tend to phase-lock to the annual cycles in the Zebiak-Cane model,with the SSTA peak at the end of a calendar year.However,El Nino events with CNOPs as initial anomalies in the linearized Zebiak-Cane model are inclined to phase-lock earlier than nonlinear El Nino events despite the existence of annual cycles in the model.It is clear that nonlinearities play an important role in El Nino’s phase-locking.In particular,nonlinear temperature advection increases anomalous zonal SST differences and anomalous westerlies,which weakens anomalous upwelling and acts on the increasing anomalous vertical temperature difference and,as a result,enhances El Nino and then delays the peak SSTA.Finally,we demonstrate that nonlinear temperature advection,together with the effect of the annual cycle,causes El Nino events to peak at the end of the calendar year. 相似文献
946.
Donna J. Peuquet Niu Duan 《International journal of geographical information science》2013,27(1):7-24
Abstract Representations historically used within GIS assume a world that exists only in the present. Information contained within a spatial database may be added-to or modified over time, but a sense of change or dynamics through time is not maintained. This limitation of current GIS capabilities has recently received substantial attention, given the increasingly urgent need to better understand geographical processes and the cause-and-effect interrelationships between human activities and the environment. Models proposed so-far for the representation of spatiotemporal data are extensions of traditional raster and vector representations that can be seen as location- or feature-based, respectively, and are therefore best organized for performing either location-based or feature-based queries. Neither form is as well-suited for analysing overall temporal relationships of events and patterns of events throughout a geographical area as a temporally-based representation. In the current paper, a new spatio-temporal data model is proposed that is based on time as its organizational basis, and is thereby intended to facilitate analysis of temporal relationships and patterns of change through time. This model is named the Event-based Spatio Temporal Data Model (ESTDM). It is shown that temporally-based queries relating to locations can be implemented in an efficient and conceptually straightforward manner using ESTDM by describing algorithms for three fundamental temporally-based retrieval tasks based on this model: (1) retrieving location(s) that changed to a given value at a given time, (2) retrieving location(s) that changed to a given value over a given temporal interval, and (3) calculation of the total area that has changed to a given value over a given temporal interval. An empirical comparison of the space efficiency of ESTDM and compressed and uncompressed forms of the ‘snapshot’ model is also given, showing that ESTDM is also a compact representation of spatio-temporal information. 相似文献
947.
Oceanic noise is the background interference in sonar performance prediction and evaluation at high sea states. Statistics of underwater ambient noise during Typhoons Soulik and Nida were analyzed on the basis of experimental measurements conducted in a deep area of the Philippine Sea and the South China Sea. Generated linear regression, frequency correlation matrix (FCM), Burr distribution and Gumbel distribution were described for the analysis of correlation with environmental parameters including wind speed (WS), significant wave height (SWH), and the inter-frequency relationship and probability density function of noise levels (NLs). When the typhoons were quite close to the receivers, the increment of NLs exceeded 10 dB. Whilst ambient noise was completely dominated by wind agitation, NLs were proportional to the cubic and quintic functions of WS and SWH, respectively. The fitted results between NLs and oceanic parameters were different for “before typhoon” and “after typhoon”. The fitted slopes of linear regression showed a linear relationship with the logarithm of frequency. The average observed typhoon-generated NLs were 5 dB lower than the Wenz curve at the same wind force due to the insufficiently developed sea state or the delay between NLs and WS. The cross-correlation coefficient of FCM, which can be utilized in the identification of noise sources in different bands, exceeded 0.8 at frequencies higher than 250 Hz. Furthermore, standard deviation increased with frequency. The kurtosis was equal to 3 at >400 Hz approximately. The characteristics of NLs showed good agreement with the results of FCM. 相似文献
948.
949.
Guoqiang Li Yanwu Duan Xiaozhong Huang Jan‐Pieter Buylaert Wei Peng David B. Madsen Zhiguo Rao Linlin She Haichao Xie Jianhui Chen Fahu Chen 《Boreas: An International Journal of Quaternary Research》2017,46(2):264-281
The lacustrine deposits of lakes in arid central Asia (ACA) potentially record palaeoclimatic changes on orbital and suborbital time scales, but such changes are still poorly understood due to the lack of reliable chronologies. Bosten Lake, the largest freshwater inland lake in China, is located in the southern Tianshan Mountains in central ACA. A 51.6‐m‐deep lacustrine succession was retrieved from the lake and 30 samples from the succession were used for luminescence dating to establish a chronology based on multi‐grain quartz OSL and K‐feldspar post‐IR IRSL (pIRIR290) dating. Quartz OSL ages were only used for samples from the upper part of the core. The K‐feldspar luminescence characteristics (dose recovery test, anomalous fading test, first IR stimulation temperature plateau test) are satisfactory and from the relationship amongst the quartz OSL, IR50 and pIRIR290 doses we infer that the feldspar signals are likely to be well bleached at deposition. Bacon age‐depth modelling was used to derive a chronology spanning the last c. 220 ka. The chronology, lithology and grain‐size proxy record indicate that Bosten Lake formed at least c. 220 ka ago and that lake levels fluctuated frequently thereafter. A stable deep lake occurred at c. 220, 210–180, c. 165, 70–60, 40–30 and 20–5 ka, while shallow levels occurred at c. 215, 180–165, 100–70, 60–40 and 30–20 ka. Bosten Lake levels decreased by at least ~29 m and possibly the lake even dried up between c. 160 and c. 100 ka. We suggest that the water‐level fluctuations in the lakes of ACA may not respond directly to climatic changes and may be affected by a number of complex factors. 相似文献
950.