全文获取类型
收费全文 | 1203篇 |
免费 | 249篇 |
国内免费 | 237篇 |
专业分类
测绘学 | 87篇 |
大气科学 | 240篇 |
地球物理 | 443篇 |
地质学 | 552篇 |
海洋学 | 106篇 |
天文学 | 45篇 |
综合类 | 84篇 |
自然地理 | 132篇 |
出版年
2024年 | 6篇 |
2023年 | 22篇 |
2022年 | 41篇 |
2021年 | 71篇 |
2020年 | 54篇 |
2019年 | 50篇 |
2018年 | 73篇 |
2017年 | 60篇 |
2016年 | 54篇 |
2015年 | 76篇 |
2014年 | 78篇 |
2013年 | 81篇 |
2012年 | 68篇 |
2011年 | 81篇 |
2010年 | 65篇 |
2009年 | 64篇 |
2008年 | 69篇 |
2007年 | 53篇 |
2006年 | 42篇 |
2005年 | 36篇 |
2004年 | 46篇 |
2003年 | 37篇 |
2002年 | 60篇 |
2001年 | 53篇 |
2000年 | 47篇 |
1999年 | 42篇 |
1998年 | 35篇 |
1997年 | 25篇 |
1996年 | 27篇 |
1995年 | 11篇 |
1994年 | 37篇 |
1993年 | 27篇 |
1992年 | 19篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 7篇 |
1981年 | 1篇 |
1980年 | 4篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1964年 | 1篇 |
1958年 | 1篇 |
1949年 | 1篇 |
排序方式: 共有1689条查询结果,搜索用时 15 毫秒
21.
医学地理研究由来已久,但地理学界血吸虫病医学地理研究相对薄弱,而且多是非地理专业人员在从事相应工作。本文首先从血吸虫病流行传播的机理出发,分析了血吸虫病医学地理的主要研究内容,认为钉螺滋生地与血吸虫病疫情的监测和预测是血吸虫病医学地理研究的主要内容;其次系统总结了气象气候因子、土壤植被因子、地貌高程因子、水文水力因子、人文经济因子对血吸虫生命史及其宿主的影响,并视其为血吸虫病医学地理研究的基础;然后回顾了近年国际、国内运用遥感、GIS技术和统计学方法(包括地统计学方法)进行钉螺滋生地与血吸虫病疫情确定的主要研究手段,归纳了7类钉螺滋生地预测方法和5类血吸虫病疫情预测方法;最后剖析了目前血吸虫病医学地理研究面临的主要挑战和未来血吸虫病医学地理研究努力方向。 相似文献
22.
Isolation of sea surface salinity maps on various timescales in the tropical Pacific Ocean 总被引:1,自引:0,他引:1
Jian Chen Ren Zhang Huizan Wang Yuzhu An Peng Peng Weitao Zhang 《Journal of Oceanography》2012,68(5):687-701
To isolate sea surface salinity (SSS) maps on seasonal, ENSO, decadal, and long-term trend timescales in the tropical Pacific Ocean, the ensemble empirical mode decomposition (EEMD) is applied on an SSS data set covering 1950–2009, concerning three key regions including the western equatorial Pacific Warm Pool (WP), South Pacific Convergence Zone (SPCZ), and Inter Tropical Convergence Zone (ITCZ); then a self-organizing map is performed on the intrinsic mode function maps decomposed by the EEMD, concerning the whole basin. The ENSO and decadal signals concern mainly the western Pacific, in contrast to the seasonal signal mostly notable in the east. (1) The modulated annual cycle has smaller (larger) amplitudes during El Ni?o (La Ni?a) years and later appearance of minima during El Ni?o years; one unique annual cycle is observed at the northwestern edge of the ITCZ lagging the well-known ITCZ cycle by ~3?months. (2) The pronounced 1999–2001 SSS-related La Ni?a event in the SPCZ was reinforced twice by the decadal shift in the 1990s; the eastern Pacific and central Pacific ENSO-related SSS features are compared. (3) The contrasted anomalies between the western equatorial and non-equatorial regions were pronounced during 1977–1996, whereas they were less pronounced during 1971–1976, 2005–2008, and a roughly opposite pattern appeared with strong and abrupt decrease shift prevailing in large areas over the southwestern basin during 1997–2004. (4) The freshening at the western equator and the saltening located east of the SPCZ SSS front together amplify the geographical SSS contrasts exhibited by the WP and SPCZ SSS fronts. 相似文献
23.
Seok-Gwan?Choi Eun-A?Yoon Doo-Hae?An Sangdeuk?Chung Jaebong?Lee Kyounghoon?LeeEmail author 《Ocean Science Journal》2018,53(4):667-677
In this study, the dB difference and characteristics of krill swarms inhabiting Subarea 48.1, which includes the west and south of the South Shetland Island and the Elephant Island peripheries, were estimated to distinguish Antarctic krill, using acoustics. From April 13 to 24, 2016, acoustic data were collected along 24 survey lines using the frequencies 38 and 120 kHz, and middle trawling was performed at 7 stations. Using the difference between the dB values of two volume backscattering strength (Sv) frequencies (38 and 120 kHz), a clear acoustic distinction could be made between Antarctic krill (4.9 to 12.0 dB) and fish (?4.0 to ?0.2 dB). The distributions and mean Sv of krill swarms in the Elephant Island peripheries and south of South Shetland Island were higher than those in the west of South Shetland Island. The mean length/ height ratio of krill swarms in the west of the South Shetland Island (64.5) was higher than that in the south (35.9) and the Elephant Island peripheries (33.8), with the length of the aggregations exceeding their height. Most krill swarms were distributed between the surface layer (less than 10 m below sea level) and within 200 m of water depth. These results are expected to serve as baseline data for evaluating krill density and biomass by distinguishing them from fish, using acoustics. 相似文献
24.
Ocean Science Journal - This study examines the different effects of high and low freshwater discharge on the water physical structure of the water column and plankton community in a tide-dominated... 相似文献
25.
For the study on the ice-induced vibration of a compliant mono-cone structure,a series of model tests were performed from 2004 to 2006.In these tests,the ice sheet before the compliant conical structure was found to be failed in two-time breaking.Based on this important finding,model tests study of the ice force on a compliant multi-cone structure were performed from 2006 to 2007.in these tests,the ice sheet broke before each single cone non-simultaneously.The exciting energy of the total ice force was found to be in a wide range of frequencies,and the structure can be easily excited with nonlinear resonance. 相似文献
26.
In Belgium a long process (1999–2005) led to the designation of several marine protected areas (MPAs). In order to analyse the designation process, the ‘policy arrangement approach’ was used as an analytical tool. Attention was given to four dimensions (actors and coalitions, arguments, rules and resources). Particular attention was paid to the switch from an authoritative to a more deliberated policy style in reaction to conflicts with stakeholders. This switch in policy style contributed to a successful legal designation, but does not guarantee the best protection of marine biodiversity. This case study linked science (setting ecological goals) to policy (analysis of a policy process) and led to some more general conclusions on the designation of MPAs and the role of participation in this process. 相似文献
27.
基于相控地质统计学反演的薄储层岩性、物性预测 总被引:1,自引:0,他引:1
针对河流相储层厚度薄、纵向叠置严重、横向变化快、常规确定性反演方法受地震频带限制难以准确刻画的特点,尝试应用基于MCMC算法的地质统计学方法进行反演,以提高分辨率和储层预测精度,并全程应用成岩储集相控制建模,以便从储层地质学的角度加强模型约束,促进概念模型向定量模型的转化,建立合理的反映地下实际情况的油藏物性模型。具体实现方法是,首先利用对岩性识别敏感的测井曲线重构拟声阻抗曲线,再以声阻抗为中间变量,对岩性、物性参数空间变异性进行双重变差函数分析,最后基于MCMC算法实现了成岩储集相控制的地质统计学反演。得到的三维岩性、阻抗及孔隙度模型,其纵向分辨率为1 ms,既可以很好地展现不同期次砂体的叠置关系、隔夹层的分布以及沉积旋回纵、横向演变规律,又可以实现物性变异方向沿河道砂体弯曲方向变化的趋势,同时可对地震资料的多解性进行有效限制。 相似文献
28.
Cheng-Bang An Shi-Chen Tao Jiaju Zhao Fa-Hu Chen Yanbin Lv Weimiao Dong Hu Li Yongtao Zhao Ming Jin Zongli Wang 《Journal of Paleolimnology》2013,49(2):145-154
Surface and fossil pollen samples were collected to reconstruct the vegetation and environment since 30.7 cal ka BP in the Lake Balikun Basin, northwest China. This record demonstrates that the region was occupied by four vegetation communities during the interval 30.7–9.0 cal ka BP, including desert steppe (30.7–25.1 cal ka BP), Amaranthaceae-Asteraceae desert (25.1–17.7 cal ka BP), Amaranthaceae-Artemisia-Asteraceae desert (17.7–12.8 cal ka BP) and Amaranthaceae desert (12.8–9.0 cal ka BP), corresponding to relatively humid, the coldest and driest, cold and dry, and colder and drier conditions, respectively. The reconstructions of vegetation and environmental changes around Lake Balikun do not support the interpretation of an extremely humid LGM in Central Asia, challenging the cold-moist (glacial), warm-dry (interglacial) pattern of late Quaternary environmental change in Central Asia. The Last Glacial Maximum was cold and dry, and similarly harsh conditions continued into the early Holocene. 相似文献
29.
30.
In order to investigate the effect of internal friction of umbilical cable on its vortex-induced vibration(VIV)responses, the experimental study on VIV of bond umbilical cable(BUC) and un-bond umbilical cable(UBUC) was carried out in an experimental tank. A current generator in the laboratory simulated the uniform current, and the current velocities were observed in real time by using a Doppler Velocimeter. In addition, different sizes of top tension were applied to the umbilical cable model. The VIV responses of the umbilical cable model were measured by using Fiber Bragg grating(FBG) strain sensors. The displacement responses of umbilical cable model were reconstructed based on the experimental strain data processed by modal superposition method. In this paper, the traveling wave characteristics, the spatial-temporal distribution characteristics of frequency and fatigue damage of the BUC and UBUC under VIV are studied. The experimental results show that there are obvious differences between BUC and UBUC in the response characteristics of VIV. The UBUC appears the traveling wave sooner than BUC, but its standing wave characteristics are more obvious than those of BUC at high velocities. Compared with BUC, the spatial-temporal distribution of UBUC frequencies appears wide-band distribution sooner, but has narrower bandwidth in the "lock-in" state. The level of fatigue damage of BUC was approximately the same as that of UBUC. 相似文献