全文获取类型
收费全文 | 1575篇 |
免费 | 322篇 |
国内免费 | 473篇 |
专业分类
测绘学 | 62篇 |
大气科学 | 372篇 |
地球物理 | 382篇 |
地质学 | 909篇 |
海洋学 | 225篇 |
天文学 | 61篇 |
综合类 | 208篇 |
自然地理 | 151篇 |
出版年
2024年 | 10篇 |
2023年 | 33篇 |
2022年 | 73篇 |
2021年 | 81篇 |
2020年 | 60篇 |
2019年 | 93篇 |
2018年 | 89篇 |
2017年 | 82篇 |
2016年 | 96篇 |
2015年 | 82篇 |
2014年 | 81篇 |
2013年 | 82篇 |
2012年 | 82篇 |
2011年 | 109篇 |
2010年 | 99篇 |
2009年 | 104篇 |
2008年 | 82篇 |
2007年 | 86篇 |
2006年 | 76篇 |
2005年 | 57篇 |
2004年 | 52篇 |
2003年 | 35篇 |
2002年 | 49篇 |
2001年 | 49篇 |
2000年 | 59篇 |
1999年 | 93篇 |
1998年 | 66篇 |
1997年 | 65篇 |
1996年 | 57篇 |
1995年 | 65篇 |
1994年 | 48篇 |
1993年 | 49篇 |
1992年 | 19篇 |
1991年 | 23篇 |
1990年 | 24篇 |
1989年 | 11篇 |
1988年 | 16篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1965年 | 1篇 |
1958年 | 2篇 |
排序方式: 共有2370条查询结果,搜索用时 218 毫秒
941.
利用LS8000闪电定位系统2009—2011年的地闪观测资料对上海及周边地区(120~122.5°E,30~32°N)的地闪活动特征进行了研究。结果表明:分析区域内正地闪的比例约占8.5%,大电流地闪(电流绝对值大于50kA)的比例约为5.6%。地闪活动主要集中在6—9月,峰值出现在8月;日变化上,12:00—19:00闪电活动最为活跃,峰值出现在14:00,凌晨闪电活动最弱。从日变化上来说,正地闪和大电流地闪比例在地闪活动较强时段低于地闪活动较弱时段;在月分布上,在地闪活动最强的夏季,正地闪比例普遍在10%以下,在地闪活动较弱的春、秋、冬季,正地闪比例普遍在10%以上。以北纬31°为界,分析区域北部地闪密度基本在6~12次·km-2·a-1以上,南部基本在2.4~4.8次·km-2·a-1。同时陆地上的地闪密度要显著高于湖泊和海洋上的地闪密度,而海洋上的正地闪比例和大电流地闪比例要显著高于陆地。闪电空间分布的时间变化说明,下午地闪活动主要出现在陆地,而凌晨地闪主要出现在水体附近,其它时段则表现出过渡特征,这与下垫面的加热作用紧密相关。 相似文献
942.
943.
Simulations of water movement and solute transport through different soil texture configurations under negative‐pressure irrigation 下载免费PDF全文
This study examined the effects of different soil texture configurations on water movement and solute transport to provide a reliable scientific basis for the application of negative‐pressure irrigation (NPI) technology. HYDRUS‐2D was used to analyse water movement and solute transport under NPI. The main results are as follows: (a) HYDRUS‐2D can be used to simulate water movement and solute transport under NPI, as there was good agreement between the simulated and measured values for water contents, NaCl concentrations, cumulative water infiltration, and wetting distances in the horizontal and vertical directions; the Nash–Sutcliffe efficiency coefficients were in the range of 0.94–0.97. (b) Layered soils have obvious effects on water movement under NPI. With the emitter position in the loam layer, when a coarse texture of loamy sand was present below the loam layer (namely, L‐LS), irrigation water accumulated in the topsoil, and this led to an increase in evaporation compared with the homogeneous loam profile. However, fine texture silty loam or silty clay loam layers beneath the loam layer (namely, L‐SiL or L‐SiCL, respectively) was more conducive to water infiltration into the lower layer, and this increased the amount of water infiltration and simultaneously reduced the surface evaporation effectively. (c) Layered soils have obvious effects on solute transport under NPI, and salt accumulation will readily occur in the clay‐rich soil layer at the interface. The maximum soil salt accumulation of L‐LS occurred above the soil interface between the two soil layers with a value of 21.80 g/kg; however, for L‐SiCL and L‐SiL, the maximum salt accumulation occurred below the soil interface between the two soil layers, with values of 23.80 g/kg and 20.08 g/kg, respectively. (d) Interlayered soils showed remarkable changes in the water infiltration characteristics and salt‐leaching intensities under NPI, and the properties for the soil profile with a silty loam interlayer were better than those for the soil profile with a silty clay loam interlayer. The soil profile with a loamy sand interlayer had the lowest amount of water infiltration, which resulted in reductions of the salt‐leaching intensities. Thus, NPI is clearly not suitable for loamy sand soil. Overall, the results demonstrated that soil texture configurations affected water movement and solute transport under NPI. Therefore, careful consideration should be given to the use of NPI to achieve target soil water and solution conditions and reduce water loss. 相似文献
944.
我国土壤中结合态硒的含量和分布规律 总被引:18,自引:3,他引:15
本文提出并建立了Jackson法系统分离土壤中不同形态硒的实验程序,研究了我国几种土壤表层(0—20cm)中硒的结合形态及其地理分布规律,并对植物可利用性作了评价。 相似文献
945.
AR6659是22周以来最重要的一个活动区,它爆发了22周最强大的高能事件。本文用云南天文台的光球、色球精细结构照片和北京天文台怀柔站的磁场速度场资料,分析了该活动区磁场速度场的二维位形和大耀斑期间的演化特征。本文分析的4个大耀斑均爆发在中性线附近的N极区磁场梯度大的地方及色球速度场的红移区。偏带观测也显示耀斑物质是向红端移动的。耀斑波沿横场传播在离本黑子群几万至十几万公里的地方激起感生耀斑,在原生耀斑与感生耀斑之间往往有耀斑环相连。此外,本文还从演化特征出发分析了耀斑爆发前活动区等离子体的宏观不稳定性。 相似文献
946.
应用综合前兆异常月频度判定地震进入中短期的研究 总被引:5,自引:3,他引:2
应用震后资料总结和震前所判定的异常资料研究了1975-1996年的44个震次,结果表明,地震孕育进入中短期时无论是近场前兆异常、近场前兆异常,还是远、近场均有的前兆异常,其综合前兆异常月频率度一般同时呈现出2个明显特征;(1)月频度累加曲线呈现为非线性加速形态;(2)同时月频度变化速率出现异常,二相结合,一般可以判定了地震孕育是否进入短期阶段。由于研究内容广泛和取得的中短期特征明显,因此所得结果 相似文献
947.
The Tuoku region in northern Tarim Basin of China is a key area for studying oil/gas reservoir rocks. The magnetic and mineralogical
parameters of well cuttings from two wells, well S7, situated on oil/gas field, and well S6, at an oil/water interface, were measured. The two wells are located in the same structure with similar strata and types
of lithology, but well S6 is a showing well of oil and gas 5 km northwest of well S7. The purpose of this paper is to evaluate the possibility and distribution of secondary magnetic alteration that may have
occurred due to hydrocarbon migration above an oil/gas accumulation. It is concluded that the magnetism of well cuttings from
major strata in well S7, including source rocks, oil reservoir rocks and cap rocks, and in Quaternary (Q) soil is higher than that from well S6. The Cambrian oil-bearing strata and cap rocks have even higher magnetism in well S7. The shape and parameters of magnetic hysteresis loops indicate that soft (H
c<20 mT,H
s<0.3 T) ferrimagnetic components dominate the magnetic carriers within the strongly magnetic strata of well S7, whereas a mixed paramagnetic and ferrimagnetic distribution occurs in well S6 (for example, low coecivityH
c and nonsaturating magnetized character). Analysis of heavy minerals shows that the contents of iron oxide (magnetite, maghemite
and hematite) in well S7 are often higher than those in well S6. The magnetite content in samples of cuttings from Cambrian rocks can reach 9.7% in oil-bearing strata in well S7, and in strata Ekm and N1j are 1.215% and1.498%, respectively. Typical spherical magnetite grains are found within the main source rocks and the soils
in well S7. By analysis of surface microtexture and of trace element contents, we infer that the spherical magnetite is composed of
aggregates of ultrafine particles that are probably authigenic magnetite formed in a hydrocarbon halo background.
Project supported by the National Natural Science Foundation of China and the Geological Industry Foundation of China (Grant
No. 49374216) and Foundation of the State Key Laboratory of Oil/Gas Reservoir Geology and Exploitation of China (Grant No.
9608). 相似文献
948.
浊流及相关重力流沉积研究综述 总被引:44,自引:2,他引:42
本文首先介绍了浊流及相关重力流的有关概念,并对浊流沉积研究历史进行了简要回顾,概括了当前浊流沉积的研究现状;随后,总结了浊积岩的沉积特征并对其成因进行了扼要解释;最后,对浊流及相关重力流沉积研究中两个有争议的问题-浊积相的划分和浊积相模式的建立进行了探讨。 相似文献
949.
950.