全文获取类型
收费全文 | 14226篇 |
免费 | 2474篇 |
国内免费 | 3747篇 |
专业分类
测绘学 | 1864篇 |
大气科学 | 1977篇 |
地球物理 | 2568篇 |
地质学 | 8535篇 |
海洋学 | 2223篇 |
天文学 | 741篇 |
综合类 | 1050篇 |
自然地理 | 1489篇 |
出版年
2024年 | 70篇 |
2023年 | 235篇 |
2022年 | 681篇 |
2021年 | 800篇 |
2020年 | 692篇 |
2019年 | 842篇 |
2018年 | 868篇 |
2017年 | 834篇 |
2016年 | 899篇 |
2015年 | 853篇 |
2014年 | 956篇 |
2013年 | 1124篇 |
2012年 | 1246篇 |
2011年 | 1302篇 |
2010年 | 1272篇 |
2009年 | 1164篇 |
2008年 | 1045篇 |
2007年 | 1140篇 |
2006年 | 987篇 |
2005年 | 687篇 |
2004年 | 499篇 |
2003年 | 385篇 |
2002年 | 411篇 |
2001年 | 388篇 |
2000年 | 289篇 |
1999年 | 172篇 |
1998年 | 79篇 |
1997年 | 92篇 |
1996年 | 61篇 |
1995年 | 68篇 |
1994年 | 38篇 |
1993年 | 36篇 |
1992年 | 39篇 |
1991年 | 19篇 |
1990年 | 25篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 20篇 |
1986年 | 10篇 |
1985年 | 8篇 |
1984年 | 17篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1976年 | 4篇 |
1969年 | 4篇 |
1958年 | 4篇 |
1957年 | 3篇 |
1954年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
91.
利用微滤和超滤技术研究了九龙江河口区表层水中铀及其同位素组成的粒级分布和地球化学行为.结果表明,溶解态(<0.4μm)中低分子量组分(<10 000 u)占主要份额,胶体态(10 000 u~0.4μm)238U所占比例不足1%,且随盐度的增加其所占份额逐渐降低.溶解态、低分子量组分和胶体态238U的比活度与盐度之间存在良好的线性正相关关系,证实它们在九龙江河口区呈现保守行为.在颗粒态(>0.4μm)中,各粒级组分238U所占份额主要受控于相应颗粒物的浓度,在盐度小于20的区域,各粒级颗粒组分238U占颗粒态的份额有如下变化次序:10~53μm>2~10μm>0.4~2μm>大于53μm,而在盐度大于30的近外海站位,该次序发生一些变化:0.4~2μm>10~53μm>2~10μm>大于53μm,最小粒级颗粒组分238U的贡献有所增加,反映了自生铀贡献的加强.九龙江河口区表层水中溶解态(包括低分子量组分和胶体态)的234U/238U)A.R.均大于1,显示出234U过剩的特征,而各粒级颗粒组分中的234U/238U)A.R.则接近于平衡值(1.0).这一现象与陆地岩石风化过程中水体对铀的淋滤释出量及234U的优先浸出有关.对232Th/238U质量比的研究显示,溶解态及其所包括的低分子量组分和胶体态的232Th/238U质量比均小于1,而颗粒态及其所包括的4个粒级组分中的232Th/238U质量比均大于1,反映了向外海输送过程中铀、钍地球化学行为的差异. 相似文献
92.
Cheng-Chien Liu Richard L. Miller Kendall L. Carder Zhongping Lee Eurico J. D’Sa James E. Ivey 《Journal of Oceanography》2006,62(3):235-248
We present a new approach that incorporates two models to estimate the underwater light field from remote sensing of ocean
color. The first employs a series of analytical, semi-analytical, and empirical algorithms to retrieve the spectrum of inherent
optical properties (IOPs), including the absorption and the backscatter coefficients, from the spectrum of remote sensing
reflectance. The second model computes the profile of photosynthetically available radiation E
0,PAR
(z) for a vertically homogeneous water column using the information of the retrieved IOPs and the ambient optical environment.
This computation is based on an improved look-up table technology that possesses high accuracy, comparable with the full solution
of the radiative transfer equation, and meets the computational requirement of remote sensing application. This new approach
was validated by in situ measurements and an extensive model-to-model comparison with a wide range of IOPs. We successfully mapped the compensation
depth by applying this new approach to process the SeaWiFS imagery. This research suggests that E
0,PAR
(z) can be obtained routinely from ocean-color data and may have significant implications for the estimation of global heat
and carbon budget. 相似文献
93.
At present the mutual influence of tropical cyclones (TCs) has been investigated to a much lesser extent than the cyclones themselves. Most frequently, such investigations are restricted to the study of the influence of the interaction of two TCs on their motion. However, actually, the mutual influence of TCs is much more diverse. For example, each cyclone alters the state of the medium (in particular, decreases the heat store of the upper ocean layer), i.e., affects the subsequent cyclones. Under conditions of a limited energy resource of the medium, cyclones, existing simultaneously, “compete,” to a degree, with each other. These and other similar circumstances have made it practical to consider tropical cyclones and the environment as interdependent elements of a complex open system. This study considers two versions of nonlinear models describing the interaction of TCs with the upper ocean layer and with one another similarly to a number of well-known and verified models of synergetics (interaction of populations, dynamics of optical quantum generators). It is shown that the models reproduce some very important qualitative features of the evolution of TC intensity. 相似文献
94.
95.
On the basis of the data of oceanographic survey in the East China Sea in four seasons during 1997-2000 (23°30'~33°00'N, 118°30'-128°E), the variation of total biomass and diet biomass of zooplankton and their spatial-temporal distribution and relationship with the fishing ground of Engraulis japonicus are approached and analyzed. The results show that the average biomass is 65.32 mg/m3 in four seasons, autumn (86.18 mg/m3) being greater than summer (69.18 mg/m3) greater than spring (55.67 mg/m3) greater than winter (50.33 mg/m3). The average value of diet zooplankton biomass is 40.9 mg/m3. The trends of horizontal distribution both in the total biomass and the diet biomass of zooplankton are similar. The high biomass region (250-500 mg/m3) is very limited, only accounting for 1% of the investigation area. Seasonal variation of the biomass is very remarkable in the west and north parts of East China Sea coastal waters ( 29°30'N,125°E). The horizontal distribution of diet zooplankton depends on the 相似文献
96.
UWAN-MAC: An Energy-Efficient MAC Protocol for Underwater Acoustic Wireless Sensor Networks 总被引:2,自引:0,他引:2
In this paper, we propose a distributed, scalable, energy-efficient media access control (MAC) protocol that works despite long, unknown propagation delays of the underwater acoustic medium. This protocol can be used for delay-tolerant applications such as underwater ecological sensor networks between energy-limited nodes. Our protocol differs significantly from ALOHA, multiple access with collision avoidance (MACA), and the media access protocol for wireless LANs (MACAW) in that energy is the main performance metric in our case rather than bandwidth utilization. We show that under a realistic underwater sensor network scenario, our MAC protocol wastes only 4% of the transmit energy and only 1.5% of the receive energy due to collisions, when the average number of neighbors is four, and the duty cycle is 0.004. This distributed, scalable MAC protocol has the potential to serve as a primer for the development of energy-efficient MAC protocols for future underwater sensor networks. 相似文献
97.
The high diversity of grain-size parameters of marine and oceanic sediments is determined by hydrodynamical and gravitational processes. Grain-size spectra of the sediments in the World Ocean are represented by two main types. Filtering organisms that passively inherit the proportions of clayey fractions in suspended matter only accelerate the deposition of fine particles, not influencing their distribution at the bottom. 相似文献
98.
以上海市向阳圩海塘围堰主堤为工程背景,在大量原位实测的基础上通过对各测试结果的统计分析研究了软土海滩围堰堤坝及地基沉降位移特性。结果表明:围堰堤坝及地基沉降具有显著的时效特性,沉降与时间的相关关系可用负指数时效特性曲线拟合;各测点位移值随时间具有明显突变变化,整个沉降时域可分为快速沉降时域区及缓慢沉降时域区。同时,在充分考察了沉降速率随时间变化特性的前提下提出了用该参量预测堤坝稳定时间及最终沉降位移的建议。 相似文献
99.
100.
沿海与内陆地下卤水对比分析 总被引:1,自引:0,他引:1
分析对比我国滨海与内陆地下卤水的分布、成因及其水文地球化学特征,指出不同成因的地下卤水具有不同的水质. 相似文献