全文获取类型
收费全文 | 1893篇 |
免费 | 265篇 |
国内免费 | 304篇 |
专业分类
测绘学 | 408篇 |
大气科学 | 219篇 |
地球物理 | 513篇 |
地质学 | 645篇 |
海洋学 | 190篇 |
天文学 | 22篇 |
综合类 | 173篇 |
自然地理 | 292篇 |
出版年
2024年 | 3篇 |
2023年 | 19篇 |
2022年 | 53篇 |
2021年 | 62篇 |
2020年 | 74篇 |
2019年 | 91篇 |
2018年 | 68篇 |
2017年 | 86篇 |
2016年 | 89篇 |
2015年 | 90篇 |
2014年 | 98篇 |
2013年 | 106篇 |
2012年 | 110篇 |
2011年 | 110篇 |
2010年 | 100篇 |
2009年 | 139篇 |
2008年 | 118篇 |
2007年 | 121篇 |
2006年 | 97篇 |
2005年 | 98篇 |
2004年 | 90篇 |
2003年 | 78篇 |
2002年 | 64篇 |
2001年 | 67篇 |
2000年 | 53篇 |
1999年 | 43篇 |
1998年 | 45篇 |
1997年 | 51篇 |
1996年 | 27篇 |
1995年 | 38篇 |
1994年 | 24篇 |
1993年 | 34篇 |
1992年 | 27篇 |
1991年 | 30篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1954年 | 2篇 |
排序方式: 共有2462条查询结果,搜索用时 15 毫秒
1.
《Astroparticle Physics》2002,16(4):183-386
Frequency distributions of local muon densities in high-energy extensive air showers (EAS) are presented as signature of the primary cosmic ray energy spectrum in the knee region. Together with the gross shower variables like shower core position, angle of incidence, and the shower sizes, the KASCADE experiment is able to measure local muon densities for two different muon energy thresholds. The spectra have been reconstructed for various core distances, as well as for particular subsamples, classified on the basis of the shower size ratio Nμ/Ne. The measured density spectra of the total sample exhibit clear kinks reflecting the knee of the primary energy spectrum. While relatively sharp changes of the slopes are observed in the spectrum of EAS with small values of the shower size ratio, no such feature is detected at EAS of large Nμ/Ne ratio in the energy range of 1–10 PeV. Comparing the spectra for various thresholds and core distances with detailed Monte Carlo simulations the validity of EAS simulations is discussed. 相似文献
2.
In order to understand the process of surface erosion and acquire basic data of conditions on hillslope without vege tation, a sprinkling experiment is conducted on a bare slope in Mt. Tanakami in the central part of Japan. Based on the mea surements of runoff, mean soil erosion depth, and sediment yield, etc. , the results suggest the following characteristics in the process of surface erosion in the experimental area. (1) The occurrence of sediment discharge is interrupted; (2) Surface runoff is a saturated overland flow; (3) The mean soil erosion depth is thick compared with other areas in Mt. Tanakami;(4) Sediment discharge process is detachment- limited. 相似文献
3.
Thomas B. Boving Mark H. Stolt Janelle Augenstern Brian Brosnan 《Environmental Geology》2008,55(3):571-582
The control of polluted surface runoff and the assessment of possible impacts on groundwater is a concern at the local and
regional scale. On this background, a study investigates possible impacts of organic and inorganic pollutants (including bacteria)
originating from a permeable asphalt parking lot on the water quality immediately beneath it. The functioning of the permeable
pavement, including clogging and restricted vertical percolation, was also evaluated. Four nested sample ports (shallow and
deep) were installed below low- and high-traffic areas, including one port outside the parking lot. At least initially there
was a good hydraulic connection between the parking surface and the shallow sample ports. The presence of a geotextile layer
at the base of the parking lot structure, however, was identified in lab tests as one factor restricting vertical percolation
to the deeper ports. Clogging of the permeable surface was most pronounced in heavy traffic areas and below snow pile storage
areas. Corroborated by high electric conductivity and chloride measurements, sand brought in by cars during winter was the
principal cause for clogging. No bacteria or BOD were found in percolating water. Polycyclic aromatic hydrocarbons (PAH) were
present at concentrations near minimum detection limit. Nutrients (nitrate and phosphate) were being leached into the ground
via the permeable parking lot surface at annual flux rates of 0.45–0.84 g/m2/year. A multi-species tracer test demonstrated a retention capacity of the permeable parking lot structure of >90% for metals
and 27% for nutrients, respectively. 相似文献
4.
对含多资料的分潮波模型的超定问题进行探讨,建立一种有限差分—最小二乘方法,该方法使得岸边资料直接进入了计算方程,并保证岸界法向流速为零条件。通过对渤海的四大分潮的试算,证实本方法可行。 相似文献
5.
根据2002年11月在亚大湾大鹏澳进行的连续30d(每日采样一次)观测资料,运用主成分分析和多元回归分析相结合方法,分析大鹏澳非养殖区中各浮游植物优势种之间的关系及影响其生长与演替的主要理化因子.建立秋季浮游植物优势种演替模型,并与春季的大鹏澳现场调查建立的浮游植物优势种演替模型进行比较,分析生境变化(降雨)对浮游植物优势种演替过程的影响。结果表明,春,秋季浮游植物优势种发生不同的演替过程。春季浮游植物对资源的竞争较为激烈,大量降雨引起海水中营养盐浓度升高,促进并维持中肋骨条藻(Skeletonema costatum)高密度生长,待营养盐被大量消耗后,中肋骨条藻数量下降,减轻了对柔弱菱形藻(Nitzschia delicatissima)的生长压力而使其成为优势种;而秋季水温较低,浮游植物细胞数量较春季大为减少,中肋骨条藻和柔弱菱形藻对资源的竞争较为缓和,使外界环境变化成为影响优势种变化的主要原因;降雨期间虽然营养盐增加,但环境变化使浮游植物的生长受到限制,雨后柔弱菱形藻数量不能恢复,水体中高营养盐浓度促使中肋骨条藻出现生长峰值。 相似文献
6.
南海东北部表层沉积中生源和矿物碎屑组分分析及其古环境意义 总被引:9,自引:3,他引:9
通过对南海东北部128个表层沉积样品的定量研究,揭示隆源和矿物碎屑各组分的分布,也显示了组分分析作为一种简单、快速的研究方法在海洋古环境研究中的潜力,研究区生源碎屑的分布明显受与水深相关的深海溶解作用所控制,水深3500-4000m的现代南海CCD以下,钙质生物的含量急剧减少,而硅质生物的含量急剧增加,研究区表层沉积中的碎屑矿物主要来源于亚洲大陆,吕宋镐的风化产物只起次要作用,且其分布主要受与离岸距离远近和海流相关的搬运作用所控制。 相似文献
7.
研究流形上的聚类分析,针对基于密度的空间聚类引入了流形概念,提出1种基于流形的密度聚类算法,该方法将流形的概念与聚类相结合,可以适用于样本为复杂分布的聚类。文中通过实例证明此算法的有效性。 相似文献
8.
对于含多资料三维分潮波模型建立了一种岸边资料运动学外推的潮波第一边值方法,并对渤海的四大原湖波进行试算。结果证实本方法可行,该方法使潮波方程既满足第一边值问题,充分利用岸边测站资料,又保证岸界法向流速为零条件,符合物理意义。并对复型病态矩阵的收敛问题提出了一种可行性处理。 相似文献
9.
本文对用于模式分类、函数逼近、参数估计的多层感知器 (MLPs)给出 1个清晰的关于内部行为的解释。作者以单隐层的 MLP为例 ,论述了关于 MLP的内部行为的半线性分析理论。对受训的MLP,将隐层单元的输出分别定义为网络输出的正、负“内部分量”;定义内部分量的连接权重集为给定问题的“内部判别模式”;建立了 MLP和模糊集相结合的新模型 ;分析了 MLP的结构为 N- 2 - 1和N- H- 1 ,给出权重初始化的方法 ;提出了 1种从受训神经 -模糊模型 (NFMs)中提取知识的全新的具有实用价值的方法。 相似文献
10.
Time series of freshwater runoff, seawater salinity, temperature and oxygen were used in transfer functions (TF) to model changes of mesozooplankton taxa in the Baltic Sea from the 1960’s to the 1990’s. The models were then compared with long term zooplankton monitoring data from the same period. The TF models for all taxa over the whole Baltic proper and at different depth layers showed statistically significant estimates in t-tests. TF models were further compared using parsimony as a criterion. We present models showing 1) r2 > 0.4, 2) the smallest residual standard error with the combination of exploratory variables, 3) the lowest number of parameters and 4) the highest proportional decrease in error term when the TF model residual standard error was compared with those of the univariate ARIMA model of the same response variable. Most often (7 taxa out of a total of 8), zooplankton taxa were dependent on freshwater runoff and/or seawater salinity. Cladocerans and estuarine copepods were more conveniently modelled through the inclusion of seawater temperature and oxygen data as independent variables. Our modelling, however, explains neither the overall increase in zooplankton abundance nor a simultaneous decrease found in the neritic copepod, Temora longicornis. Therefore, biotic controlling agents (e.g. nutrients, primary production and planktivore diets) are suggested as independent variables for further TF modelling. TF modelling enabled us to put the controlling factors in a time frame. It was then possible, despite the inherent multiple correlation among parameters studied to deduce a chain-of-events from the environmental controls and biotic feedback mechanisms to changes in zooplankton species. We suggest that the documented long-term changes in zooplankton could have been driven by climatic regulation only. The control by climate could be mediated to zooplankton through marine chemical and physical factors, as well as biotic factors if all of these were responding to the same external control, such as changes in the freshwater runoff. Increased runoff would explain both the increasing eutrophication, causing the overall increase of zooplankton, and the changes in selective predation, contributing to decline of Temora. 相似文献