全文获取类型
收费全文 | 635篇 |
免费 | 28篇 |
国内免费 | 149篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 42篇 |
地球物理 | 368篇 |
地质学 | 217篇 |
海洋学 | 78篇 |
天文学 | 29篇 |
综合类 | 17篇 |
自然地理 | 42篇 |
出版年
2023年 | 12篇 |
2022年 | 6篇 |
2021年 | 6篇 |
2020年 | 19篇 |
2019年 | 13篇 |
2018年 | 13篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 10篇 |
2014年 | 13篇 |
2013年 | 14篇 |
2012年 | 47篇 |
2011年 | 70篇 |
2010年 | 7篇 |
2009年 | 12篇 |
2008年 | 48篇 |
2007年 | 32篇 |
2006年 | 40篇 |
2005年 | 37篇 |
2004年 | 42篇 |
2003年 | 36篇 |
2002年 | 30篇 |
2001年 | 21篇 |
2000年 | 15篇 |
1999年 | 24篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 13篇 |
1995年 | 10篇 |
1994年 | 11篇 |
1993年 | 24篇 |
1992年 | 22篇 |
1991年 | 7篇 |
1990年 | 15篇 |
1989年 | 13篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1980年 | 3篇 |
1948年 | 3篇 |
1947年 | 3篇 |
1943年 | 3篇 |
1942年 | 3篇 |
1937年 | 3篇 |
1935年 | 12篇 |
1934年 | 4篇 |
1925年 | 7篇 |
1923年 | 5篇 |
排序方式: 共有812条查询结果,搜索用时 15 毫秒
131.
Boitsov S Petrova V Jensen HK Kursheva A Litvinenko I Chen Y Klungsøyr J 《Marine environmental research》2011,71(5):357-368
Subsurface sediments from a pockmark area in South-Western Barents Sea have been earlier found to contain elevated levels of petroleum-related polycyclic aromatic hydrocarbons. This work describes a comprehensive analysis of various biomarkers, including the highly source-specific hopanes, in a 4.5?m long gravity core from the same area, together with subsurface sediment samples from other areas in the region without pockmarks present ("background samples"). A clear difference between the pockmark gravity core and the background sediment cores was found, both with regard to genesis and the level of transformation of organic matter. A number of indicator parameters, such as methylphenanthrene index (MPI-1), point towards a significantly higher maturity of hydrocarbons in the pockmark core throughout its length as compared to the other sampled locations. Higher contents of microbial hopanoids (hopenes) may indicate the former presence of petroleum. These findings confirm the hypothesis of a natural hydrocarbon source in the deeper strata present in the studied location with pockmarks. 相似文献
132.
Background
Induced precipitation of phosphate minerals to scavenge trace elements from groundwater is a potential remediation approach for contaminated aquifers. The success of engineered precipitation schemes depends on the particular phases generated, their rates of formation, and their long term stability. The purpose of this study was to examine the precipitation of calcium phosphate minerals under conditions representative of a natural groundwater. Because microorganisms are present in groundwater, and because some proposed schemes for phosphate mineral precipitation rely on stimulation of native microbial populations, we also tested the effect of bacterial cells (initial densities of 105 and 107 mL-1) added to the precipitation medium. In addition, we tested the effect of a trace mixture of propionic, isovaleric, formic and butyric acids (total concentration 0.035 mM).Results
The general progression of mineral precipitation was similar under all of the study conditions, with initial formation of amorphous calcium phosphate, and transformation to poorly crystalline hydroxylapatite (HAP) within one week. The presence of the bacterial cells appeared to delay precipitation, although by the end of the experiments the overall extent of precipitation was similar for all treatments. The stoichiometry of the final precipitates as well as Rietveld structure refinement using x-ray diffraction data indicated that the presence of organic acids and bacterial cells resulted in an increasing a and decreasing c lattice parameter, with the higher concentration of cells resulting in the greatest distortion. Uptake of Sr into the solids was decreased in the treatments with cells and organic acids, compared to the control.Conclusions
Our results suggest that the minerals formed initially during an engineered precipitation application for trace element sequestration may not be the ones that control long-term immobilization of the contaminants. In addition, the presence of bacterial cells appears to be associated with delayed HAP precipitation, changes in the lattice parameters, and reduced incorporation of trace elements as compared to cell-free systems. Schemes to remediate groundwater contaminated with trace metals that are based on enhanced phosphate mineral precipitation may need to account for these phenomena, particularly if the remediation approach relies on enhancement of in situ microbial populations. 相似文献133.
134.
在台湾,一般认为地貌学是地理学的分支。尽管许多地理学家和工程学家在这一领域作出很大贡献,但仍然是地貌学家作了大量的工作。在各个综合大学及师范大学的地理系,地貌学被作为一个必不可少的独立课程来进行教学。台湾现已有20个职业地貌学家,他们中的大多数正在一些高等院校,中学和研究所从事教学和科研工作。一、战前时期的研究从十九世纪到1946年这一时期,台湾才开始地形学的研究。但这方面的工作主要是由外国学者做的。这一阶段前期,研究工作主要围绕对台湾岛主要地形特征的勘探和形态描述。这时常常强调了陆地表面起伏和自然地理的描述,却忽视了对这两方面的起源及其发展历史的原因的探讨。 相似文献
135.
Ground water management models require the repeated solution of a simulation model to identify an optimal solution to the management problem. Limited precision in simulation model calculations can cause optimization algorithms to produce erroneous solutions. Experiments are conducted on a transient field application with a streamflow depletion control management formulation solved with a response matrix approach. The experiment consists of solving the management model with different levels of simulation model solution precision and comparing the differences in optimal solutions obtained. The precision of simulation model solutions is controlled by choice of solver and convergence parameter and is monitored by observing reported budget discrepancy. The difference in management model solutions results from errors in computation of response coefficients. Error in the largest response coefficients is found to have the most significant impact on the optimal solution. Methods for diagnosing the adequacy of precision when simulation models are used in a management model framework are proposed. 相似文献
136.
Kunihiro T Miyazaki T Uramoto Y Kinoshita K Inoue A Tamaki S Hama D Tsutsumi H Ohwada K 《Marine pollution bulletin》2008,57(1-5):68-77
We monitored seasonal changes of the abundance and composition of microorganisms in the fish-farm sediment in Kusuura Bay, Amakusa, Japan, using the quinone profiling technique, during bioremediation by introducing cultured colonies of polychaete, Capitella sp. I. In November 2004, approximately 9.2 million cultured worms were transferred to the fish-farm sediment, which increased rapidly, and reached 458.5 gWW/m(2) (528,000 indiv./m(2)) in March 2005. During this fast-increasing period of Capitella, the microbial quinone content of the surface sediment (0-2 cm) also increased markedly, and reached 237 micromol/m(2) in January 2005, although the water temperature decreased to the lowest levels in the year. Particularly, the mole fraction of ubiquinone-10 in total quinones in the sediment, indicating the presence of alpha subclass of Proteobacteria, increased by 9.3%. These facts suggest that the bacterial growth was enhanced markedly by the biological activities of worms in the sediment, and the bacteria played an important role in the decomposition of the organic matter in the sediment. 相似文献
137.
The investigations were carried out at 6 tidal flats located on the eastern part of the Seto Inland Sea, Japan. This study was focused on physical characteristics of sediments, namely as particle size of sediment and difference in elevation, and generalizes the relationship between sediments and macrobenthos. A total of 192 species were collected at 187 stations at 6 tidal flats. Physical characteristics of sediment were classified into 9 groups by cluster analysis in relation to sediment particle size and difference in elevation. Those groups had also significant difference in physical characteristics of sediments, and were characterized by some specific macrobenthos species. Distribution of macrobenthos can be explained by the classification of physical characteristics of sediment. These findings show the possibility to predict the variety of macrobenthos community using the physical characteristics of sediment. 相似文献
138.
Variability in rainfall threshold for debris flow after the Chi-Chi earthquake in central Taiwan, China 总被引:6,自引:0,他引:6
The purpose of this study is to analyze variability in rainfall threshold for debris flow (critical rainfall for debris flow triggering) after the ML 7.3 Chi-Chi earthquake in central Taiwan in 1999. Two study sites with different geological conditions were surveyed in the earthquake area. Streambed surveys were conducted to continuously monitor debris flows between 1999 and 2006. During the 7-year study period, every debris flow event was identified, and the streambed characterized. Results show that the rainfall threshold for debris flow was remarkably lower just after the Chi-Chi Earthquake, but gradually recovered. To date, this rainfall threshold is still lower than the original level prior to the earthquake. This variability in rainfall threshold is closely related to the mount of sediment material in the initiation area of debris flow, which increased rapidly due to landslides resulting from the earthquake. With the increase in sediment material, the rainfall threshold was lowered severely during the first year following the Chi-Chi earthquake. However, heavy rainfalls mobilized the sediment material, causing debris flows and transporting sediment downstream. With the decrease in sediment material, the rainfall threshold recovered gradually over time. Furthermore, debris flows occurred only in the subbasins that had sufficient sediment material to cause significant movement. Hence, these results confirm that the sediment material in the initiation area of debris flow is a crucial component of the rainfall threshold for debris flow. 相似文献
139.
通过反演研究2008年日本岩手—宫城内陆地震(M~6.9),说明了用1Hz全球定位系统(GPS)观测数据推导中强震(M6级)震源过程的能力。比较了此次地震采样率为1Hz的GPS观测波形,发现与强震仪记录波形有很好的一致性。由于震源区及其周围有足够多的GPS台站很好地记录到了地面运动,因而使我们仅仅采用1Hz的GPS观测数据就可进行波形反演。反演的结果与大地测量和强震仪数据联合反演的结果相当一致,如地震矩(Mw~6.9)、累积滑动量分布和破裂传播等。这种一致性证明,采样率为1Hz的GPS观测数据不仅能用来推导M6级中强震破裂过程的动态特征,而且能用来推导其持久(或静态)的滑动。 相似文献
140.