全文获取类型
收费全文 | 1702篇 |
免费 | 174篇 |
国内免费 | 216篇 |
专业分类
测绘学 | 98篇 |
大气科学 | 174篇 |
地球物理 | 547篇 |
地质学 | 438篇 |
海洋学 | 295篇 |
天文学 | 149篇 |
综合类 | 62篇 |
自然地理 | 329篇 |
出版年
2024年 | 7篇 |
2023年 | 13篇 |
2022年 | 46篇 |
2021年 | 47篇 |
2020年 | 55篇 |
2019年 | 63篇 |
2018年 | 44篇 |
2017年 | 69篇 |
2016年 | 69篇 |
2015年 | 70篇 |
2014年 | 79篇 |
2013年 | 104篇 |
2012年 | 60篇 |
2011年 | 75篇 |
2010年 | 83篇 |
2009年 | 100篇 |
2008年 | 108篇 |
2007年 | 107篇 |
2006年 | 115篇 |
2005年 | 95篇 |
2004年 | 88篇 |
2003年 | 71篇 |
2002年 | 56篇 |
2001年 | 56篇 |
2000年 | 79篇 |
1999年 | 44篇 |
1998年 | 52篇 |
1997年 | 37篇 |
1996年 | 25篇 |
1995年 | 19篇 |
1994年 | 33篇 |
1993年 | 19篇 |
1992年 | 21篇 |
1991年 | 17篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1980年 | 1篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有2092条查询结果,搜索用时 93 毫秒
971.
Mummichogs, Fundulus heteroclitus, an estuarine fish with a relatively small home range found along the eastern coast of the United States are well-suited to monitoring contaminant effects, including those of polycyclic aromatic hydrocarbons (PAHs). One of the common PAHs in estuaries is pyrene. We report here on efforts to develop multiple biomarkers of pyrene exposure in this species. Adult male mummichogs were exposed in the laboratory to the weak aryl hydrocarbon receptor (AhR) agonist pyrene at 0, 30, or 50 microg/L in 7-day static renewal exposures. The RNA was extracted from livers and alterations in mRNA expression were assessed by subtractive hybridization and differential display in order to produce multiple biomarkers of pyrene exposure. Genes demonstrating differential expression were confirmed by quantitative-PCR (Q-PCR) and include cytochrome P-450 1A (CYP1A), a putative hepatocyte growth factor activator, a X-ray inducible retrotransposon, and several expressed sequenced tags (ESTs). Some of these genes represent new biomarkers of pyrene exposure and potential biomarkers of PAH exposure. Therefore, similar changes were investigated at a Superfund site in Charleston, SC. Mummichogs from a creosote contaminated site and from a reference site (North Inlet National Estuarine Research Reserve near Georgetown, SC) were trapped, RNA extracted from the livers, and Q-PCR performed. Many of the genes differentially expressed following pyrene exposure were not altered at the creosote contaminated site in comparison to the reference site. However, CYP1A and an EST were induced. CYP1A induction at Diesel Creek indicates that this population of fish does not demonstrate refractory CYP1A phenotypes observed at several sites with high levels of AhR agonists. Ultimately, we anticipate that the use of multiple biomarkers of PAH exposure will provide useful information on the potential effects of toxicants. 相似文献
972.
文章在充分分析已有地质和水文地质资料的基础之上,综合大面积水文地质调绘和多种勘探成果,对石太客运专线太行山隧道和南梁隧道所在区域的水文地质条件进行了系统分析。将隧道区域划分为娘子关泉域系统、威州泉域系统和散泉系统三大地下水含水系统,同时将太行山隧道区划分为5个水文地质单元,将南梁隧道区划分为两个水文地质单元。另外,论述了隧道开挖对地下水和地表水环境的影响,主要表现在对断层或岩脉与隧道相交地段的水位、水量产生影响,这为进一步解决特长隧道施工过程中的突水、涌水等问题提供了可靠的基础性资料,加深了对河北、山西两省交界处水文地质条件薄弱地带的了解。 相似文献
973.
974.
山东半岛水资源开发和利用中的环境效应研究 总被引:2,自引:0,他引:2
山东半岛不仅是我国经济发展和对外开放的重要地区 ,也是我国水资源短缺比较严重的地区。分析山东半岛的水资源的现状 ,研究水资源开发利用中的环境效应 ,并提出山东半岛水资源可持续利用的 7点策略。 相似文献
975.
976.
使用探索性空间分析技术,对首都圈制造业空间分布及其变化的地域与行业特征进行了研究,并建立空间计量模型重点分析了中心城市的空间溢出效应对产业空间格局变化的影响。2001-2009 年,多数制造业空间集聚程度表现出不断地加强的趋势,而且分行业的发展热点区在空间上有变化和迁移的现象,总体上工业发展表现出由中心向外围扩散的趋势,临近中心城市的外围郊县区成为工业集聚与发展的热点地区。中心城市发展对区域内的空间溢出效应是推动首都圈产业空间分布变化的重要因素。北京、天津两个中心城市对外围地区产业发展具有一定的空间溢出效应,但这种影响也因产业特征与中心城市产业结构及发展阶段的差异而有所不同。北京整体的发展环境相对有利于对资本、技术、人才需求较高的科技、资本密集型产业,北京本身这类产业的发展会袭夺外围地区的生产要素,引致负面的回流效应;而天津还处于工业化后期阶段,无论是经济总量增长还是分行业的发展对外围地区的空间作用并不具有明显的一致性。 相似文献
977.
震电耦合产生的伴随震电场(CS)和震电界面场(IR)具有不同的强度,且能够反映地下地质体界面信息的震电界面响应一般较弱,而震电界面响应场可能在薄互层地质背景下能够出现较强的振幅值.目前还未有针对薄互层震电波场响应特征分析,且纵波引起的震电界面响应场是否对薄互层敏感有待探究.本文基于体积电荷密度的震电耦合理论数值模拟了平行薄互层(VTI)和倾斜薄互层(HTI)地质背景下的震电波场响应特征.结果表明,伴随纵波传播而产生的CS震电场几乎不受平行薄互层和楔形薄互层的影响.纵波在界面处诱导产生的IR震电场对垂直薄互层不敏感,但水平薄互层和楔形薄互层模型的厚度和薄互层的层数均能影响IR震电信号,薄互层单层厚度减小和薄互层层数增加会导致IR信号的增强.结果有力证明了纵波引起的震电界面响应场在薄互层地质体处会产生振幅异常,而水平和垂直薄互层的震电波场差异也有将助于探究层理方向有显著差异的页岩等储层中产生的震电信号.
相似文献978.
Livestock behaviour in the Pyrenees includes free grazing and a long resting period that provokes the accumulation of dung and urine in some places,so-called camping areas. The aims of this study were (i) to analyze any change in floral composition,and in nutritional and chemical contents of plants in a livestock camping area; and (ii) to relate the floral composition with soil chemical properties. In a linear transect,five sampling zones were established,from the centre of the camping area to the surrounding Nardus stricta-dominant pasture. The above ground plant biomass and the topsoil were sampled in each zone with 6 replicates per zone. Plant species were classified and weighed to calculate above ground biomass,nutritional and chemical contents,and Shannon diversity and evenness indices. Additionally,soils were sampled in two periods,at the beginning and at the end of grazing period. Soil available nutrients (nitrate,ammonium,phosphorus,potassium,calcium and magnesium),total nitrogen,organic carbon and pH were measured.
Plant chemical contents (protein,lignin and others) were significantly related to the proportions of grasses,legumes and other plants; so,the protein content is positively correlated with legumes plant biomass while lignin content is negatively correlated with grasses. Both plant and soil nutrients increased linearly towards the centre of the camping area. However,the relationship among plant species richness,diversity and evenness relative to its position along the studied transect was bell-shaped. From the outskirts to the centre of the camping area,plants with low nutrient demand were progressively replaced by those with medium and high nutrients demand and by pioneers.
Nardus stricta-dominant pasture has low plant diversity and plant nutrient content as well as a poor soil nutrient availability. The presence of the camping area introduced patches with more soil nutrients and new species in the large spatial scale. However at a small spatial scale,the strong soil nutrien 相似文献
Plant chemical contents (protein,lignin and others) were significantly related to the proportions of grasses,legumes and other plants; so,the protein content is positively correlated with legumes plant biomass while lignin content is negatively correlated with grasses. Both plant and soil nutrients increased linearly towards the centre of the camping area. However,the relationship among plant species richness,diversity and evenness relative to its position along the studied transect was bell-shaped. From the outskirts to the centre of the camping area,plants with low nutrient demand were progressively replaced by those with medium and high nutrients demand and by pioneers.
Nardus stricta-dominant pasture has low plant diversity and plant nutrient content as well as a poor soil nutrient availability. The presence of the camping area introduced patches with more soil nutrients and new species in the large spatial scale. However at a small spatial scale,the strong soil nutrien 相似文献
979.
980.
A simplified analytical model including the coupled effects of the wheel–rail–soil system and geometric irregularities of the track is proposed for evaluation of the moving train load. The wheel–rail–soil system is simulated as a series of moving point loads on an Euler–Bernoulli beam resting on a visco-elastic half-space, and the wave-number transform is adopted to derive the 2.5D finite element formulation. The numerical model is validated by published data in the literature. Numerical predictions of ground vibrations by using the proposed method are conducted at a site on the Qin-Shen Line in China. 相似文献