全文获取类型
收费全文 | 2120篇 |
免费 | 388篇 |
国内免费 | 532篇 |
专业分类
测绘学 | 302篇 |
大气科学 | 640篇 |
地球物理 | 308篇 |
地质学 | 1215篇 |
海洋学 | 259篇 |
天文学 | 14篇 |
综合类 | 143篇 |
自然地理 | 159篇 |
出版年
2024年 | 14篇 |
2023年 | 32篇 |
2022年 | 50篇 |
2021年 | 85篇 |
2020年 | 81篇 |
2019年 | 126篇 |
2018年 | 67篇 |
2017年 | 138篇 |
2016年 | 157篇 |
2015年 | 139篇 |
2014年 | 192篇 |
2013年 | 138篇 |
2012年 | 132篇 |
2011年 | 132篇 |
2010年 | 114篇 |
2009年 | 138篇 |
2008年 | 119篇 |
2007年 | 142篇 |
2006年 | 132篇 |
2005年 | 99篇 |
2004年 | 77篇 |
2003年 | 76篇 |
2002年 | 103篇 |
2001年 | 85篇 |
2000年 | 55篇 |
1999年 | 53篇 |
1998年 | 59篇 |
1997年 | 61篇 |
1996年 | 41篇 |
1995年 | 24篇 |
1994年 | 20篇 |
1993年 | 23篇 |
1992年 | 21篇 |
1991年 | 24篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 14篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 2篇 |
排序方式: 共有3040条查询结果,搜索用时 15 毫秒
971.
Measurement of the major and trace elements were carried out on the Lower Cretaceous limestones interbedded in the volcano-sedimentary Alisitos Formation, northwestern México to understand the source of rare earth elements (REEs) and paleo-redox conditions. The five limestone beds (from the base up, Unit 5 to Unit 9) of the Alisitos Formation show large variations in SiO2 content (0.9–27.9%). A low concentration of CaO is observed in Unit 6 and Unit 8, and high content of CaO is observed in Unit 5, Unit 7 and Unit 9. The limestones are depleted in many trace elements with respect to Post-Archaean Australian Shale (PAAS), whereas Sr shows slight enrichment when compared to PAAS. The concentrations of ΣREE are higher in Unit 6 and Unit 8 (37.4 ± 7.5; 46.6 ± 19.4; respectively) than Unit5, Unit7, and Unit 9 (9.1 ± 3.2; 11.3 ± 9.4; 4.2 ± 2.5; respectively). The limestones of the Alisitos Formation show a non-seawater-like REE + Y pattern with positive Eu anomalies relative to PAAS (0.95–2.47). Variations in ΣREE, Al2O3, Zr, Sc, REE + Y patterns, and Y/Ho ratios are influenced mainly by the amount of terrigenous materials. The variations in the Eu/Eu*, La/Sc and La/Co suggest that the terrigenous materials included in the lower four limestone beds (from Unit 5, Unit 6, Unit 7 and Unit 8) were likely contributed by intermediate to felsic rocks whereas terrigenous materials from Unit 9 were derived from mafic to intermediate source rocks. The slightly negative to slightly positive Ce anomalies in the studied limestones resulted from variations in the bottom water oxygenation. This was also corroborated by V/Cr and Ni/Co ratios suggesting that the depositional environments experienced large fluctuations in oxygenation conditions ranging from oxic to anoxic conditions during the deposition of limestones of the Alisitos Formation. 相似文献
972.
地理国情监测是全面了解国情、把握国事、制定国策的基础性工作。基于2018年度基础性地理国情监测项目,结合基础性地理国情监测的生产实际,探讨了基础性地理国情监测中的地表覆盖分类和地理国情要素成果的质量控制方法,针对生产中出现的典型质量问题提出了应对策略,为常态化基础性地理国情监测项目提供了技术支撑。 相似文献
973.
基于"三调"外业核查成果,利用FME、Python和ArcMap进行了数据筛选和ETL流程处理,形成符合基础性地理国情监测要求的外业核查成果。除了需要人工挑选照片及拍照点外,本方法实现了转换过程的自动化处理,转换过程不需要任何人工干预,避免了一些常见的人为错误,如错填、漏填信息等。 相似文献
974.
基于地理国情信息编制自然资源资产负债表,以广安市为研究区开展了编制自然资源资产负债表技术方法研究,从数量、质量和价值3个方面,运用数值统计与空间分布格局相结合的方法探究编制广安市自然资源资产负债表。研究结果表明:(1)地理国情矢量数据作为制作自然资源资产数据集的基础数据,在统计自然资源资产面积和空间分布上有极大的优势,可以直观地反映上述两项指标。(2)多光谱遥感影像数据,可用于部分质量和价值指标核算。(3)自然资源资产负债表的编制,需要多部门专业数据支持。 相似文献
975.
Observed trends in severe weather conditions based on public alert statements issued by Environment Canada are examined for Canada. Changes in extreme heat and extreme cold events represented by various humidex and wind chill indices are analyzed for 1953–2012 at 126 climatological stations. Changes in heavy rainfall events based on rainfall amounts provided by tipping bucket rainfall gauges are analyzed for 1960–2012 at 285 stations. The results show that extreme heat events, defined as days with at least one hourly humidex value above 30, have increased significantly at more than 36% of the stations, most of which are located south of 55°N; days with nighttime hourly humidex values remaining above 20 have increased significantly at more than 52% of the stations, most of which are located south of 50°N. Extreme cold events represented by days with at least one hourly wind chill value below ?30 have decreased significantly at more than 76% of the stations across the country. No consistent changes were found in heavy rainfall events. Because city residents are very vulnerable to severe weather events, detailed results on changes in extreme heat, extreme cold, and heavy rainfall events are also provided for ten urban centres. 相似文献
976.
Changes in available fresh water resources, together with changes in water use, force our society to adapt continuously to water scarcity conditions. Although several studies assess the role of long-term climate change and socioeconomic developments on global water scarcity, the impact of inter-annual climate variability is less understood and often neglected. This paper presents a global scale water scarcity assessment that accounts for both temporal changes in socioeconomic conditions and hydro-climatic variability over the period 1960–2000. We thereby visualized for the first time possible over- and underestimations that may have been made in previous water scarcity assessments due to the use long-term means in their analyses. Subsequently, we quantified the relative contribution of hydro-climatic variability and socioeconomic developments on changing water scarcity conditions. We found that hydro-climatic variability and socioeconomic changes interact and that they can strengthen or attenuate each other, both regionally and at the global scale. In general, hydro-climatic variability can be held responsible for the largest share (>79%) of the yearly changes in global water scarcity, whilst only after six to ten years, socioeconomic developments become the largest driver of change. Moreover, our results showed that the growth in the relative contribution of socioeconomic developments to changing water scarcity conditions stabilizes towards 2000 and that the impacts of hydro-climatic variability remain significantly important. The findings presented in this paper could be of use for water managers and policy makers coping with water scarcity issues since correct information both on the current situation and regarding the relative contribution of different mechanisms shaping future conditions is key to successful adaptation and risk reduction. 相似文献
977.
利用广义伊藤公式证明了混杂随机时滞微分方程(SDDE)在局部Lipschitz和广义Khasminskii条件下存在唯一解,从而涵盖了一大类非线性混杂SDDE.最后给出实例说明了理论的可行性. 相似文献
978.
为了增加塌陷区耕地资源,有效地提高土地利用率,减少环境污染,本文通过对海孜煤矿塌陷区进行实地勘查,室内测试和实验,分析塌陷区煤矸石充填复垦条件。分析结果表明:塌陷区的地形地貌条件有利于土地复垦工程的实施,较高的地下水位可以为重构土壤提供充足的水分;塌陷区塌陷深度小,且绝大部分塌陷区已达到稳沉状态,有益于煤矸石充填复垦;大量的煤矸石为充填复垦提供了充足的物质基础,煤矸石中有害元素含量低,复垦后不会对地下水和土壤造成显著的污染,且煤矸石的块度组分有利于水分的保持和上移。 相似文献
979.
980.
快速城镇化导致我国城市消防应急服务基础设施与城市发展不同步的问题日益凸显,城市消防救援覆盖率评估是提升消防服务质量与优化消防资源配置的重要手段。本文提出一种基于实时路况的城市消防救援覆盖率评估模型,通过考虑消防站管辖区域的空间限制,在2020年9月连续三周时间内利用高德地图API获取消防站到达历史火灾事件的实时出行救援时间,对南京市消防救援覆盖率的时空模式进行分析与挖掘。结果显示:① 南京城市火灾密集区域(简称火灾密集区)消防站的平均出行救援时间约10 min,非火灾密集区约16 min,均比国家规定的5 min到达时间标准明显要长,导致南京市消防站在5 min到达标准下的覆盖率仅为8.2%;② 由于南京火灾密集区消防站的平均行车救援距离仅为非火灾密集区的37%,导致火灾密集区火灾事件等待救援时间明显低于非火灾密集区,尤其火灾密集区西南部、东北部及部分消防站周边火灾事件等待救援时间相对较短,但南京全区火灾事件等待救援时间在5 min以内的比例不足7%,且等待救援时间在5~10 min之间的短距离火灾事件受早晚交通出行高峰期交通拥堵影响最大;③ 南京市消防站救援覆盖率受早晚交通出行高峰影响呈现早晚交通出行高峰期明显低于其他时段的“W”形变化模式,火灾密集区消防站在5 min到达标准下的救援覆盖率从非交通出行高峰期的11.5%降低到交通出行高峰期的8.4%,而非火灾密集区从6.1%降低到5%,火灾密集区东南部石门坎与东山交界区域和北部汉中门与迈皋桥周边区域早晚交通出行高峰时段等待救援时间超过15 min的火灾事件比非交通出行高峰时段明显增多;④ 在5 min到达标准下,南京市消防站救援覆盖率“W”形模式波动最小,10 min到达标准下的平均覆盖率为43.5%且波动最明显,15 min到达标准下的平均覆盖率达到75%。最后根据分析结果给出了南京市消防未来建设发展意见。 相似文献