全文获取类型
收费全文 | 466篇 |
免费 | 86篇 |
国内免费 | 128篇 |
专业分类
测绘学 | 69篇 |
大气科学 | 83篇 |
地球物理 | 57篇 |
地质学 | 245篇 |
海洋学 | 82篇 |
天文学 | 4篇 |
综合类 | 77篇 |
自然地理 | 63篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 27篇 |
2021年 | 36篇 |
2020年 | 17篇 |
2019年 | 34篇 |
2018年 | 34篇 |
2017年 | 32篇 |
2016年 | 36篇 |
2015年 | 28篇 |
2014年 | 43篇 |
2013年 | 56篇 |
2012年 | 19篇 |
2011年 | 20篇 |
2010年 | 23篇 |
2009年 | 30篇 |
2008年 | 16篇 |
2007年 | 25篇 |
2006年 | 15篇 |
2005年 | 24篇 |
2004年 | 12篇 |
2003年 | 12篇 |
2002年 | 12篇 |
2001年 | 9篇 |
2000年 | 17篇 |
1999年 | 27篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 9篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1980年 | 2篇 |
排序方式: 共有680条查询结果,搜索用时 640 毫秒
291.
本文研究了南极布兰斯菲尔德海峡表层沉积物中C5-C15烃类化合物的分布特征,其中包括正构烷烃、异构烷烃、环烷烃和芳烃化合物。所有试样中均含有大量的芳烃物苯、甲苯、二甲苯和三甲苯,其中甲苯占绝对优势,为总烃量的75-81%。对一些有机化合物进行了多元统计分析,如Q和R模式系统聚类分析和主因子分析。Q模式的最长距离法反映了901航次和首次南大洋样的差异。901航次样经旋转后三个因子的等值线趋向上,各站位间有着明显的差别,其中G-27站均趋向极端值 相似文献
292.
Using a 3D lightning radiation source locating system (LLS), three pairs of associated lightning discharges (two or more adjacent lightning discharges following an arbitrary rule that their space-gap was less than 10 km and their time-gap was less than 800 ms) were observed, and the interaction between associated lightning discharges was analyzed. All these three pairs of associated lightning discharges were found to involve three or more charge regions (the ground was considered as a special charge region). Moreover, at least one charge region involved two lightning discharges per pair of associated lightning discharges. Identified from electric field changes, the subsequent lightning discharges were suppressed by the prior lightning discharges. However, it is possible that the prior lightning discharge provided a remaining discharge channel to facilitate the subsequent lightning discharge. The third case provided evidence of this possibility. Together, the results suggested that, if the charges in the main negative charge region can be consumed using artificial lightning above the main negative charge regions, lightning accidents on the ground could be greatly reduced, on the condition that the height of the main negative charge region and the charge intensity of the lower positive charge region are suitable. 相似文献
293.
利用河源及其周边地区共24个国家气象站1970-2018年气象资料和地理信息数据,通过分析河源猕猴桃主产区气候特征及影响猕猴桃生长的关键因素,选取气象、地形和土壤三大类共12个评价因子指标,在GIS技术的支持下,采用层次分析法和加权指数求和法,构建河源地区猕猴桃种植生态适宜性评价指标体系,将种植区域划分为不适宜、次适宜、适宜和最适宜4个等级,并绘制出猕猴桃种植综合区划图。分析结果表明:连平、和平、龙川北部的零散区域为猕猴桃种植最适宜区,约占总区划面积的3%;中部偏西和紫金南部的零散区域为不适宜区,约占总区划面积的2%,主要受土壤、降水和气温等因素的限制;其他区域则处于适宜和次适宜种植区。 相似文献
294.
太平洋洋底虽然有大规模的洋底高原和大量大大小小的海山,但西北太平洋的Shatsky海隆不管在构造位置上,还是在理解洋底高原成因所特有的重要证据上,都是独一无二的,对研究太平洋各隆起形成的机制有重要参考价值.目前,对Shatsky海隆的形成有3种主要假说:(1)地幔柱头假说;(2)洋中脊假说;(3)陨星撞击假说.这些假说... 相似文献
295.
296.
Sui Linhui Holman Gordon D. Dennis Brian R. Krucker Säm Schwartz Richard A. Tolbert Kim 《Solar physics》2002,210(1-2):245-259
We have analyzed a C7.5 limb flare observed by RHESSI on 20 February 2002. The RHESSI images appear to show two footpoints
and a loop-top source. Our goal was to determine if the data are consistent with a simple steady-state model in which high-energy
electrons are continuously injected at the top of a semicircular flare loop. A comparison of the RHESSI images with simulated
images from the model has made it possible for us to identify spurious sources and fluxes in the RHESSI images. We find that
the RHESSI results are in many aspects consistent with the model if a thermal source is included between the loop footpoints,
but there is a problem with the spectral index of the loop-top source. The thermal source between the footpoints is likely
to be a low-lying loop interacting with the northern footpoint of a higher loop containing the loop-top source. 相似文献
297.
俄罗斯贝加尔湖区伸展构造及与中国东部伸展构造对比 总被引:4,自引:0,他引:4
在晚白垩世-始新世夷平面基础上,由于断裂作用形成了贝加尔裂谷系。断裂作用最大幅度超过10 km。在裂谷系中心部位发育的断层长度最大、最深、最早,并以准对称形式向四周扩展。贝加尔裂谷系是在地幔隆起和印度-欧亚大陆碰撞双重作用下形成。贝加尔裂谷系与中国东部新生代断陷盆地和汾渭裂谷系同时形成,并有密切的成因联系。它们的形成不仅受太平洋板块的俯冲和印度-欧亚大陆碰撞的制约和影响,而且位于中国西南部的地幔流发散中心,呈扇状向太平洋区流动,可能是它们在更深层次上的共同场源基础。 相似文献
298.
鄂尔多斯市糜黍产量风险评估 总被引:1,自引:0,他引:1
糜黍生育期短,具有其他作物少有的耐旱、耐瘠薄、耐风沙、耐盐碱等特性,也是很好晚田备荒作物。以历年减产率指标、减产率变异系数、减产率概率指标和综合风险指数等为评估标准,结合小网格资源推算的方法,分析了鄂尔多斯市糜黍产量风险规律。同时对影响糜黍产量波动的风险成因进行综合评述,结果为气象灾害的频繁发生是引起糜黍产量波动的主要因子之一。 相似文献
299.
300.
Determining a wellhead protection area (WHPA) is a basic and important step in protecting groundwater from contamination. Based on availability of data and complexity of hydrogeological conditions, different delineation methods can be adopted. This study’s objective was to examine the relation and difference of the results calculated using six WHPA delineation methods, which ranged from simplified shapes to stochastic models with superior sampling means. All the methods were applied to a well field in Beijing, China. The WHPA probability distribution from a stochastic simulation was used as reference for comparison with other methods. WHPAs calculated using simplified and analytical approaches turned out to be smaller than reference WHPA. Those calculated using the numerical model provided better results but still cannot include a considerable area of reference WHPA. The stochastic approach based on efficient orthogonal Latin hypercube sampling provided an estimation of the probability distribution of WHPA delineation with different degrees of uncertainty, which played an important role in the WHPA delineation of the well field in Beijing. To attain a realistic WHPA delineation in a complex aquifer system, various delineation approaches can be adopted and all results that validate each other must be considered. 相似文献