全文获取类型
收费全文 | 169篇 |
免费 | 19篇 |
国内免费 | 19篇 |
专业分类
测绘学 | 6篇 |
大气科学 | 13篇 |
地球物理 | 40篇 |
地质学 | 60篇 |
海洋学 | 33篇 |
天文学 | 18篇 |
综合类 | 6篇 |
自然地理 | 31篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 4篇 |
2020年 | 8篇 |
2019年 | 7篇 |
2018年 | 5篇 |
2017年 | 9篇 |
2016年 | 7篇 |
2015年 | 18篇 |
2014年 | 7篇 |
2013年 | 9篇 |
2012年 | 9篇 |
2011年 | 7篇 |
2010年 | 3篇 |
2009年 | 14篇 |
2008年 | 5篇 |
2007年 | 13篇 |
2006年 | 9篇 |
2005年 | 5篇 |
2004年 | 9篇 |
2003年 | 5篇 |
2002年 | 6篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 7篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
排序方式: 共有207条查询结果,搜索用时 12 毫秒
81.
Hospital service areas (HSAs) are increasingly adopted as a basic analysis unit for health care studies. The popular Dartmouth HSAs were produced more than two decades ago, and the process was far from automated. This research uses a Huff-based model automated in a geographic information systems (GIS) environment to delineate HSAs. Based on the Florida State Inpatient Database (SID) in 2011, a best-fitting distance decay function is derived from the actual travel patterns of hospitalization and then fed into the Huff model to strengthen the model's theoretical foundation in individual spatial behavior. The HSAs derived from the Huff-based model are then compared to the traditional flow-based HSAs defined by the Dartmouth method and assessed in terms of self-containment and heterogeneity of internal socioeconomic structure and urbanicity. The Huff-based model requires fewer data and is easy to implement as an automated toolkit and thus has great potential for replication in other regions to define large-scale and consistent HSAs. 相似文献
82.
《International Geology Review》2012,54(7):905-914
Twenty-first century advances in both the analytical procedures and instrumentation used in geochronology promise age accuracy better than ±1‰, but realizing this potential requires knowledge of decay constants (λ) that exceed this level. Given the paucity of improved recent measurements of λ, the community has experimented with hybrid methodologies utilizing data largely generated during the 1970s. In this article, we perform a systematic review of laboratory decay constant determinations relevant to geochronology (i.e. 87Rb, 147Sm, 176Lu, 230Th, 232Th, 235U, and 238U), focusing on methodological consistency. For radioisotopes for which multiple studies are available, results are combined through a random effects model to yield the best available values and associated uncertainties. Unfortunately, despite its vital role in modern geochronology, only one experimental determination of 238U decay met our criteria for consideration, significantly limiting the ability to assess its reliability. Thus, utilizing λ238 as an anchor for establishing other decay constants (e.g. 40K, 176Lu, and spontaneous 238U fission) places an unverified result at the core of geochronology. For geochronology to attain its greatest potential, more and better laboratory determinations of decay constants are required, along with a community methodology that permits us to continuously take advantage of new data. 相似文献
83.
84.
85.
在人为选定的模拟无限边界的有限位置处设置无界单元。在该单元上配置带有衰减特征的线性插值函数,进而可建立按有限元法或不规则网格有限差分法解算地下水流问题所需要的单元刚度矩阵。该单元刚度矩阵可很容易地进入总刚度矩阵。无界单元的应用较好地解决了地下水流数值模拟中无限边界不易确定的问题。计算实例表明本方法具有较高的精度。 相似文献
86.
雷娟 《南京气象学院学报》2011,(2):172-183
利用黎曼几何方法和乘子技巧,讨论了带变系数和非线性边界扩散反馈项的紧耦合波动方程组的镇定,得到了解能量的衰减估计. 相似文献
88.
《The Professional geographer》2013,65(3):296-297
Edward Jarvis in 1850 first demonstrated that admission rates to mental hospitals decrease with increasing residential distance, a relationship known today as “Jarvis's Law.” His original data are presented, mapped, and examined by regression analysis to better understand spatial and temporal patterns of mid-19th century mental hospital utilization. Distance substantially affected admission rates to a radius of about 60 miles from the institution in Massachusetts; and there was strong distance decay in the other states examined. For all twelve states, there was a positive association between age of the hospitals and admission rates, which also decreased with increasing residential distance. 相似文献
89.
城际集装箱交流枢纽的识别及其物流特征——以中国铁路运输为例 总被引:5,自引:3,他引:2
集装箱流反映了城市间的经济联系,枢纽城市在区域经济网络中具有重要地位,主导着物流网络的运行。依据图论原理,构筑了枢纽城市的识别模型,融合了城市交流的直接联系和间接联系,对集装箱枢纽进行识别,发现中国形成了21 个集装箱枢纽,以沿海港口城市为主,枢纽牵引的集装箱交流体系显示“区域-门户”空间结构明显。然后分析了枢纽集装箱流的距离衰减规律,采用Pareto 和Newling 模型进行拟合,确定了主要衰减参数,Pareto 模型的衰减系数b 显示其数值越高,集装箱牵系的城市影响越限于近距离,Newling 模型的递增系数b1为正值说明枢纽附近存在影响塌陷区,最高影响不邻近枢纽。同时依据有向图的矢量属性揭示了枢纽集装箱流的集散规律,发现枢纽城市的辐射力和吸引力呈明显的非均衡性,邻近地区形成影响塌陷区,一定距离范围内形成主影响区,各枢纽的本底决定了其主影响区明显不同,虽然各城市的辐射峰值和吸引峰值不同,但其集聚和扩散通道往往一致。 相似文献
90.
渗透率是油气藏储层评价、产能计算及制定合理的开发方案所需关键参数之一。针对常规稳态法测试致密储层岩石渗透率效率低下、试验过程易受环境温度影响等缺点,使用脉冲衰减渗透率测试仪对39块致密岩石稳态法克氏渗透率与非稳态脉冲衰减渗透率进行系统的对比研究,分析岩样渗透率、试验操作方法及有效应力组合方式等因素对测试结果的影响。研究结果表明,相同净围压条件下致密岩石脉冲渗透率小于克氏渗透率,脉冲渗透率平均约为克氏渗透率的47.26%,岩样渗透率越小,则两种渗透率之间的差别越大。误差分析表明,脉冲渗透率试验初期岩样短时间承受的9 MPa高围压和其高围压、高孔隙压力组合的有效应力施加方式对脉冲渗透率测试结果具有一定程度的影响,但仍无法完全解释两种渗透率的总体误差。数学拟合表明,取自同一区块的露头砂岩岩样脉冲与克氏渗透率相对误差与脉冲渗透率具有较好地对数函数关系,由数学推导获得该露头砂岩储层岩石脉冲渗透率与克氏渗透率的转换关系。 相似文献