全文获取类型
收费全文 | 1137篇 |
免费 | 164篇 |
国内免费 | 190篇 |
专业分类
测绘学 | 204篇 |
大气科学 | 187篇 |
地球物理 | 268篇 |
地质学 | 383篇 |
海洋学 | 100篇 |
天文学 | 6篇 |
综合类 | 103篇 |
自然地理 | 240篇 |
出版年
2024年 | 1篇 |
2023年 | 22篇 |
2022年 | 43篇 |
2021年 | 57篇 |
2020年 | 69篇 |
2019年 | 74篇 |
2018年 | 49篇 |
2017年 | 74篇 |
2016年 | 60篇 |
2015年 | 65篇 |
2014年 | 72篇 |
2013年 | 78篇 |
2012年 | 75篇 |
2011年 | 74篇 |
2010年 | 74篇 |
2009年 | 63篇 |
2008年 | 59篇 |
2007年 | 71篇 |
2006年 | 63篇 |
2005年 | 48篇 |
2004年 | 39篇 |
2003年 | 31篇 |
2002年 | 22篇 |
2001年 | 26篇 |
2000年 | 23篇 |
1999年 | 28篇 |
1998年 | 17篇 |
1997年 | 8篇 |
1996年 | 16篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 16篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有1491条查询结果,搜索用时 321 毫秒
71.
Tiexin Gu Zhong Zhang Chunshu Wang Weidong Yan 《Geostandards and Geoanalytical Research》2001,25(1):153-158
Two types of gold deposit with both good homogeneity and a high-grade of gold were selected to prepare four gold ore reference materials (GAu-19,GAu-20,GAu-21 and GAu-22) by the Institute of Geophysical and Geochemical Exploration (IGGE),China.Seven laboratories participated in the certification programme. Volumetric methods for GAu19-21 and atomic absorption spectrometry for GAu-22 were used for the homogeneity testing,the coefficient of variation being found to be less than 3%.After sample digestion and preconcentration,the samples were analysed by flame atomic absorption spectrometry (AAS),colorimetry,neutron activation analysis (NAA)and volumetric analysis. The certified values for the gold concentration in GAu19-22 are 18.3 μg g -1, 32.3 μg g -1, 53.0 μg g -1 and 5.72 μg g -1,respectively. 相似文献
72.
73.
J. H. Barrett 《Stochastic Hydrology and Hydraulics》1992,6(3):151-165
An extreme value analysis of the flow of Burbage Brook is carried out by modelling peaks over a high threshold. The aims are to illustrate recently developed statistical techniques and to report on interesting features of the flow of the brook over a 58-year period. Peak flows are found to show marked seasonal variation and a downward trend. Then-year return level is estimated for various values ofn, and the reliability of the estimates is assessed. 相似文献
74.
本文主要研究了a、b值的变化对震级特征图的影响。当a、b值的变化幅度不太大时,仍然可从震级特征图上识别出最小均匀震级等震级阈值;当a、b值的变化达到相当的幅度后,震级阈值点变模糊了。 相似文献
75.
Statistic and typical-year composition methods are used to study the northwest Pacific typhoon activities in relation with the EI Niño and La Niña events. The result indicates that the typhoon tends to be inactive in the EI Niño years and active in the La Niña years and it is also dependent on the onset and ending time and intensity of the events and areas of genesis of typhoons. With statistic features of the frequency of typhoon activity in the EI Niño and La Niña years and the time-lag correlation between the frequency and sea surface temperature(SST), useful information is provided for the prediction of typhoon occurrence. In addition, the singular values disassemble(SVD)method is applied to study the correlation between the geopotential field and SST field. The result shows that the air-sea coupling in the EI Niño years is unfavorable for the typhoon to develop. Which take place with a smaller number. Opposite situations are found with the La Niña years. 相似文献
76.
湘西南石英脉型金矿矿物流体包裹体pH值和Eh值的估算 总被引:2,自引:0,他引:2
以湘西南石英脉型金矿床为例,研究p H、Eh 估算值的结果表明利用围岩蚀变反应方程以及采用碱金属离子、卤素元素离子估算pH 值是不完善的。本文根据热力学原则( 相平衡、电价中和) 提出了矿物流体包裹体成分体系计算pH 估算值的方法,并且讨论了测试包裹体时气体成分外逸是影响pH 和Eh 估算值的主要因素之一。 相似文献
77.
78.
阜康典型荒漠C3植物稳定碳同位素值的环境分析 总被引:26,自引:3,他引:23
通过对阜康典型荒漠C3植物稳定碳同位素值的分析。叶片炭同位素值在-23‰和-29‰之间变化,其中主要在-27‰附近波动。这与前人报道的世界上其他地区荒漠植物碳同位素值的变化非常一致。降水可以改变叶片碳同位素值的大小,降水越多,叶片碳同位素值越负,它们的变化幅度有物种的依赖性。叶片碳同位素值也受植物生长形式或期望寿命的影响,木本植物或寿命长的植物叶片碳同位素值要高。分析表明,利用该区土壤或陆相沉积中有机质碳同位素值可以判断气候的干湿变化:土壤或陆相沉积中有机质碳同位素值越高,气候则越干燥。 相似文献
79.
Gully erosion of cultural sites in Grand Canyon National Park is an urgent management problem that has intensified in recent decades, potentially related to the effects of Glen Canyon Dam. We studied 25 gullies at nine sites in Grand Canyon over the 2002 monsoon–erosion season to better understand the geomorphology of the gully erosion and the effectiveness of erosion‐control structures (ECS) installed by the park under the direction of the Zuni Conservation Program. Field results indicate that Hortonian overland flow leads to concentrated flow in gullies and erosion focused at knickpoints along channels as well as at gully heads. Though groundcover type, soil shear strength and permeability vary systemat‐ically across catchments, gradient and, to a lesser degree, contributing drainage area seem to be the first‐order controls on gully extent, location of new knickpoints, and ECS damage. The installed ECS do reduce erosion relative to reaches without them and initial data suggest woody checkdams are preferable to rock linings, but maintenance is essential because damaged structures can exacerbate erosion. Topographic data from intensive field surveys and detailed photogrammetry provide slope–contributing area data for gully heads that have a trend consistent with previous empirical and theoretical formulations from a variety of landscapes. The same scaling holds below gully heads for knickpoint and ECS topographic data, with threshold coefficients the lowest for gully heads, slightly higher for knickpoints, and notably higher for damaged ECS. These topographic thresholds were used with 10‐cm digital elevation models to create simple predictive models for gully extent and structure damage. The model predictions accounted for the observed gullies but there are also many false‐positives. Purely topographical models are probably inadequate at this scale and application, but models that also parameterize the variable soil properties across sites would be useful for predicting erosion problems and ECS failure. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
80.
长三角经济高速发展地区土壤pH时空变化及其影响因素 总被引:13,自引:0,他引:13
通过调查和分析长三角地区张家港市2004年和第二次土壤普查时(1980)的土壤pH,探讨了该市近20年来基于经济高速发展影响下的土壤pH变化及影响因素。结果表明,自第二次土壤普查以来,该市土壤pH变化明显。南部人为土地区绝大部分土壤pH值都下降了一个单位,平均值由7.39降至6.33;北部雏形土区,两个时期的土壤pH值分别为7.92和7.98。土壤pH的降低可能同该地区长期施用化学肥料、酸雨及工业酸性“三废”排放的增加有关。此外,土地利用和田间管理也对土壤pH变化起着较为重要的作用,而土壤地球化学性质差异则是导致南北地区土壤pH变化不同的内在因素。 相似文献