全文获取类型
收费全文 | 3052篇 |
免费 | 410篇 |
国内免费 | 645篇 |
专业分类
测绘学 | 549篇 |
大气科学 | 240篇 |
地球物理 | 699篇 |
地质学 | 1237篇 |
海洋学 | 614篇 |
天文学 | 94篇 |
综合类 | 273篇 |
自然地理 | 401篇 |
出版年
2024年 | 8篇 |
2023年 | 34篇 |
2022年 | 55篇 |
2021年 | 61篇 |
2020年 | 95篇 |
2019年 | 115篇 |
2018年 | 101篇 |
2017年 | 140篇 |
2016年 | 130篇 |
2015年 | 134篇 |
2014年 | 176篇 |
2013年 | 202篇 |
2012年 | 202篇 |
2011年 | 219篇 |
2010年 | 171篇 |
2009年 | 196篇 |
2008年 | 193篇 |
2007年 | 213篇 |
2006年 | 224篇 |
2005年 | 175篇 |
2004年 | 163篇 |
2003年 | 150篇 |
2002年 | 141篇 |
2001年 | 118篇 |
2000年 | 117篇 |
1999年 | 89篇 |
1998年 | 59篇 |
1997年 | 60篇 |
1996年 | 57篇 |
1995年 | 43篇 |
1994年 | 43篇 |
1993年 | 36篇 |
1992年 | 19篇 |
1991年 | 21篇 |
1990年 | 19篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1976年 | 3篇 |
1905年 | 3篇 |
1900年 | 3篇 |
1897年 | 7篇 |
1877年 | 7篇 |
1875年 | 3篇 |
排序方式: 共有4107条查询结果,搜索用时 15 毫秒
61.
介绍了一种新型齿形的复合片钻头,在硬致密泥岩中钻进,与常规复合片钻头相比,具有很高的钻进效率和高的钻头寿命,解决了泥岩钻进效率低的难题。 相似文献
62.
63.
ABSTRACT. This study examines altitudinal residential segregation by race in 146 cities in the U.S. South. It begins by embedding the topic in recent theorizations of the social construction of nature, the geography of race, and environmental justice. Second, it focuses on how housing markets, particularly in the South, tend to segregate minorities in low‐lying, flood‐prone, and amenity‐poor segments of urban areas. It tests empirically the hypothesis that blacks are disproportionately concentrated in lower‐altitude areas using gis to correlate race and elevation by digital elevation‐model block group within each city in 1990 and 2000. The statistical results confirm the suspected trend. A map of coefficients indicates strong positive associations in cities in the interior South‐where the hypothesis is confirmed‐and an inverse relationship near the coast, where whites dominate higher‐valued coastal properties. Selected city case studies demonstrate these relationships connecting the broad dynamics of racial segregation to the particularities of individual places. 相似文献
64.
65.
随着空间科学技术和计算机技术的发展,以“3S”技术为代表的测绘高新技术已普遍应用于测绘生产中。作为培养测绘实用型人才的测绘职业学校,测绘专业如何重新划分、测绘教育与教学内容如何改革,是测绘教育界普遍关注的问题,本文从教学的角度对有关问题进行了探讨。 相似文献
66.
The electrical aerosol spectrometer (EAS) of the parallel measuring principle at Tartu University is an efficient instrument for rapid measurement of the unstable size spectrum of aerosol particles. The measuring range from 10 nm to 10 μm is achieved by simultaneously using a pair of differential mobility analyzers with two different particle chargers. The particle spectrum is calculated and measurement errors are estimated in real time by using a least-squares method. Experimental calibration ensures reliability of measurement. The instrument is well suited for continuous monitoring of atmospheric aerosol. 相似文献
67.
The paper presents a numerical investigation aimed at evaluating the improvements achievable through devices for passive seismic protection of buildings based on the use of shape memory alloys (SMA) in place of conventional steel or rubber devices. To get some generality in the results, different resisting reinforced concrete plane frames were analysed, either protected or not. ‘New’ and ‘existing’ buildings were considered depending on whether seismic provisions are adopted in the building design or not. Base isolation and energy dissipation were equally addressed for both conventional and innovative SMA‐based devices. Fragility analyses were performed using specific damage measures to account for comparisons among different damage types; the results were then used to estimate quantitatively the effectiveness of the various protection systems. More specifically, the assessment involved a direct comparison of the damage reduction provided by each protection system with respect to the severe degradation experienced by the corresponding non‐protected frame. Structural damage, non‐structural damage and damage to contents were used on purpose and included in a subsequent phase of cost analysis to evaluate the expected gains also in terms of economic benefits and life loss prevention. The results indicate that base isolation, when applicable, provides higher degrees of safety than energy dissipation does; moreover, the use of SMA‐based devices generally brings about better performances, also in consideration of the reduced functional and maintenance requirements. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
68.
Celestite formation, bacterial sulphate reduction and carbonate cementation of Eocene reefs and basinal sediments (Igualada, NE Spain) 总被引:5,自引:0,他引:5
Conxita Taberner James D. Marshall† James P. Hendry‡ Catherine Pierre§ & M. F. Thirlwall¶ 《Sedimentology》2002,49(1):171-190
Petrographic and geochemical studies of an Upper Eocene reef and associated basinal sediments from the mixed carbonate–siliciclastic fill of the south‐eastern Pyrenean foreland basin near Igualada (NE Spain) provide new insights into the evolution of subsurface hydrology during the restriction of a marine basin. The reef deposits are located on delta‐lobe sandstones and prodelta marls, which are overlain by hypersaline carbonates and Upper Eocene evaporites. Authigenic celestite (SrSO4) is an important component in the observed diagenetic sequences. Celestite is a significant palaeohydrological indicator because its low solubility constrains transportation of Sr2+ and SO42? in the same diagenetic fluid. Stable isotopic analyses of carbonates in the reef indicate that meteoric recharge was responsible for aragonite stabilization and calcite cementation. Sulphur and oxygen isotope geochemistry of the celestite demonstrates that it formed from residual sulphate after bacterial sulphate reduction, but also requires that there was a prior episode of sulphate recycling. Meteoric water reaching the reef and basinal areas was most probably charged with SO42? from the dissolution of younger Upper Eocene marine evaporites. This sulphate, combined with organic matter present in the sediments, fuelled bacterial sulphate reduction in the meteoric palaeoaquifer. Strontium for celestite precipitation was partly derived in situ from dissolution of aragonite corals in the reef and basinal counterparts. However, 87Sr/86Sr data also suggest that Sr2+ was partly derived from dissolution of overlying evaporites. Mixing of these two fluids promoted celestite formation. The carbonate stable isotopic data suggest that the local meteoric water was enriched in 18O compared with that responsible for stabilization of other reefs along the basin margin. Furthermore, meteoric recharge at Igualada post‐dated evaporite deposition in the basin, whereas other parts of the same reef complex were stabilized before evaporite formation. This discrepancy resulted from the spatial distribution of continental siliciclastic units that acted as groundwater conduits. 相似文献
69.
Abstract: Carbonate rocks of Cambrian (18 samples) and lower-middle Ordovician (11 samples) ages from South Korea were analyzed for sulfur contents of structurally substituted sulfate (SSS) and sulfides and their δ34 S values. The δ34 S values of SSS ranging from +25.9 to +45.2 permil, are averaged as +33.6 and +33.5 permil for the Cambrian and Ordovician rocks, respectively, which indicate high δ34 S values of the Cambro-Ordovician seawater. The SSS contents in the carbonate rocks are low being 2.9 to 17.3 ppm S (averaged as 7.0 ppm S). Sulfide sulfur, on the contrary, is much abundant containing 3 to 1,880 ppm S and the δ34 S values range widely between –17.6 and +31.1 permil. Sulfide sulfur of the studied rocks excluding impure carbonates has an average content of 187 ppm S and δ34 S value of +12.8 permil (n=24). The estimated δ34 S (sulfate–sulfide) values, which range from 13.8 to 25.4 permil in general with a few exceptions from 36.5 up to 52.3 permil for some impure carbonates, may provide evidence for the persistent oceanic anoxia with its temporary recovery during the Cambro-Ordovician time.
The SSS and sulfide sulfurs have often higher δ34 S values than the Mesozoic-Cenozoic ore sulfur (Ishihara et al., 2000). Since carbonate rocks are very reactive with circulating hydrothermal ore solution, high δ34 S values of the Korean ore deposits might be caused to some extent by 34 S enrichment from the host carbonates, resulting in the low SSS contents observed. 相似文献
The SSS and sulfide sulfurs have often higher δ
70.