全文获取类型
收费全文 | 381篇 |
免费 | 41篇 |
国内免费 | 79篇 |
专业分类
测绘学 | 65篇 |
大气科学 | 44篇 |
地球物理 | 97篇 |
地质学 | 187篇 |
海洋学 | 46篇 |
天文学 | 8篇 |
综合类 | 31篇 |
自然地理 | 23篇 |
出版年
2024年 | 3篇 |
2023年 | 3篇 |
2022年 | 9篇 |
2021年 | 12篇 |
2020年 | 5篇 |
2019年 | 18篇 |
2018年 | 14篇 |
2017年 | 18篇 |
2016年 | 23篇 |
2015年 | 17篇 |
2014年 | 23篇 |
2013年 | 32篇 |
2012年 | 23篇 |
2011年 | 20篇 |
2010年 | 13篇 |
2009年 | 17篇 |
2008年 | 24篇 |
2007年 | 21篇 |
2006年 | 36篇 |
2005年 | 19篇 |
2004年 | 20篇 |
2003年 | 16篇 |
2002年 | 13篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 17篇 |
1998年 | 13篇 |
1997年 | 10篇 |
1996年 | 4篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
排序方式: 共有501条查询结果,搜索用时 15 毫秒
71.
Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks. 相似文献
72.
土壤重金属连续提取方法的优化 总被引:2,自引:0,他引:2
重金属在污染土壤中的形态分布决定着重金属的迁移性和危害的程度。土壤重金属形态分析应用最多的是Tessier和BCR连续提取法。Tessier和BCR连续提取法没有考虑土壤样品的特征。美国环保署危险废物浸出毒性鉴别标准法虽然考虑土壤pH值,但没有划分形态。采集不同pH土样,结合Tessier、BCR和毒性浸出鉴别方法的特征,优化出针对不同pH值土壤的连续提取方法,将土壤中重金属划分为活性态、次生碳酸盐结合态、次稳定态和稳定态。用优化的连续提取方法对土样进行连续提取,重金属回收率为85%~115%。优化连续提取方法的结果与Tessier和BCR结果对比显示优化连续提取方法克服了Tessier连续提取法对非石灰质土壤提取过量BCR连续提取法对石灰质土壤提取不足的缺点。优化后的连续提取方法数据稳定可靠,可作为重金属形态分析方法使用。 相似文献
73.
74.
Performance based design becomes an effective method for estimating seismic demands of buildings. In asymmetric plan tall building the effects of higher modes and torsion are crucial. The consecutive modal pushover (CMP) procedure is one of the procedures that consider these effects. Also in previous studies the influence of soil-structure interaction (SSI) in pushover analysis is ignored. In this paper the CMP procedure is modified for one-way asymmetric plan mid and high-rise buildings considering SSI. The extended CMP (ECMP) procedure is proposed in order to overcome some limitations of the CMP procedure. In this regard, 10, 15 and 20 story buildings with asymmetric plan are studied considering SSI assuming three different soil conditions. Using nonlinear response history analysis under a set of bidirectional ground motion; the exact responses of these buildings are calculated. Then the ECMP procedure is evaluated by comparing the results of this procedure with nonlinear time history results as an exact solution as well as the modal pushover analysis procedure and FEMA 356 load patterns. The results demonstrate the accuracy of the ECMP procedure. 相似文献
75.
A baseline might be used as a point of reference to monitor change from some specific data without concern for whether the
baseline determination is natural or has been changed by human activity. We selected 326 sediment samples from Dexing area,
South China, and analyzed for 17 chemical elements. The geochemical baseline was predicted with the method of the normalization
procedure combined with the relative cumulative frequency curve. The results indicate Al was the best reference element for
the normalization procedure among four potential reference elements (Al, Fe, Ti, and Mn). The baseline value range obtained
from the normalization procedure method included both the regional geochemical background of the sediment and the median value
of the measured contents. The median value of baselines obtained from relative cumulative frequency method was lower than
that obtained from normalization procedure method. In contrast to the geochemical patterns of heavy metals in 1987, the spatial
distribution of anomalies sprawled in 2004 in study area, especially for Cu, Pb, Zn, Cd, As, Fe, and Cr. 相似文献
76.
基于性能的碳纤维抗震加固设计 总被引:2,自引:1,他引:1
本文探讨了基于性能的加固设计的基本思路和设计过程,并给出了一个基于性能的碳纤维加固设计的工程实例,采用pushover静力推覆分析的方法分析对比了结构加固前后的性能,表明原结构的抗震性能不足,经碳纤维加固后的结构抗震性能明显提高,满足8度抗震设防烈度要求。 相似文献
77.
大跨预应力混凝土框架结构的静力弹塑性(pushover)分析 总被引:4,自引:1,他引:3
基于美国规范ATC-40和N2方法,建立了大跨预应力混凝土框架结构竖向地震反应的静力弹塑性(pushover)分析方法。编制了预应力混凝土构件截面弯矩-曲率的计算程序。这些为评价大跨预应力混凝土框架结构的竖向抗震能力提供了有益的参考。结合具体工程实例进一步说明了该方法的实施步骤。 相似文献
78.
We describe the development and application of a management procedure (decision rule) that resulted in a voluntary reduction in the commercial catch of spiny rock lobster (Jasus edwardsii) in the lower east coast of North Island of New Zealand. The management procedure was developed from an accepted assessment of the CRA 4 (Wellington‐Hawke's Bay) fishery, which used an integrated length‐based assessment model fitted to commercial fishery catch‐per‐unit‐effort (CPUE) biomass indices, commercial length‐frequency data, and tag‐recapture data. The assessment model had been Bayesian, and used the joint posterior distribution of parameters to predict the effect of 384 alternative harvest control rules on the future size of the CRA 4 stock. The harvest control rules all used CPUE as their input, and generated annual changes in catch, which were then simulated by the population dynamics of the operating model. Uncertainty was added to evaluations through observation error, added to the simulated CPUE observations, and stochastic serial auto‐correlation variation in recruitment. We describe how this management procedure was used to effect a voluntary reduction in catch to address the problem of a rapidly declining population. 相似文献
79.
80.
The Wulanmulun site found in 2010 is an important Paleolithic site in Ordos (China), from which lots of stone and bone artifacts and mammalian fossils have been recovered. It was previously dated by radiocarbon and optically stimulated luminescence (OSL) techniques on quartz. To further confirm the reliability of the chronology constructed based on OSL ages and test the applicability of the recently developed pIRIR procedure on sediments from northern China, twenty-four sediment samples (including eolian, lacustrine and fluvio-eolian sands) from the site were determined using the multi-elevated-temperature post-IR IRSL (MET-pIRIR or pIRIR) procedure on potassium feldspar. The results show that the studied samples have two MET-pIRIR De preheat plateaus (280–320 and 340–360 °C), and the bleaching rates of the luminescence signals are associated with sample ages and stimulation temperatures. All the pIRIR ages (7–155 ka) corrected for anomalous fading and residual dose obtained after solar bleaching for 15 h are larger than the corresponding quartz OSL ages (4–66 ka) previously determined, even for the young eolian samples (<10 ka). But the corrected IRSL(50 °C) ages (6–85 ka) are broadly consistent with the quartz ages. It appears that the IRSL(50 °C) ages are more reliable, although this contradicts the previously results obtained by other people. On the other hand, we also obtained an extended age plateau between the stimulation temperatures of 50 and 290 °C in the plot of age versus stimulation temperature (A-T plot) by subtracting different residual doses obtained after different bleaching times. The reliability of the plateau ages requires further investigation. For the sediment samples from this site, quartz should be more suitable for dating than K-feldspar, and the quartz OSL ages of 50–65 ka for its cultural layer should be reliable. 相似文献