全文获取类型
收费全文 | 661篇 |
免费 | 168篇 |
国内免费 | 137篇 |
专业分类
测绘学 | 4篇 |
大气科学 | 23篇 |
地球物理 | 227篇 |
地质学 | 461篇 |
海洋学 | 205篇 |
天文学 | 10篇 |
综合类 | 13篇 |
自然地理 | 23篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 16篇 |
2021年 | 12篇 |
2020年 | 16篇 |
2019年 | 21篇 |
2018年 | 24篇 |
2017年 | 51篇 |
2016年 | 48篇 |
2015年 | 31篇 |
2014年 | 56篇 |
2013年 | 36篇 |
2012年 | 47篇 |
2011年 | 94篇 |
2010年 | 33篇 |
2009年 | 57篇 |
2008年 | 44篇 |
2007年 | 46篇 |
2006年 | 52篇 |
2005年 | 31篇 |
2004年 | 36篇 |
2003年 | 30篇 |
2002年 | 19篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 17篇 |
1998年 | 16篇 |
1997年 | 13篇 |
1996年 | 16篇 |
1995年 | 18篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 1篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1978年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有966条查询结果,搜索用时 953 毫秒
71.
南海含天然气水合物地层地震反射特征及可能分布研究 总被引:4,自引:0,他引:4
天然气水合物是当前能源与环境领域的研究热点之一,南中国海是天然气水合物赋存的有利区域。天然气水合物的存在改变了沉积地层的声学特征,这一性质使多道地震勘探成为发现海洋天然气水合物的主要手段。本文首先根据地震成像结果定性分析南海可能含天然气水合物沉积地层的地震反射特征,初步确定天然气水合物存在的可能性并指出地震成像关键技术。在无井条件下,构建虚拟井进行波阻抗反演得到定量的地层速度参数进一步证实这一可能性,最后将反演获得的速度场与ODP184 航次在此区域获得的地球物理、地球化学信息综合分析,可以确定此区域天然气水合物的存在及其空间展布。 相似文献
72.
利用静态箱/气相色谱(GC)法,对华北平原冬小麦拔节-成熟期间麦田土壤CH4气体通量进行了测定,得出华北平原典型冬麦田土壤是大气CH4的弱吸收汇。试验期间土壤CH4通量存在明显的季节变化和日变化,麦田拔节-成熟期间土壤CH4通量日平均值为-18.3μg·m-2·h-1,波动范围为-4.3-24.4μg·m-2·h-1;在土壤CH4通量的日变化中,观测到麦田土壤在午间和夜间都有一个吸收峰,峰值出现的时间因生育期不同而有所不同。试验期间CH4通量日平均值与土壤温度关系不明显,而与土壤水分呈负相关(α=0.01);日变化中土壤CH4通量与地表温度的相关性较差,而与5cm地温相关密切。麦田拔节-成熟期间土壤CH4通量日平均值随NH4+-N施用量的增加呈递减规律,农田秸秆还田后不利于土壤对CH4的吸收。 相似文献
73.
The relaxed eddy accumulation (REA) method allows the measurement of trace gas fluxes when no fast sensors are available for eddy covariance measurements. The flux parameterisation used in REA is based on the assumption of scalar similarity, i.e., similarity of the turbulent exchange of two scalar quantities. In this study changes in scalar similarity between carbon dioxide, sonic temperature and water vapour were assessed using scalar correlation coefficients and spectral analysis. The influence on REA measurements was assessed by simulation. The evaluation is based on observations over grassland, irrigated cotton plantation and spruce forest.Scalar similarity between carbon dioxide, sonic temperature and water vapour showed a distinct diurnal pattern and change within the day. Poor scalar similarity was found to be linked to dissimilarities in the energy contained in the low frequency part of the turbulent spectra ( < 0.01 Hz).The simulations of REA showed significant change in b-factors throughout the diurnal course. The b-factor is part of the REA parameterisation scheme and describes a relation between the concentration difference and the vertical flux of a trace gas. The diurnal course of b-factors for carbon dioxide, sonic temperature and water vapour matched well. Relative flux errors induced in REA by varying scalar similarity were generally below ± 10%. Systematic underestimation of the flux of up to − 40% was found for the use of REA applying a hyperbolic deadband (HREA). This underestimation was related to poor scalar similarity between the scalar of interest and the scalar used as proxy for the deadband definition. 相似文献
74.
Underground coal mines have always had to control the presence of different gases in the mining environment. Among these gases, methane is the most important one, since it is inherent to coal. Despite of the technical developments in recent decades, methane hazards have not yet been fully avoided. This is partly due to the increasing depths of modern mines, where methane emissions are higher, and also to other mining-related circumstances, such as the increase in production rates and its consequences: difficulties in controlling the increasing methane levels, increasing mechanization, the use of explosives and not paying close attention to methane control systems.The main purposes of this paper are to establish site measurements using some critical parameters that are not part of the standard mining-control methods for risk assessment and to analyze the gas behavior of subvertical coal seams in deep mines in order to prevent gas incidents from occurring. The ultimate goal is the improvement in mining conditions and therefore in safety conditions.For this purpose, two different mines were instrumented for mine control and monitoring. Both mines belong to the Riosa–Olloniego coalfield, in the Asturias Central Basin, Spain and the areas instrumented are mined via subhorizontal sublevels at an actual depth of around 1000 m under the overburden of Mount Lusorio.During this research, a property favoring gas outbursts was site measured for the first time in an outburst-prone coal (8th Coalbed), gas pressure and its variations, which contributed to complete the data available from previous characterizations and to set some guidelines for assessing the potential outburst-prone areas. A gas-measurement-tube set has been designed for measuring gas pressure as well as its variation over time as a result of nearby workings and to calculate permeability.The paper establishes the effect of overlapping of works, but it also shows the efficacy of two preventive measures to be applied: high pressure water infusion and the exploitation of a protective coal seam (7th Coalbed), that must be mined preferably two complete sublevels before commencing the advance in the outburst-prone coalbed. Both measures constitute an improvement in the mining sequence and therefore in safety, and should be completed with a systematic measurement to control the risk: gas pressure in the 8th Coalbed in the area of influence of other workings, to establish the most suitable moment to renew the advance. Further researches could focus on ascertaining the permeability, not only in mined areas but also in areas of the mine that are still not affected by mining work and on tuning more finely the ranges of influence of overstress time and overlap distance of the workings of the 7th Coalbed in the 8th Coalbed. 相似文献
75.
Pit crater structure and processes governing persistent activity at Masaya Volcano, Nicaragua 总被引:1,自引:1,他引:0
Hazel Rymer Benjamin van Wyk de Vries John Stix Glyn Williams-Jones 《Bulletin of Volcanology》1998,59(5):345-355
Persistent activity at Masaya Volcano, Nicaragua, is characterised by cycles of intense degassing, lava lake development
and pit crater formation. It provides a useful site to study the processes which govern such activity, because of its easy
accessibility and relatively short cycles (years to decades). An understanding of the present activity is important because
Masaya is visited by large numbers of tourists, is located close to major cities and has produced voluminous lavas, plinian
eruptions and ignimbrites in the recent past. We provide structural and geophysical data that characterise the "normal" present
state of activity. These indicate that the ongoing degassing phase (1993 to present) was not caused by fresh magma intrusion.
It was associated with shallow density changes within the active Santiago pit crater. The activity appears to be associated
predominantly with shallow changes in the pit crater structure. More hazardous activity will occur only if there are significant
departures from the present gravity, deformation and seismic signatures.
Received: 16 May 1997 / Accepted: 29 October 1997 相似文献
76.
复杂地质条件下气层的多参数识别 总被引:1,自引:0,他引:1
在复杂地质条件下,应用单一或两种孔隙测井资料直接识别气层,灵敏度不高,为了准确地识别复杂地质条件下的气层,需从不同角度,应用多参数进行综合识别,本文介绍了声波法,中子-密度法,双孔隙度法,中子-伽玛法,等效弹性模量法等,并应用它们对哈盆地两口井进行了重点分析,效果良好。 相似文献
77.
本文在C-O-H体系流体相平衡基础上,利用现有的热力学数据和新的p-V-T资料,在pT=∑pi假定下,计算了高温、高压条件下流体相组成。结果表明,该体系主要存在五种流体相,在不同温压条件下各流体相所占比例不同。在相对较低的温、压条件下,CH4是体系中占主要的流体相(约占70%),且随温、压和氧逸度的升高,它所占比例却明显降低,所获结果为探讨无机成因天然气形成的可能性、存在的量比和稳定存在的物理化学条件提供了充分的理论依据。 相似文献
78.
Petrographic investigations of serial ply samples from five high- to medium-volatile bituminous coal seams from Australia (4) and Canada (1) reveal substantial in-seam variations in the reflectance and monochromatic microfluorescence intensities of the maceral subgroup telovitrinite. The variations consist of one case of reflectance enhancement and fluorescence suppression, and four cases of reflectance suppression and fluorescence enhancement. The single case of reflectance enhancement and fluorescence suppression is due to the oxidation of the vitrinite nuclei at the sequence boundary between the Bayswater and Upper Wynn seams in New South Wales. The four cases of reflectance suppression and fluorescence enhancement result from the syn- and epigenetic absorption by the vitrinite nuclei of hydrogen donated by, presumably, anaerobic bacteria-generated lipids. Two of the coals are marine-influenced: the Liskeard Seam from the Bowen Basin by combined syngenetic and epigenetic effects, and the Greta Seam from the Sydney Basin mainly by epigenetic contact with sea water. For both coals, the results are strong vitrinite reflectance suppression and fluorescence enhancement. The remaining two coals, the Bulli Seam from the Sydney Basin and a coal seam from the Gates Formation in British Columbia, show moderate epigenetic effects on the optical properties of telovitrinite by fresh-water. In the Bulli Seam which was studied in two adjacent localities, the reflectance suppression and fluorescence enhancement of telovitrinite are stronger under sandstone roof than under shale roof. In some cases, the epigenetic effects are superimposed on syngenetic telovitrinite reflectance and fluorescence variations resulting from the cogeneration and mixing of different telovitrinite precursors, for example, autochthonous roots and hypautochthonous or allochthonous shoots. A measure of the degree of dispersal and mixing is the coefficient of variation of telovitrinite reflectance and/or fluorescence. This coefficient correlates well with detrital minerals and dispersed macerals, e.g., inertodetrinite and, to a lesser extent, sporinite. Some comments are made on slitted so-called pseudovitrinite which is regarded as a telovitrinite that was subjected to very weak post-coalification desiccation and possibly oxidation without losing much of its thermoplastic properties. 相似文献
79.
In order to understand the paleoenvironment of the Early Cambrian black shale deposition in the western part of the Yangtze Block, geochemical and organic carbon isotopic studies have been performed on two wells that have drilled through the Qiongzhusi Formation in the central and southeastern parts of Sichuan Basin. It shows that the lowest part of the Qiongzhusi Formation has high TOC abundance, while the middle and upper parts display relative low TOC content. Redox-sensitive element (Mo) and trace elemental redox indices (e.g., Ni/Co, V/Cr, U/Th and V/(V + Ni)) suggest that the high-TOC layers were deposited under anoxic conditions, whereas the low-TOC layers under relatively dysoxic/oxic conditions. The relationship of the enrichment factors of Mo and U further shows a transition from suboxic low-TOC layers to euxinic high-TOC layers. On the basis of the Mo-TOC relationship, the Qiongzhusi Formation black shales were deposited in a basin under moderately restricted conditions. Organic carbon isotopes display temporal variations in the Qiongzhusi Formation, with a positive excursion of δ13Corg values in the lower part and a continuous positive shift in the middle and upper parts. All these geochemical and isotopic criteria indicate a paleoenvironmental change from bottom anoxic to middle and upper dysoxic/oxic conditions for the Qiongzhusi Formation black shales. The correlation of organic carbon isotopic data for the Lower Cambrian black shales in different regions of the Yangtze Block shows consistent positive excursion of δ13Corg values in the lower part for each section. This excursion can be ascribed to the widespread Early Cambrian transgression in the Yangtze Block, under which black shales were deposited. 相似文献
80.
Caprock has the most important role in the long term safety of formation gas storage. The caprocks trap fluid accumulated beneath, contribute to lateral migration of this fluid and impede its upward migration. The rapid upward passage of invasive plumes due to buoyancy pressure is prevented by capillary pressure within these seal rocks. In the present study, two main seal rocks, from the Zagros basin in the southwest of Iran, a shale core sample of Asmari formation and an anhydrite core sample of Gachsaran formation, were provided. Absolute permeabilities of shale and anhydrite cores, considering the Klinkenberg effect, were measured as 6.09 × 10−18 and 0.89 × 10−18 m2, respectively. Capillary sealing efficiency of the cores was investigated using gas breakthrough experiments. To do so, two distinct techniques including step by step and residual capillary pressure approaches were performed, using carbon dioxide, nitrogen and methane gases at temperatures of 70 and 90 °C, under confining pressures in the range 24.13–37.92 MPa. In the first technique, it was found that capillary breakthrough pressure of the cores varies in the range from 2.76 to 34.34 MPa. Moreover, the measurements indicated that after capillary breakthrough, gas effective permeabilities lie in range 1.85 × 10−21 – 1.66 × 10−19 m2. In the second technique, the minimum capillary displacement pressure of shale varied from 0.66 to 1.45 MPa with the maximum effective permeability around 7.76 × 10−21 – 6.69 × 10−20 m2. The results indicate that anhydrite caprock of the Gachsaran formation provides proper capillary sealing efficacy, suitable for long term storage of the injected CO2 plumes, due to its higher capillary breakthrough pressure and lower gas effective permeability. 相似文献