全文获取类型
收费全文 | 2730篇 |
免费 | 149篇 |
国内免费 | 33篇 |
专业分类
测绘学 | 88篇 |
大气科学 | 310篇 |
地球物理 | 695篇 |
地质学 | 865篇 |
海洋学 | 233篇 |
天文学 | 368篇 |
综合类 | 6篇 |
自然地理 | 347篇 |
出版年
2024年 | 7篇 |
2022年 | 13篇 |
2021年 | 51篇 |
2020年 | 61篇 |
2019年 | 61篇 |
2018年 | 78篇 |
2017年 | 84篇 |
2016年 | 108篇 |
2015年 | 94篇 |
2014年 | 107篇 |
2013年 | 201篇 |
2012年 | 133篇 |
2011年 | 166篇 |
2010年 | 126篇 |
2009年 | 152篇 |
2008年 | 146篇 |
2007年 | 145篇 |
2006年 | 138篇 |
2005年 | 109篇 |
2004年 | 97篇 |
2003年 | 99篇 |
2002年 | 82篇 |
2001年 | 52篇 |
2000年 | 58篇 |
1999年 | 40篇 |
1998年 | 37篇 |
1997年 | 31篇 |
1996年 | 29篇 |
1995年 | 32篇 |
1994年 | 21篇 |
1993年 | 22篇 |
1992年 | 15篇 |
1991年 | 18篇 |
1990年 | 17篇 |
1989年 | 28篇 |
1988年 | 17篇 |
1987年 | 25篇 |
1986年 | 12篇 |
1985年 | 24篇 |
1984年 | 20篇 |
1983年 | 17篇 |
1982年 | 13篇 |
1981年 | 21篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 8篇 |
1974年 | 9篇 |
排序方式: 共有2912条查询结果,搜索用时 0 毫秒
101.
Sumanta Dandapath Bishwajit Chakraborty Nicolas Maslov Siddaiah M. Karisiddaiah Dhrupad Ghosh William Fernandes Andrew Menezes 《Marine and Petroleum Geology》2012,29(1):115-128
Here we apply quantitative technique to describe the seafloor seepages based on the multi-beam backscatter and bathymetric investigations to characterize the pockmark morphology. The variable seafloor backscatter strength for coarser seafloor sediments are related to the diagenesis derived from biodegraded seepages. In this regard, box counting method is used to estimate ‘fractal dimension’ for backscatter imagery data of 398 blocks. These blocks are further sub-grouped into six classes depending on the spread of pockmark related seepages. The study area lies 102 km west off Marmagao along the central west coast of India which contains pre-dominantly (70%) gas-charged sediments. Comparison between the estimated self-similar fractals reveals that there is approximately 97% correlation between the box (Dbox) and information (Dinfo) dimensions. Box dimension–derived fractal dimension values, suggest that the seepages are more along the fault trace in deeper waters, in comparison to sparsely distributed shallow water seepages. Besides, this poor seepage is confined within the smooth to moderately rough seafloor. It is established that the high backscatter strength along the upper slope of the pockmark region having higher fractal dimensions reflects multifractal behavior of seepage distribution. Entire area indicates patchy seepage patterns as supported by estimated fractal values showing intermittent fluctuations, which emphasizes non-linear behavior. Estimated self organizing criticality (SOC) parameters for six representative blocks reveal that the nature of pockmark, fault trace, sediment nature coupled with slumping of pockmark’s wall, sediment movement due to bottom currents are controlling the dynamic balance in the area seepage system. Further, our study emphasizing the multifractal behavior of seepage blocks, clearly depicts the drift in the seepage pattern. 相似文献
102.
Andrew J. Turner John Woodward Stuart A. Dunning Adrian J. Shine Chris R. Stokes Colm Ó Cofaigh 《第四纪科学杂志》2012,27(2):221-232
We present results from three geophysical campaigns using high‐resolution sub‐bottom profiling to image sediments deposited in Loch Ness, Scotland. Sonar profiles show distinct packages of sediment, providing insight into the loch's deglacial history. A recessional moraine complex in the north of the loch indicates initial punctuated retreat. Subsequent retreat was rapid before stabilisation at Foyers Rise formed a large stillstand moraine. Here, the calving margin produced significant volumes of laminated sediments in a proglacial fjord‐like environment. Subsequent to this, ice retreated rapidly to the southern end of the loch, where it again deposited a sequence of proglacial laminated sediments. Sediment sequences were then disturbed by the deposition of a thick gravel layer and a large turbidite deposit as a result of a jökulhlaup from the Spean/Roy ice‐dammed lake. These sediments are overlain by a Holocene sheet drape. Data indicate: (i) a former tributary of the Moray Firth Ice Stream migrated back into Loch Ness as a major outlet glacier with a calving margin in a fjord‐like setting; (ii) there was significant sediment supply to the terminus of this outlet glacier in Loch Ness; and (iii) that jökulhlaups are important for sediment supply into proglacial fjord/lake environments and may compose >20% of proglacial sedimentary sequences. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
103.
104.
105.
Civil society is a critical arena both for exploring Sustainability itself and for sustaining trajectories towards it through innovation, experimentation and debate. Innovations can be mould breaking and can challenge local institutions. Concurrently, initiatives may be fragile due to the development of new working relationships, reliance on voluntary labour and goodwill, and dependence on grant funding. Here we examine different aspects of what it takes to sustain grassroots trajectories for ‘communal growing’, given the pressures that groups and intermediary organisations practicing and supporting this activity experience, and the consequential need to build qualities like ‘resilience’. Attending carefully to the definition of this otherwise slippery concept, a particular focus is given to how contrasting aspects of temporality and agency lead to divergent constructions of ‘resilience’ and strategies for sustaining growing. We draw on fieldwork that explores the practice and support of communal growing in East Sussex, England, and directly associated activities at a national level.We find important interdependencies between communal growing projects and the intermediary organisations supporting them. Additionally there is huge diversity within and between both projects and the organisations that support them, including with respect to the ends to which growing is seen as a means. These ends link growing initiatives – both antagonistically and synergistically – to food, education and health systems. This diversity can be seen positively as: a source of innovation; facilitating the open and bottom up nature of growing; and, enabling the securing of greater financial support for the endeavour. What is less clear is how this plays into framing and configuring communal growing specifically in relation to achieving a more Sustainable and localised food system. We discuss the conceptual and methodological implications of these empirically derived observations with regards future research on grassroots innovations. 相似文献
106.
Andrew J. Davies Gerard C.A. Duineveld Tjeerd C.E. van Weering Furu Mienis Andrea M. Quattrini Harvey E. Seim John M. Bane Steve W. Ross 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(2):199-212
The Lophelia pertusa community at Viosca Knoll (VK826) is the most extensive found to date in the Gulf of Mexico. As part of a multi-disciplinary study, the physical setting of this area was described using benthic landers, CTD transects and remotely operated vehicle observations. The site was broadly characterised into three main habitats: (1) dense coral cover that resembles biogenic reef complexes, (2) areas of sediment, and (3) authigenic carbonate blocks with sparse coral and chemosynthetic communities. The coral communities were dominated by L. pertusa but also contained numerous solitary coral species. Over areas that contained L. pertusa, the environmental conditions recorded were similar to those associated with communities in the north-eastern Atlantic, with temperature (8.5–10.6 °C) and salinity (~35) falling within the known species niche for L. pertusa. However, dissolved oxygen concentrations (2.7–2.8 ml l?1) and density (σΘ, 27.1–27.2 kg m?3) were lower and mass fluxes from sediment trap data appeared much higher (4002–4192 mg m?2 d?1). Yet, this species still appears to thrive in this region, suggesting that L. pertusa may not be as limited by lower dissolved oxygen concentrations as previously thought. The VK826 site experienced sustained eastward water flow of 10–30 cm s?1 over the 5-day measurement period but was also subjected to significant short-term variability in current velocity and direction. In addition, two processes were observed that caused variability in salinity and temperature; the first was consistent with internal waves that caused temperature variations of 0.8 °C over 5–11 h periods. The second was high-frequency variability (20–30 min periods) in temperature recorded only at the ALBEX site. A further pattern observed over the coral habitat was the presence of a 24 h diel vertical migration of zooplankton that may form part of a food chain that eventually reaches the corals. The majority of detailed studies concerning local environmental conditions in L. pertusa habitats have been conducted within the north-eastern Atlantic, limiting most knowledge of the niche of this species to a single part of an ocean basin. Data presented here show that the corals at VK826 are subjected to similar conditions in temperature, salinity, and flow velocity as their counterparts in the north-east Atlantic, although values for dissolved oxygen and density (sigma-theta: σΘ) are different. Our data also highlight novel observations of short-term environmental variability in cold-water coral habitat. 相似文献
107.
Roger L. Nielsen Paula M. Davidson Timothy L. Grove 《Contributions to Mineralogy and Petrology》1988,100(3):361-373
An updated model for pyroxene-melt equilibria at 1 atm has been developed and calibrated using new and existing experimental data in order to refine calculations of liquid lines of descent, which simulate the effect of igneous differentiation processes. We combine the Davidson and Lindsley (1985) model for activities of components in clinopyroxene and orthopyroxene solid solutions, a i p , where i represents a quadrilateral endmember, with the Nielsen and Drake (1979) expressions for component activities in the melt, a i L (two-lattice melt model). The chemical potential differences for pyroxene-melt equilibria are expressed in the form: $$\Delta \mu _{\iota } = 0 = In \left( {{{a_i^p } \mathord{\left/{\vphantom {{a_i^p } {a_i^L }}} \right.\kern-\nulldelimiterspace} {a_i^L }}} \right) + A_i + {{B_i } \mathord{\left/{\vphantom {{B_i } T}} \right.\kern-\nulldelimiterspace} T}$$ Pyroxene compositions were projected to quadrilateral compositions with the method of Lindsley and Anderson (1983). The regression constants A i and B i were calculated from experimental data that consists of 282 pyroxene-melt pairs, including 83 orthopyroxene-melt pairs. These experiments were all performed at 1 atm and represent compositions ranging from basalts (alkali to lunar) to dacites (42–66 wt% SiO2). The model is calibrated for 1000相似文献
108.
Andrew G. Christy 《Physics and Chemistry of Minerals》1989,16(4):343-351
The concept of short-range interlayer interactions, fundamental to spin-analogue models for polytypism, is examined in the case of sapphirine. Consideration of interactions out to fourth-nearest neighbours provides a rationale for the difference between the polytype suites observed for sapphirine and wollastonite. In each case, the observed long-period structures are consistent with those predicted to be stable by the appropriate mappings onto the axial next-nearest neighbour model. Short-range interaction parameters may also be used to express stacking fault energies. This approach, combined with a simple nucleation-andgrowth model, is used to examine the possibility of metastable generation of complex polytypes in sapphirine. Statistical analysis of defect distributions and frequencies in sapphirine suggests that interactions over several hundred Ångstroms must be considered if the stacking energetics are to be accurately modelled. 相似文献
109.
110.
Andrew Warren 《GeoJournal》1993,31(1):11-14
Compared to other environmental issues, such as Global Warming/Climatic Change, and the Ozone Hole, Desertification has been neglected by both scientists and funding agencies. The reasons are the complexity of the problem and the poverty and marginality of the arid lands. The author suggests a policy-oriented definition and draws attention to the differences between drought, desiccation and dry-land degradation. 相似文献