全文获取类型
| 收费全文 | 2314篇 |
| 免费 | 504篇 |
| 国内免费 | 396篇 |
专业分类
| 测绘学 | 23篇 |
| 大气科学 | 40篇 |
| 地球物理 | 679篇 |
| 地质学 | 1482篇 |
| 海洋学 | 370篇 |
| 天文学 | 16篇 |
| 综合类 | 78篇 |
| 自然地理 | 526篇 |
出版年
| 2024年 | 8篇 |
| 2023年 | 40篇 |
| 2022年 | 89篇 |
| 2021年 | 104篇 |
| 2020年 | 94篇 |
| 2019年 | 102篇 |
| 2018年 | 89篇 |
| 2017年 | 95篇 |
| 2016年 | 114篇 |
| 2015年 | 100篇 |
| 2014年 | 132篇 |
| 2013年 | 173篇 |
| 2012年 | 112篇 |
| 2011年 | 133篇 |
| 2010年 | 111篇 |
| 2009年 | 127篇 |
| 2008年 | 145篇 |
| 2007年 | 169篇 |
| 2006年 | 140篇 |
| 2005年 | 98篇 |
| 2004年 | 139篇 |
| 2003年 | 98篇 |
| 2002年 | 90篇 |
| 2001年 | 78篇 |
| 2000年 | 90篇 |
| 1999年 | 56篇 |
| 1998年 | 75篇 |
| 1997年 | 57篇 |
| 1996年 | 65篇 |
| 1995年 | 64篇 |
| 1994年 | 46篇 |
| 1993年 | 38篇 |
| 1992年 | 30篇 |
| 1991年 | 20篇 |
| 1990年 | 7篇 |
| 1989年 | 21篇 |
| 1988年 | 17篇 |
| 1987年 | 9篇 |
| 1986年 | 8篇 |
| 1985年 | 8篇 |
| 1984年 | 7篇 |
| 1983年 | 3篇 |
| 1982年 | 3篇 |
| 1981年 | 1篇 |
| 1979年 | 3篇 |
| 1976年 | 1篇 |
| 1971年 | 2篇 |
| 1954年 | 3篇 |
排序方式： 共有3214条查询结果，搜索用时 218 毫秒
31.
32.
东、黄海典型海区分粒级浮游植物叶绿素a的周日波动及影响因子 总被引：3，自引：0，他引：3
本文分析了东、黄海典型海区3个测站在2000~2003年间4个航次的叶绿素a周日变动特征,结果表明由于地理环境、水文特征以及季节差异,各海区叶绿素a表现出各自不同的变动特点.在东海陆架区,日变化上表层各粒级主要以半日周期为主,受黑潮入侵程度不同而略有变化;长江口由于受到潮汐的影响,各粒级的日变化同潮汐的涨落相对应,主要为半日周期;黄海中部海区叶绿素a尤其是NANO级浮游植物在周日变化上以全日周期为主,受黄海冷水团强弱不同而不同.本文进一步应用渐近回归初步分析了多种环境因子对叶绿素a周日变化的影响. 相似文献
33.
34.
Theoretical aspects of cap-rock and fault seals for single- and two-phase hydrocarbon columns 总被引：6，自引：0，他引：6
Cap-rock seals can be divided genetically into those that fail by capillary leakage (membrane seals) and those whose capillary entry pressures are so high that seal failure preferentially occurs by fracturing and/or wedging open of faults (hydraulic seals). A given membrane seal can trap a larger oil column than gas column at shallow depths, but below a critical depth (interval), gas is more easily sealed than oil. This critical depth increases with lower API gravity, lower oil GOR and overpressured conditions (for the gas phase). These observations arise from a series of modelling studies of membrane sealing and can be conveniently represented using pressure/ depth (P/D) profiles through sealed hydrocarbon columns. P/D diagrams have been applied to the more complex situation of the membrane sealing of a gas cap underlain by an oil rim; at seal capacity, such a two-phase column will be always greater than if only oil or gas occurs below the seal.These conclusions contrast with those for hydraulic seals where the seal capacity to oil always exceeds that for gas. Moreover, a trapped two-phase column, at hydraulic seal capacity will be less than the maximum-allowed oil-only column, but more than the maximum gas-only column. Unlike membrane seals, hydraulic seal capacity should be directly related to cap-rock thickness, in addition to the magnitude of the minimum effective stress in the sealing layer and the degree of overpressure development in the sequence as a whole.Fault-related seals are effectively analogous to membrane cap-rocks which have been tilted to the angle of the fault plane. Consequently, all of the above conclusions derived for membrane cap-rocks apply to both sealing faults sensu stricto (fault plane itself seals) and juxtaposition faults (hydrocarbon trapped laterally against a juxtaposed sealing unit). The maximum-allowed two-phase column trapped by a sealing fault is greater than for equivalent oil-only and gas-only columns, but less than that predicted for a horizontal membrane cap-rock under similar conditions. Where a two-phase column is present on both sides of a sealing fault (which is at two-phase seal capacity), a deeper oil/water contact (OWC) in one fault block is associated with a deeper gas/oil contact (GOC) compared with the adjacent fault block. If the fault seal is discontinuous in the gas leg, however, the deeper OWC is accompanied by a shallower GOC, whereas a break in the fault seal in the oil leg results in a common OWC in both fault blocks, even though separate GOC's exist. Schematic P/D profiles are provided for each of the above situations from which a series of fundamental equations governing single- and two-phase cap-rock and fault seal capacities can be derived. These relationships may have significant implications for exploration prospect appraisal exercises where more meaningful estimates of differential seal capacities can be made.The membrane sealing theory developed herein assumes that all reservoirs and seals are water-wet and no hydrodynamic flow exists. The conclusions on membrane seal capacity place constraints on the migration efficiency of gas along low-permeabiligy paths at depth where fracturing, wedging open of faults and/or diffusion process may be more important. Contrary to previous assertions, it is speculated that leakage of hydrocarbons through membrane seals occurs in distinct pulses such that the seal is at or near the theoretically calculated seal capacity, once this has been initially attained.Finally, the developed seal theory and P/D profile concepts are applied to a series of development geological problems including the effects of differential depletion, and degree of aquifer support, on sealing fault leakage, and the evaluation of barriers to vertical cross-flow using RFT profiles through depleted reservoirs. It is shown that imbibition processes and dynamic effects related to active cross-flow across such barriers often preclude quantitative analysis and solution of these problems for which simulation studies are usually required. 相似文献
35.
At hatching the larvae of flatfish closely resemble the bilateral symmetric larvae of other teleosts, especially perciforms. Literature data show that transformation to asymmetric benthic juveniles normally occurs at body lengths between 10 and 25 mm. Unexpectedly, minimal size at its completion (including eye migration) can be 4.1 mm SL and maximal size is over 72 mm. In this paper we consider the functional requirements for a successful switch from a symmetric pelagic larva to a typical asymmetric juvenile benthic flatfish partly based on evidence from other teleosts. The unfavourable period of eye migration and transition to a benthic habitat requires some food reserves and rewiring and/or recalibration of vision and gravity-associated structures utilised previously by the still symmetric larvae for e.g. food detection. Binocular fixation of the prey probably occurs in that stage. Critical or sensitive periods occurring during development of fish larvae suggest that a completely functional symmetric stage of development must precede transformation. The normal size range in flatfish larvae at transformation seems to confirm our considerations. Recent data on temperature effects during development provide an explanation for metamorphosis at the minimal size. Some evidence for paedomorphic heterochrony in flatfish larvae is presented. 相似文献
36.
37.
38.
39.
40.