全文获取类型
收费全文 | 3249篇 |
免费 | 121篇 |
国内免费 | 34篇 |
专业分类
测绘学 | 73篇 |
大气科学 | 243篇 |
地球物理 | 791篇 |
地质学 | 1098篇 |
海洋学 | 311篇 |
天文学 | 475篇 |
综合类 | 14篇 |
自然地理 | 399篇 |
出版年
2024年 | 6篇 |
2023年 | 7篇 |
2022年 | 10篇 |
2021年 | 56篇 |
2020年 | 50篇 |
2019年 | 63篇 |
2018年 | 70篇 |
2017年 | 79篇 |
2016年 | 94篇 |
2015年 | 92篇 |
2014年 | 97篇 |
2013年 | 201篇 |
2012年 | 148篇 |
2011年 | 196篇 |
2010年 | 147篇 |
2009年 | 187篇 |
2008年 | 168篇 |
2007年 | 166篇 |
2006年 | 147篇 |
2005年 | 132篇 |
2004年 | 131篇 |
2003年 | 113篇 |
2002年 | 109篇 |
2001年 | 75篇 |
2000年 | 76篇 |
1999年 | 69篇 |
1998年 | 59篇 |
1997年 | 43篇 |
1996年 | 49篇 |
1995年 | 38篇 |
1994年 | 38篇 |
1993年 | 36篇 |
1992年 | 29篇 |
1991年 | 28篇 |
1990年 | 30篇 |
1989年 | 32篇 |
1988年 | 29篇 |
1987年 | 29篇 |
1986年 | 26篇 |
1985年 | 37篇 |
1984年 | 35篇 |
1983年 | 21篇 |
1982年 | 33篇 |
1981年 | 28篇 |
1980年 | 20篇 |
1979年 | 20篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1974年 | 7篇 |
1973年 | 5篇 |
排序方式: 共有3404条查询结果,搜索用时 0 毫秒
31.
We aim to show how some of the important interdisciplinary fixes or solutions to diverse problems observed in fisheries can complement each other. This can be achieved through methodical allocation of the rights pertaining to fisheries and simultaneous implementation of policy instruments to correct for market failures and equity concerns. We emphasize via a roadmap that there are some general principles that should be invoked when choosing between alternative structures of rights. Our examples from Sweden provide evidence of the flexibility of fishing rights and how they can be adapted to integrate fixes from different disciplines into practical fisheries management. 相似文献
32.
Bacteria and bovine faecal matter were introduced into cultures of pathogenic free‐living amoebae, with chlorine or chlorine dioxide. The bacteria and faeces were to simulate natural conditions for a more exacting test of disinfectant demand when compared to axenic conditions. Both chlorine and chlorine dioxide were effective disinfectants under all conditions tested. Axenically and monoxenically cultured amoebae were used, the latter exerted a greater disinfectant demand. 相似文献
33.
34.
Rosalind C. King Guillaume Backé Christopher K. Morley Richard R. Hillis Mark R.P. Tingay 《Marine and Petroleum Geology》2010
Recent GPS measurements demonstrate that NW Borneo undergoes 4–6 mm of plate-scale shortening a year, which is not accommodated by plate-scale structures. The only geological structure in NW Borneo described to accommodate on-going shortening is the Baram Delta System located on the outer shelf to basin floor. Delta toe fold-thrust belts are commonly thought to be caused by margin-normal compressional stresses generated by margin-parallel upslope gravitational extension. 相似文献
35.
Melissa M. Foley Benjamin S. Halpern Fiorenza Micheli Matthew H. Armsby Margaret R. Caldwell Caitlin M. Crain Erin Prahler Nicole Rohr Deborah Sivas Michael W. Beck Mark H. Carr Larry B. Crowder J. Emmett Duffy Sally D. Hacker Karen L. McLeod Stephen R. Palumbi Charles H. Peterson Helen M. Regan Mary H. Ruckelshaus Paul A. Sandifer Robert S. Steneck 《Marine Policy》2010
The declining health of marine ecosystems around the world is evidence that current piecemeal governance is inadequate to successfully support healthy coastal and ocean ecosystems and sustain human uses of the ocean. One proposed solution to this problem is ecosystem-based marine spatial planning (MSP), which is a process that informs the spatial distribution of activities in the ocean so that existing and emerging uses can be maintained, use conflicts reduced, and ecosystem health and services protected and sustained for future generations. Because a key goal of ecosystem-based MSP is to maintain the delivery of ecosystem services that humans want and need, it must be based on ecological principles that articulate the scientifically recognized attributes of healthy, functioning ecosystems. These principles should be incorporated into a decision-making framework with clearly defined targets for these ecological attributes. This paper identifies ecological principles for MSP based on a synthesis of previously suggested and/or operationalized principles, along with recommendations generated by a group of twenty ecologists and marine scientists with diverse backgrounds and perspectives on MSP. The proposed four main ecological principles to guide MSP—maintaining or restoring: native species diversity, habitat diversity and heterogeneity, key species, and connectivity—and two additional guidelines, the need to account for context and uncertainty, must be explicitly taken into account in the planning process. When applied in concert with social, economic, and governance principles, these ecological principles can inform the designation and siting of ocean uses and the management of activities in the ocean to maintain or restore healthy ecosystems, allow delivery of marine ecosystem services, and ensure sustainable economic and social benefits. 相似文献
36.
A geochemical investigation of oils in sandstone core plugs and drill stem test oils was carried out on samples from a North Sea reservoir. A sample of diesel used as a constituent of the drilling fluids was also analysed. The aliphatic and aromatic hydrocarbons and polar non-hydrocarbons were isolated using solid phase extraction methods. GC analysis of the hydrocarbon fraction of the core extract indicated that contamination may be diesel derived. From analysis of diesel some compound classes are less likely to be affected by contamination from diesel itself including: steranes, hopanes, aromatic steroid hydrocarbons, benzocarbazoles and C0–C3-alkylphenols. 相似文献
37.
Previous stable isotope studies at Lizzies Basin revealed that metasedimentary rocks are 18O-depleted relative to protolith values, particularly in the lower parts of the section (Lower Zone) where the rocks are also
isotopically homogeneous on a scale of hundreds of meters (quartz δ18O=+9.0 to +9.6 per mil). In contrast, metasedimentary rocks at higher levels at Lizzies Basin (Upper Zone) are less 18O-depleted and more heterogeneous in δ18O. In order to understand more fully the isotopic evolution of this terrane, a series of detailed, meter-scale traverses across
various metamorphic and igneous lithologies were completed at Lizzies Basin, and at the structurally higher Angel Lake locality.
Traverses in the Lizzies Basin Lower Zone and in the lower parts of Angel Lake (Angel Lake Lower Sequence) across various
silicate lithologies, including abundant granitoids, reveal similar degrees of homogeneity, although the average δ18O values are higher at Angel Lake. In contrast, traverses which include substantial thicknesses of marble and calc-silicate
gneiss and very little granitoid have more heterogeneous quartz δ18O values (+11.9 to +13.4 per mil), and also have a higher average δ18O (+12.9 per mil), than observed elsewhere. The scale of 18O/16O homogeneity in quartz observed at Lizzies Basin and Angel Lake (meters to hundreds of meters) requires fluid-mediated isotope
exchange, which accompanied Tertiary metamorphism. There is a correlation between the degree of 18O-depletion in metasedimentary rocks, 18O/16O homogenization between lithologies, and the proportion of granitoids (leucogranites in particular) within any part of the
section, and a corresponding anticorrelation with the proportion of marble. This points to a causal relationship, whereby
the leucogranites (as well as the Tertiary hornblende diorite and biotite monzogranite) acted as both a relatively low-18O reservoir and a source of fluids to enhance exchange, while the marbles hindered isotope depletion and homogenization by
acting as relatively high-18O reservoirs and impermeable layers. Material balance calculations help delineate the plausible mechanisms of exchange between
granitoids and metasediments. Single-pass infiltration of magmatic fluids from the granitoids is not capable of reproducing
all of the observations. Fluid-mediated exchange by convective recirculation of magmatic fluids on a scale of meters is the
mechanism which explains all of the observations. The generalized model for the isotopic evolution of the East Humboldt Range
core complex provides an excellent opportunity to establish the main causes and controlling factors of 18O-depletion and 18O/16O homogenization during regional metamorphism.
Received: 27 July 1993 / Accepted: 1 July 1994 相似文献
38.
Mark Taylor 《International Journal of Coal Geology》1981,1(3):213-233
Pennsylvanian strata of the Hazard coalfield, Eastern Kentucky, contain fluvial, upper-delta plain facies characterized by thick localized coals, bay-fill shales, levee silt- and sandstones, and channel-fill sands and gravels. Although the deltaic nature of these sediments has long been established, mining and exploration activities in the district require a thorough understanding of small areas within the delta environment. Coal quantity and quality trends in the Hazard #7 seam, the major producer in the area, have been examined in detail.The #7 coal accumulated in a peat swamp restricted laterally by a major fluvial channel. Three types of non-coal parting are recognized. Thin, tabular, fine-grained partings resulted from periods of increased terrigenous influx into the swamp. Lenticular crevasse-splay deposits locally split the coal. Wedge-shaped, fine- to coarse-grained partings, of probable levee origin, are found along the channel margin. Post-swamp deposits consist of thick bay-fill shales, thin shales, silt- and sandstones deposited on floodplains, and channel-fill sandstones and gravels.The modeling technique discussed provides guidance for mine development and regional exploration by prediction of coal seam quantity and quality trends from local geologic features. The thickest #7 coal is split by thin tabular partings and is overlain by thick shale sequences. Coal overlain by silt and sandstone is thinner and unsplit, and typically of higher heat value. Regional seam thinning due to channel scour is recognized. Factors which control the configuration of the coal seam include position within the peat swamp, proximity to the fluvial channel, swamp burial processes, and paleochannel sinuosity. 相似文献
39.
Comprehensive risk assessments are fundamental to effective emergency management. These assessments need to identify the range
of hazards (or perils) an entity is exposed to and quantify the specific threats associated with each of those hazards. While
hazard identification is commonly, if not formally, conducted in most circumstances, specific threat analysis is often overlooked
for a variety of reasons, one of which is poor communication with subject matter experts. This poor communication is often
attributable to an adherence to scientific jargon and missed opportunities to simplify information. In Canada, for example,
earthquake hazard calculations have been readily available to engineers and scientists for decades. This hazard information,
however, is expressed in terms of peak ground accelerations (PGA) or spectral accelerations (SA) that are foreign concepts
to most emergency managers, community decision-makers and the public-at-large. There is, therefore, a need to more clearly,
simply and effectively express seismic hazard information to the non-scientific community. This paper provides crustal, sub-crustal
and subduction interface earthquake shaking probabilities, expressed as simple percentages for each of 57 locations across
Vancouver Island, British Columbia, Canada. Calculations present the likelihood of earthquake shaking on Vancouver Island
as the probabilities of exceeding each of three shaking intensity thresholds (“widely felt”; onset of “non-structurally damaging”
shaking; and onset of “structurally damaging” shaking) over four timeframes (10, 25, 50 and 100 years). Results are based
on the latest Geological Survey of Canada hazard models used for the 2010 national building code and are presented in both
tabular and graphic formats. This simplified earthquake hazard information is offered to aid local residents, organizations
and governments in understanding and assessing their risk and to encourage and facilitate sound earthquake preparedness funding
decisions. 相似文献
40.
Atmospheric composition is a key control on climate and the habitability of planetary surfaces. Ablation of infalling micrometeorites has been recognised as one way in which atmospheric chemistry can be changed, especially at times in solar system history when the infall rates of exogenous material were high. Despite its potential to influence climate and habitability, extraterrestrial sulphur dioxide is currently an unquantified contribution to the atmospheres of the terrestrial planets. We have used flash pyrolysis to simulate the atmospheric entry of micrometeorites and Fourier-transform infrared spectroscopy to identify and quantify the sulphur dioxide produced from the carbonaceous meteorites Orgueil (CI1), ALH 88045 (CM1), Cold Bokkeveld (CM2), Murchison (CM2) and Mokoia (CV3). We have used this approach to understand the introduction of sulphur dioxide to the atmospheres of Earth and Mars from infalling micrometeorites. Sulphates, present in carbonaceous chondrites at a few wt.%, are resistant to thermal decomposition, limiting the yields of sulphur dioxide from unmelted micrometeorites. Infalling micrometeorites are a minor source of present-day sulphur dioxide on Earth and Mars, calculated to be up to around 2400 tonnes and about 350 tonnes, respectively. During the Late Heavy Bombardment (LHB), the much greater infall rates of micrometeoritic dust are calculated to be associated with average production rates of sulphur dioxide of around 20 Mt yr−1 for the early Earth and 0.5 Mt yr−1 for early Mars, for a LHB of 100 Myr. These rates of delivery of sulphur dioxide at high altitudes would have reduced the solar energy reaching the surfaces of these planets, via scattering of sunlight by stratospheric sulphate aerosols, and may have had detrimental effects on developing biospheres by promoting cooler climates and reducing the probability of liquid water on planetary surfaces. 相似文献