全文获取类型
收费全文 | 756篇 |
免费 | 67篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 24篇 |
大气科学 | 55篇 |
地球物理 | 147篇 |
地质学 | 307篇 |
海洋学 | 69篇 |
天文学 | 102篇 |
综合类 | 1篇 |
自然地理 | 125篇 |
出版年
2024年 | 4篇 |
2023年 | 4篇 |
2022年 | 6篇 |
2021年 | 22篇 |
2020年 | 29篇 |
2019年 | 24篇 |
2018年 | 34篇 |
2017年 | 48篇 |
2016年 | 43篇 |
2015年 | 32篇 |
2014年 | 32篇 |
2013年 | 43篇 |
2012年 | 45篇 |
2011年 | 56篇 |
2010年 | 51篇 |
2009年 | 82篇 |
2008年 | 52篇 |
2007年 | 46篇 |
2006年 | 28篇 |
2005年 | 15篇 |
2004年 | 25篇 |
2003年 | 26篇 |
2002年 | 16篇 |
2001年 | 12篇 |
2000年 | 10篇 |
1999年 | 2篇 |
1998年 | 9篇 |
1997年 | 1篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1974年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有830条查询结果,搜索用时 15 毫秒
151.
Numerical flow models can be a useful tool for dimensioning water wells and to investigate the hydraulics in their near‐field. Fully laminar flow can be assumed for all models calculated up to the screen. Therefore models can be used to predict—at least qualitatively, neglecting turbulent losses inside the well—the spatial distribution of inflow into the well and the overall hydraulic performance of different combinations of aquifer parameters and technical installations. Models for both horizontal (plan view) and vertical flow (cross section) to wells were calculated for a variety of setups. For the latter, this included variations of hydraulic conductivity of the screen, pump position, and aquifer heterogeneity. Models of suction flow control devices showed that they indeed can homogenize inflow, albeit at the cost of elevated entrance losses. 相似文献
152.
153.
Blogging, social networking, and other Web 2.0 practices have sparked widespread debate about the status and future of privacy. This paper examines an explicitly geographical aspect of Web 2.0 with respect to these debates: the geospatial web, or ‘geoweb’. As part of fundamental shifts in the kinds of geographic information available, its circulation, and representative forms it assumes, the geoweb implies new objects of privacy concern and subsequent privacy-related negotiations over the aggregate of its component information, technologies, and data praxes. Thus we argue that privacy must not only be revisited, but indeed re-conceptualized. Whereas prior research on privacy vis-à-vis geographic information technologies has tended to question what privacy ‘is’, we focus instead on the constitutive outcomes of societal struggles over privacy. We examine how privacy is being negotiated around two geoweb services - Google Street View and the Twitter GeoAPI - to illustrate that these contestations produce privacy as a social object in particular ways. We show that public discourse around actual or anticipated privacy harms stemming from geoweb services and their uses, as well as the preventatives and remedies proposed or implemented to address such harms, reconstitute the objects and practices of privacy concern, and alter the roles and relationships of state, civil and corporate actors in the construction of privacy. Finally we suggest that the geoweb raises new privacy concerns because some of its representational forms - namely geo-tagged images and self-authored texts - facilitate identification and disclosure with more immediacy and less abstraction. 相似文献
154.
Sarah A. S. Dare Sarah-Jane Barnes Hazel M. Prichard Peter C. Fisher 《Mineralium Deposita》2011,46(4):381-407
Magmatic sulfide deposits consist of pyrrhotite, pentlandite, chalcopyrite (± pyrite), and platinum-group minerals (PGM).
Understanding the distribution of the chalcophile and platinum-group element (PGE) concentrations among the base metal sulfide
phases and PGM is important both for the petrogenetic models of the ores and for the efficient extraction of the PGE. Typically,
pyrrhotite and pentlandite host much of the PGE, except Pt which forms Pt minerals. Chalcopyrite does not host PGE and the
role of pyrite has not been closely investigated. The Ni–Cu–PGE ores from the South Range of Sudbury are unusual in that sulfarsenide
PGM, rather than pyrrhotite and pentlandite, are the main carrier of PGE, probably as the result of arsenic contribution to
the sulfide liquid by the As-bearing metasedimentary footwall rocks. In comparison, the North Range deposits of Sudbury, such
as the McCreedy East deposit, have As-poor granites in the footwall, and the ores commonly contain pyrite. Our results show
that in the pyrrhotite-rich ores of the McCreedy East deposit Os, Ir, Ru, Rh (IPGE), and Re are concentrated in pyrrhotite,
pentlandite, and surprisingly in pyrite. This indicates that sulfarsenides, which are not present in the ores, were not important
in concentrating PGE in the North Range of Sudbury. Palladium is present in pentlandite and, together with Pt, form PGM such
as (PtPd)(TeBi)2. Platinum is also found in pyrite. Two generations of pyrite are present. One pyrite is primary and locally exsolved from
monosulfide solid solution (MSS) in small amounts (<2 wt.%) together with pyrrhotite and pentlandite. This pyrite is unexpectedly
enriched in IPGE, As (± Pt) and the concentrations of these elements are oscillatory zoned. The other pyrite is secondary
and formed by alteration of the MSS cumulates by late magmatic/hydrothermal fluids. This pyrite is unzoned and has inherited
the low concentrations of IPGE and Re from the pyrrhotite and pentlandite that it has replaced. 相似文献
155.
A spatially explicit degree-day model was used to evaluate the risk of Rift Valley fever virus (RVFV) transmission by mosquitoes
to humans and livestock within five target states in the continental United States: California, Minnesota, Nebraska, New York,
and Texas. A geographic information system was used to model potential virus transmission based on a 12-day moving window
assessment of the extrinsic incubation period theorized for RVFV in the United States. Risk of potential virus transmission
in each state was spatially evaluated on a 10-km grid using average historical daily temperature data from 1994 to 2003. The
highest levels of transmission risk occur in California and Texas, with parts of these states at risk of RVFV transmission
for up to 8 months per year. Northern Minnesota, central New York, and most of coastal and high-elevation California are at
low to null risk. Risk of impact to the livestock industry is greatest in California, Texas, and Nebraska. A standard global
climate model was used to evaluate future risk in the year 2030 in Nebraska, and showed an increase of transmission risk days
from approximately 3 to 4 months per year. 相似文献
156.
Mobilisation of people living with HIV/AIDS in the 1980s was tied to social identity and the public rendering of AIDS as a
gay disease. This activism has continued in large cities alongside new streams of mobilization addressing the needs of groups
such as women and Aboriginal peoples living with HIV. We draw on interviews conducted with six people living with HIV/AIDS
in a small Ontario city where the population does not allow for services tailored to specific groups. We investigate experiences
of social engagement amongst people living with, and affected by, HIV/AIDS and examine why efforts to establish a community-based
participatory research project was unsuccessful. Drawing from indicators of community capacity developed by Jackson et al.
(Health Promot Int 18(4): 339–350, 2003) we identify the role of stigma both amongst different groups living with HIV/AIDS
and the wider public as undermining a common sense of community and, potentially, our efforts to establish a community-based
participatory research process. 相似文献
157.
The Padu system of South Asia has received growing attention as an example of customary marine tenure that has survived despite rapid development and change throughout the region’s fisheries. This paper describes the Padu system as it functions at Pulicat lagoon, India, where it has enjoyed decades of legitimacy amongst its members, and has contributed to sustainable fishing. Recently, however, the Padu system has become unstable, driven by pressures of an expanding fishing population, reduced access to fishing grounds and a growing ‘shared poverty’. In spite of this, fisher loyalty to uphold the Padu system remains strong. This raises questions about the broader social, political and cultural meanings of Padu, which extend beyond access to a lucrative fishery. The paper highlights a trade-off between the benefits received through Padu membership at a societal level through collective action, and the individual costs of partaking in ‘shared poverty’, which is inherently distributed unequally amongst fishing families. The paper concludes with a discussion on the future of the Padu system in Pulicat, drawing from evolutionary pathways of other Padu systems in the region. It is suggested that, in the Pulicat case, the high social values attributed to the Padu system, alongside complex power structures, may hinder institutional adaptation. 相似文献
158.
Magalie Castelin Nicolas PuillandrePierre Lozouet Alexander SysoevBertrand Richer de Forges Sarah Samadi 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(6):637-646
Seamounts were often considered as ‘hotspots of diversity’ and ‘centers of endemism’, but recently this opinion has been challenged. After 25 years of exploration and the work of numerous taxonomists, the Norfolk Ridge (Southwest Pacific) is probably one of the best-studied seamount chains worldwide. However, even in this intensively explored area, the richness and the geographic patterns of diversity are still poorly characterized. Among the benthic organisms, the post-mortem remains of mollusks can supplement live records to comprehensively document geographical distributions. Moreover, the accretionary growth of mollusk shells informs us about the life span of the pelagic larva. To compare diversity and level of endemism between the Norfolk Ridge seamounts and the continental slopes of New Caledonia we used species occurrence data drawn from (i) the taxonomic literature on mollusks and (ii) a raw dataset of mainly undescribed deep-sea species of the hyperdiverse Turridae. Patterns of endemism and species richness were analyzed through quantitative indices of endemism and species richness estimator metrics. To date, 403 gastropods and bivalves species have been recorded on the Norfolk Ridge seamounts. Of these, at least 38 species (∼10%) are potentially endemic to the seamounts and nearly all of 38 species have protoconchs indicating lecithotrophic larval development. Overall, our results suggest that estimates of species richness and endemism, when sampling effort is taken into account, were not significantly different between slopes and seamounts. By including in our analyses 347 undescribed morphospecies from the Norfolk Ridge, our results also demonstrate the influence of taxonomic bias on our estimates of species richness and endemism. 相似文献
159.
Aggregate size distribution affects the soil’s physical quality and fertility. Widely used aggregation indices include the size fractions <0.25 mm, >2 mm, and >8 mm, and the mean weight diameter of aggregates. A recent study from the semi-arid hilly rangeland of the Negev desert, Israel revealed considerable impact of the type of surface cover (shrub patches, trampling routes, and the remainder of the interpatch spaces) on these indices, but only limited impact of hillside aspect (north- vs. south-facing) and a small impact of livestock grazing. The aim of this study was to address the impact of these factors on the poorly studied mid-size aggregate fraction of 1-5 mm as a complementary index of the soil’s physical quality. The content and mean weight diameter of this fraction were analyzed by using the same set of soil samples. As a general trend, compared with the widely used aggregation indices, the content of this fraction was found to be more responsive to livestock grazing. Furthermore, both the content and mean weight diameter of this fraction were found to be highly responsive to type of cover. These effects reveal positive relationships between this fraction and the soil’s physical quality. 相似文献
160.
Sarah E. Gelman Chad D. Deering Francisco J. Gutierrez Olivier Bachmann 《Contributions to Mineralogy and Petrology》2013,166(5):1355-1374
The 20 ka ~0.1 km3 Omega dacite, which erupted shortly after the 26.5 ka Oruanui super-eruption, compositionally stands out among Taupo Volcanic Zone (TVZ) magmas, which are overwhelmingly characterized by rhyolites (>90 % by volume). The previously reported presence of inherited zircons in this zircon-undersaturated magma has provided unequivocal evidence for the involvement of upper-crustal material in a 1–10 year timescale prior to the Omega eruption. However, whether this crustal involvement is characterized by wholesale, melting of preexisting crust or subordinate bulk assimilation into an already differentiated magma body remains unclear. To disentangle these processes, we describe the mineral chemistry of the major phases present in the Omega dacite and determine intensive parameters describing magma chamber conditions. Dominantly unimodal populations of plagioclase (An50–60), orthopyroxene (Mg# from 58 to 68), and clinopyroxene (Mg# from 65 to 73), along with coexisting equilibrium pairs of Fe–Ti oxides, constrain pre-eruptive temperatures to 850–950 °C, a pressure between ~3 and 7 kbars, and an oxygen fugacity of ~NNO. MELTS thermodynamic modeling suggests that this phase assemblage is in equilibrium with the bulk rock and glass compositions of the Omega dacite at these estimated P–T–fO2 pre-eruptive conditions. Combining these petrological observations with insights into conductive thermal models of magma–crust interactions, we argue that the Omega dacite more likely formed in the mid-to-lower crust via protracted processing through fractional crystallization coupled with some assimilation (AFC). Incorporation of crustal material is likely to have occurred at various stages, with the inherited zircons (and potentially parts of glomerocrysts) representing late and subordinate upper-crustal assimilants. This petrogenetic model is consistent with the presence of a differentiating crustal column, consisting of a polybaric fractional crystallization and assimilation history. On the basis of petrological, thermal, and geophysical considerations, upper-crustal reservoirs, which feed large-scale rhyolitic volcanism in the TVZ, most likely take the form of large, long-lived crystal mush zones. Following large eruptions, such as the Oruanui event, this mush is expected to crystallize significantly (up to 70–80 vol% crystals) due to syn-eruptive decompression. Hence, the Omega dacite, immediately post-dating the Oruanui event, potentially represents incoming deeper recharge of less-evolved magma that was able to penetrate the nearly solidified upper-crustal mush. Over the past 20,000 years, similar intermediate recharge magmas have incrementally reheated, reconstructed, and reactivated the upper-crustal mush zone, allowing a gradual return to rhyolitic volcanism at the Taupo Volcanic Center. 相似文献