全文获取类型
收费全文 | 2676篇 |
免费 | 142篇 |
国内免费 | 33篇 |
专业分类
测绘学 | 112篇 |
大气科学 | 229篇 |
地球物理 | 611篇 |
地质学 | 993篇 |
海洋学 | 227篇 |
天文学 | 404篇 |
综合类 | 14篇 |
自然地理 | 261篇 |
出版年
2023年 | 15篇 |
2022年 | 23篇 |
2021年 | 66篇 |
2020年 | 89篇 |
2019年 | 79篇 |
2018年 | 90篇 |
2017年 | 108篇 |
2016年 | 130篇 |
2015年 | 94篇 |
2014年 | 116篇 |
2013年 | 165篇 |
2012年 | 127篇 |
2011年 | 173篇 |
2010年 | 149篇 |
2009年 | 153篇 |
2008年 | 136篇 |
2007年 | 99篇 |
2006年 | 95篇 |
2005年 | 97篇 |
2004年 | 82篇 |
2003年 | 80篇 |
2002年 | 66篇 |
2001年 | 51篇 |
2000年 | 45篇 |
1999年 | 33篇 |
1998年 | 27篇 |
1997年 | 32篇 |
1996年 | 30篇 |
1995年 | 28篇 |
1994年 | 14篇 |
1993年 | 16篇 |
1992年 | 22篇 |
1991年 | 18篇 |
1990年 | 22篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 19篇 |
1986年 | 8篇 |
1985年 | 21篇 |
1984年 | 26篇 |
1983年 | 18篇 |
1982年 | 19篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 8篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 16篇 |
排序方式: 共有2851条查询结果,搜索用时 0 毫秒
121.
Ernest S. GLADNEY David B. CURTIS Daniel R. PERRIN 《Geostandards and Geoanalytical Research》1984,8(1):43-46
Concentrations of boron in 35 silicate reference materials are measured by thermal neutron capture gamma-ray spectrometry. Results are compared for NBS, USGS, and CCRMP reference materials with values from the literature. The use of two prompt gamma facilities at Los Alamos are discussed. 相似文献
122.
Daniel J. Fornari 《Bulletin of Volcanology》1986,48(5):291-298
High-resolution, side-looking sonar surveys of the East Pacific rise and seamounts in the eastern Pacific have revealed the common presence of lava tubes and channels in seafloor volcanic terrains. Tube and channel systems commonly issue from small volcanic cones or domes, are continuous for distances of 1–3 km, and are considered to be important in distributing lavas around seafloor extrusive sites and creating characteristic morphology. Eruption rate and volume are probably the most important controlling factors in determining whether tubes or channels will form. Volatile content and state, slope angle, and preeruption surface morphology are secondary factors that influence the ease with which a tube or channel may form and its direction and shape. Seafloor tubes and channels may also have a profound influence on the structure and evolution of the upper oceanic crust. Tubes and channels are likely to form intracrustal horizontal pathways for circulating hydrothermal fluids. Flow differentiation processes acting within tubes and channels may affect the chemical composition of lavas and could be partially responsible for the chemical diversity of rocks along accretionary boundaries. Seafloor tubes and channels that are either partially water-filled or contain volcaniclastics which are then buried within the volcanic pile can lower the compressional velocity and seismic response of the upper oceanic crustal layer along spreading axes. 相似文献
123.
Michael J. Barcelona Jie Lu Daniel M. Tomczak 《Ground Water Monitoring & Remediation》1995,15(2):114-124
Evidence for the natural microbial remediation of subsurface fuel contamination situations should include identification and analysis of transformation or degradation products. In this way. u mass balance between fuel constituents and end products may be approached to monitor cleanup progress. Application of advanced organic acid metabolite derivatization techniques to several known sites of organic compounds and fuel mixture contamination provide valuable information on the pathways and progress of microbial transformation. Good correlation between observed metabolites and transformation pathways of aromatic fuel constituents were observed at the sites. 相似文献
124.
125.
Compositional and thermal convection in magma chambers 总被引:7,自引:1,他引:7
Daniel Martin Ross W. Griffiths Ian H. Campbell 《Contributions to Mineralogy and Petrology》1987,96(4):465-475
Magma chambers cool and crystallize at a rate determined by the heat flux from the chamber. The heat is lost predominantly through the roof, whereas crystallization takes place mainly at the floor. Both processes provide destabilizing buoyancy fluxes which drive highly unsteady, chaotic convection in the magma. Even at the lowest cooling rates the thermal Rayleigh number Ra is found to be extremely large for both mafic and granitic magmas. Since the compositional and thermal buoyancy fluxes are directly related it can be shown that the compositional Rayleigh number Rs (and therefore a total Rayleigh number) is very much greater than Ra. In the case of basaltic melt crystallizing olivine Rs is up to 106 times greater than Ra. However compositional and thermal buoyancy fluxes are roughly equal. Therefore thermal and compositional density gradients contribute equally to convection velocities in the interior of the magma. Effects of thermal buoyancy generated by latent heat release at the floor are included.The latent heat boundary layer at the floor of a basaltic chamber is shown to be of the order of 1 m thick with very low thermal gradients whereas the compositional boundary layer is about 1 cm thick with large compositional gradients. As a consequence, the variation in the degree of supercooling in front of the crystal-liquid interface is dominated by compositional effects. The habit and composition of the growing crystals is also controlled by the nature of the compositional boundary layer. Elongate crystals are predicted to form when the thickness of the compositional boundary layer is small compared with the crystal size (as in laboratory experiments with aqueous solutions). In contrast, equant crystals form when the boundary layer is thicker than the crystals (as in most magma chambers). Instability of the boundary layer in the latter case gives rise to zoning within crystals. Diffusion of compatible trace elements through the boundary layer can also explain an inverse correlation, observed in layered intrusions, between Ni concentration in olivine and the proportion of Ni-bearing phases in the crystallizing assemblage. 相似文献
126.
Multi-borehole pumping and tracer tests on the 10 to 100-m scale were conducted in a fractured chalk aquitard in the Negev
Desert, Israel. Outcrop and core fracture surveys, as well as slug tests in packed-off intervals, were carried out at this
site to obtain the parameters needed for construction of a stochastic discrete fracture network (DFN). Calibration of stochastic
DFNs directly to the multiple borehole test data was inadequate. Instead, two equivalent deterministic DFN flow models were
used: the vertical-fractures (VF) model, consisting of only vertical fractures, and the fractures’ intersections (INT) model,
consisting of vertical and horizontal fractures with enhanced transmissivity at their intersections. Both models were calibrated
against the multi-borehole response of one pumping test and their predictions were tested against three other independent
pumping tests. The average accuracies of all transient drawdown predictions of the VF and INT models were 65 and 66%, respectively.
In contrast to this equality in average drawdown predictions of both models, the INT model predicted better important breakthrough
curve features (e.g., first and peak arrival times), than the VF model. This result is in line with previously assumed channeled
flow, derived from analytical analysis of these pumping and tracer tests.
Ronit Nativ, deceased, may her memory be blessed. 相似文献
127.
Daniel Bernoulli Lukas Hottinger Silvia Spezzaferri Peter Stille 《Swiss Journal of Geoscience》2007,100(2):215-225
Shallow-water limestones of presumed Late Cretaceous and Eocene age, interbedded with basaltic lavas, were described by earlier authors from São Nicolau in the northwestern part of the Cabo Verde archipelago. If confirmed, these ages would imply late Mesozoic shallow-marine and subaerial volcanic activity in the Cabo Verde archipelago, and document a geological history very different from that known so far from other Cabo Verde Islands, from which no subaerial volcanic activity before the mid-Cenozoic is known. Our re-investigation of the foraminiferal fauna indicates a Late Miocene age for the presumed Late Cretaceous and Eocene limestones. The hypothesis of a long-lived hot spot, active by the Early Cretaceous, and of a major island-building stage in the Cabo Verde Islands during this period, is therefore not supported by the present bio- or chronostratigraphic data. 相似文献
128.
Air was injected through a well in a thin transparent tank filled with saturated glass beads to study how the size and air
saturation of air sparging plumes are affected by particle size and gradation; operational parameters such as injection pressure,
well depth, injection pressure pulsing; and well outlet configuration. V-shaped air plumes with an apex between 40° and 60°
were obtained for all tests. The air pressure required to initiate sparging agreed closely with the sum of the air entry pressure
and the hydrostatic pressure, with higher initiation pressures required in the fine and well-graded beads. Higher air flow
rates and air saturations were obtained in coarser beads at a given pressure, and the variation in flow rate was consistent
with estimated air permeabilities. Peak average air saturations were 28–56% for the coarse-medium beads, 10% for the well-graded
beads, and 8% for the fine beads. Air saturation and the radius of influence increased modestly (<40%) as the normalized injection
pressure exceeded 0.1. Radius of influence increased by approximately a factor of two as the well depth increased, but leveled
off once the ratio of radius of influence to well depth reached 0.60–1.05. Pulsing of injection pressure had no effect on
the initiation pressure, air flow rate, or air saturation, but increased the size of the air plume and the radius of influence
slightly (<15%). Well outlet configuration had only a slight affect the radius of influence (<10%), air saturation (<10%),
or air flow rate (<12%). Dye testing showed that water surrounding the air plume circulated during continuous and pulsed sparging.
However, pulsed sparging resulted in greater and more defined circulation of water within and adjacent to the air plume, which
should reduce mass transfer limitations during sparging. 相似文献
129.
Morphologic variability of exposed mass-transport deposits on the eastern slope of Gela Basin (Sicily channel) 总被引:1,自引:0,他引:1
Daniel Minisini§ Fabio Trincardi Alessandra Asioli† Marcello Canu‡ Federica Foglini 《Basin Research》2007,19(2):217-240
The NE portion of Gela Basin in the Sicily Channel is affected by multiple slope failures originated during the late‐Quaternary. Basin sequences show evidence of stacked acoustically transparent and/or chaotic units, characterized by irregular upper surfaces, interpreted as mass‐transport deposits. The seafloor morphology also shows evidence of both old, partially buried, as well as recent slide products. Two recent slides exposed at seafloor, only 6 km apart (Twin Slides), are similar in geomorphological parameters, age and multistage evolution. Multistage failure of Twin Slides evolved from mud flows, derived from the extensive failure of less consolidated post‐glacial units, to localized slides (second stage of failure) affecting older and more consolidated materials. Although Twin Slides are very close to each other and have similar runout and fall height, they produced very dissimilar organization of the displaced masses, likely reflecting the distinct source units affected by failures. Integrating geophysical, sedimentological, structural and palaeontological data, a detailed investigation was conducted to determine the size and internal geometry of this mass‐transport complex, to explain the differentiated product and to shed light on its predisposing factors, triggers and timing. 相似文献
130.
W. Douglas Thompson Daniel Wartenberg 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(5):635-646
Much research in environmental epidemiology relies on aggregate-level information on exposure to potentially toxic substances
and on relevant covariates. We compare the use of additive (linear) and multiplicative (log-linear) regression models for
the analysis of such data. We illustrate how both additive and multiplicative models can be fit to aggregate-level data sets
in which disease incidence is the dependent variable, and contrast these results with similar models fitted to individual-level
data. We find (1) that for aggregate-level data, multiplicative models are more likely than additive models to introduce bias
into the estimation of rates, an effect not found with individual-level data; and (2) that under many circumstances multiplicative
models reduce the precision of the estimates, an effect also not found in individual-level models. For both additive and multiplicative
models of aggregate-level data, we find that, in the presence of covariates, narrow confidence interval are obtained only
when two or more antecedent factors are strongly related to the measured covariate and/or the exposure of primary substantive
interest. We conclude that the equivalency of fitting additive versus multiplicative models in studies with individual-level
binary data does not carry over to studies that analyze aggregate-level information. For aggregate data, we strongly recommend
use of additive models.
Supported by Grant #1 U19 EH000102 from the National Center for Environmental Health, Centers for Disease Control and Prevention,
Atlanta, GA. 相似文献