全文获取类型
收费全文 | 6536篇 |
免费 | 285篇 |
国内免费 | 81篇 |
专业分类
测绘学 | 235篇 |
大气科学 | 515篇 |
地球物理 | 1548篇 |
地质学 | 2185篇 |
海洋学 | 523篇 |
天文学 | 1191篇 |
综合类 | 30篇 |
自然地理 | 675篇 |
出版年
2022年 | 30篇 |
2021年 | 75篇 |
2020年 | 87篇 |
2019年 | 122篇 |
2018年 | 179篇 |
2017年 | 162篇 |
2016年 | 233篇 |
2015年 | 174篇 |
2014年 | 197篇 |
2013年 | 418篇 |
2012年 | 259篇 |
2011年 | 344篇 |
2010年 | 296篇 |
2009年 | 387篇 |
2008年 | 339篇 |
2007年 | 294篇 |
2006年 | 274篇 |
2005年 | 263篇 |
2004年 | 261篇 |
2003年 | 212篇 |
2002年 | 216篇 |
2001年 | 115篇 |
2000年 | 144篇 |
1999年 | 104篇 |
1998年 | 115篇 |
1997年 | 81篇 |
1996年 | 89篇 |
1995年 | 85篇 |
1994年 | 91篇 |
1993年 | 74篇 |
1992年 | 88篇 |
1991年 | 67篇 |
1990年 | 57篇 |
1989年 | 52篇 |
1988年 | 57篇 |
1987年 | 52篇 |
1986年 | 56篇 |
1985年 | 70篇 |
1984年 | 66篇 |
1983年 | 77篇 |
1982年 | 62篇 |
1981年 | 66篇 |
1980年 | 56篇 |
1979年 | 67篇 |
1978年 | 53篇 |
1977年 | 37篇 |
1976年 | 28篇 |
1975年 | 29篇 |
1974年 | 28篇 |
1973年 | 31篇 |
排序方式: 共有6902条查询结果,搜索用时 546 毫秒
471.
Geochemical modeling of magma mixing allows for evaluation of volumes of magma storage reservoirs and magma plumbing configurations.
A new analytical expression is derived for a simple two-component box-mixing model describing the proportions of mixing components
in erupted lavas as a function of time. Four versions of this model are applied to a mixing trend spanning episodes 3–31 of
Kilauea Volcano’s Puu Oo eruption, each testing different constraints on magma reservoir input and output fluxes. Unknown
parameters (e.g., magma reservoir influx rate, initial reservoir volume) are optimized for each model using a non-linear least
squares technique to fit model trends to geochemical time-series data. The modeled mixing trend closely reproduces the observed
compositional trend. The two models that match measured lava effusion rates have constant magma input and output fluxes and
suggest a large pre-mixing magma reservoir (46±2 and 49±1 million m3), with little or no volume change over time. This volume is much larger than a previous estimate for the shallow, dike-shaped
magma reservoir under the Puu Oo vent, which grew from ∼3 to ∼10–12 million m3. These volumetric differences are interpreted as indicating that mixing occurred first in a larger, deeper reservoir before
the magma was injected into the overlying smaller reservoir.
Electronic Supplementary Material Supplementary material is available at and is accessible for authorized users. 相似文献
472.
Michael Tiefelsdorf 《Stochastic Environmental Research and Risk Assessment (SERRA)》2007,21(5):615-624
Analytical disease mapping aims at linking control variables, the exposure to potential risk factors and to protective elements
with the geographical distribution of disease events. Its goals are (a) to identify areas of atypical low and high disease
risk and (b) to enhance our understanding of the underlying disease generating process. However, along with many other obstacles
in analytical disease mapping, interregional migration masks for degenerative diseases with a long latency period any direct
regional association between the observed disease rates and risks factors. Prostate gland cancer mortality rates of the 508
State Economic Areas (SEA) for the reporting period 1970–1994 are linked in an ecological regression model to proxy variables
of putatively influential factors. A semi-parametric geographical filtering approach that uses the 1965–1970 interregional
census migration flows among the SEA as underlying spatial structure is applied to control for confounding migration effects.
This approach allows, despite some simplifying assumptions, to assess direct effects of potential risk factors on the spatial
variability of prostate cancer and to visualize underlying spatial dimensions of the disease generating process. Results indicate
a shift in relevance of the incorporated exogenous variables. 相似文献
473.
474.
Michael H. Cosh Jery R. Stedinger Steve C. Ou Kuo-Nan Liou Wilfried Brutsaert 《Advances in water resources》2007
This study focuses on how the variability of land surface temperature and vegetation density at the SGP ARM-CART site changes over episodic (day to day) and seasonal time scales using AVHRR satellite data. Four drying periods throughout the year are analyzed. Land surface temperature had an erratic relationship with time exhibiting no deterministic pattern from day-to-day or season-to-season. Furthermore, it did not exhibit spatial pattern persistence. On the other hand, vegetation density had a consistent spatial pattern and temporal decay during average length drying periods (less than 7 days) as well as within a season. However, there were distinct differences in the seasonal pattern of variation between winter and growing seasons. In addition, the paper highlights a methodology to quantify the relationships that exist at the land surface between the primary parameter of interest and the controlling variables. 相似文献
475.
476.
477.
Gert Lube Shane J. Cronin Thomas Platz Armin Freundt Jonathan N. Procter Cargill Henderson Michael F. Sheridan 《Journal of Volcanology and Geothermal Research》2007
Small-volume pyroclastic density currents (PDCs) are generated frequently during explosive eruptions with little warning. Assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and/or deposits. To this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small-volume PDC deposits from the 1975 eruption of Ngauruhoe volcano, whose emplacement were also witnessed during eruption. Using high-resolution GPS surveys, a series of lateral excavations across the deposits, and bulk sedimentological analysis we constrained the geomorphology, internal structure and texture of the deposits with respect to laterally varying modes of deposition. 相似文献
478.
Use of GPS tracking data from different dual-frequency receiver types (cross-correlating vs. codeless) has revealed satellite-dependent
biases in pseudorange observables P1 (Y-code) and C1 (C/A, Clear Acquisition code). These biases can have a direct effect
on clock estimates, carrier phase bias fixing, and other parameters estimated in GPS data processing. A set of satellite-specific
compensatory pseudorange offsets is calculated, and each is applied to a wee of daily global network analyses in which satlellite,
receiver, atmospheric, and Earth rotation parameters are estimated. Results from these analyses are then compared to those
from corresponding baseline cases in which no biases were applied. There is also some evidence that suggests that the pseudorange
biases differ even among codeless receiver models. Hence, a second set of offsets is computed on a different basis, and compared
with the baseline model in a similar manner. A preliminary examination of C1-P1 variations over time is presented. Finally,
recommendations are made for the use of the calculated offsets, and consideration is given to a future dissemination of updates
to these values as necessary. ? 2001 John Wiley & Sons, Inc. 相似文献
479.
Analysis And Simulation Of Surface-Layer Winds Using Multiplicative Cascade Models With Self-Similar Probability Densities 总被引:1,自引:0,他引:1
Statistical analysis techniques based on multiplicative cascades are investigated for use with surface-layer wind data sets collected in the atmospheric boundary layer over flat farm land. The data were found to exhibit multiscaling statistics, allowing the surface-layer winds to be simulated with the use of multiplicative random cascades. The study found evidence that, for the surface-layer at least, these cascade models (andhence the methods of multifractal analysis) should be applied in separate ways to the microscale inertial range, and the mesoscale. This is at odds with the view found in the existing literature, which proposes a `universal multifractal' model to replace the widely held view that there exists separate microscale, mesoscale and synoptic scales for which the processes governing each are different. At least two separate ranges of scaling are suggested for surface-layer wind data, corresponding to the microscale inertial range and the mesoscale. For the case of the mesoscale range, a self-similar distribution of weighting factors was found for the wind speed data themselves, rather than for an intermediate (dissipation) field, as is required for themicroscale data. 相似文献
480.
The karst landscape in the interior of the Philippines' Bohol Province represents one of the world's premier kegelkarst (cone karst) environments. Government efforts to protect some of this karst, exemplified by the establishment of the Rajah Sikatuna National Park and the Chocolate Hills Natural Monument, have proven to be significant catalysts of social conflict. In Bohol there is a long history of traditional land tenure, which has recently been supplanted by a Westernized model. Protected area establishment is a response to deforestation, agricultural exploitation and uncontrolled quarrying. However, the imposition of protective legislation to prevent further degradation has disenfranchized and marginalized many local farmers and residents. The conflict between the obligation of the State to ensure environmental protection and the perceived property rights of landowners and farmers has provoked an escalation in civil unrest and armed conflict. 相似文献