全文获取类型
收费全文 | 969篇 |
免费 | 27篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 25篇 |
大气科学 | 36篇 |
地球物理 | 231篇 |
地质学 | 438篇 |
海洋学 | 70篇 |
天文学 | 107篇 |
综合类 | 7篇 |
自然地理 | 83篇 |
出版年
2021年 | 6篇 |
2020年 | 7篇 |
2019年 | 10篇 |
2018年 | 22篇 |
2017年 | 16篇 |
2016年 | 24篇 |
2015年 | 12篇 |
2014年 | 24篇 |
2013年 | 47篇 |
2012年 | 35篇 |
2011年 | 37篇 |
2010年 | 44篇 |
2009年 | 57篇 |
2008年 | 50篇 |
2007年 | 39篇 |
2006年 | 42篇 |
2005年 | 52篇 |
2004年 | 43篇 |
2003年 | 50篇 |
2002年 | 25篇 |
2001年 | 22篇 |
2000年 | 22篇 |
1999年 | 12篇 |
1998年 | 17篇 |
1997年 | 14篇 |
1996年 | 20篇 |
1995年 | 8篇 |
1994年 | 5篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 15篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 12篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1973年 | 4篇 |
1970年 | 5篇 |
1969年 | 5篇 |
排序方式: 共有997条查询结果,搜索用时 828 毫秒
161.
Bart Christiaen Rebecca J. Bernard Behzad Mortazavi Just Cebrian Alice C. Ortmann 《Marine pollution bulletin》2014
Many studies show that seagrass δ15N ratios increase with the amount of urbanization in coastal watersheds. However, there is little information on the relationship between urbanization and seagrass δ15N ratios on a global scale. We performed a meta-analysis on seagrass samples from 79 independent locations to test if seagrass δ15N ratios correlate with patterns of population density and fertilizer use within a radius of 10–200 km around the sample locations. Our results show that seagrass δ15N ratios are more influenced by intergeneric and latitudinal differences than the degree of urbanization or the amount of fertilizer used in nearby watersheds. The positive correlation between seagrass δ15N ratios and latitude hints at an underlying pattern in discrimination or a latitudinal gradient in the 15N isotopic signature of nitrogen assimilated by the plants. The actual mechanisms responsible for the correlation between δ15N and latitude remain unknown. 相似文献
162.
Barbara J. Thompson Sarah E. Gibson Peter C. Schroeder David F. Webb Charles N. Arge Mario M. Bisi Giuliana de Toma Barbara A. Emery Antoinette B. Galvin Deborah A. Haber Bernard V. Jackson Elizabeth A. Jensen Robert J. Leamon Jiuhou Lei Periasamy K. Manoharan M. Leila Mays Patrick S. McIntosh Gordon J. D. Petrie Simon P. Plunkett Liying Qian Peter Riley Steven T. Suess Munetoshi Tokumaru Brian T. Welsch Thomas N. Woods 《Solar physics》2011,274(1-2):29-56
We present an overview of the data and models collected for the Whole Heliosphere Interval, an international campaign to study the three-dimensional solar?Cheliospheric?Cplanetary connected system near solar minimum. The data and models correspond to solar Carrington Rotation 2068 (20 March??C?16 April 2008) extending from below the solar photosphere, through interplanetary space, and down to Earth??s mesosphere. Nearly 200 people participated in aspects of WHI studies, analyzing and interpreting data from nearly 100 instruments and models in order to elucidate the physics of fundamental heliophysical processes. The solar and inner heliospheric data showed structure consistent with the declining phase of the solar cycle. A closely spaced cluster of low-latitude active regions was responsible for an increased level of magnetic activity, while a highly warped current sheet dominated heliospheric structure. The geospace data revealed an unusually high level of activity, driven primarily by the periodic impingement of high-speed streams. The WHI studies traced the solar activity and structure into the heliosphere and geospace, and provided new insight into the nature of the interconnected heliophysical system near solar minimum. 相似文献
163.
J. Christopher L’Esperance Bernard P. Boudreau Mark A. Barry Bruce D. Johnson 《Geo-Marine Letters》2013,33(1):75-81
Poisson’s ratio (static) appears as a parameter in models of soft-sediment geomechanical processes such as gas bubble formation and bioturbation, and in a number of practical applications, including acoustic imaging, but is rarely measured in soft near-surface (upper 20 cm) marine sediments. A new uniaxial apparatus was developed to assess Poisson’s ratios of cylindrical samples of soft cohesive sediments under finite-strain, unconfined, undrained conditions. No constraining membrane around the sample is necessary in the instrument. By monitoring the fluid level in a capillary, changes in sample volume can be determined to high precision (reproducibility). Measurements on reference materials, i.e., gelatine and polyurethane, also indicate high accuracy (veracity). For natural sediments from Nova Scotia, Canada, Poisson’s ratios range from 0.4951 to 0.5 with no indication of anisotropy; therefore, such sediments act as incompressible solids to short-term/small-magnitude stresses, as occur during bubble formation and infaunal movement. 相似文献
164.
Abstract On the basis of the Atterberg indexes of various sediments collected in the Mediterranean Sea, we select two major parameters indicative of the mechanical behavior of marine soils: the sedimentary facies (i.e., mechanism of deposition expressed by silt content) and the clay mineral association characterized by smectite content in the fraction finer than 2 μm. Activity and real thixotropic ability appear to be particularly important parameters in marine engineering; better values are obtained by a double test operation in distilled and marine water. 相似文献
165.
Space- and ground-based measurements of sulphur dioxide emissions from Turrialba Volcano (Costa Rica) 总被引:2,自引:0,他引:2
Robin Campion Maria Martinez-Cruz Thomas Lecocq Corentin Caudron Javier Pacheco Gaia Pinardi Christian Hermans Simon Carn Alain Bernard 《Bulletin of Volcanology》2012,74(7):1757-1770
Remotely sensed measurements of sulphur dioxide (SO2) emitted by Turrialba Volcano (Costa Rica) are reported for the period September 2009–January 2011. These measurements were obtained using images from Advanced Spaceborne Thermal Emission and Reflexion radiometer, Ozone Monitoring Instrument and a ground-based UV camera. These three very different instruments provide flux measurements in good agreement with each other, which demonstrate that they can be integrated for monitoring SO2 fluxes. Fluxes from Turrialba increased fourfold in January 2010, following a phreatic explosion that formed a degassing vent in the W crater of Turrialba. Since then, the SO2 flux has remained high (30–50?kg/s) but seems to be showing a slowly decreasing trend. We interpret this evolution as the start of open vent degassing from a recently intruded magma body. The opening of the degassing vent decreased the confining pressure of the magma body and allowed the gases to bypass the hydrothermal system. 相似文献
166.
Bruce D. Johnson Mark A. Barry Bernard P. Boudreau Peter A. Jumars Kelly M. Dorgan 《Geo-Marine Letters》2012,32(1):39-48
This study reports the first in situ measurements of tensile fracture toughness, K
IC, of soft, surficial, cohesive marine sediments. A newly developed probe continuously measures the stress required to cause
tensile failure in sediments to depths of up to 1 m. Probe measurements are in agreement with standard laboratory methods
of K
IC measurements in both potter’s clay and natural sediments. The data comprise in situ depth profiles from three field sites
in Nova Scotia, Canada. Measured K
IC at two muddy sites (median grain size of 23–50 μm) range from near zero at the sediment surface to >1,800 Pa m1/2 at 0.2 m depth. These profiles also appear to identify the bioturbated/mixed depth. K
IC for a sandy site (>90% sand) is an order of magnitude lower than for the muddy sediments, and reflects the lack of cohesion/adhesion.
A comparison of K
IC, median grain size, and porosity in muddy sediments indicates that consolidation increases fracture strength, whereas inclusion
of sand causes weakening; thus, sand-bearing layers can be easily identified in K
IC profiles. K
IC and vane-measured shear strength correlate strongly, which suggests that the vane measurements should perhaps be interpreted
as shear fracture toughness, rather than shear strength. Comparison of in situ probe-measured values with K
IC of soils and gelatin shows that sediments have a K
IC range intermediate between denser compacted soils and softer, elastic gelatin. 相似文献
167.
Mixed estimation methods for Halphen distributions with applications in extreme hydrologic events 总被引:3,自引:1,他引:2
Fateh Chebana Salaheddine El Adlouni Bernard Bobée 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(3):359-376
The Halphen family of distributions is a flexible and complete system to fit sets of observations independent and identically
distributed. Recently, it is shown that this family of distributions represents a potential alternative to the generalized
extreme value distributions to model extreme hydrological events. The existence of jointly sufficient statistics for parameter
estimation leads to optimality of the method of maximum likelihood (ML). Nevertheless, the ML method requires numerical approximations
leading to less accurate values. However, estimators by the method of moments (MM) are explicit and their computation is fast.
Even though MM method leads to good results, it is not optimal. In order to combine the advantages of the ML (optimality)
and MM (efficiency and fast computations), two new mixed methods were proposed in this paper. One of the two methods is direct
and the other is iterative, denoted respectively direct mixed method (MMD) and iterative mixed method (MMI). An overall comparison
of the four estimation methods (MM, ML, MMD and MMI) was performed using Monte Carlo simulations regarding the three Halphen
distributions. Generally, the MMI method can be considered for the three Halphen distributions since it is recommended for
a majority of cases encountered in hydrology. The principal idea of the mixed methods MMD and MMI could be generalized for
other distributions with complicated density functions. 相似文献
168.
169.
170.
Marie Boucher Guillaume Favreau Marc Descloitres Jean-Michel Vouillamoz Sylvain Massuel Yahaya Nazoumou Bernard Cappelaere Anatoli Legchenko 《Comptes Rendus Geoscience》2009,341(10-11):800-809
Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling. 相似文献