全文获取类型
收费全文 | 1142篇 |
免费 | 72篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 46篇 |
大气科学 | 103篇 |
地球物理 | 247篇 |
地质学 | 398篇 |
海洋学 | 87篇 |
天文学 | 242篇 |
综合类 | 3篇 |
自然地理 | 102篇 |
出版年
2023年 | 3篇 |
2022年 | 3篇 |
2021年 | 16篇 |
2020年 | 22篇 |
2019年 | 31篇 |
2018年 | 45篇 |
2017年 | 39篇 |
2016年 | 45篇 |
2015年 | 40篇 |
2014年 | 35篇 |
2013年 | 78篇 |
2012年 | 56篇 |
2011年 | 64篇 |
2010年 | 48篇 |
2009年 | 66篇 |
2008年 | 63篇 |
2007年 | 56篇 |
2006年 | 61篇 |
2005年 | 40篇 |
2004年 | 48篇 |
2003年 | 60篇 |
2002年 | 31篇 |
2001年 | 23篇 |
2000年 | 27篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 17篇 |
1996年 | 14篇 |
1995年 | 12篇 |
1994年 | 12篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 11篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1971年 | 5篇 |
排序方式: 共有1228条查询结果,搜索用时 15 毫秒
851.
Isotopic biogeochemistry of dissolved organic nitrogen: A new technique and application 总被引:1,自引:0,他引:1
We present a new technique for isolating and isotopically characterizing dissolved organic nitrogen (DON) for non-marine waters, δ15N values for DON from lacustrine samples and data suggesting that this technique will be applicable to marine samples. Our technique involves preconcentration of DON via rotary evaporation and removal of dissolved inorganic nitrogen (DIN) via dialysis using a membrane that retains material above 100 Da. Results demonstrate quantitative removal of DIN, complete recovery of DON (95% or greater) and retention of isotopic integrity (isotope effect less than 0.4‰) for a solution containing a DON standard (tripeptide) and DIN in deionized water. Reproducibility of carbon and nitrogen isotope values and elemental concentrations is demonstrated for DOM from Chefswet Basin, Lake Superior and Grand Traverse Bay, Lake Michigan. The applicability of this technique to marine samples is suggested by demonstrating 99% removal of DIN from a sample of Gulf Stream water amended with ammonium and nitrate. 相似文献
852.
853.
Effects of fractures on seismic-wave velocity and attenuation 总被引:1,自引:0,他引:1
The effects of fractures on the seismic velocity and attenuation of a rock are investigated using theoretical results and experimental data. Fractures in a rock mass influence the traveltimes and amplitudes of seismic waves that have propagated through them. The displacement discontinuity model, recently employed in fracture investigations, is modified to describe the effect of fractures on seismic-wave velocity and attenuation. This new model, the modified displacement discontinuity model (MDD), is formulated in a way analogous to transmission-line analysis. The fractures are treated as transmission lines for the passage of seismic waves. The MDD takes into consideration realistic fracture parameters which include the fracture length, the fractional area of a fracture surface in contact, and the nature of the infilling material. A single fracture of varying geometric and material properties is shown to affect dramatically the transmission properties of a propagating waveform, and hence the seismic velocity and attenuation. These effects have been shown to result in a frequency-dependent velocity and attenuation. The sensitivity of the fracture parameters to seismic-wave velocity and attenuation was investigated and interesting results were obtained. Fracture parameters used in designing experimental models consisting of synthetically manufactured cracks were fed into the MDD and a well-known crack model, Hudson's model, for comparison. Velocities as a function of the incident-wave angle were obtained from both numerical models and were compared with the results from the experimental modelling. For P waves, the MDD model results show better agreement with those of the experimental model for all crack densities investigated than those from Hudson's model. 相似文献
854.
Kelly Fast Theodor KostiukPaul Romani Fred EspenakTilak Hewagama Albert BetzRita Boreiko Timothy Livengood 《Icarus》2002,156(2):485-497
Infrared emission lines of stratospheric ammonia (NH3) were observed following the collisions of the fragments of Comet Shoemaker-Levy 9 with Jupiter in July of 1994 at the impact sites of fragments G and K. Infrared heterodyne spectra near 10.7 μm were obtained by A. Betz et al. (in Abstracts for Special Sessions on Comet Shoemaker-Levy 9, The 26th Meeting of the Division for Planetary Sciences, Washington DC, 31 Oct.-4 Nov. 1994, p. 25) using one of the Infrared Spatial Interferometer telescope systems on Mount Wilson. Lineshapes of up to three different NH3 emission lines were measured at a resolving power of ∼107 at multiple times following the impacts. We present here our radiative transfer analysis of the fully resolved spectral lineshapes of the multiple rovibrational lines. This analysis provides information on temperature structure and NH3 abundance distributions and their temporal changes up to 18 days after impact. These results are compared to photochemical models to determine the role of photochemistry and other mechanisms in the destruction and dilution of NH3 in the jovian stratosphere after the SL9 impacts.One day following the G impact, the inferred temperature above 0.001 mbar altitude is 283±13 K, consistent with a recent plume splashback model. Cooling of the upper stratosphere to 204 K by the fourth day and to quiescence after a week is consistent with a simple gray atmosphere radiative flux calculation and mixing with cold jovian air. During the first 4 days after impact, NH3 was present primarily at altitudes above 1 mbar with a column density of (7.7±1.6)×1017 cm−2 after 1 day and (3.7±0.8)×1017 cm−2 after 4 days. (Errors represent precision.) We obtained >2.5 times more NH3 than can be supplied by nitrogen from a large cometary fragment, suggesting a primarily jovian source for the NH3. By 18 days postimpact, a return to quiescent upper stratospheric temperature is retrieved for the G region, with an NH3 column density of 7.3×1017 cm−2 or more in the lower stratosphere, possibly supplied by NH3 upwelling across an impact-heated and turbulent tropopause, which may have been masked by initial dust and haze. Above the 1-mbar level, the maximum retrieved column density decreased to 6.5×1016 cm−2. Comparison to photochemical models indicates that photolysis alone is not sufficient to account for the loss of NH3 above 1 mbar by that time, even when chemical reformation of NH3 is ignored. We speculate that the dispersion of plume material at high altitudes (above 1 mbar) is responsible for the change in the spectra observed a few days postimpact. Data on the K impact region provide qualitatively consistent results. 相似文献
855.
Reliable and up-to-date urban land cover information is valuable in urban planning and policy development. Due to the increasing demand for reliable land cover information there has been a growing need for robust methods and datasets to improve the classification accuracy from remotely sensed imagery. This study sought to assess the potential of the newly launched Landsat 8 sensor’s thermal bands and derived vegetation indices in improving land cover classification in a complex urban landscape using the support vector machine classifier. This study compared the individual and combined performance of Landsat 8’s reflective, thermal bands and vegetation indices in classifying urban land use-land cover. The integration of Landsat 8 reflective bands, derived vegetation indices and thermal bands overall produced significantly higher accuracy classification results than using traditional bands as standalone (i.e. overall, user and producer accuracies). An overall accuracy above 89.33% and a kappa index of 0.86, significantly higher than the one obtained with the use of the traditional reflective bands as a standalone data-set and other analysis stages. On average, the results also indicate high producer and user accuracies (i.e. above 80%) for most of the classes with a McNemar’s Z score of 9.00 at 95% confidence interval showing significant improvement compared with classification using reflective bands as standalone. Overall, the results of this study indicate that the integration of the Landsat 8’s OLI and TIR data presents an invaluable potential for accurate and robust land cover classification in a complex urban landscape, especially in areas where the availability of high resolution datasets remains a challenge. 相似文献
856.
ABSTRACTObtaining useful geographic information from the flood of user-generated geographic content (UGGC) data is of significant interest, but comes with challenges in an age of crowdsourcing. To address this issue, we adopt Maslow’s model of human needs to frame UGGC. Synthesizing across literature about volunteered geographic information and participatory GIS, we present a conceptual framework for characterizing collection of UGGC. The framework is composed of multiple dimensions and levels that include perspectives about motivations, participatory methods, and participants. We also articulate four levels of participants (stakeholders, core-engaged participants, semi-core-engaged participants, and engaged citizens) together with four work principles (communication, cooperation, coordination, and collaboration). Finally, a case study describes an example we use to evaluate the conceptual framework for its suitability in design of a UGGC system. We conclude by reflecting on how and why this framework offers a valuable perspective for deriving useful geographic information and offer future research directions. 相似文献
857.
Constraining provenance,thickness and erosion of nappes using low‐temperature thermochronology: the Northland Allochthon,New Zealand 下载免费PDF全文
Ruohong Jiao Diane Seward Timothy A. Little Frédéric Herman Barry P. Kohn 《Basin Research》2017,29(1):81-95
The Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto the in situ Mesozoic and early Cenozoic rocks (predominantly Late Eocene–earliest Miocene) in northwestern New Zealand. Using low‐temperature thermochronology, we investigate the sedimentary provenance, burial and erosion histories of the rocks from both the hanging and footwalls of the allochthon. In central Northland (Parua Bay), both the overlying allochthon and underlying Early Miocene autochthon yield detrital zircon and partially reset apatite fission‐track ages that were sourced from the local Jurassic terrane and perhaps Late Cretaceous volcanics; the autochthon contains, additionally, material sourced from Oligocene volcanics. Thermal history modelling indicates that the lower part of the allochthon together with the autochthon was heated to ca. 55–100°C during the Late Oligocene and Early Miocene, most likely due to the burial beneath the overlying nappe sequences. From the Mesozoic basement exposed in eastern Northland, we obtained zircon fission‐track ages tightly bracketed between 153 and 149 Ma; the apatite fission‐track ages on the other hand, generally young towards the northwest, from 129 to 20.9 Ma. Basement thermochronological ages are inverted to simulate the emplacement and later erosion of the Northland Allochthon, using a thermo‐kinematic model coupled with an inversion algorithm. The results suggest that during the Late Oligocene, the nappes in eastern Northland ranged from ca. 4–6‐km thick in the north to zero in the Auckland region (over a distance >200 km). Following the allochthon emplacement, eastern Northland was uplifted and unroofed during the Early Miocene for a period of ca. 1–6 Myr at the rate of 0.1–0.8 km/Myr, leading to rapid erosion of the nappes. Since Middle Miocene, the basement uplift ceased and the erosion of the nappes and the region as a whole slowed down (ca. 0–0.2 km/Myr), implying a decay in the tectonic activity in this region. 相似文献
858.
Rethinking economy requires rethinking the relationship between economics and its object. The economy is a recent product of socio-technical practice, including the practice of academic economics. Previously, the term “economy” referred to ways of managing resources and exercising power. In the mid-twentieth century, it became an object of power and knowledge. Rival metrological projects brought the economy into being. The development of the modern electricity industry illustrates the kind of work involved. It required new technical processes, new forms of distribution, addressing, and monitoring, new forms of calculation that were simultaneously electrical, chemical, economic, and social. Analyses of how the economic is embedded in social ties or in cultural meanings cannot understand these intersecting projects. The projects that form the economy involve the work of economics. Economic knowledge does not represent the economy from some place outside. It participates in making sites where its facts can survive. The case of an economic research project on property rights in Peru illustrates how this happens. Economic facts were established in a world that was organized, through specific projects, such as the property titling programs of Hernando de Soto, to enable economic knowledge to be made. There is no simple divide between a virtual world of economic theory and a real world outside it. Every economic project involves multiple arrangements of the simulated and that to which it refers. 相似文献
859.
Identification of food web linkages is a major aim in ecology because it provides basic information on trophic flows and the
potential for interspecific interactions. In addition, policy and restoration measures mandated to conform to ecosystem-based
management principals can benefit from information on temporal and spatial variability in community-level interactions. Here,
we analyzed guild structure of the demersal fish assemblage in Puget Sound, WA, a temperate estuarine system on the US west
coast. Using diet information from 2,401 stomachs collected across three seasons (fall, winter, and summer), we identified
guild membership for 21 fish species, examined seasonal guild switching, and tested for seasonal shifts in predation and for
differences in the degree of diet overlap at the assemblage level. We accounted for ontogenetic variation in diet by dividing
species into large (L) and small (S) size classes when appropriate. Using cluster analysis and a permutation approach, we
identified seven significant guilds that were typified by predation on benthic invertebrates, pelagic invertebrates, and piscivory.
Of the 18 species with more than one season of diet information, six switched guilds (Pacific sanddab L, sturgeon poacher,
Pacific tomcod S, speckled sanddab, rex sole, and rock sole S). At the assemblage level, we tested for seasonal differences
in prey use between seasons by performing an analysis of similarities based on Bray–Curtis diet similarities and found no
significant difference. However, diet overlap was significantly higher in the summer than the fall and winter (with summer > fall > winter)
indicating that diets within the assemblage converged in the summer. These results indicate that analyses of guild structure
and diet overlap can reveal seasonal variation in community trophic structure and highlight intra-annual food web variation
in the Puget Sound demersal fish community. 相似文献
860.
Silvania Avelar Tessa S. van der Voort Timothy I. Eglinton 《Carbon balance and management》2017,12(1):10