全文获取类型
收费全文 | 1959篇 |
免费 | 132篇 |
国内免费 | 91篇 |
专业分类
测绘学 | 105篇 |
大气科学 | 123篇 |
地球物理 | 505篇 |
地质学 | 999篇 |
海洋学 | 91篇 |
天文学 | 170篇 |
综合类 | 38篇 |
自然地理 | 151篇 |
出版年
2024年 | 4篇 |
2023年 | 17篇 |
2022年 | 66篇 |
2021年 | 74篇 |
2020年 | 79篇 |
2019年 | 90篇 |
2018年 | 171篇 |
2017年 | 144篇 |
2016年 | 194篇 |
2015年 | 92篇 |
2014年 | 200篇 |
2013年 | 194篇 |
2012年 | 105篇 |
2011年 | 119篇 |
2010年 | 73篇 |
2009年 | 88篇 |
2008年 | 68篇 |
2007年 | 45篇 |
2006年 | 60篇 |
2005年 | 35篇 |
2004年 | 26篇 |
2003年 | 28篇 |
2002年 | 27篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 10篇 |
1998年 | 18篇 |
1997年 | 8篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 12篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1990年 | 8篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1972年 | 2篇 |
1971年 | 2篇 |
排序方式: 共有2182条查询结果,搜索用时 372 毫秒
941.
Kamal A. Ali Robert J. Stern William I. Manton Peter R. Johnson Sumit K. Mukherjee 《International Journal of Earth Sciences》2010,99(4):705-726
The Neoproterozoic Atud diamictite in Wadi Kareim and Wadi Mobarak in the Eastern Desert of Egypt and the Nuwaybah formation
in NW Saudi Arabia consist of poorly sorted, polymictic breccia, with clasts up to 1 m of granitoid, quartz porphyry, quartzite,
basalt, greywacke, marble, arkose, and microconglomerate in fine-grained matrix. Stratigraphic relations indicate that the
diamictite was deposited in a marine environment. Integrated field investigation, petrographic study and U–Pb SHRIMP zircon
ages demonstrate that the Atud and Nuwaybah are correlative. The distribution of zircon ages indicate that ~750 Ma ages are
dominant with a significant component of older materials, characterized by minor Mesoproterozoic and more abundant Paleoproterozoic
and Neoarchean ages. Some matrix and metasedimentary clast zircons yield ages that are a few 10s of Ma younger than the age
of the youngest clast (754 ± 15 Ma), suggesting Atud/Nuwaybah diamictite deposition ~750 Ma or slightly later, broadly consistent
with being deposited during the Sturtian glaciation (740–660 Ma). The Paleoproterozoic and Neoarchean clasts have no source
within the ensimatic Arabian–Nubian Shield. The distribution of the pre-Neoproterozoic ages are similar to the distribution
of the pre-Neoproterozoic ages in Yemen and Saharan Metacraton, suggesting that these clasts have been transported hundreds
of kilometers, maybe by ice-rafting. The Atud diamictite may represent important evidence for Cryogenian “Snowball Earth”
in the Arabian–Nubian Shield. 相似文献
942.
Mineralogical and spectroscopic characterization,and potential environmental use of limestone from the Abiod formation,Tunisia 总被引:1,自引:0,他引:1
Ali Sdiri Teruo Higashi Tamao Hatta Fakher Jamoussi Norio Tase 《Environmental Earth Sciences》2010,61(6):1275-1287
Limestone beds of the Late Cretaceous Abiod formation (Campanian-Maastrichtian system) are fundamentally important for the
economic growth of the raw material sector in Tunisia. However, little attention has been paid to the detailed physical and
chemical properties of the Abiod limestone. Nine limestone samples collected from the Abiod formation outcropping in the areas
of Bizerte, Gafsa and Gabes, Tunisia, as well as their separated clay fractions, were characterized using different techniques,
such as XRF, XRD, FTIR and TG/DTA. XRF showed the chemical composition of the limestone in which calcium carbonate was the
main constituent, and silica, iron and magnesium were the impurities. XRD also confirmed the presence of small amounts of
clay minerals and quartz along with sharp peaks of calcite. FTIR spectra indicated that the limestone was mainly composed
of CaCO3 in the form of calcite, as identified by its main characteristic absorption bands. These data were in agreement with XRD
and XRF analysis data. The TG/DTA curves of the limestone samples, showing a close similarity to that of pure calcium carbonate,
exhibited an endothermic peak between 600 and 760°C, with the maximum near 750°C. Moreover, FTIR spectra of clay fraction
samples indicated high silica content in some samples. Especially the samples SD1 and SD2 collected in the northern area showed
higher amounts of silica compared with those of AS1, AS2, CHB, ZNC, SND, MKM and GBS collected from southern districts. However,
among the latter seven samples, one could recognize two groups based on the clay mineral investigations: (1) limestone with
minor amounts of smectite and mixed layer minerals of smectite/illite (AS1 and 2, CHB, ZNC, SND and GBS) and (2) limestone
with smectite, kaolinite and apatite (SND and MKM). Differences in these mineralogical and chemical characteristics should
be considered when limestone from the Abiod formation is utilized as a medium for heavy metal removal from wastewater. 相似文献
943.
Ahmed I. Rushdi Khalid F. Al-Mutlaq Bernd R. T. Simoneit Adnan Al-Azri Ali A. Z. DouAbul Sheikha Al-Zarban Faiza Al-Yamani 《Arabian Journal of Geosciences》2010,3(2):113-131
River runoff and atmospheric fallout (dust and air particulate matter) are major input sources of natural and anthropogenic
terrestrial organic and inorganic components to the Arabian seas. In this study, we report on the various lipid tracer compounds
that might be transported to the Arabian Gulf by rivers, dust, and air particulate matter. These are based on geochemical
analysis of sediment, dust, and particulate samples collected from Iraq, Kuwait, and Saudi Arabia. The samples were extracted
with a dichloromethane/methanol mixture and analyzed by gas chromatography-mass spectrometry. The extractable organic compounds
(lipids) in the samples include n-alkanes, n-alkanoic acids, n-alkanols, methyl n-alkanoates, steroids, triterpenoids, carbohydrates, and petroleum hydrocarbons. The steroids and triterpenoids were major
components in river and wetland samples. The major sources of these lipids were from natural vegetation, microbial (plankton
and bacteria) residues in the sediments, sand, and soils, with some contribution from anthropogenic sources. Accordingly,
these sources could be major inputs to the Arabian seas besides the autochthonous marine products. Future studies of the organic
and inorganic biogeochemistry on river, dust, and coastal areas are needed to characterize the various regional sources, transformation,
and diagenetic processes of the organic matter en route to the marine environment. 相似文献
944.
Ali Harzallah 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(4):459-470
The vertical structure of the flow variability through the Strait of Gibraltar is studied based on the Gibraltar Experiment and the Word Ocean Circulation Experiment data sets. An analysis of the leading modes of velocity and density variability at the Strait of Gibraltar showed an adjustment of the water masses exchanged through the Strait. Mediterranean mass variations resulting from the water exchanged by barotropic tidal oscillations generate changes of the baroclinic component of the flow that damp these mass variations. This adjustment explains the previously observed fortnightly variation of the shear. Moreover, the adjustment is found to operate for the subinertial time scale flow variability forced by the atmospheric pressure. An analytical model aimed at reproducing variations of the velocity with time and in the vertical is derived. The model includes a depth-varying parameterisation of friction and takes into account density gradient fluctuations across the Strait. The model reproduces the main features of the flow, in particular the shear and the interface depth variations with the tide phase. 相似文献
945.
Abbas El‐Zein 《国际地质力学数值与分析法杂志》2008,32(3):265-287
Finite‐element models of contaminant transport through composite landfill liners require highly refined meshes around the interface between the geomembrane and the clay layer, especially if leakage through holes in the geomembrane is considered. In addition, no general formulation for transport through leaking geomembranes can be found in the literature. The paper develops a general approach to time‐dependent contaminant migration through composite liners with intact or leaking geomembranes. Equations are derived for various combinations of system conditions including Dirichlet and Neumann boundary conditions in the waste, constant mass of contaminants in the waste, steady state or transient transport in the geomembrane, and steady state or transient seepage velocities in the mineral liner. The effect of the geomembrane on transport in the soil is converted into an equivalent boundary condition applicable at the top of the clay layer. Hence, only the media underlying the top geomembrane are explicitly represented in the numerical model, yielding a computationally efficient algorithm. The new formulation is validated in conjunction with finite‐layer, finite‐element and boundary‐element methods, by comparing its predictions to those of more conventional approaches which represent the geomembrane explicitly. The scope of the method is illustrated by modelling a landfill liner with a geomembrane leaking in five locations. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
946.
Sam Boggs David H. Krinsley Gordon G. Goles Abbas Seyedolali Henning Dypvik 《Meteoritics & planetary science》2001,36(6):783-791
Abstract— Quartz grains subjected to high‐strain‐rate shock waves owing to meteorite or cometary impact on Earth's surface commonly display shock lamellae. These lamellae appear as remarkably straight, thin, planar features (microstructures) in sets within which lamellae are essentially parallel to each other and spaced ≤ 20 μm apart. Two or more intersecting sets are typically present. Shock lamellae are commonly recognized and identified by optical methods, by use of the transmission electron microscope (TEM), and by etching polished sections and subsequent examination with a scanning electron microscope (SEM) operated in the secondary electron mode. We present here a method for observing planar microstructures in shocked quartz by using a cathodoluminescence (CL) detector attached to a SEM. The method relies on the fact that planar microstructures in quartz arising as a result of shock display no CL whatever; thus, they show up as distinct, thin, black lines on otherwise luminescent quartz grains. We used scanning CL imaging to study shocked quartz from the Ries Crater, Germany, a well‐known impact crater of Miocene age. We demonstrate that shock‐produced planar microstructures are clearly displayed in SEM‐CL images and can be distinguished from microfractures generated by tectonism, and subsequently filled with quartz, and other similar features not related to impact events. The SEM‐CL method provides a powerful supplement to other methods of identifying shocked quartz. It commonly provides better spatial resolution than does standard optical methods, and does not require etching of quartz grains. Further, it is easier and faster to use than are TEM methods, although it is not capable of the fine‐scale defect analysis possible with TEM. 相似文献
947.
综述了脉冲星星际闪烁观测研究的进展,对脉冲星星际闪烁现象,星际介质中电子密度涨落谱,散射等离子体在银河系中的分面等方面的最新研究结果作了介绍。星际闪烁现象和昨际介质的深入理解,使脉冲星星际闪烁已成为研究诸如脉冲星辐射区结构和脉冲星速度等脉冲星本身性质的重要工具。 相似文献
948.
A rigorous theoretical investigation is carried out in analyzing the excitation of electrostatic ion acoustic (IA) solitary wave (SW) structures in two dimensional negative ion magneto-plasmas with superthermal electrons (following κ type distribution). The Zakharov-Kuznetsov (ZK) equation is derived by employing the well known reductive perturbation method, and the analytical solution of ZK equation assists to find out the SW profiles along with their properties. The consequences of different plasma parameters (regarding our considered plasma system) variation on SW structures has been studied. It is found that magnetic field intensity, superthermal parameter κ and temperature of positive and negative ions as well as their densities significantly modify the basic characteristics (amplitude, width, etc.) of the SW waves. A comparison of the SW structures is also presented when the electrons are Maxwellian to when they are superthermal. The relevance of the findings of this work with astrophysical plasmas is briefly pointed out. 相似文献
949.
Mohammad S. Khan Mohamed H. Abdullah Gamal B. Ali 《Astrophysics and Space Science》2014,351(1):289-297
We derive analytical expression for the velocity dispersion of galaxy clusters, using the statistical mechanical approach. We compare the observed velocity dispersion profiles for 20 nearby (z≤0.1) galaxy clusters with the analytical ones. It is interesting to find that the analytical results closely match with the observed velocity dispersion profiles only if the presence of the diffuse matter in clusters is taken into consideration. This takes us to introduce a new approach to detect the ratio of diffuse mass, M diff , within a galaxy cluster. For the present sample, the ratio f=M diff /M, where M the cluster’s total mass is found to has an average value of 45±12 %. This leads us to the result that nearly 45 % of the cluster mass is impeded outside the galaxies, while around 55 % of the cluster mass is settled in the galaxies. 相似文献
950.
Taufiq Hidayat Achmad Munir Budi Dermawan Anton Timur Jaelani Stéphane Léon Dading Hadi Nugroho Andriyan Bayu Suksmono Putra Mahasena Premana Wardayanti Premadi Dhani Herdiwijaya Chatief Kunjaya Zadrach Ledoufij Dupe Budi Brahmantyo Denny Mandey Muhammad Yusuf Hesti Retno Tri Wulandari Falahuddin Arief Muhammad Irfan Agus Triono Puri Jatmiko Evan Irawan Akbar Hery Leo Sianturi Jehunias Leonidas Tanesib Ali Warsito Judhistira Aria Utama 《Experimental Astronomy》2014,37(1):85-108
We report the first measurements of radio frequency spectrum occupancy performed at sites aimed to host the future radio astronomy observatory in Indonesia. The survey is intended to obtain the radio frequency interference (RFI) environment in a spectral range from low frequency 10 MHz up to 8 GHz. The measurements permit the identification of the spectral occupancy over those selected sites in reference to the allocated radio spectrum in Indonesia. The sites are in close proximity to Australia, the future host of Square Kilometre Array (SKA) at low frequency. Therefore, the survey was deliberately made to approximately adhere the SKA protocol for RFI measurements, but with lower sensitivity. The RFI environment at Bosscha Observatory in Lembang was also measured for comparison. Within the sensitivity limit of the measurement equipment, it is found that a location called Fatumonas in the surrounding of Mount Timau in West Timor has very low level of RFI, with a total spectrum occupancy in this measured frequency range being about 1 %, mostly found at low frequency below 20 MHz. More detailed measurements as well as a strategy for a radio quiet zone must be implemented in the near future. 相似文献