全文获取类型
收费全文 | 24399篇 |
免费 | 172篇 |
国内免费 | 916篇 |
专业分类
测绘学 | 1410篇 |
大气科学 | 1977篇 |
地球物理 | 4498篇 |
地质学 | 11589篇 |
海洋学 | 1002篇 |
天文学 | 1631篇 |
综合类 | 2161篇 |
自然地理 | 1219篇 |
出版年
2020年 | 1篇 |
2018年 | 4761篇 |
2017年 | 4037篇 |
2016年 | 2576篇 |
2015年 | 234篇 |
2014年 | 80篇 |
2013年 | 24篇 |
2012年 | 988篇 |
2011年 | 2728篇 |
2010年 | 2014篇 |
2009年 | 2310篇 |
2008年 | 1888篇 |
2007年 | 2360篇 |
2006年 | 52篇 |
2005年 | 194篇 |
2004年 | 402篇 |
2003年 | 409篇 |
2002年 | 249篇 |
2001年 | 47篇 |
2000年 | 51篇 |
1999年 | 13篇 |
1998年 | 21篇 |
1993年 | 1篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1976年 | 6篇 |
1966年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Well Zheng-1 is located in the combined area of the central uplift and the north Tianshan piedmont depression in the Junggar Basin. Two oil-bearing beds are recognized at 4788–4797 m of the Lower Cretaceous Tugulu Formation (K1tg) and 4808.5–4812.5 m of the Lower Jurassic Sangonghe Formation (J1s). The geochemical characteristics of family composition, carbon isotopic composition, saturated hydrocarbons, sterane and terpane biomarkers and carotane of two crude oils are described in this paper. The results show that the geochemical characteristics of the two crude oils are basically similar to each other, indicating they were all derived mainly from the high mature, brine, algae-rich lake facies sediments. Oil-source correlation revealed that crude oils of the two beds were derived mainly from the source rocks of Permian and mixed by the oil derived from the source rocks of Jurassic and Triassic. This is consistent with the geological background with several sets of source rocks in the area studied. 相似文献
892.
Defining the reference state of some perturbed streams and rivers is challenging since their non-altered state is undocumented.
Despite the near-absence of pristine sites in highly anthropogenic areas, information about aquatic communities that existed
prior to human disturbance can be obtained from historic sources. Sediment coring is commonly used in paleolimnology to estimate
past environmental conditions of lakes, but this technique is often not appropriate for studies of lotic systems. An alternative
is to use diatom assemblages present in the guts of fish that were captured in the streams prior to significant human disturbance.
The purpose of the present study was to evaluate the biological integrity of several streams in Ontario and Québec during
the early twentieth century based on “paleo” diatom assemblages extracted from the guts of fish stored in museums. The Eastern
Canadian diatom index (IDEC: Indice Diatomées de l’Est du Canada) was used to evaluate the biological status of “paleo” and
“modern” diatom assemblages. The IDEC shows the position of diatom assemblages on a general pollution gradient. The comparison
of IDEC values calculated for the 1925–1948 and the 2003–2007 periods showed that several streams were severely polluted in
the early 1900s. In general, present water quality has declined compared to the early 1900s. The biological integrity of only
three of the 22 sites has increased. IDEC values were not influenced by the species of fish studied. 相似文献
893.
In climate and weather prediction models the near-surface turbulent fluxes of heat and momentum and related transfer coefficients are usually parametrized on the basis of Monin–Obukhov similarity theory (MOST). To avoid iteration, required for the numerical solution of the MOST equations, many models apply parametrizations of the transfer coefficients based on an approach relating these coefficients to the bulk Richardson number \(Ri_{b}\). However, the parametrizations that are presently used in most climate models are valid only for weaker stability and larger surface roughnesses than those documented during the Surface Heat Budget of the Arctic Ocean campaign (SHEBA). The latter delivered a well-accepted set of turbulence data in the stable surface layer over polar sea-ice. Using stability functions based on the SHEBA data, we solve the MOST equations applying a new semi-analytic approach that results in transfer coefficients as a function of \(Ri_{b}\) and roughness lengths for momentum and heat. It is shown that the new coefficients reproduce the coefficients obtained by the numerical iterative method with a good accuracy in the most relevant range of stability and roughness lengths. For small \(Ri_{b}\), the new bulk transfer coefficients are similar to the traditional coefficients, but for large \(Ri_{b}\) they are much smaller than currently used coefficients. Finally, a possible adjustment of the latter and the implementation of the new proposed parametrizations in models are discussed. 相似文献
894.
Reena De S. G. Gaonkar B. V. Srirama Sagina Ram J. R. Kayal 《Journal of Earth System Science》2003,112(3):413-419
A 12-station temporary microearthquake network was established by the Geological Survey of India for aftershock monitoring
of the January 26th, 2001 Bhuj earthquake (M
w 7.6) in the Kutch district of Gujarat state, western India. The epicentres of the aftershocks show two major trends: one
in the NE direction and the other in the NW direction. Fault-plane solutions of the best-located and selected cluster of events
that occurred along the NE trend, at a depth of 15–38 km, show reverse faulting with a large left-lateral strike-slip motion,
which are comparable with the main-shock solution. The NW trending upper crustal aftershocks at depth <10 km, on the other
hand, show reverse faulting with right-lateral strike-slip motion, and the mid crustal and lower crustal aftershocks, at a
depth of 15–38 km, show pure reverse faulting as well as reverse faulting with right-lateral and left-lateral strike-slip
motions; these solutions are not comparable with the main-shock solution. It is inferred that the intersection of two faults
has been the source area for stress concentration to generate the main shock and the aftershocks. 相似文献
895.
Xiaopeng Gong Shengfeng Gu Yidong Lou Fu Zheng Maorong Ge Jingnan Liu 《Journal of Geodesy》2018,92(7):797-809
Global navigation satellite systems (GNSS) are acting as an indispensable tool for geodetic research and global monitoring of the Earth, and they have been rapidly developed over the past few years with abundant GNSS networks, modern constellations, and significant improvement in mathematic models of data processing. However, due to the increasing number of satellites and stations, the computational efficiency becomes a key issue and it could hamper the further development of GNSS applications. In this contribution, this problem is overcome from the aspects of both dense linear algebra algorithms and GNSS processing strategy. First, in order to fully explore the power of modern microprocessors, the square root information filter solution based on the blocked QR factorization employing as many matrix–matrix operations as possible is introduced. In addition, the algorithm complexity of GNSS data processing is further decreased by centralizing the carrier-phase observations and ambiguity parameters, as well as performing the real-time ambiguity resolution and elimination. Based on the QR factorization of the simulated matrix, we can conclude that compared to unblocked QR factorization, the blocked QR factorization can greatly improve processing efficiency with a magnitude of nearly two orders on a personal computer with four 3.30 GHz cores. Then, with 82 globally distributed stations, the processing efficiency is further validated in multi-GNSS (GPS/BDS/Galileo) satellite clock estimation. The results suggest that it will take about 31.38 s per epoch for the unblocked method. While, without any loss of accuracy, it only takes 0.50 and 0.31 s for our new algorithm per epoch for float and fixed clock solutions, respectively. 相似文献
896.
A three-dimensional regional ocean model is used to examine the impact of positive Indian ocean dipole (pIOD) events on the coastal upwelling features at the southwest coast of India (SWCI). Two model experiments are carried out with different surface boundary conditions that prevailed in the normal and pIOD years from 1982 to 2010. Model experiments demonstrate the weakening of coastal upwelling at the SWCI in the pIOD years. The reduced southward meridional wind stress off the SWCI leads to comparatively lower offshore Ekman transport during August–October in the pIOD years to that in normal years. The suppressed coastal upwelling results in warmer sea surface temperature and deeper thermocline in the pIOD years during June–September. The offshore spatial extent of upwelled colder (<?22 °C) water was up to 75.5° E in August–September in normal years that was limited up to 76.2° E in pIOD years. The heat budget analysis reveals the decreased contribution of vertical entrainment process to the mixed layer cooling in pIOD years which is almost half of that of normal years in October. The net heat flux term shows warming tendency during May–November with a higher magnitude (+?0.4 °C day?1) in normal years than pIOD years (+?0.28 °C day?1). The biological productivity is found to reduce during the pIOD years as the concentration of phytoplankton and zooplankton decreases over the region of coastal upwelling at SWCI. Nitrate concentration in the pIOD years dropped by half during August–September and dropped by an order of magnitude in October as compared to its ambient concentration of 13 μmol L?1 in normal years. 相似文献
897.
Rafael Moniz Caixeta Diniz Tamantini Ribeiro João Felipe Coimbra Leite Costa Péricles Lopes Machado 《Natural Resources Research》2017,26(2):213-221
Multiple Random Walk Simulation consists of a methodology adapted to run fast simulations if close-spaced data are abundant (e.g., short-term mining models). Combining kriging with the simulation of random walks attempts to approximate traditional simulation algorithm results but at a computationally faster way when there is a large amount of conditioning samples. This paper presents this new algorithm illustrating the situations where the method can be used properly. A synthetic study case is presented in order to illustrate the Multiple Random Walk Simulation and to analyze the speed and goodness of its results against the ones from using Turning Bands Simulation and Sequential Gaussian Simulation. 相似文献
898.
Over the past few years, OSL and TCN datings of glacial material from High Asia have come into fashion. To this day, however,
these techniques do not permit safe calibration. The intensity of the cosmic ray flux is being modulated by the solar and
terrestrial magnetic fields and their secular fluctuations in the past. So far, these variations cannot be converted into
the respective local TCN production rates for High Asia. We have reason to believe that the ages that are being calculated
despite these uncertainties are generally overestimated. This assessment is supported by conventional radiocarbon dates and
above all by the glacial chronology developed independently on the basis of the Quaternary geological method. The strongly
emerging evidence for a much more extensive LGM glaciation of High Asia is, however, either being ignored or rejected by many
authors, solely on the basis of the above-mentioned uncalibrated datings. This self-conceit based on the “dating fallacy”,
as we call it, should be avoided since it goes decidedly against the standards of the scientific method established in Quaternary
geology and makes a fundamental scientific discussion impossible. 相似文献
899.
Tian-shun Hou Xin-gang Wang Sibel Pamukcu 《Geotechnical and Geological Engineering》2016,34(1):237-249
To investigate the formation mechanism and the stability of Wanjia middle school slope in Wenchuan Earthquake Area, the macroscopic geological characteristics and the failure process of the landslide are researched by engineering geology analysis method, limit equilibrium method, and finit element method. The results show that after the Wenchuan Earthquake, retaining walls, houses and other infrastructure on the foot of Wanjia middle school slope were severely destroyed, 10 cm wide tension fracture appeared at the trailing edge of the slope. Wanjia middle school slope is a type of medium-sized soil landslide. The area of the deformation body is about 19,314 m2, the total volume of the deformation body is about 23 × 104 m3. There may be two potential sliding surfaces in the unstable slope: shallow and deep landslide. The analysis results of the limit equilibrium method and the finite element method show that: under dead weight, dead weight + rainstorm, dead weight + earthquake conditions, the plastic zone occurs mainly at the middle part or the trailing edge of the slope, and it doesn’t fully cut through the deep landslide body, so the deep landslide is stable. However, under rainstorm or earthquake conditions, the plastic zone almost completely cut thorough the shallow landslide body, it shows that the shallow landslide is in the understable–basic stable state. It is found that the results of finite element method is concordant with the results of the limit equilibrium method (F s = 1.06–1.29, the shallow landslide is in the basic stable–stable state). The calculation results show that shallow landslides are likely to occur in Wanjia middle school slope during a rainstorm or an earthquake, so monitoring and control of the slope should be strengthened. The shallow landslide should be managed by some measures, such as anti slide pile retaining structures and drainage works, and the dangerous rock bodies on the slope surface should be cleaned up. 相似文献
900.
Min Wang Wei Wang Benlin He Mingliang Sun Yansheng Yin Lan Liu Wuyuan Zou Xuefei Xu 《中国海洋大学学报(英文版)》2010,9(4):376-380
TiO2 films were formed on metallic titanium substrates by the anodic oxidation method in H2SO4 solution under the 80V D.C.. Phase component and microstructure were characterized by X-ray diffraction (XRD) and scanning
electron microscopy (SEM). Water contact angles on titanium oxide film surface were measured under both dark and sunlight
illumination conditions. Corrosion tests were carried out in seawater under different illumination conditions by electrochemistry
impedance spectrum (EIS) and polarization curves. The result showed that the TiO2 film prepared by the anodic oxidation method was anatase with a uniform structure and without obvious pores or cracks on
its surface. The average water contact angle of the film was 116.4° in dark, in contrast to an angle of 42.7° under the UV
illumination for 2 hours, which demonstrates good hydrophobic property. The anti-corrosion behavior of the TiO2 film was declining with the extended immersion time. Under dark conditions, however, the hydrophobic TiO2 film retarded the water infiltrating into the substrate. The impedance changed slowly and the corrosion current density was
2 orders of magnitude lower than that with the film illuminated by sunlight. All of those mentioned above indicate that the
TiO2 film possesses much better performance under dark condition, and it can be applied as an engineering material under dark
seawater environment. 相似文献