全文获取类型
收费全文 | 29886篇 |
免费 | 467篇 |
国内免费 | 942篇 |
专业分类
测绘学 | 1567篇 |
大气科学 | 2390篇 |
地球物理 | 6607篇 |
地质学 | 13333篇 |
海洋学 | 1263篇 |
天文学 | 2545篇 |
综合类 | 2178篇 |
自然地理 | 1412篇 |
出版年
2022年 | 52篇 |
2021年 | 114篇 |
2020年 | 116篇 |
2019年 | 91篇 |
2018年 | 4914篇 |
2017年 | 4197篇 |
2016年 | 2887篇 |
2015年 | 507篇 |
2014年 | 358篇 |
2013年 | 400篇 |
2012年 | 1292篇 |
2011年 | 2918篇 |
2010年 | 2228篇 |
2009年 | 2541篇 |
2008年 | 2078篇 |
2007年 | 2476篇 |
2006年 | 228篇 |
2005年 | 346篇 |
2004年 | 530篇 |
2003年 | 546篇 |
2002年 | 346篇 |
2001年 | 148篇 |
2000年 | 151篇 |
1999年 | 80篇 |
1998年 | 113篇 |
1997年 | 84篇 |
1996年 | 49篇 |
1995年 | 79篇 |
1994年 | 85篇 |
1993年 | 52篇 |
1992年 | 48篇 |
1991年 | 48篇 |
1990年 | 63篇 |
1989年 | 44篇 |
1988年 | 41篇 |
1987年 | 36篇 |
1986年 | 46篇 |
1985年 | 48篇 |
1984年 | 40篇 |
1983年 | 48篇 |
1982年 | 52篇 |
1981年 | 63篇 |
1980年 | 63篇 |
1979年 | 40篇 |
1978年 | 51篇 |
1977年 | 42篇 |
1976年 | 34篇 |
1975年 | 41篇 |
1973年 | 43篇 |
1971年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
221.
The origin and biogeochemistry of organic matter in surface sediments of Lake Shihwa and Lake Hwaong
To understand the origin and biogeochemistry of the organic matter in surface sediments of Lake Shihwa and Lake Hwaong, organic
nitrogen, inorganic nitrogen, labile organic carbon, and residual organic carbon contents as well as stable isotope ratios
for carbon and nitrogen were determined by KOBr-KOH treatment. Ratios of organic carbon to organic nitrogen (Corg/Norg) (mean = 24) were much higher than ratios of organic carbon to total nitrogen (Corg/Ntot) (mean=12), indicating the presence of significant amounts of inorganic nitrogen in the surface sediments of both lakes.
Stable isotope ratios for organic nitrogen were, on average, 5.2‰ heavier than ratios of inorganic nitrogen in Lake Shihwa,
but those same ratios were comparable in Lake Hwaong. This might be due to differences in the origin or the degree of degradation
of sedimentary organic matter between the two lakes. In addition, stable isotope ratios for labile organic carbon were, on
average, 1.4‰ heavier than those for residual organic carbon, reflecting the preferential oxidation of13C-enriched organic matter. The present study demonstrates that KOBr-KOH treatment of sedimentary organic matter can provide
valuable information for understanding the origin and degradation state of organic matter in marine and brackish sediments.
This also suggests that the ratio of Corg/Norg and stable isotope ratios for organic nitrogen can be used as indexes of the degree of degradation of organic matter. 相似文献
222.
Background signal of the scintillation detector routinely used for atmospheric Kr-85 counting follows fluctuations, and their connection with cosmic ray variations is searched for. A strong correlation between the 27-day moving average of sunspot numbers and the background is obtained from the 1988 data. 相似文献
223.
Elena Dzifčáková 《Solar physics》1992,140(2):247-267
The ionization equilibrium of the Fe in the solar corona for a non-Maxwellian electron distribution with an enhanced number of particles in the high-energy tail is presented. A parametric form of the distribution function is used to demonstrate the changes in the ionization equilibrium with changes in the shape of the distribution. The results over the range of temperature 105 K T 108 K for different deviations of the distribution from a Maxwellian are given in tabular form. The results can be used for specific applications in the solar corona, especially in the active corona, where deviations from the Maxwellian distribution can be significant. 相似文献
224.
The H analysis of the development of the strong impulsive and faint gradual phase of the June 26, 1983 flare indicates the following: (1) The flare originated from two microprominences on the southeast border of NOAA 4227. Several similar events are summarized in Table II. (2) The main flare structure was a flare cone, which consisted of a bright surge-like stream, elevated above two flare ribbons (located in the cone's base). The flare cone had a height of about 40 × 103 km and lasted 4 min in H. The upper part of the cone was terminated by a very fine loop, which was bent to the west, where later a chromospheric brightening occurred at the footpoint of a flaring arch. A 300 keV burst and radio spikes were observed during the maximum flare phase. (3) The flaring arch system, with its apex at a height of about 48 × 103 km, formed the skeleton for the coronal helmet structure (Figure 7(c)). The velocity of the plasma moving along the flaring arch was between 3500 km s–1} and 6900 km s–1} during the first brightening (14:07 UT). 相似文献
225.
J A Vargas-Guzmán 《中国地质大学学报(英文版)》2008,19(1):85-92
An analysis of statistical expected values for transformations is performed in this study to quantify the effect of heterogeneity on spatial geological modeling and evaluations. Algebraic transformations are frequently applied to data from logging to allow for the modeling of geological properties. Transformations may be powers, products, and exponential operations which are commonly used in well-known relations (e.g., porosity-permeability transforms). The results of this study show that correct computations must account for residual transformation terms which arise due to lack of independence among heterogeneous geological properties. In the case of an exponential porosity-permeability transform, the values may be positive. This proves that a simple exponential model back-transformed from linear regression underestimates permeability. In the case of transformations involving two or more properties, residual terms may represent the contribution of heterogeneous components which occur when properties vary together, regardless of a pair-wise linear independence. A consequence of power- and product-transform models is that regression equationswithin those transformations need corrections via residual cumulants. A generalization of this result isthat transformations of multivariate spatial attributes require multiple-point random variable relations. This analysis provides practical solutions leading to a methodology for nonlinear modeling using correct back transformations in geology. 相似文献
226.
Error sources which decrease the accuracy of GPS in absolute velocity determination have been changed since SA was turned off. Firstly, quantities of all kinds of error sources that influence velocity determination are analyzed. The potential accuracy of GPS absolute velocity determination is derived from both theory and field GPS data simulation. After that, two tests were carried out to evaluate the performance of GPS absolute velocity determination in the case of a static and an airborne GPS receiver and INS (Inertial Navigation System) instrument in kinematic mode. In static mode, the receiver velocity has been estimated to be several mm/s with the carrier-phase derived Doppler measurements, and several cm/s with the receiver generated Doppler measurements. In kinematic mode, GPS absolute velocity estimates are compared with the synchronized measurements from the high accuracy INS. The root mean square statistics of the velocity discrepancies between GPS and INS come up to dm/s. Moreover, it has a strong correlation with the acceleration or jerk of the aircraft. 相似文献
227.
Geochronological studies on the crust-derived Tianmenshan pluton were undertaken by SHRIMP zircon U-Pb dating and Ar-Ar dating of biotite, muscovite and K-feldspar, giving a petrogenetic age of 167 Ma. Owing to the closure systems in different minerals, the cooling history of the pluton can be determined with an age-temperature diagram. The late hydrothermal event has been recognized, which is related probably with mineralization. In terms of the comparative geochronological and petrologic records, it is concluded that there are some constrains on tectonic evolution and that the formation of the Tianmenshan pluton proceeded in a transition period from Indosinian post-orogeny extension to strong compressive tectonics. And the timing of the hydrothermal event matches the compressive climax of the Yanshanian orogeny. The temporal gap between granite emplacement and wolframite mineralization could last 10-20 Ma owing to the low cooling rate of the pluton. 相似文献
228.
Studies on rain-runoff process in the peripheral mountainous area of the Sichuan Basin, which is regarded as a key ecological shelter, will contribute to flood control and environmental protection for the Upper Yangtze River Basin. In two typical catchments--the Fujiang River Catchment and the Wujiang River Catchment, rainfall simulations have been conducted to study the rain-runoff processes of yellow soil and limestone soil in three types of land use--forestland, farmland and grassland. Results showed that (1) within the same rainfall process, overland flow occurs first on farmland, then on grassland, and finally on forestland; (2) soil surface coverage has a great impact on the occurrence and amount of overland flow. The runoff amount can increase 2-4 times after the coverage is removed; (3) the infiltration before the occurrence of overland flow will decrease because of higher gravel contents of soil, but it takes no effect on infiltration once overland flow becomes stable; (4) the runoff coefficient of the limestone soil forestland is greater than that of the yellow soil forest land, but less than that of the farmland; (5) three empirical infiltration models, including Horton' model, Kostiakov' model, and modified Kostiakov' model, were compared by using the observed results under rainfall simulation. The results showed that the Kostiakov' model performed better than both the Horton' model and modified Kostiakov model. According to the results of this research, the Kostiakov's model can be used to simulate rainfall infiltration when water erosion is modeled in the peripheral mountainous area of the Sichuan Basin. 相似文献
229.
230.
Airborne LiDAR has found application in an increasing number of mapping and Geo-data acquisition tasks. Apart from terrain
information generation, applications such as automatic detection and modeling of objects like buildings or vegetation for
the generation of 3-D city models have been explored. Besides the height itself, height texture defined by local variations
of the height is a significant parameter for object recognition. The paper explores the potential of the analysis of height
texture as a cue for the automatic detection of objects in LiDAR datasets. A number of texture measures were computed. Based
on their definition and computation these measures were used as bands in a classification algorithm, and objects like buildings,
single trees, and roads could be recognized. 相似文献