全文获取类型
收费全文 | 1288篇 |
免费 | 42篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 61篇 |
大气科学 | 100篇 |
地球物理 | 269篇 |
地质学 | 420篇 |
海洋学 | 90篇 |
天文学 | 250篇 |
综合类 | 5篇 |
自然地理 | 143篇 |
出版年
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 12篇 |
2018年 | 29篇 |
2017年 | 29篇 |
2016年 | 31篇 |
2015年 | 19篇 |
2014年 | 24篇 |
2013年 | 74篇 |
2012年 | 47篇 |
2011年 | 50篇 |
2010年 | 48篇 |
2009年 | 59篇 |
2008年 | 59篇 |
2007年 | 56篇 |
2006年 | 41篇 |
2005年 | 42篇 |
2004年 | 34篇 |
2003年 | 38篇 |
2002年 | 37篇 |
2001年 | 23篇 |
2000年 | 41篇 |
1999年 | 22篇 |
1998年 | 22篇 |
1997年 | 20篇 |
1996年 | 21篇 |
1995年 | 21篇 |
1994年 | 13篇 |
1993年 | 22篇 |
1992年 | 20篇 |
1991年 | 14篇 |
1990年 | 23篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 18篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 22篇 |
1983年 | 26篇 |
1982年 | 19篇 |
1981年 | 15篇 |
1980年 | 16篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1974年 | 10篇 |
1973年 | 9篇 |
1972年 | 19篇 |
1971年 | 12篇 |
1969年 | 7篇 |
排序方式: 共有1338条查询结果,搜索用时 31 毫秒
991.
Modeling Sediment Suspensions in an Idealized Tidal Embayment: Importance of Tidal Asymmetry and Settling Lag 总被引:1,自引:0,他引:1
Suspended sediment transport processes in a short tidal embayment with a simple geometry are investigated using analytic and
numerical models. On the basis of numerical results, the horizontal gradient of depth-averaged suspended sediment concentration
can be parameterized with a combination of the first harmonic and mean. Using the parameterization, the solution of the analytic
model is obtained. Evaluation of the major terms from the solution of the analytic model shows that a quarter-diurnal frequency
is significant near the mouth while a semidiurnal component dominates the interior area. The settling lag consists of local
and nonlocal components. The local phase lag is a function of the ratio between tidal period and settling time. The nonlocal
phase lag is determined by the phase difference between tidal velocity and the horizontal gradient of sediment concentration
and by the strength of erosion and horizontal advection. 相似文献
992.
P. Brindley J. Goulding M. L. Wilson 《International journal of geographical information science》2018,32(3):498-523
Neighbourhoods have been described as ‘the building blocks of public services society’. Their subjective nature, however, and the resulting difficulties in collecting data, means that in many countries there are no officially defined neighbourhoods either in terms of names or boundaries. This has implications not only for policy but also business and social decisions as a whole. With the absence of neighbourhood boundaries many studies resort to using standard administrative units as proxies. Such administrative geographies, however, often have a poor fit with those perceived by residents. Our approach detects these important social boundaries by automatically mining the Web en masse for passively declared neighbourhood data within postal addresses. Focusing on the United Kingdom (UK), this research demonstrates the feasibility of automated extraction of urban neighbourhood names and their subsequent mapping as vague entities. Importantly, and unlike previous work, our process does not require any neighbourhood names to be established a priori. 相似文献
993.
In the present paper, an inverse problem of radiative transfer in an inhomogeneous plane medium of scattering albedo =0 exp(-/s) is solved by a moment method. The results are compared with those obtained by Dunn (1983) using Monte Carlo method.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
994.
Radial integrals have been calculated under the one-electron hydrogenic model. Two different values of the effective charge parameter, one for the initial state and the other for the final state, are retained in these formulae. The model is able to reasonably reproduce the existing dipole oscillator strength values with little effort. The dipole oscillator strength values are given for many ions for the first time. 相似文献
995.
Robert M. Wilson 《Solar physics》1988,117(1):179-186
Examined are associational aspects as they relate the maximum amplitude R
M
for the sunspot cycle to the rate of rise R
t
during the ascending phase, where R
M
is the smoothed sunspot number at cycle maximum and R
t
is the sum of the monthly mean sunspot numbers for selected 6-month intervals (t) measured from cycle onset. One finds that, prior to about 2 yr into the cycle, the rate of rise is not a reliable predictor for maximum amplitude. Only during the latter half of the ascent do the fits display strong linearity, having a coefficient of correlation r 0.9 and a standard error S
yx
20. During the first four intervals, the expected R
M
and the observed R
M
were found to differ by no more than 20 units of smoothed sunspot number only 25, 42, 50, and 58 % of the time; during the latter four intervals, they differed by no more than 20 units 67, 83, 92, and 100% of the time. 相似文献
996.
Isotopic and thermochronologic evidence of extremely cold lithosphere associated with a slab flattening in the Central Andes of Argentina 下载免费PDF全文
Gilda Collo Federico M. Dávila Wilson Teixeira Julieta C. Nóbile Lucy G. Sant′ Anna Andrew Carter 《Basin Research》2017,29(1):16-40
We present mineralogic, isotopic and thermochronologic analyses on psammopelitic and tuffaceous levels from the Bermejo and Vinchina basins – both foreland depocentres of the Central Andes of Argentina – that define a low‐temperature regime for the crust akin to a slab shallowing and flattening process. The contents of illite in illite/smectite interstratified (I/S) show a progressive illitization into the deeper parts of both basins. The distribution of I/S is compatible with theoretical simulations and predicted heat flow values of ca. 26 mW m?2 in the 8–3.4 Ma interval for the Vinchina Basin and ca. 42 mW m?2 since 9 Ma for the Bermejo Basin. The latter shows heat flow values that are comparable to those reported by magnetotelluric analysis (36–40 mW m?2) in agreement with previously published heat flow calculations along the modern Andean foreland. The Rb–Sr isochrones in psammopelites (<2 μm fractions) show ages between 125 and 165 Ma, whereas the K–Ar ages decrease as the grain size is smaller (136–224 Ma for 1–2 μm, 112–159 Ma for 0.2–1 μm, 76–116 Ma for <0.2 μ and 39.3–42 Ma for <0.1 μm). These ages are significantly older than the sedimentation in the basins (ca. 16 Ma for the Vinchina Basin; U–Pb age), and can be explained by the presence of a significant amount of detrital components, mainly illite, even in the finer fractions. The preservation of detrital ages is consistent with the shallow diagenesis related to a low‐temperature regime, proposed here for the basins. Younger K–Ar ages (21.3–12 Ma) were obtained for a basal tuffaceous level. Clay mineralogy and R0 ordering in the deepest part of the Vinchina Basin, together with the evolution model of I/S with depth, suggest that the burial temperatures would have not exceeded ca. 100°C in agreement with (U–Th)/He analyses performed on apatite extracted from two tuffaceous units. Thermal indicators from both studied basins confirm the existence of a low‐temperature regime during flat subduction. 相似文献
997.
Dorothea S. Macholdt Klaus Peter Jochum Stephen A. Wilson Laura M. Otter Brigitte Stoll Ulrike Weis Meinrat O. Andreae 《Geostandards and Geoanalytical Research》2016,40(4):493-504
Manganese‐ and iron‐rich materials are of major geoscientific and economic interest, many of which contain microscopic features that provide valuable information. To obtain accurate results, a homogeneous microanalytical reference material for calibration is needed. Several researchers have used the Mn‐ and Fe‐rich RMs, JMn‐1, NOD‐A‐1, NOD‐P‐1 and FeMn‐1, for this purpose; therefore, they were tested in this study to determine their suitability for microanalysis. Their homogeneity was investigated by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) with two different types of lasers (nano‐ and femtosecond), with spot and line scan analyses and with different operating parameters, such as spot size, pulse repetition rate and fluence. As the established manganese nodule RMs revealed inhomogeneities for picogram to microgram test portions, we also investigated the new synthetic Fe‐ and Mn‐rich RM, FeMnOx‐1. FeMnOx‐1 was found to be homogeneous for large (ø 40 μm: 2% RSD repeatability) and small (ø 8–10 μm: 10% RSD repeatability) spot sizes. This homogeneity is in the range of the homogeneous NIST SRM 610 and GSE‐1G reference glasses. Furthermore, FeMnOx‐1 revealed a large‐scale homogeneity within uncertainties of a few per cent, using test portions in the ng range, when measuring four individual mounts of this material. 相似文献
998.
Kátia Kellem da Rosa Cláudio Wilson Mendes Jr. Jorge Arigony-Neto Jefferson Cardia Simões 《国际地球制图》2016,31(9):1032-1043
This work analysed the spatio-temporal variation of snow cover on the Kraków Ice Field, located in the King George Island, Antarctica. High spatial resolution images of COSMO-SkyMed were used in this study. These X-band images are vertically and horizontally co-polarized and their intensity data were converted to amplitude (dB). The COSMO-SkyMed images were classified by a minimum distance algorithm and post-classified based on knowledge of adjacency relationships of snow zones. Hypsometric, slope, aspect and solar radiation maps to support the interpretation of backscatter patterns in the COSMO-SkyMed images. Three radar zones were classified in these images: percolation, slush and wet snow radar zone. Positive surface air temperatures and rainfall events, registered from a meteorological station, lead to increase in wet snow and slush zones. The COSMO-SkyMed images and minimum distance algorithm were adequate to discriminate the snow cover and to assess the supraglacial melting pattern during the ablation season in the study area. 相似文献
999.
1000.
Abdulmohsin Al-Sofyani N. Marimuthu J. Jerald Wilson Arulazhagan Pugazhendi Jeyakumar Dhavamani 《中国海洋大学学报(英文版)》2016,15(3):465-470
Three species of sponges and a tunicate were collected from Obhur creek of Jeddah coast for this bioactivity study. In order to assess the antifouling efficacy of selected marine organisms, methanolic extracts of these organisms were tested against different fouling bacterial forms and II-instar stage of the barnacle, Balanus amphitrite. Antibiosis, bioactivity and followed by multivariate analyses were carried out to check the efficacy of antifouling effect of the selected marine organisms. Principal component analysis revealed the exemplary antifouling efficacy of the sponge extracts of Stylissa sp. observed followed by Hyrtios sp. against bacterial forms in the laboratory study. De-trended correspondence analysis confirmed that the contribution of antifouling efficacy of the selected sponge extracts was observed to be more towards Bacillus sp., Vibrio sp. and Alteromonas sp. Moreover, the efficacy of Hyrtios sp. extract(20.430 μg m L~(-1)) followed by Stylissa sp.(30.945 μg m L~(-1)) showed higher against barnacle instar compared with other extracts in the bioactivity assay. Bray-Curtis cluster analysis under paired linkage categorized all the sponge extracts into one major cluster with 75% similarity, and one outlier tunicate. More than 80% similarity observed between Hyrtios sp. and Stylissa sp. Fourier transform infrared spectroscopy(FTIR) showed that the contribution of major peaks found in the marine organisms were towards sulfones, sulfoxides, cyanates and ketones. 相似文献