全文获取类型
收费全文 | 25647篇 |
免费 | 253篇 |
国内免费 | 959篇 |
专业分类
测绘学 | 1476篇 |
大气科学 | 2058篇 |
地球物理 | 4894篇 |
地质学 | 12182篇 |
海洋学 | 1079篇 |
天文学 | 1697篇 |
综合类 | 2172篇 |
自然地理 | 1301篇 |
出版年
2024年 | 4篇 |
2023年 | 4篇 |
2022年 | 54篇 |
2021年 | 76篇 |
2020年 | 73篇 |
2019年 | 51篇 |
2018年 | 4816篇 |
2017年 | 4087篇 |
2016年 | 2679篇 |
2015年 | 303篇 |
2014年 | 210篇 |
2013年 | 196篇 |
2012年 | 1065篇 |
2011年 | 2790篇 |
2010年 | 2047篇 |
2009年 | 2352篇 |
2008年 | 1919篇 |
2007年 | 2382篇 |
2006年 | 87篇 |
2005年 | 207篇 |
2004年 | 424篇 |
2003年 | 427篇 |
2002年 | 262篇 |
2001年 | 64篇 |
2000年 | 66篇 |
1999年 | 22篇 |
1998年 | 28篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 26篇 |
1980年 | 19篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1976年 | 9篇 |
1973年 | 2篇 |
1972年 | 4篇 |
1971年 | 5篇 |
1968年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
The sediments from three stratigraphic levels in the Bababudan schist belt of Dharwar craton exhibit great diversity in major,
trace and rare earth element (REE) geochemistry and thus interpreted to represent significant compositional variation in the
source rocks. Detailed geological and geochemical studies have been carried out on clastic rocks constituting the Archaean
Sargur supracrustals and the Bababudan belt of Dharwar craton (DC), southern India for understanding the geochemical characteristics
and to define the Archaean-Proterozoic Boundary (APB/QPC) in southern India. There is significant contrast in the geochemical
signatures for the sediments from these stratigraphic levles. The Sargur enclave population is characterised by slight LREE
enrichment with (La/Sm)N ranging from 1.45 to 3.58, almost flat HREE with (Gd/Yb)N ranging from 0.65 to 1.29 with Eu/Eu* ranging from 0.49 to 0.91 suggesting mafic-ultramafic source rocks in the provenance.
On the other hand, the Post QPC (PQPC) rocks are characterised by LREE enrichment with (La/Sm)N ranging from 2.66 to 7.07, nearly flat HREE with (Gd/Yb)N ranging from 0.58 to 0.95 and significant depletion of Eu with Eu/Eu* ranging from 0.34 to 0.85, indicating felsic province
in the source area. The conglomerates and quartzites representing the QPC are showing mixed nature of these, reflecting the
transitional character in depositional environment. Increase in abundance of REE, K2O/Na2O, Th/Sc, La/Sc, Th/U, Hf/Ta and Zr/Y ratios are characteristic of the QPC. The PQPC sediments are enriched in Th, U and HFSE
like Hf, Nb, Zr and Y, and depleted in Co and Eu than their older counterparts. These geochemical signatures signify the dominance
of mafic-ultramafic rocks in the source area for Sargur rocks and the existence of granite-granodiorite for PQPC clastics.
Thus, the unconformity related oligomictic quartz pebble conglomerates (QPC) and quartzites at the base of Bababudan Group
resembling the QPC of Witswaterand, South Africa signifies that a stable continental crust had already developed in southern
India prior to ∼3.0Ga. 相似文献
942.
Surface water gross primary production potential (pGPP), respiration (RESP), metabolism potential (pMET), and CO2 fluxes in Hilo Bay, Hawai’i, USA, were examined along two river plumes during storm (high-flow) and non-storm (low-flow)
conditions. Significant differences in pGPP, RESP, and pMET were found between low- and high-flow conditions, with lowest
rates of all processes occurring during high-flow conditions. CO2 fluxes were influenced by metabolic processes at all but one site, with the bay’s surface waters being autotrophic and a
sink for atmospheric CO2 during low-flow conditions and less autotrophic and a source of atmospheric CO2 during high-flow conditions. Significant differences in pMET were found between the two river plumes during low-flow conditions
at spatial scales of 1.5 km; however, no differences between river plumes were found during high-flow conditions. Our study
suggests that an increase in storms associated with global climate change could impact surface water metabolic dynamics of
tropical estuaries. 相似文献
943.
Jiban K. Sarker Mehedi Ahmed Ansary Md. S. Rahman A. M. M. Safiullah 《Environmental Earth Sciences》2010,60(3):643-653
Mymensingh municipality lies in one of the most earthquake-prone areas of Bangladesh. The town was completely destroyed during
the Great Indian Earthquake of 12 June 1897, for which the surface-wave magnitude was 8.1. In this study the 1897 Great Indian
Earthquake was used as a scenario event for developing seismic microzonation maps for Mymensingh. For microzonation purposes
SPT data from 87 boreholes were collected from different relevant organizations. To verify those data ten boreholes of depth
up to 30 m were drilled. Intensity values obtained for different events were calibrated against attenuation laws to check
applicability to the study area. Vibration characteristics at diverse points of the study area were estimated by employing
the one-dimensional wave-propagation software SHAKE. SHAKE discretizes the soil profile into several layers and uses an iterative
technique to represent the non-linear behavior of the soil by adjusting the material properties at each iteration step. The
required input information includes depth, shear wave velocity, damping factor, and unit weight of each soil layer. The liquefaction
resistance factor and the resulting liquefaction potential were estimated to quantify the severity of liquefaction. Quantification
of secondary site effects and the weighting scheme for combining the various seismic hazards were heuristic, based on judgment
and expert opinion. 相似文献
944.
Gary Sands 《GeoJournal》2010,75(6):539-552
Canada’s three large city regions—Montréal, Toronto and Vancouver—have been relatively successful in retaining their Manufacturing
bases, while at the same time adding substantial numbers of New Economy job opportunities. Deindustrialization appears to
have occurred primarily in the Montréal CMA (Census Metropolitan Area) and in some smaller communities where motor vehicle
manufacturing was significant. Growth in New Economy jobs has occurred in the larger CMAs; the smaller urban areas have had
little success in attracting these jobs. For the most part, these trends appear to be path dependent: prosperous communities
with substantial proportions of New Economy jobs are likely to remain prosperous and to attract more such jobs. The lack of
significant correlation between Economic Health and the presence of immigrants, same sex couples, performing and visual artists
suggests that local economic development strategies based on these factors may have limited success. 相似文献
945.
Francesco La Vigna Simone Ciadamidaro Roberto Mazza Laura Mancini 《Environmental Earth Sciences》2010,60(6):1267-1279
Chemical, physical, and biological features of streams and ground water of the North-Eastern area of Rome are jointly analyzed
in order to assess the status of water resources. Ground water was investigated with classical survey methods (pH, temperature,
and electric conductivity). Microbiological pollution, faunal composition, and stream surrounding area conditions of surface
waters were studied, in order to quantify the residual value of these ecosystems from both a human and an environmental point
of view. Results show a general impairment of the system and the comparison between superficial waters and shallow ground
waters makes it possible to detect the presence of a connection between the two levels. This relationship occurs as an exchange
from superficial waters (streams and rivers) to the shallow aquifers. Where superficial waters are contaminated, as in the
Tor Sapienza stream, pollutants move to the shallow aquifers too, due to the decreased pressure of the over-exploited aquifer.
Moreover, uncontrolled drilling activity, diffused in urban areas, makes it possible the connection between shallow and deep
ground water. Notwithstanding this, the mixing between superficial and deep ground water system in Rome is not very widespread
and, apparently, limited to restricted areas. 相似文献
946.
Recent studies of Chesapeake Bay hypoxia suggest higher susceptibility to hypoxia in years after the 1980s. We used two simple
mechanistic models and Bayesian estimation of their parameters and prediction uncertainty to explore the nature of this regime
shift. Model estimates show increasing nutrient conversion efficiency since the 1980s, with lower DO concentrations and large
hypoxic volumes as a result. In earlier work, we suggested a 35% reduction from the average 1980–1990 total nitrogen load
would restore the Bay to hypoxic volumes of the 1950s–1970s. With Bayesian inference, our model indicates that, if the physical
and biogeochemical processes prior to the 1980s resume, the 35% reduction would result in hypoxic volume averaging 2.7 km3 in a typical year, below the average hypoxic volume of 1950s–1970s. However, if the post-1980 processes persist the 35% reduction
would result in much higher hypoxic volume averaging 6.0 km3. Load reductions recommended in the 2003 agreement will likely meet dissolved oxygen attainment goals if the Bay functions
as it did prior to the 1980s; however, it may not reach those goals if current processes prevail. 相似文献
947.
Sixty-five sediment samples and 25 water samples were collected from Al-Mujib reservoir, central Jordan, in order to investigate
the heavy metal and ionic contamination assessment. Therefore, to achieve this aim, water and sediment samples were collected
during winter and summer seasons (2007) from Al-Mujib reservoir and the areas surrounding it. The study shows that there are
elevated levels of SO4
2−, Cl− and Na+ in reservoir water, which might originate from anthropogenic activities in the reservoir catchment area. In addition, the
reservoir water has higher total hardness (TH) values together with high Ca and Mg contents. This might be attributed to pH
of reservoir water and the nature of the rocks exposed in the catchment area. The average levels of heavy metals in reservoir
sediments are Fe = 14,888.1, Cu = 17.8, Zn = 88.6, Ni = 38.7, Cd = 4.4, Mn = 337.9 and Pb = 6.1 mg/kg, which are lower than
that observed in Wadi Al-Arab reservoir, northern Jordan. The values of enrichment factor are Cd = 35.5, Ni = 3.02, Zn = 2.54,
Cu = 1.26, Mn = 1.2 and Pb = 0.57; these values indicate that heavy metals in sediments of Al-Mujeb reservoir have a different
anthropogenic incrimination inputs. The study showed that the sediments are polluted with Cd, relatively contaminated with
Ni and Zn and uncontaminated with respect to Mn, Pb and Cu. 相似文献
948.
Aliasger E. Haiderali Gopal Madabhushi 《Geotechnical and Geological Engineering》2016,34(5):1453-1473
The p–y method is one of the most popular methods for the analysis and design of laterally loaded piles. The mathematical relationship it provides between the bending moment, which can be easily measured at strain gauges along the pile, and the soil resistance and lateral pile displacement, facilitates the construction of p–y curves. Numerical techniques are required to fit smooth continuous curves to the discrete bending moment data in order to improve the accuracy of subsequent differentiation and integration operations. Due to the lack of guidance on the optimum positioning of strain gauges and the reliability and accuracy of curve fitting methods, a unifying study, inclusive of small (0.61 m) and large (3.8 and 7.5 m) diameter piles in clay, was carried out using 18 strain gauge layouts and cubic spline, cubic to quintic B-spline and 3rd to 10th degree global polynomial techniques. Bending moment data was obtained using 3D finite element analysis. Through a comprehensive evaluation, the cubic and cubic B-spline methods were found to be consistently accurate in deriving p–y curves for both the small and large diameter piles. 相似文献
949.
Yaxiong Peng Li Wu Ying Su Ruifeng Zhou 《Geotechnical and Geological Engineering》2016,34(6):1923-1932
Water or mud inrush can not only hamper the normal construction of tunnel, but also cause severe casualties and property losses. Through the cause analysis and statistical theory, a total of 9 predictors based on 3 factors, the engineering geology, hydrogeology and construction, were put forward to control the happening of water or mud inrush. Based on the classification principles of forewarning, 3 alert levels, red, orange and yellow, were established. Using the AHP–TOPSIS evaluation theory, the risk prediction model of water or mud inrush was built based on the classification of disaster forewarning. The model was used in the diversion project of Fujian Long Jin–xi and achieved good effects. 相似文献
950.