全文获取类型
收费全文 | 69920篇 |
免费 | 920篇 |
国内免费 | 519篇 |
专业分类
测绘学 | 1781篇 |
大气科学 | 5353篇 |
地球物理 | 13535篇 |
地质学 | 23712篇 |
海洋学 | 5839篇 |
天文学 | 16402篇 |
综合类 | 151篇 |
自然地理 | 4586篇 |
出版年
2020年 | 507篇 |
2019年 | 518篇 |
2018年 | 1226篇 |
2017年 | 1173篇 |
2016年 | 1501篇 |
2015年 | 1047篇 |
2014年 | 1655篇 |
2013年 | 3422篇 |
2012年 | 1685篇 |
2011年 | 2360篇 |
2010年 | 2129篇 |
2009年 | 2879篇 |
2008年 | 2607篇 |
2007年 | 2605篇 |
2006年 | 2423篇 |
2005年 | 2176篇 |
2004年 | 2134篇 |
2003年 | 2016篇 |
2002年 | 1909篇 |
2001年 | 1747篇 |
2000年 | 1632篇 |
1999年 | 1535篇 |
1998年 | 1499篇 |
1997年 | 1503篇 |
1996年 | 1200篇 |
1995年 | 1167篇 |
1994年 | 1101篇 |
1993年 | 992篇 |
1992年 | 957篇 |
1991年 | 905篇 |
1990年 | 1008篇 |
1989年 | 902篇 |
1988年 | 826篇 |
1987年 | 987篇 |
1986年 | 836篇 |
1985年 | 1102篇 |
1984年 | 1252篇 |
1983年 | 1202篇 |
1982年 | 1115篇 |
1981年 | 1061篇 |
1980年 | 966篇 |
1979年 | 929篇 |
1978年 | 936篇 |
1977年 | 842篇 |
1976年 | 806篇 |
1975年 | 747篇 |
1974年 | 796篇 |
1973年 | 780篇 |
1972年 | 492篇 |
1971年 | 461篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
922.
Non-linear response of the soil is investigated by comparing the spectral ratios (uphole/downhole) using weak and strong motions. Data from seven vertical arrays in Japan are analysed in this study. The frequency-dependent transfer function of soil is calculated as a ratio of the spectrum at uphole to the spectrum at downhole, considering the horizontal component of shear wave. In spectral ratio analysis auto- and cross-spectra are employed. The reduction in the predominant frequency of the transfer function with increases in excitation level reflects the non-linear response of the soil. Results of analysis demonstrate a significant non-linear ground response at six sites with surface PGA exceeding 90 gal. However, the results of one site show the linear response up to 130 gal surface PGA. Furthermore, the in situ strain-dependent soil behaviour is examined through the shear modulus – shear strain relationship. When compared, the actual and laboratory results of the shear strain – shear modulus relationship are in agreement. Additionally, a good consistency between the tendency of reduction in shear modulus ratio with shear strain increases, and reduction of predominant frequency with ground motion increases, confirms the significance of non-linearity in site effects study. 相似文献
923.
David R. Geist Mark C. Joy David R. Lee Tom Gonser 《Ground Water Monitoring & Remediation》1998,18(1):78-82
An impact drive point method is described for emplacing piezometers in a cobble river bottom where this has previously been difficult without the use of drilling rigs. To force the drive point piezometers through coble, the vibrational impact of an air-powered hammer was carried directly to the drive point by the use of an internal drive rod. After insertion to depth, the drive rod was removed from the lower portion of the piezometer and a standpipe was added to extend the piezometer above the river level. Piezometers installed in this way have permitted water quality analysis and dynamic measurement of vertical potentials in cobble sediments ranging in size from 2.5 to >30 cm and the method has been successfully used in the Columbia River, USA, and Töss River, Switzerland. This innovative method provides information on the hydrodynamics of pore water in highly permeable, cobble deposits that are common in high-energy river and lake bottoms. Piezometers installed using the internal drive rod method facilitate the assessment of the temporal and spatial dynamics of recharge and discharge at the ground water/surface water interface and analyses of the ecological connectivity between the hyporheic zone and surface water of rivers and streams. This information will lead to improved management decisions related to our nation's ground water and surface water supplies. 相似文献
924.
Geochemical processes controlling the spatial distribution of selenium in soil and water,west central South Dakota,USA 总被引:8,自引:0,他引:8
Selenium (Se) is essential in the human diet, but has a low threshold for toxic concentration. It is recommended that nutrients
such as Se should be consumed through foods as part of a normal diet. Se concentrations in crops and meat depend on the amount
of labile Se in the soil where crops and forage are grown. Therefore, managing agriculture for optimal Se in grain crops and
forage requires an understanding of the distribution and mobility of Se. Elevated concentrations of Se occur in waters, soils,
and forage 120 km west of Pierre in west central South Dakota, USA. The research site lies in an elevated, dissected plain
where soils developed on gently dipping Pierre Shale. Soils were sampled along catena transects and waters collected from
soil, ponds, and shallow borings in areas of known elevated forage and crop Se. Soil extracts from saturated-paste extraction
and acid (aqua regia and hydrofluoric acid) extraction were analyzed. Selenate was the dominant Se species in both acid and
saturated-paste extracts; selenite and organic Se were below detection (<0.2 ppb) in the same soil extracts. On average, 98% of soil Se was not water-soluble. The distribution of total Se shows much less spatial variation than water-soluble
Se in the areas sampled. Se shows correlation with organic carbon in soils and waters, suggesting its association with organic
carbon. Ca shows some correlation to Se in acid extracts, but not in saturated-paste extracts or in waters. Total Se shows
no significant correlation to Na, Mg, and total S in the soils. Se in saturated-paste extracts and water samples shows good
correlations with Na, Mg, and SO4, suggesting that evaporitic Na–Mg–sulfate minerals may temporarily concentrate water-soluble Se in shallow soils. The dissolution
and precipitation of these Na–Mg–sulfate salts together with pH and oxidation–reduction conditions apparently control water-soluble
Se distribution and mobilization in shallow subsurface environments. 相似文献
925.
M. A. Short S. Lamontagne P. G. Cook R. Cranswick 《Australian Journal of Earth Sciences》2013,60(2):319-331
The spatial and temporal distribution of near-shore fresh submarine groundwater discharge (SGD) was characterised from the coastal aquifers of the Willunga Basin, South Australia, an extensive aquifer system that supports an important viticultural region. Measurements of electrical conductivity (EC) and 222Rn (radon) activity were collected at 19 sites along the coastline during the Southern Hemisphere spring (2011) and summer (2013). At each site, samples were collected from the surf zone as well asporewater from beach sediment in the intertidal zone. Surf-zone radon activity ranged from <5 to 70mBq L–1, and intertidal porewater radon ranged over two orders of magnitude (220–36 940 mBq L–1) along the Willunga Basin coastline during both surveys. Overall, surf-zone and porewater EC was lower in the spring 2011 survey than in the summer 2013 survey. Porewater EC was similar to that of coastal water at most sites along the coastline, except at three sites where porewater EC was found to be lower than coastal water during both surveys, and three sites where evaporated seawater was observed in the summer survey. Based on the patterns in radon and EC along the coastline, two sites of localised fresh SGD were identified, in addition to a groundwater spring that is known to discharge to the coast. The results indicate that near-shore fresh SGD occurs as localised seeps rather than diffuse seepage along the entire coastline. The apparent absence of groundwater discharge at most locations is also consistent with current evidence suggesting that extensive groundwater pumping within the basin has resulted in seawater intrusion across much of the coastline. These observations also suggest that previous studies are likely to have over-estimated SGD rates from the Willunga Basin because they assumed that SGD occurred along the entire coastline. 相似文献
926.
Lateritic soils near Calicut, Kerala, contain halloysite of intermediate hydration, kaolinite, goethite, gibbsite and quartz. The presence of halloysite is responsible for relatively high plasticity and cation-exchange capacity. Fe-hydroxide colloids along with halloysite contribute to significant phosphate uptake by this soil. Composition of local groundwater is consistent with weathering of sodic plagioclase to gibbsite, kaolinite and metastable halloysite. 相似文献
927.
Nivedita Sanwlani Prakash Chauhan Ranganath R. Navalgund 《Journal of the Indian Society of Remote Sensing》2010,38(4):617-626
Space-borne ocean-colour remote sensor-detected radiance is heavily contaminated by solar radiation backscattered by the atmospheric
air molecules and aerosols. Hence, the first step in ocean-colour data processing is the removal of this atmospheric contribution
from the sensor-detected radiance to enable detection of optically active oceanic constituents e.g. chlorophyll-a, suspended sediment etc. In standard atmospheric correction procedure for OCEANSAT-1 Ocean Colour Monitor (OCM) data, NIR
bands centered at 765 and 865 nm wavelengths were used for aerosol characterization. Due to high absorption by water molecules,
ocean surface in these two wavelengths acts as dark background, therefore, sensor detected radiance can be assumed to have
major contribution from atmospheric scattering. For coastal turbid waters this assumption of dark surface fails due to the
presence of highly scattering sediments which causes sufficient water-leaving radiance in NIR bands and lead to over-estimation
of aerosol radiance resulting in negative water leaving radiance for λ < 700 nm. In the present study, for the turbid coastal
waters in the northern Bay of Bengal, the concept of spatial homogeneity of aerosol and water leaving reflectance has been
applied to perform atmospheric correction of OCAEANSAT-1 OCM data. The results of the turbid water atmospheric correction
have also been validated using in-situ measured water-leaving radiance. Comparison of satellite derived water-leaving radiance for five coastal stations with in-situ measured radiance spectra, indicates an improvement over the standard atmospheric correction algorithm giving physically
realistic and positive values. Root Mean Square Error (RMSE) between the in-situ measured and satellite derived water leaving radiance for wavelengths 412 nm, 443 nm, 490 nm, 512 nm and 555 nm was found
to be 1.11, 0.718, 0.575, 0.611 and 0.651%, respectively, using standard atmospheric correction procedure. By the use of spatial
homogeneity concept, this error was reduced to 0.125, 0.173, 0.176, 0.225, and 0.290 and the correlation coefficient arrived
at 0.945, which is an improvement over the standard atmospheric correction procedure. 相似文献
928.
929.
930.
R.W. Marjoribanks 《Tectonophysics》1976,33(1-2)
A major, linear, west-trending deformed zone (The Redbank Zone), 350 km long and up to 20 km wide, can be identified in the Arunta Block immediately north of the Amadeus Basin. The marked linearity of this zone and of the coincident gravity anomaly probably result from thrust-fault movement during the Carboniferous Alice Springs Orogeny. However, in the Ormiston area, there is evidence that the zone originated prior to 1070 m.y. and acted as a major crustal feature controlling the later orogenic event.The Alice Springs Orogeny affected the overlying Proterozoic and Lower Palaeozoic cover rocks as well as the Arunta Block basement. During the orogeny, steep north-dipping thrusts within the Redbank Zone were reactivated causing uplift to the north. These faults penetrated the Heavitree Quartzite—the basal unit of the cover sequence—to drive wedges of basement, with attached veneers of Heavitree Quartzite, for up to 20 km southward within the overlying Bitter Springs Formation. The nappes did not reach the surface or penetrate formations above the Bitter Springs. Accompanying nappe emplacement the Basin to the south rapidly deepened to receive a thick wedge of synorogenic molasse sediments.Gravity, sedimentary and structural features combine to suggest that the Alice Springs orogeny movements reached their maximum on the central part of the northern margin of the Amadeus Basin, in the Ormiston area. 相似文献