全文获取类型
收费全文 | 1176篇 |
免费 | 45篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 42篇 |
大气科学 | 121篇 |
地球物理 | 253篇 |
地质学 | 390篇 |
海洋学 | 85篇 |
天文学 | 184篇 |
综合类 | 13篇 |
自然地理 | 139篇 |
出版年
2022年 | 6篇 |
2021年 | 14篇 |
2020年 | 16篇 |
2019年 | 23篇 |
2018年 | 39篇 |
2017年 | 20篇 |
2016年 | 38篇 |
2015年 | 23篇 |
2014年 | 40篇 |
2013年 | 89篇 |
2012年 | 40篇 |
2011年 | 48篇 |
2010年 | 48篇 |
2009年 | 68篇 |
2008年 | 71篇 |
2007年 | 56篇 |
2006年 | 45篇 |
2005年 | 41篇 |
2004年 | 38篇 |
2003年 | 45篇 |
2002年 | 42篇 |
2001年 | 30篇 |
2000年 | 17篇 |
1999年 | 19篇 |
1998年 | 17篇 |
1997年 | 24篇 |
1996年 | 19篇 |
1995年 | 17篇 |
1994年 | 7篇 |
1993年 | 13篇 |
1992年 | 8篇 |
1991年 | 15篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 12篇 |
1986年 | 5篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 9篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 12篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1975年 | 8篇 |
1973年 | 4篇 |
1972年 | 5篇 |
1969年 | 4篇 |
排序方式: 共有1227条查询结果,搜索用时 171 毫秒
11.
A code of practice for ocean mining has been developed by the International Marine Minerals Society. The code provides general guiding Principles for development of marine mineral resources and delineates Operating Guidelines for application, as appropriate, at specific mining sites. In total 11 operating guidelines are provided to serve as benchmarks for industry and targets for regulatory agencies and other stakeholders. They include the following commitments: •To sustainable development, •To environmentally responsible corporate ethics, •To the development of effective community partnership, •To implementation of environmental risk management, •To the integration of environmental management into all phases of development, •To the establishment of corporate environmental performance targets, •To timely environmental improvements and upgrading, •To responsible rehabilitation of operations sites and decommissioning of facilities, and •To accurate and timely, reporting and documentation, archiving, and performance review. 相似文献
12.
Valeria Mattiangeli Anthony W. Ryan Paul Galvin Jarle Mork & Thomas F. Cross 《Marine Ecology》2003,24(4):247-258
Abstract. Nine allozyme and two minisatellite loci were used to investigate potential genetic differentiation among three samples of Mediterranean poor cod, Trisopterus minutus capelanus, from the Gulf of Lion, the Tuscan Archipelago and the Aegean Sea. Both types of markers showed consistent results, with FST values of 0.0262 and 0.0296 (P < 0.0015, after Bonferroni correction for multiple tests) for allozymes and minisatellites, respectively. Allele frequency heterogeneity tests between pairs of samples showed a clear separation between the two western Mediterranean samples (Gulf of Lion, Tuscan Archipelago) and the eastern one (Aegean Sea). The results indicate that at least two reproductively isolated populations of poor cod occur in the Mediterranean. 相似文献
13.
Anthony H. Knap Peter J. LeB. Williams Elizabeth Lysiak 《Marine environmental research》1982,7(4):235-249
The sediments of Southampton Water were analysed for petroleum hydrocarbons to determine the fate of petrochemical refinery waste in the estuary. Much of the petroleum hydrocarbons appears to be removed by adsorption onto estuarine sediments close to source. Surface sediment concentrations range from 3·1mg/g dry weight near a refinery to 0·5 mg/g dry weight at locations distant from the refinery.The absence of any change with depth in total hydrocarbon concentrations from some sediment cores and the presence of distinct ‘oil’ horizons in others suggests that mixing of sediments in the area is very random. Sedimentation rates in areas where a distinct oil horizon was observed ranged from 1 to 3cm a year. 相似文献
14.
The morphology, bedforms and hydrodynamics of Merlimont beach, in northern France, characterised by intertidal bars and a spring tidal range of 8.3 m, were surveyed over a 10-day experiment with variable wave conditions that included a 2-day storm with significant wave heights of up to 2.8 m. The beach exhibited two pronounced bar-trough systems located between the mean sea level and low neap tide level. Waves showed a cross-shore depth modulation, attaining maximum heights at high tide. The mean current was characterised dominantly by strong tide-induced longshore flows significantly reinforced by wind forcing during the storm, and by weaker, dominantly offshore, wave-induced flows. Vertical tidal water-level variations (tidal excursion rates) showed a bimodal distribution with a peak towards the mid-tide position and low rates near low and high water. The two bar-trough systems in the mid-tide zone remained stable in position during the experiment but showed significant local change. The absence of bar migration in spite of the relatively energetic context of this beach reflects high macro-scale bar morphological lag due to a combination of the large vertical tidal excursion rates in the mid-tide zone, the cross-shore wave structure, and the pronounced dual bar-trough system. The profile exhibited a highly variable pattern of local morphological change that showed poor correlation with wave energy levels and tidal excursion rates. Profile change reflected marked local morphodynamic feedback effects due mainly to breaks in slope associated with the bar-trough topography and with trough activity. Change was as important during low wave-energy conditions as during the storm. Strong flows in the entrenched troughs hindered cross-shore bar mobility while inducing longshore migration of medium-sized bedforms that contributed in generating short-term profile change. The large size and location of the two pronounced bars in the mid-tide zone of the beach are tentatively attributed respectively to the relatively high wave-energy levels affecting Merlimont beach, and to the cross-shore increase in wave height hinged on tidal modulation of water depths. These two large quasi-permanent bars probably originated as essentially breakpoint bars and are different from a small bar formed by swash and surf processes in the course of the experiment at the mean high water neap tide level, which is characterised by a certain degree of tidal stationarity and larger high-tide waves. 相似文献
15.
16.
Barotropic tidal mixing effects in a coupled climate model: Oceanic conditions in the Northern Atlantic 总被引:3,自引:0,他引:3
Impacts of mixing driven by barotropic tides in a coupled climate model are investigated by using an atmosphere–ocean–ice–land coupled climate model, the GFDL CM2.0. We focus on oceanic conditions of the Northern Atlantic. Barotropic tidal mixing effects increase the surface salinity and density in the Northern Atlantic and decrease the RMS error of the model surface salinity and temperature fields related to the observational data. 相似文献
17.
Agricultural Census data is summarised over spatially coarse reporting units for reasons of farm confidentiality. This is problematic for research at a local level. This article describes an approach combining dasymetric and volume preserving techniques to create a national land use dataset at 1 km2 resolution. The results for an English county are compared with contemporaneous aggregated habitat data. The results show that the accurate estimates of local agricultural land use (Arable and Grass) patterns can be estimated when individual 1 km squares are combined into blocks of > 9 squares, thereby providing local estimates of agricultural land use. This in turn allows more detailed modelling of land uses related to specific livestock and cropping activities. The dataset created by this work has been subject to extensive external validation through its incorporation into a number of other national models: nitrate leaching (e.g. MAGPIE, NEAP‐N), waste, and pathogen modelling related to agricultural activity. 相似文献
18.
Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Québec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of 0.4GPa at 1090–1070Ma. Fluid infiltration produced exoskarns, calcite+periclase+forsterite±diopside±orthoclase assemblages in the marbles, and quartz±calcite±wollastonite±diopside±anorthite assemblages in the calc-silicates. Phase-equilibria in the CaO–MgO–Al2O3–SiO2–H2O–CO2 system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715–815°C) and that the fluids hadXCO20.15. In the metasediments, 18O values of calcite (Cc) are as low as 8.6, suggesting that the fluids were in isotopic equilibrium with the syenites (18O =8.8–10.2). Marble 13C(Cc) values are-0.1 to-3.2; the lack of correlation between 13C(Cc) and 18O(Cc) is consistent with the infiltration of water-rich fluids. The resetting of stable isotopes and the mineralogical changes can be explained by time-integrated fluid fluxes of up to 110 m3/m2 (4×106 mol/m2), corresponding to actual fluxes of 3×10-11 to 3×10-12 m3/m2-s and intrinsic permeabilities of 10-18 to 10-20 m2 for fluid flow lasting 0.1-1Ma. Marble 18O(Cc) values do not correlate well with distance from the syenite, and fluids were probably channelled across lithological layering. The correlation between the degree of resetting of marble 18O(Cc) values with the abundance of submillimetre-wide veins, suggests that fluid focussing may have resulted from variations in fracture density. Late, lower temperature (<500°C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. 18O(Br) and 18O(Scrp) values correlate with 18O(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that 18O(Cc-Br) and 18O(Cc-Serp) values are higher in marbles that have lower 18O(Cc) values is interpreted as indicating that fluid flow persisted to lower temperatures in those rocks due to higher intrinsic permeabilities. Calcite in the syenite was also formed by the influx of fluids during cooling. Syenite 18O(Cc) values are approximately in isotopic equilibrium with the high-temperature silicate minerals, suggesting that again only minor volumes of fluid were involved. In detail fluid flow was prolonged and complex, creating problems for the application of quantitative fluid flow models.This paper is a contribution to IGCP 304, Lower Crustal Processes 相似文献
19.
利用主成分分析改善土地利用变化的遥感监测精度——以珠江三角洲城市用地扩张为例 总被引:41,自引:7,他引:34
近年来,珠江三角洲由于经济的快速发展,城市用地急剧增加,利用多时相的遥感图,可以定量地监测这种城市化的现象。但理,由一般的遥感动态监测方法所得的结果往往夸大变化的程度,以及获得一些不合理的结论.该文提出主成分分析的方法来改善遥感动态监测的精度。将该方法应用应用于珠江三角洲发展最快的东莞市,获得了较满意的结果。 相似文献
20.
Properties of acoustic-gravity waves in the upper atmosphere of Venus are studied using a two-fluid model which includes the effects of wave-induced diffusion in a diffusively separated atmosphere. In conjunction with neutral mass spectrometer data from the Pioneer Venus orbiter, the theory should provide information on the distribution of wave sources in the Venus upper atmosphere. Observed wave structure in species density measurements should generally have periods ?30–35 min, small N2, CO, and O amplitudes, and highly variable phase shifts relative to CO2. A near resonance may exist between downward phase-propagating internal gravity and diffusion waves near the 165-km level at periods near 29 min. As a result, if very large He wave amplitudes are observed near this level, it will indicate that the wave source is below the 150- to 175-km level and that the exospheric temperature is close to 350°K. Wave energy dissipation may be an important mechanism for heating of the nightside Venus thermosphere. Large-density oscillations in stratospheric cloud layer constituents are also possible and may be detectable by the Pioneer Venus large probe neutral mass spectrometer. 相似文献