全文获取类型
收费全文 | 1949篇 |
免费 | 68篇 |
国内免费 | 15篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 149篇 |
地球物理 | 399篇 |
地质学 | 741篇 |
海洋学 | 134篇 |
天文学 | 344篇 |
综合类 | 9篇 |
自然地理 | 216篇 |
出版年
2023年 | 9篇 |
2022年 | 7篇 |
2021年 | 24篇 |
2020年 | 28篇 |
2019年 | 20篇 |
2018年 | 44篇 |
2017年 | 31篇 |
2016年 | 52篇 |
2015年 | 42篇 |
2014年 | 48篇 |
2013年 | 117篇 |
2012年 | 56篇 |
2011年 | 98篇 |
2010年 | 84篇 |
2009年 | 103篇 |
2008年 | 98篇 |
2007年 | 74篇 |
2006年 | 82篇 |
2005年 | 75篇 |
2004年 | 78篇 |
2003年 | 78篇 |
2002年 | 79篇 |
2001年 | 52篇 |
2000年 | 42篇 |
1999年 | 53篇 |
1998年 | 51篇 |
1997年 | 28篇 |
1996年 | 24篇 |
1995年 | 26篇 |
1994年 | 29篇 |
1993年 | 30篇 |
1992年 | 25篇 |
1991年 | 22篇 |
1990年 | 25篇 |
1989年 | 30篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 27篇 |
1983年 | 19篇 |
1982年 | 17篇 |
1981年 | 25篇 |
1980年 | 22篇 |
1979年 | 16篇 |
1977年 | 10篇 |
1976年 | 11篇 |
1975年 | 7篇 |
1974年 | 12篇 |
1970年 | 7篇 |
排序方式: 共有2032条查询结果,搜索用时 15 毫秒
121.
Ian Jackson 《Earth and Planetary Science Letters》1983,62(1):91-103
The physical properties(?, K, K′) of the adiabatically decompressed lower mantle are interpreted in terms of an (Mg,Fe)SiO3 perovskite + magnesiowüstite mineralogy. The approach employed in this paper involves the removal of the relatively better characterised magnesiowüstite component from the two-phase mixture in order to highlight the physical properties required of the perovskite phase for consistency between the seismological data and any proposed compositional model. It is concluded that a wide tradeoff (emphasized by Davies [1]) between composition, temperature and the physical properties (especially thermal expansion) of the perovskite phase accommodates most recently proposed compositional models including Ringwood's [2] pyrolite and the more silicic models of Burdick and Anderson [3], Anderson [4], Sawamoto [5], Butler and Anderson [6], Liu [7,8] and Watt and Ahrens [9]. 相似文献
122.
Chironomid remains from Big Lake, British Columbia were analysed and paleosalinities were estimated using a pre-existing transfer function and several developed using new regression methods. A two component partial-least-squares model (PLS-2) had the highest coefficient of determination (R2
(Jackknifed) = 0.75) and lowest root-mean-squared error-of-prediction (RMSEP). As compared to the pre-existing model, it was also less sensitive to the influence of rare taxa. Nevertheless, the marginally larger R2
(Jackknifed) and lower RMSEP do not clearly identify a single best model. The models were applied to Big, Mahoney and Kilpoola lakes, revealing the sensitivity of paleosalinity inferences to model selection. A synopsis of chironomid-based paleosalinities in British Columbia and their correspondence with other paleoclimatic data are presented and discussed. 相似文献
123.
A non-hydrostatic numerical model, the Regional Atmospheric Modeling System (RAMS), has been used to investigate the development of katabatic jumps in Coats Land, Antarctica. In the control run with a 5 m s-1downslope directed initial wind, a katabatic jump develops near the foot of the idealized slope. The jump is manifested as a rapid deceleration of the downslope flow and a change from supercritical to subcritical flow, in a hydraulic sense, i.e., the Froude number (Fr) of the flow changes from Fr > 1 to Fr> 1. Results from sensitivity experiments show that an increase in the upstream flow rate strengthens the jump, while an increase in the downstream inversion-layer depth results in a retreat of the jump. Hydraulic theory and Bernoulli's theorem have been used to explain the surface pressure change across the jump. It is found that hydraulic theory always underestimates the surface pressure change, while Bernoulli's theorem provides a satisfactory estimation. An analysis of the downs balance for the katabatic jump indicates that the important forces are those related to the pressure gradient, advection and, to a lesser extent, the turbulent momentum divergence. The development of katabatic jumps can be divided into two phases. In phase I, the t gradient force is nearly balanced by advection, while in phase II, the pressure gradient force is counterbalanced by turbulent momentum divergence. The upslope pressure gradient force associated with a pool of cold air over the ice shelf facilitates the formation of the katabatic jump. 相似文献
124.
Ian Knowles Michael Teubner Aimin Yan Paul Rasser Jong Wook Lee 《Hydrogeology Journal》2007,15(6):1107-1118
A new inverse technique for modelling groundwater flow, based on a functional minimization technique, has been used to calibrate
a groundwater flow model of a subregion of the Port Willunga aquifer within the Willunga Basin in South Australia. The Willunga
Basin is the location of extensive viticulture, irrigated primarily by groundwater, the levels and quality of which have declined
significantly over the last 40 years. The new method is able to generate estimates of transmissivity, storativity and groundwater
recharge over the whole subregion as a time-varying continuous surface; previous methods estimate local discrete parameter
values at specific times. The new method has also been shown to produce accurate head values for the subregion and very good
estimates of groundwater recharge. Its ultimate goal will be to provide a new and invaluable tool for significantly improved
groundwater resource management.
Supported in part by US National Science Foundation grants, DMS-0107492 and DMS-0079478. 相似文献
125.
126.
Ian Robson 《Astronomy& Geophysics》2007,48(4):4.30-4.30
IYA2009 will be a great opportunity for all of us working in astronomy (and space) to make an impact; certainly locally and nationally and, we hope, globally. So what is going on and how can you participate? Ian Robson has the answers. 相似文献
127.
Mike Hapgood and Ian Crawford assess the relevance of the BNSC UKSEWG Report for the Royal Astronomical Society. 相似文献
128.
Costas Dounas Ian Davies George Triantafyllou Panayota Koulouri George Petihakis Christos Arvanitidis George Sourlatzis Anastasios Eleftheriou 《Continental Shelf Research》2007
Disturbance of the seabed resulting from bottom trawling affects ecosystem processes, such as the rate and magnitude of nutrient regeneration. The potential responses of the plankton community arising from such effects can be modelled, provided that reliable data on the effects on nutrient fluxes are available. In a north Cretan outer continental shelf and upper slope fishing ground (Heraklion Bay, Crete, Eastern Mediterranean) we applied a new field instrument which can simulate the passage of trawl groundropes across the sea floor and made direct seasonal measurements of the rate of dissolved and particulate nutrient releases resulting from seabed disturbance. These observational data were then integrated in a 3D ecosystem model. Results revealed that bottom trawling may trigger off considerable productivity pulses, in addition to pulses from the natural seasonal cycle. 相似文献
129.
Steve Dury Brian Turner Bill Foley Ian Wallis 《International Journal of Applied Earth Observation and Geoinformation》2001,3(4):328
In this study we assess the feasibility of remotely measuring canopy biochemistry, and thus the potential for conducting large-scale mapping of habitat quality. A number of studies have found nutrient composition of eucalypt foliage to be a major determinant of the distribution of folivorous marsupials. More recently it has been demonstrated that a specific group of secondary plant chemicals, the diformylphloroglucinols (DFPs), are the most important feeding deterrents, and are thus vital determinants of habitat quality. We report on the use of laboratory spectroscopy to attempt to identify one such DFP, sideroxylonal-A, in the foliage of Eucalyptus melliodora, one of the few eucalypt species browsed by folivorous marsupials. Reflectance spectra were obtained for freeze-dried, ground leaves using near infrared spectroscopy (NIRS) and for both oven-dried and fresh whole leaves using a laboratory-based (FieldSpec) spectroradiometer. Modified partial least squares (MPLS) regression was used to develop calibration equations for sideroxylonal-A concentration based on the reflectance spectra transformed as both the first and second difference of absorbance (Log 1/R). The predictive ability of the calibration equations was assessed using the standard error of calibration statistic (SECV). Coefficients of determination (r2) were highest for the ground leaf spectra (0.98), followed by the fresh leaf and dry leaf spectra (0.94 and 0.87, respectively). When applied to independent validation sub-sets, sideroxylonal-A was most accurately predicted from the ground leaf spectra (r2 = 0.94), followed by the dry leaf and fresh leaf spectra (0.72 and 0.53, respectively). Two spectral regions, centred on 674 nm and 1394 nm, were found to be highly correlated with sideroxylonal-A concentration for each of the three spectral data sets studied. Results from this study suggest that calibration equations derived from modified partial least squares regression may be used to predict sideroxylonal-A concentration, and hence leaf palatability, of Eucalyptus melliodora trees, thereby indicating that the remote estimation of habitat quality of eucalypt forests for marsupial folivores is feasible. 相似文献
130.
Results of laboratory and numerical experiments on both homogeneous and density-stratified flow over single, bluff obstacles of various shapes are presented. The obstacle height is in most cases of the same order as the base diameter and the major controlling (flow) parameter is the Froude number, defined here as Fh=U/Nh, where U is the (uniform) upstream velocity, h the obstacle height and N is the buoyancy frequency. Attention is concentrated, firstly, on the case of homogeneous flows over rather weakly tapered obstacles and, secondly, for bodies whose height is similar to their base width, on the case Fh=0.1, representing stratification sufficiently strong that lee-wave motions do not play a significant role in the flow dynamics. For right-circular cones it is shown that the sectional contributions to the total fluctuating side force (lift) show significant phase variations up the height of the obstacle, which are not always reflected in the developed vortex street further downstream. For some obstacle shapes, the vortex lines linking the von Karman eddies at different heights can be significantly tilted, particularly in the upper part of the wake. Vortex convection speeds do not appear generally to vary greatly with height and, as found in previous work, the shedding frequency remains constant with height, despite the strong variation of cross-stream obstacle width. By comparison with the homogeneous results, it is suggested that the stratification enhances the shedding instability, which would otherwise be very weak for squat obstacles, but does not annihilate the ability of the flow at one level to influence that at another. 相似文献