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201.
Estuaries and Coasts - Effects of the herbicide, atrazine, on the submersed vascular plant,Potamogeton perfoliatus, were monitored for 4 wk in 700 l microcosms containing water, sediments and... 相似文献
202.
The satellites of the outer solar system planets are thought to be mixtures of ices and rocky material, in which decay of radioactive nuclides can lead to internal melting and solid-state convection. Time-dependent models indicate that melting will reach its maximum extent approximately 2.0 GYr after formation; bodies of radius <500 km will never melt, and those <750 km in radius will be totally refrozen by present. Surface water flows are not expected for bodies of <1500-km radius. However, even small (100 km) bodies may be unstable against solid-state convection, and their surfaces may show signs of tectonism. Other processes altering the surfaces include sublimation and photolysis of ices. Sublimation likely explains the absence of CH4 ices on any Saturnian satellite except Titan; photolysis explains the absence of NH3 ices on these bodies, and possibly the absence of water ice on the surface of Callisto. The photolysis rate of CH4 also implies a crustal reservoir of CH4 on Titan. 相似文献
203.
204.
Stuart G Wakeham Cynthia M Lewis Stefan Schouten 《Geochimica et cosmochimica acta》2003,67(7):1359-1374
We evaluate anaerobic oxidation of methane (AOM) in the Black Sea water column by determining distributions of archaea-specific glyceryl dialkyl glyceryl tetraethers (GDGTs) and 13C isotopic compositions of their constituent biphytanes in suspended particulate matter (SPM), sinking particulate matter collected in sediment traps, and surface sediments. We also determined isotopic compositions of fatty acids specific to sulfate-reducing bacteria to test for biomarker and isotopic evidence of a syntrophic relationship between archaea and sulfate-reducing bacteria in carrying out AOM. Bicyclic and tricyclic GDGTs and their constituent 13C-depleted monocyclic and bicyclic biphytanes (down to −67‰) indicative of archaea involved in AOM were present in SPM in the anoxic zone below 700 m depth. In contrast, GDGT-0 and crenarchaeol derived from planktonic crenarchaeota dominated the GDGT distributions in the oxic surface and shallow anoxic waters. Fatty acids indicative of sulfate-reducing bacteria (i.e., iso- and anteiso-C15) were not strongly isotopically depleted (e.g., −32 to −25‰), although anteiso-C15 was 5‰ more depleted in 13C than iso-C15. Our results suggest that either AOM is carried out by archaea independent of sulfate-reducing bacteria or those sulfate-reducing bacteria involved in a syntrophy with methane-oxidizing archaea constitute a small enough fraction of the total sulfate-reducing bacterial community that an isotope depletion in their fatty acids is not readily detected. Sinking particulate material collected in sediment traps and the underlying sediments in the anoxic zone contained the biomarker and isotope signature of upper-water column archaea. AOM-specific GDGTs and 13C-depleted biphytanes characteristic of the SPM in the deep anoxic zone are not incorporated into sinking particles and are not efficiently transported to the sediments. This observation suggests that sediments may not always record AOM in overlying euxinic water columns and helps explain the absence of AOM-derived biomarkers in sediments deposited during past periods of elevated levels of methane in the ocean. 相似文献
205.
The 750-Ma Seychelles granites have whole-rock '18O values that range from -1.2 to +7.5. Differences in '18O values between quartz, feldspar and whole-rock for samples that have a range of '18O values suggest that these minerals are in magmatic equilibrium, and that the whole-rock '18O values were little affected by post-crystallization interaction with fluids. Two suites of granites (the Mahé type and the Praslin type) have previously been recognized on the basis of chemical and radiogenic isotope composition. The former have a mean whole-rock '18O value of 5.25ǂ.65 (1C), whereas the latter are much more variable, with a mean whole-rock '18O value of 3.00Dž.27. Biotite and amphibole separates from both granite types range in 'D from -69 to -116, and show a positive correlation with whole-rock '18O values. Dolerite dykes that intruded the granites during or shortly after their crystallization also have whole-rock '18O values (mean 2.24ǃ.93) lower than that expected for mantle-derived basaltic rocks, but these values appear to be the result of fluid-rock interaction. We suggest that the Mahé-type granites are derived mainly from juvenile mafic to intermediate crust with 'normal' 'D and '18O values, whereas the Praslin-type granites are mixtures of this source and older crust that acquired its low 'D and '18O values by extensive interaction with meteoric water at high temperature. It is unlikely that meteoric water sufficiently depleted in D and 18O was available at 750 Ma because of the relatively low latitude of the region at that time. We suggest that alteration of the source took place significantly before production of the granite magmas. Depletion in 18O of the Seychelles granites does not necessarily require a regional extensional tectonic setting at 750 Ma, as has been proposed. 相似文献
206.
207.
Béla Paláncz Joseph L. Awange Piroska Zaletnyik Robert H. Lewis 《Journal of Geodesy》2010,84(1):79-95
A fundamental task in geodesy is solving systems of equations. Many geodetic problems are represented as systems of multivariate
polynomials. A common problem in solving such systems is improper initial starting values for iterative methods, leading to
convergence to solutions with no physical meaning, or to convergence that requires global methods. Though symbolic methods
such as Groebner bases or resultants have been shown to be very efficient, i.e., providing solutions for determined systems
such as 3-point problem of 3D affine transformation, the symbolic algebra can be very time consuming, even with special Computer
Algebra Systems (CAS). This study proposes the Linear Homotopy method that can be implemented easily in high-level computer languages like C++ and Fortran that are faster than CAS by at
least two orders of magnitude. Using Mathematica, the power of Homotopy is demonstrated in solving three nonlinear geodetic problems: resection, GPS positioning, and affine
transformation. The method enlarging the domain of convergence is found to be efficient, less sensitive to rounding of numbers,
and has lower complexity compared to other local methods like Newton–Raphson. 相似文献
208.
ABSTRACTAir travel has become central to Australian academic practice, with flying increasingly seen as crucial to the progression of successful academic careers. This paper seeks to understand why academic air travel has come to be perceived as so necessary, particularly given its significant environmental, social, and economic implications. Drawing on the mobilities paradigm and social practice theory, we seek to understand how air travel practices are a key part of contemporary academic careers. We conducted an online survey of 301 academics and 24 in depth semi-structured interviews to understand academic air travel in Australia, finding that it is seen as a means to connect and collaborate within and between academic communities, and to counteract the effects of ‘remoteness'. This remoteness is experienced domestically and internationally, by Australian academics who feel they must travel to perceived centres of knowledge production to further their careers. Academics often frame their ‘need' for connections as necessarily involving physical co-presence. Air travel offers career benefits for those who can take advantage of it, and corresponding drawbacks for those ‘stuck on the ground’. We conclude by discussing the need for valuing localised forms of collaboration as markers of success, and developing imaginative alternatives to academic flying. 相似文献
209.
This paper describes N-body simulations of two regions of the saturnian ring system and examines what we might expect the Cassini orbiter to see in those areas. The first region is the edge of the Encke gap in the A ring that is perturbed by the satellite, Pan. Our previous simulations of this region neglected particle self-gravity [Lewis and Stewart, 2000a, Bull. Am. Astron. Soc. 34, 883]. Here we examine the interactions of the wakes caused by Pan with the wakes that form from local gravitational instabilities. We find that the two phenomena do not normally coexist and predict that measurements of particle sizes between the moon wakes should reflect the true particle size distribution of the region and not what is caused by gravitational aggregation. The region between the Encke gap edge and the first wake peak is an exception to this rule because our simulations exhibit the formation of exceptionally large gravity-induced wakes in this region. We also describe simulations of the F ring and explain the nature of braid-like structures that form naturally when the ring is perturbed by a single moon on an eccentric orbit. Finally, we discuss the very dynamic nature of the F ring system and how this should be taken into account when interpreting observations and even when planning future observations of this system. 相似文献
210.
Neil Terry Frederick D. Day‐Lewis Judith L. Robinson Lee D. Slater Keith Halford Andrew Binley John W. Lane Jr Dale Werkema 《Ground water》2017,55(6):885-890
Geophysical tools have much to offer users in environmental, water resource, and geotechnical fields; however, techniques such as electrical resistivity imaging (ERI) are often oversold and/or overinterpreted due to a lack of understanding of the limitations of the techniques, such as the appropriate depth intervals or resolution of the methods. The relationship between ERI data and resistivity is nonlinear; therefore, these limitations depend on site conditions and survey design and are best assessed through forward and inverse modeling exercises prior to field investigations. In this approach, proposed field surveys are first numerically simulated given the expected electrical properties of the site, and the resulting hypothetical data are then analyzed using inverse models. Performing ERI forward/inverse modeling, however, requires substantial expertise and can take many hours to implement. We present a new spreadsheet‐based tool, the Scenario Evaluator for Electrical Resistivity (SEER), which features a graphical user interface that allows users to manipulate a resistivity model and instantly view how that model would likely be interpreted by an ERI survey. The SEER tool is intended for use by those who wish to determine the value of including ERI to achieve project goals, and is designed to have broad utility in industry, teaching, and research. 相似文献