全文获取类型
收费全文 | 1324篇 |
免费 | 83篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 45篇 |
大气科学 | 81篇 |
地球物理 | 378篇 |
地质学 | 508篇 |
海洋学 | 101篇 |
天文学 | 166篇 |
综合类 | 6篇 |
自然地理 | 135篇 |
出版年
2023年 | 8篇 |
2022年 | 6篇 |
2021年 | 23篇 |
2020年 | 24篇 |
2019年 | 32篇 |
2018年 | 45篇 |
2017年 | 43篇 |
2016年 | 50篇 |
2015年 | 38篇 |
2014年 | 36篇 |
2013年 | 77篇 |
2012年 | 59篇 |
2011年 | 61篇 |
2010年 | 42篇 |
2009年 | 73篇 |
2008年 | 64篇 |
2007年 | 68篇 |
2006年 | 62篇 |
2005年 | 50篇 |
2004年 | 58篇 |
2003年 | 29篇 |
2002年 | 48篇 |
2001年 | 25篇 |
2000年 | 38篇 |
1999年 | 30篇 |
1998年 | 33篇 |
1997年 | 16篇 |
1996年 | 20篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 12篇 |
1992年 | 16篇 |
1991年 | 14篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 11篇 |
1980年 | 7篇 |
1979年 | 12篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 12篇 |
1975年 | 6篇 |
1974年 | 6篇 |
排序方式: 共有1420条查询结果,搜索用时 31 毫秒
201.
The TerraSAR-X (TSX) synthetic aperture radar (SAR) marks the recent emergence of a new generation of spaceborne radar sensors
that can for the first time lay claim to localization accuracies in the sub-meter range. The TSX platform’s extremely high
orbital stability and the sensor’s hardware timing accuracy combine to enable direct measurements of atmospheric refraction
and solid Earth movements. By modeling these effects for individual TSX acquisitions, absolute pixel geolocation accuracy
on the order of several centimeters can be achieved without need for even a single tiepoint. A 16-month time series of images
was obtained over a fixed test site, making it possible to validate both an atmospheric refraction and a solid Earth tide
model, while at the same time establishing the instrument’s long-term stability. These related goals were achieved by placing
trihedral corner reflectors (CRs) at the test site and estimating their phase centers with centimeter-level accuracy using
differential GPS (DGPS). Oriented in pairs toward a given satellite track, the CRs could be seen as bright “points” in the
images, providing a geometric reference set. SAR images from the high-resolution spotlight (HS) mode were obtained in alternating
ascending and descending orbit configurations. The highest-resolution products were selected for their small sample dimensions,
as positions can be more precisely determined. Based on the delivered product annotations, the CR image positions were predicted,
and these predictions were compared with their measured image positions both before and after compensation for atmospheric refraction and systematic solid Earth deviations. It was possible to show that when the atmospheric
distortion and Earth tides are taken into account, the TSX HS products have geolocation accuracies far exceeding the specified
requirements. Furthermore, this accuracy was maintained for the duration of the 16-month test period. It could be demonstrated
that with a correctly calibrated sensor, and after accounting for atmospheric and tidal effects, tiepoint-free geolocation
is possible with TSX with an absolute product accuracy of about 5 cm. 相似文献
202.
Sea-ice cover over the Hudson Bay (HB) exhibits large variability in the freeze-up season normally starting in November. Its influence on the climate over eastern Canada has been studied with the Canadian Regional Climate Model (CRCM) in three steps. First, a 30-year continuous simulation from 1970 to 1999 was performed as a control run to evaluate the simulated climate variability over eastern Canada, in particularly variability associated with the North Atlantic oscillation (NAO). Then, 50 additional 1 month experiments were performed with modified sea-surface conditions prescribed over the HB. These integrations allowed us to quantify the contribution of HB sea-ice anomalies versus large scale NAO atmospheric variability (as defined by prescribed lateral boundary conditions) in inducing climate variability over eastern Canada. Results show that the NAO is the dominant factor controlling climate variability over eastern Canada. The contribution of HB sea-ice anomalies is significant only in the immediate coastal region. Under the influence of different phases of NAO, HB sea-ice anomalies do co-vary with temperature and precipitation anomalies downstream of the HB over eastern Canada. The ultimate cause of this co-variability is NAO variability which forces variability in both HB sea-ice cover as well as temperature/precipitation over eastern Canada. 相似文献
203.
204.
205.
We consider periodic halo orbits about artificial equilibrium points (AEP) near to the Lagrange points L
1 and L
2 in the circular restricted three body problem, where the third body is a low-thrust propulsion spacecraft in the Sun–Earth
system. Although such halo orbits about artificial equilibrium points can be generated using a solar sail, there are points
inside L
1 and beyond L
2 where a solar sail cannot be placed, so low-thrust, such as solar electric propulsion, is the only option to generate artificial
halo orbits around points inaccessible to a solar sail. Analytical and numerical halo orbits for such low-thrust propulsion
systems are obtained by using the Lindstedt Poincaré and differential corrector method respectively. Both the period and minimum
amplitude of halo orbits about artificial equilibrium points inside L
1 decreases with an increase in low-thrust acceleration. The halo orbits about artificial equilibrium points beyond L
2 in contrast show an increase in period with an increase in low-thrust acceleration. However, the minimum amplitude first
increases and then decreases after the thrust acceleration exceeds 0.415 mm/s2. Using a continuation method, we also find stable artificial halo orbits which can be sustained for long integration times
and require a reasonably small low-thrust acceleration 0.0593 mm/s2. 相似文献
206.
Matrix micrites are a commonly used carbonate archive for the reconstruction of past environmental parameters, but one that is submitted to known limitations. Main reasons for the often ambiguous value of many micrite-based isotope data sets are the unknown origin of the micrite components and their poorly resolved diagenetic history. Here we present carbon and oxygen-isotope data retrieved from Oxfordian to Tithonian Ammonitico Rosso nodular micrites sampled from three sections in the Betic Cordillera (Southern Spain). All three sections were correlated and sampled using a rigorous biostratigraphic framework. A noteworthy feature is that analyzed matrix micrites are more conservative in terms of their isotopic composition than other carbonate materials commonly considered to resist diagenetic alteration under favourable circumstances. Remarkably, this refers not only to δ13C ratios, which reflect the typical Late Jurassic global trend, but also to δ18O ratios that range around 0.3‰. The 18O-enriched oxygen-isotope ratios are considered to represent diagenetic stabilization of carbonate ooze under the influence of marine porewaters within the sediment–water interphase (i.e., the immature sedimentary section, usually submitted to biogenic activity). This interpretation agrees with the very early lithification of micrite nodules with cements precipitated from marine porewaters, enriched by the dissolution of aragonite skeletals (i.e., ammonite shells). According to the model proposed, low sedimentation rates as well as rapid early marine differential cementation, under the influence of currents and seawater pumping, affected the sediment–water interphase of epioceanic swells where deposition resulted in early lithified Ammonitico Rosso facies. The data obtained show that special care must be taken to prevent oversimplified interpretations of carbonate archives, particularly in the context of epioceanic settings. 相似文献
207.
Spatial geochemistry of a Carboniferous platform-margin-to-basin transect: Balancing environmental and diagenetic factors 总被引:1,自引:0,他引:1
Bram van der Kooij Adrian Immenhauser Anita Csoma Juan Bahamonde Thomas Steuber 《Sedimentary Geology》2009,219(1-4):136-150
Conventional (one-dimensional) chemostratigraphy of marine carbonates assesses the chemical archive of individual stratigraphic sections and their correlation in space and time. Whereas this approach has shown to be of value when linking isobathymetric domains, usually characterised by similar facies, more caution is needed when correlations are extended across different physiographic settings and hence different facies belts. Here, the spatial geochemical record of Pennsylvanian platform-margin-to-basin transects across a bathymetric range of about 800 m is documented and discussed in a process-oriented context. Particularly, the presence of layered palaeo-water masses and their potential control on slope facies distribution and geochemical properties requires attention. Whereas Carboniferous thermo- and/or chemo-clines most likely affected depth-related slope facies zonation, it was facies change and hence, variances in porosity–permeability properties, that controlled differential early burial diagenetic alteration. Specifically, the lower-slope related breccia facies is characterised by higher volumes of early burial carbonate cements. This implies that these sediments entered the shallow-burial domain with a considerable open pore space and gave way to an increased rock:fluid ratio. Whereas the δ13C record is invariant with bathymetry, the more diagenesis-sensitive δ18O proxy, records pronounced shifts observed across major facies boundaries. From this it is concluded that although the primary controlling factor of slope facies distribution with depth is probably palaeoceanographic in nature, it is differential rock:fluid ratios that control the first-order, spatial shifts in δ18O composition. These findings show that one-dimensional chemostratigraphy will severely underestimate the complexity of three-dimensional (bathymetric) data sets across platform margins. This is of relevance for the interpretation of the geochemical archive of fossil carbonate platforms in general. 相似文献
208.
Yeşim Çoban-Yildiz Alec F. Gaines Pat E. Keating Gordon D. Love Dennis McLoughlin Colin E. Snape 《Organic Geochemistry》2009,40(2):270-282
The suspended particulate organic matter, SPOM, in the autumnal Black Sea has been characterised using catalytic hydropyrolysis (HyPy) of the total (bound plus free) lipid material. The technique, which generates maximum yields of volatile products from sediments, kerogens and phytoplankton, was followed using gas chromatography and gas chromatography/mass spectrometry. The generated alkanes were dominated by n-C18, hypothesised to arise predominantly from unsaturated C18 fatty acids. Steranes were generated from reductive conversion of free and bound sterols. The generation of branched alkanes and especially of hopanes provided formal evidence for the participation of bacteria in the mineralisation of the SPOM. Whereas similar distributions of n-alkanes were generated from SPOM sampled from different depths of the comparatively well-stirred Rhodes Gyre (eastern Mediterranean), mineralisation of the SPOM at each depth of the central Black Sea produced characteristic changes in the composition and concentration of the HyPy products. Depth profiles of the n-alkanes generated from SPOM in the region of the Rim Current were affected by the local hydrography. Polynuclear aromatic hydrocarbons (PAHs) of anthropogenic origin were present in the surface waters of the central Black Sea. Some methyl benzenes, thiophenes and pyrroles were also generated. 相似文献
209.
Colin K. Ballantyne John O. Stone L. Keith Fifield 《Proceedings of the Geologists' Association. Geologists' Association》2009,120(2-3):139-144
Exposure dating using cosmogenic 36Cl demonstrates that the summit plateau of Scafell Pike (978 m) in the SW Lake District escaped erosion by glacier ice during the last glacial maximum (LGM; c. 26–21 kyr) and probably throughout the Devensian Glacial Stage (MIS 5d-2). Exposure ages obtained for ice-moulded bedrock on an adjacent col at 750–765 m confirm over-riding and erosion of bedrock by warm-based glacier ice during the LGM. The contrast between the two sites is interpreted in terms of preservation of tors, frost-shattered outcrops and blockfields on terrain above 840–870 m under cold-based ice. An exposure age of 17.3 ± 1.1 kyr for the col at 750–765 m suggests that substantial downwastage of the last ice sheet had occurred by c. 17 kyr, consistent with deglacial exposure ages obtained for other high-level sites in the British Isles. An exposure age of 12.5 ± 0.8 kyr obtained for a glacially transported rockfall boulder within the limits of later corrie glaciation confirms that the final episode of local glaciation in the Lake District occurred during the Loch Lomond Stade (c. 12.9–11.7 kyr). This research also demonstrated the difficulties of obtaining reliable exposure ages from rhyolite and andesite bedrock that has proved resistant to glacial abrasion. 相似文献
210.
One significant difference between the atmospheres of stars and exoplanets is the presence of condensed particles (clouds or hazes) in the atmosphere of the latter. In current 1D models clouds and hazes are treated in an approximate way by raising the surface albedo, or adopting measured Earth cloud properties. The former method introduces errors to the modeled spectra of the exoplanet, as clouds shield the lower atmosphere and thus modify the spectral features. The latter method works only for an exact Earth-analog, but it is challenging to extend to other planets.The main goal of this paper is to develop a self-consistent microphysical cloud model for 1D atmospheric codes, which can reproduce some observed properties of Earth, such as the average albedo, surface temperature, and global energy budget. The cloud model is designed to be computationally efficient, simple to implement, and applicable for a wide range of atmospheric parameters for planets in the habitable zone.We use a 1D, cloud-free, radiative–convective, and photochemical equilibrium code originally developed by Kasting, Pavlov, Segura, and collaborators as basis for our cloudy atmosphere model. The cloud model is based on models used by the meteorology community for Earth’s clouds. The free parameters of the model are the relative humidity and number density of condensation nuclei, and the precipitation efficiency. In a 1D model, the cloud coverage cannot be self-consistently determined, thus we treat it as a free parameter.We apply this model to Earth (aerosol number density 100 cm?3, relative humidity 77%, liquid cloud fraction 40%, and ice cloud fraction 25%) and find that a precipitation efficiency of 0.8 is needed to reproduce the albedo, average surface temperature and global energy budget of Earth. We perform simulations to determine how the albedo and the climate of a planet is influenced by the free parameters of the cloud model. We find that the planetary climate is most sensitive to changes in the liquid water cloud fraction and precipitation efficiency.The advantage of our cloud model is that the cloud height and the droplet sizes are self-consistently calculated, both of which influence the climate and albedo of exoplanets. 相似文献