全文获取类型
收费全文 | 171篇 |
免费 | 3篇 |
专业分类
测绘学 | 26篇 |
大气科学 | 4篇 |
地球物理 | 42篇 |
地质学 | 34篇 |
海洋学 | 6篇 |
天文学 | 31篇 |
自然地理 | 31篇 |
出版年
2021年 | 1篇 |
2018年 | 1篇 |
2017年 | 1篇 |
2016年 | 2篇 |
2015年 | 7篇 |
2014年 | 8篇 |
2013年 | 3篇 |
2012年 | 2篇 |
2011年 | 6篇 |
2010年 | 2篇 |
2009年 | 13篇 |
2008年 | 10篇 |
2007年 | 5篇 |
2006年 | 11篇 |
2005年 | 12篇 |
2004年 | 14篇 |
2003年 | 13篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有174条查询结果,搜索用时 15 毫秒
131.
Five exposed microtidal sandy beaches of North-eastern Buenos Aires (Argentina) were analysed to evaluate the influence of coastal dynamics on the abundance of the intertidal clams Donax hanleyanus and Mesodesma mactroides over a 1-year period. The physical characterisation of each beach was performed using 15 environmental variables and six morphodynamic indices. The population densities of both clam species were estimated by the number of individuals per strip-transect. Analysis of similarities revealed a significant multivariate environmental difference among beaches, which could be aggregated into a northern and a southern group. The former group was characterised by an intermediate morphodynamic state and by having, in general, high density values for both clam species. The latter was composed of beaches with an intermediate reflective state and showed, in general, low density values. Beach Index and Area showed the lowest temporal variation. Total density values, as well as density values of recruits and adults of D. hanleyanus and M. mactroides did not follow a north–south latitudinal gradient. Spatial and temporal fluctuations of density values were best explained by changes in individual physical variables rather than by changes in composite indices. 相似文献
132.
GPS zero-difference applications with a sampling rate up to 1 Hz require corresponding high-rate GPS clock corrections. The
determination of the clock corrections in a full network solution is a time-consuming task. The Center for Orbit Determination
in Europe (CODE) has developed an efficient algorithm based on epoch-differenced phase observations, which allows to generate
high-rate clock corrections within reasonably short time (< 2 h) and with sufficient accuracy (on the same level as the CODE
rapid or final clock corrections, respectively). The clock determination procedure at CODE and the new algorithm is described
in detail. It is shown that the simplifications to speed up the processing are not causing a significant loss of accuracy
for the clock corrections. The high-rate clock corrections have in essence the same quality as clock corrections determined
in a full network solution. In order to support 1 Hz applications 1-s clock corrections would be needed. The computation time,
even for the efficient algorithm, is not negligible, however. Therefore, we studied whether a reduced sampling is sufficient
for the GPS satellite clock corrections to reach the same or only slightly inferior level of accuracy as for the full 1-s
clock correction set. We show that high-rate satellite clock corrections with a spacing of 5 s may be linearly interpolated
resulting in less than 2% degradation of accuracy. 相似文献
133.
Neodymium (Nd) isotope profiles were analyzed on two Baltic Mn/Fe precipitates (99/2 and TL1) from shallow water (20 m) of the Mecklenburg Bay. The age range of these Mn/Fe precipitates determined by 226Raex/Ba dating reaches from recent growth back to ∼4300 and 1000 yr BP, respectively. Over this time range, the Nd isotope composition varies from εNd (0) = −13.1 to −17.5 in the selected Baltic precipitates indicating substantial changes in the Nd isotope composition of the Baltic Sea. The lowest εNd values were recorded during the time interval of the Little Ice Age (LIA, AD ∼1350 to 1850). These minimum values indicate either an increase of the input of less radiogenic Nd from Scandinavian Archean-Proterozoic sources (εNd about −22) to the Baltic Sea or a decrease of the input of more radiogenic Nd from continental European sources (εNd about −12) and/or North Sea water (εNd about −10). Variations of both, erosive continental input and North Sea inflow may indicate a direct response of the Nd isotope signal in the Baltic Sea to climate changes during the LIA and be related to cyclic shifts in the atmospheric circulation triggered by the North Atlantic Oscillation (NAO). Another aspect that possibly influenced the input of trace elements and Nd isotopes into the Baltic Sea is the population development in the circum Baltic area during the LIA. The lowest εNd values also correspond to the medieval demographic crises that led to a significant decrease of agricultural activity and farmland. The reduction of soil erosion and enhanced regrowth of natural vegetation may have changed the amount and proportions of dissolved and suspended particulate matter transported into the Baltic Sea by rivers which in turn may have resulted in a change of the Nd isotope composition of Baltic Sea water. 相似文献
134.
J. Hinderer J. Pfeffer M. Boucher S. Nahmani C. De Linage J.-P. Boy P. Genthon L. Seguis G. Favreau O. Bock M. Descloitres 《Pure and Applied Geophysics》2012,169(8):1391-1410
This paper is devoted to the first results from the GHYRAF (Gravity and Hydrology in Africa) experiment conducted since 2008 in West Africa and is aimed at investigating the changes in water storage in different regions sampling a strong rainfall gradient from the Sahara to the monsoon zone. The analysis of GPS vertical displacement in Niamey (Niger) and Djougou (Benin) shows that there is a clear annual signature of the hydrological load in agreement with global hydrology models like GLDAS. The comparison of GRACE solutions in West Africa, and more specifically in the Niger and Lake Chad basins, reveals a good agreement for the large scale annual water storage changes between global hydrology models and space gravity observations. Ground gravity observations done with an FG5 absolute gravimeter also show signals which can be well related to measured changes in soil and ground water. We present the first results for two sites in the Sahelian band (Wankama and Diffa in Niger) and one (Djougou in Benin) in the Sudanian monsoon region related to the recharge–discharge processes due to the monsoonal event in summer 2008 and the following dry season. It is confirmed that ground gravimetry is a useful tool to constrain local water storage changes when associated to hydrological and subsurface geophysical in situ measurements. 相似文献
135.
136.
137.
GNSS processing at CODE: status report 总被引:26,自引:19,他引:7
Rolf Dach Elmar Brockmann Stefan Schaer Gerhard Beutler Michael Meindl Lars Prange Heike Bock Adrian Jäggi Luca Ostini 《Journal of Geodesy》2009,83(3-4):353-365
Since May 2003, the Center for Orbit Determination in Europe (CODE), one of the analysis centers of the International GNSS Service, has generated GPS and GLONASS products in a rigorous combined multi-system processing scheme, which promises the best possible consistency of the orbits of both systems. The resulting products, in particular the satellite orbits and clocks, are easily accessible by the user community. In the first part of this article, we focus on the generation of the combined global products at CODE, where we put emphasis not only on accuracy, but also on completeness. We study the impact of GLONASS on the CODE products, and the benefit of using them. Last, but not least, we introduce AGNES (Automated GNSS Network for Switzerland), a regional tracking network of small extensions (roughly 400 km East–West, 200 km North–South), which consequently tracks all GNSS satellites and analyzes their measurements using the CODE products. 相似文献
138.
Yariv?HamielEmail author Vladimir?Lyakhovsky Yehuda?Ben-Zion 《Pure and Applied Geophysics》2011,168(12):2199-2210
Laboratory and field data indicate that rocks subjected to sufficiently high loads clearly deviate from linear behavior. Non-linear
stress–strain relations can be approximated by including third and higher-order terms of the strain tensor in the elastic
energy expression (e.g., the Murnaghan model). Such classical non-linear models are successful for calculating deformation
of soft materials, for example graphite, but cannot explain with the same elastic moduli small and large non-linear deformation
of stiff rocks, such as granite. The values of the third (higher-order) Murnaghan moduli estimated from acoustic experiments
are one to two orders of magnitude above the values estimated from stress–strain relations in quasi-static rock-mechanics
experiments. The Murnaghan model also fails to reproduce an abrupt change in the elastic moduli upon stress reversal from
compression to tension, observed in laboratory experiments with rocks, concrete, and composite brittle material samples, and
it predicts macroscopic failure at stress levels lower than observations associated with granite. An alternative energy function
based on second-order dependency on the strain tensor, as in the Hookean framework, but with an additional non-analytical
term, can account for the abrupt change in the effective elastic moduli upon stress reversal, and extended pre-yielding deformation
regime with one set of elastic moduli. We show that the non-analytical second-order model is a generalization of other non-classical
non-linear models, for example “bi-linear”, “clapping non-linearity”, and “unilateral damage” models. These models were designed
to explain the abrupt changes of elastic moduli and non-linearity of stiff rocks under small strains. The present model produces
dilation under shear loading and other non-linear deformation features of the stiff rocks mentioned above, and extends the
results to account for gradual closure of an arbitrary distribution of initial cracks. The results provide a quantitative
framework that can be used to model simultaneously, with a small number of coefficients, multiple observed aspects of non-linear
deformation of stiff rocks. These include, in addition to the features mentioned above, stress-induced anisotropy and non-linear
effects in resonance experiments with damaged materials. 相似文献
139.
At present, reliable ambiguity resolution in real-time GPS precise point positioning (PPP) can only be achieved after an initial observation period of a few tens of minutes. In this study, we propose a method where the incoming triple-frequency GPS signals are exploited to enable rapid convergences to ambiguity-fixed solutions in real-time PPP. Specifically, extra-wide-lane ambiguity resolution can be first achieved almost instantaneously with the Melbourne-Wübbena combination observable on L2 and L5. Then the resultant unambiguous extra-wide-lane carrier-phase is combined with the wide-lane carrier-phase on L1 and L2 to form an ionosphere-free observable with a wavelength of about 3.4 m. Although the noise of this observable is around 100 times the raw carrier-phase noise, its wide-lane ambiguity can still be resolved very efficiently, and the resultant ambiguity-fixed observable can assist much better than pseudorange in speeding up succeeding narrow-lane ambiguity resolution. To validate this method, we use an advanced hardware simulator to generate triple-frequency signals and a high-grade receiver to collect 1-Hz data. When the carrier-phase precisions on L1, L2 and L5 are as poor as 1.5, 6.3 and 1.5 mm, respectively, wide-lane ambiguity resolution can still reach a correctness rate of over 99 % within 20 s. As a result, the correctness rate of narrow-lane ambiguity resolution achieves 99 % within 65 s, in contrast to only 64 % within 150 s in dual-frequency PPP. In addition, we also simulate a multipath-contaminated data set and introduce new ambiguities for all satellites every 120 s. We find that when multipath effects are strong, ambiguity-fixed solutions are achieved at 78 % of all epochs in triple-frequency PPP whilst almost no ambiguities are resolved in dual-frequency PPP. Therefore, we demonstrate that triple-frequency PPP has the potential to achieve ambiguity-fixed solutions within a few minutes, or even shorter if raw carrier-phase precisions are around 1 mm. In either case, we conclude that the efficiency of ambiguity resolution in triple-frequency PPP is much higher than that in dual-frequency PPP. 相似文献
140.