全文获取类型
收费全文 | 2472篇 |
免费 | 128篇 |
国内免费 | 23篇 |
专业分类
测绘学 | 102篇 |
大气科学 | 165篇 |
地球物理 | 856篇 |
地质学 | 869篇 |
海洋学 | 165篇 |
天文学 | 292篇 |
综合类 | 15篇 |
自然地理 | 159篇 |
出版年
2022年 | 20篇 |
2021年 | 31篇 |
2020年 | 56篇 |
2019年 | 44篇 |
2018年 | 81篇 |
2017年 | 78篇 |
2016年 | 117篇 |
2015年 | 102篇 |
2014年 | 102篇 |
2013年 | 139篇 |
2012年 | 118篇 |
2011年 | 138篇 |
2010年 | 123篇 |
2009年 | 151篇 |
2008年 | 126篇 |
2007年 | 98篇 |
2006年 | 87篇 |
2005年 | 66篇 |
2004年 | 54篇 |
2003年 | 64篇 |
2002年 | 50篇 |
2001年 | 55篇 |
2000年 | 44篇 |
1999年 | 44篇 |
1998年 | 37篇 |
1997年 | 31篇 |
1996年 | 30篇 |
1995年 | 21篇 |
1994年 | 21篇 |
1993年 | 14篇 |
1992年 | 23篇 |
1991年 | 15篇 |
1990年 | 23篇 |
1989年 | 23篇 |
1988年 | 21篇 |
1987年 | 14篇 |
1986年 | 26篇 |
1985年 | 25篇 |
1984年 | 20篇 |
1983年 | 29篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 15篇 |
1979年 | 19篇 |
1978年 | 17篇 |
1977年 | 17篇 |
1976年 | 16篇 |
1975年 | 18篇 |
1973年 | 16篇 |
1972年 | 10篇 |
排序方式: 共有2623条查询结果,搜索用时 15 毫秒
951.
Commonly, the most costly part of geomorphological distribution modelling studies is gathering the data. Thus, guidance for researchers concerning the quantity of field data needed would be extremely practical. This paper scrutinises the relationship between the sample size (the number of observations varied from 20 to 600) and the predictive ability of the generalized linear model (GLM), generalized additive model (GAM), generalized boosting method (GBM) and artificial neural network (ANN) in two data settings, i.e., independent and split-sample approaches. The study was performed using empirical data of periglacial processes from an area of 600 km2 in northernmost Finland at grid resolutions of 1 ha (100 × 100 m) and 25 ha (500 × 500 m). A rather sharp increase in the predictive ability of the models was observed when the number of observations increased from 20 to 100, and the level of robust predictions was reached with 200 observations. The result indicates that no more than a few hundred observations are needed in geomorphological distribution modelling at a medium scale resolution (ca. 0.01–1 km2). 相似文献
952.
A. Pepliski P. Artymowicz G. Mellema 《Monthly notices of the Royal Astronomical Society》2008,387(3):1063-1079
We present a numerical study of rapid, so-called type III migration for Jupiter-sized planets embedded in a protoplanetary disc. We limit ourselves to the case of outward migration, and study in detail its evolution and physics, concentrating on the structure of the corotation and circumplanetary regions, and processes for stopping migration. We also consider the dependence of the migration behaviour on several key parameters. We perform this study using global, two-dimensional hydrodynamical simulations with adaptive mesh refinement. We find that the outward-directed type III migration can be started if the initial conditions support the initial average non-dimensional migration rate bigger than one. Unlike the inward-directed migration, in the outward migration the migration rate increases due to the growing of the volume of the co-orbital region. We find the migration to be strongly dependent on the rate of the mass accumulation in the circumplanetary disc, leading to two possible regimes of migration, fast and slow. The structure of the co-orbital region and the stopping mechanism differs between these two regimes. 相似文献
953.
Sedimentary regimes in Arctic's Amerasian and Eurasian Basins: Clues to differences in sedimentation rates 总被引:2,自引:0,他引:2
A standard lithostratigraphic model based on cores retrieved 1963–1973 from the ice island T-3 was developed by Clark et al. [Stratigraphy and glacial-marine sediments of the Amerasian Basin, central Arctic Ocean. Geological Society of America Special Paper, 181, 1–57, 1980] for the Amerasian Basin of the Arctic Ocean. We have investigated whether or not it is possible to apply this lithostratigraphy to cores from the Lomonosov Ridge, which can be correlated to Eurasian Basin cores, for the purpose of correlating the Amerasian and Eurasian stratigraphies. Published averaged sedimentary proxies from a selected set of T-3 cores are used to correlate with the identical published proxies for the included Lomonosov Ridge cores. The standard lithostratigraphic classification could not be applied to the Lomonosov Ridge cores, which is interpreted to result from differences in sedimentary regimes in the Amerasian and Eurasian Basins. These differences also apply to the barrier between the two basins, the Lomonosov Ridge. The general sedimentation rates are three to four times lower in the Amerasian Basin than in the Eurasian Basin if the first down-core paleomagnetic inclination change is used to correlate between the two basins whereas correlation based on sediment coarse fraction suggests only two times lower rates in the Amerasian Basin. 相似文献
954.
MonaLisa Azam A. Khwaja M. Qasim Jan Robert S. Yeats Ahmad Hussain Shahid A. Khan 《Journal of Seismology》2009,13(3):339-361
This paper deals with the data obtained from local networks in northern Pakistan for 251 earthquakes of magnitude ≥4.0 for
October 8, 2005 to December 31, 2006 period. The study presents focal mechanism solutions (FMS) of 12 pre- (1904–2005) and
17 post- (October 8, 2005–December, 2005) Muzaffarabad Earthquake, their detailed tectonic interpretation, and correlation
with surface evidence of co-seismic rupture with published synthetic aperture radar data. Distribution of landslides obtained
from National Engineering Services of Pakistan and the earthquake damages are also discussed. Aftershock distribution, which
is more prominent in the crystalline zone (northwest of Muzaffarabad), defines a 50-km-wide NW–SE trending zone that extends
for 200 km from the main mantle thrust to the center of the Hazara–Kashmir Syntaxis. The FMS of the main shock and 16 aftershocks
having magnitude ≥4.0 indicate thrusting to be the dominant mechanism with rupture planes having NW–SE trend and NE dip. In
addition, 12 FMS of pre-Muzaffarabad Earthquake (1904–2004) from the same area have been determined and results are compared.
This leads to the conclusion that the wedge-shaped NW–SE trending blind zone, referred to by earlier workers as the Indus
Kohistan Seismic Zone (IKSZ), has been activated during the Muzaffarabad earthquake. The right-lateral component in all FMS,
supported by the surface evidences, suggests the involvement of Balakot–Bagh Fault (BBF). We propose that the IKSZ is the
source of the October 8, 2005 Muzaffarabad earthquake that reactivated the BBF. Furthermore, the IKSZ does not end at the
nose of the syntaxis but extends further southeast of it. Tectonic complexity seems to be due to a variety of factors. Also,
thrust and reverse solutions near the northern collisional boundary (main mantle thrust) have mostly NE/SW-directed P-axis
orientations. From the detailed FMS analysis, three conclusions have been drawn: (1) Shallow events (depth ≤10 km) with prominent
strike slip solutions (earlier earthquakes) are associated with the surface strike slip faults (e.g., Muzaffarabad Fault)
and/or the Besham domal structure; (2) moderate depth events (depth 10–25 km) with thrust/reverse solutions but having minor
right-lateral strike slip component (all Muzaffarabad earthquakes and two earlier) are associated with the IKSZ; (3) deeper
earthquakes (depth below IKSZ) with pure thrust/reverse solutions may be related to the under-thrusting of the Indian plate
beneath the IKSZ, which represents a major thrust zone. Imbricate thrusting and breaking and thickening of the crust are considered
to be caused by steep bending of the under-thrusting plate at the collisional boundary.
An erratum to this article can be found at 相似文献
955.
Beata Dziak-Jankowska Jacek Leliwa-Kopystyński Konrad J. Kossacki 《Acta Geophysica》2009,57(2):509-535
In this paper we intend to analyze how the sublimation of ice from cometary nuclei affects changes of the moments of inertia.
Our aim is to show general trends for different orientations of cometary nucleus’ rotation axis. Thus we apply numerical model
of a hypothetical homogeneous and initially spherical nucleus composed of water ice and dust. As an example we present simulations
for a model comet of the orbital elements and the nucleus size the same as determined for C/1995 O1 Hale-Bopp, a widely analyzed
long-period comet. We calculated water production from the nucleus and changes of the shape (initially spherical) and of the
moment of inertia versus time. Simulations are performed for the full range (0–90°) of inclinations I of the rotation axis. The second paramater related to the orientation of the rotation axis is the argument Φ (0–360°). The heat conductivity of the nucleus spans over the vast range, 0.04–4 W m−1 K−1. 相似文献
956.
Prominent longitudinal features are often reported on the surfaces of mass movement deposits. However, the genesis and implications of these have not hitherto been considered, and herein we present preliminary observations of their occurrence both in the field and in the laboratory. Elongated ridges are often oriented (sub-) parallel to the flow direction and aligned radially from the source due to debris spreading. They are particularly prominent in large (> 106m3) rock avalanches emplaced onto deformable substrates and are also found in the proximal reaches of volcanic debris avalanches. Flowbands, which are longer and thinner expressions of longitudinal ridges, are continuous along the entire flow length and are observed in rock avalanches emplaced onto glaciers, in snow and some ice avalanches, in pyroclastic flows and some block-and-ash flows, in ejecta sheets, in extraterrestrial landslides, and in some volcanic debris avalanches. Other volcanic debris avalanches and the distal areas of rock avalanches often display hummocks that are aligned radially from the source; we propose that these aligned hummocks are remnants of longitudinal ridges. The formation of elongate ridges (and their expressions as flowbands, aligned hummocks, or distal lobes and digits) in qualitatively-similar fashion in both laboratory and field environments suggests they represent an intrinsic tendency of granular flows in a wide range of situations. 相似文献
957.
958.
Michal Horsák Jind?iška Bojková Světlana Zahrádková Marie Omesová Jan Helešic 《Limnologica》2009,39(2):140-151
In order to assess and compare the ecological impacts of channelization and shallow lowland reservoirs, macroinvertebrate communities of a lowland metapotamal river below reservoirs with epilimnial release were studied. The study was carried out in the Dyje River (Czech Republic) at five sites located from 1.5 to 22.5 km downstream of the reservoir outfall. The five sites differed in the degree of channel modification from natural muddy banks to riprap regulation. Seven samples were collected during the years 1998 and 1999 at each site using a semiquantitative method. The data were processed using multivariate analyses and methods for assessing the ecological and functional structure of communities. Altogether, 261 species of benthic macroinvertebrates were recorded including several rare and threatened taxa. Based on the results of principal component analysis (PCA), most of the variability within the species data (the first PCA axis) was explained by the degree of channel modification, from natural muddy banks with aquatic vegetation to a man-made riprap. The second axis was strongly correlated with current velocity. The sites differed in species richness, total abundances, proportion of individual functional feeding groups, pattern of the distribution of the current preference groups, and values of several biotic indexes, all of which also corresponded to the degree of channel modification. Thus, the morphological man-made modifications of the river channel were found to be the main factor affecting lowland river macroinvertebrates and their biodiversity. Our results suggest that the biggest threat to benthic macroinvertebrate diversity of lowland rivers comes from channelization. The impact of reservoirs can be completely overwhelmed by the impact of channelization, especially when muddy banks with aquatic vegetation present a substantial part of habitat diversity and significantly contribute to the total species pool. 相似文献
959.
The satellite missions CHAllenging Minisatellite Payload (CHAMP) and Gravity Recovery And Climate Experiment (GRACE) provide
accurate data that are routinely inverted into spherical harmonic coefficients of the geopotential forming a global geopotential
model (GGM). Mean square errors of these coefficients, in some cases even entire covariance matrices, are included in the
GGM. Due to estimation procedures with a large redundancy and insufficiently propagated observation errors, they often do
not represent the actual accuracy of the harmonic coefficients, thus also gravity field parameters synthesized from the respective
GGM. Since in most cases standard methods validating the GGMs reached their limits, new procedures and independent data are
being currently sought. This article discusses an alternative validation procedure based on comparison of the GGMs with independent
data represented by a set of GPS/leveling stations. Due to a different spectral content of the height anomalies synthesized
from the GGMs and of those derived by combination of GPS-based ellipsoidal and leveled normal heights, the GGM-based low frequency
height anomaly is enhanced for a high frequency component computed from local ground gravity and elevation data. The methodology
is applied on a set of selected points of the European Vertical Reference Network and Czech trigonometric stations. In accordance
with similar tests based on entirely independent data of cross-over altimetry, obtained results seem to indicate low-frequency
deficiencies in the current GGMs, namely in those estimated from data of single-satellite missions. 相似文献
960.
Jarosław Zawadzki Tadeus Magiera Piotr Fabijańczyk 《Studia Geophysica et Geodaetica》2009,53(1):133-149
The goal of the study was the geostatistical evaluation of quantitative magnetic measures, which can be used for effective
delineation of the extent of the area polluted with heavy metals. Several parameters of magnetic susceptibility, measured
in the soil profile, were proposed as magnetic indicators of soil pollution and analyzed in detail. The following parameters
were calculated: maximum magnetic susceptibility, magnetic susceptibility at the depth of 3 cm and 5 cm, and the area under
the curve of magnetic susceptibility. Measurements were performed at two forested study areas, located in Upper Silesian Industrial
Area (Poland). Analyses were performed using geostatistical methods, and the results were verified using dense chemical measurements.
The results showed that the area under curve of magnetic susceptibility was the most effective magnetic indicator of soil
contamination with heavy metals. It was possible to detect the entire polluted area, and only about 16% of the study area
was assumed to be contaminated while being unpolluted. The results obtained with maximum magnetic susceptibility and magnetic
susceptibility at the depth of 3 cm and 5 cm were less effective in comparison with the area under curve of magnetic susceptibility. 相似文献