全文获取类型
收费全文 | 349篇 |
免费 | 11篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 18篇 |
地球物理 | 80篇 |
地质学 | 71篇 |
海洋学 | 55篇 |
天文学 | 86篇 |
综合类 | 1篇 |
自然地理 | 42篇 |
出版年
2020年 | 6篇 |
2019年 | 8篇 |
2018年 | 14篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 7篇 |
2013年 | 20篇 |
2012年 | 14篇 |
2011年 | 18篇 |
2010年 | 18篇 |
2009年 | 21篇 |
2008年 | 13篇 |
2007年 | 16篇 |
2006年 | 7篇 |
2005年 | 10篇 |
2004年 | 9篇 |
2003年 | 9篇 |
2002年 | 12篇 |
2001年 | 11篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 13篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 8篇 |
1974年 | 2篇 |
1969年 | 3篇 |
1968年 | 1篇 |
1966年 | 1篇 |
1965年 | 1篇 |
1928年 | 1篇 |
1922年 | 1篇 |
1912年 | 1篇 |
排序方式: 共有361条查询结果,搜索用时 31 毫秒
51.
The relationship between hydrological conditions and riparian helophyte vegetation was studied in two freshwater estuaries that differed in tidal regulation in order to assess the effects of large-scale hydrological regulation on the fringe vegetation. Vegetation and environmental variables were sampled for 206 sites in the Rhine-Meuse estuary (146 sites) and the IJsselmeer region (60 sites) in the Netherlands. These samples were classified into 10 vegetation types, all of which were dominated by tall helophytes. The most common vegetation types were dominated byPhragmites australis andTypha angustifolia, which formed both monospecific stands and mixtures. Tall Cyperaceae dominated three vegetation types (dominated individually bySchoenoplectus lacustris, Bolboschoenus maritimus, andSchoenoplectus tabernaemontani).Acorus calamus. Principal components analysis of the species composition of vegetation fringing open-water areas and associated environmental data revealed complex gradients incorporating differences in water depth, water-level fluctuation, were exposure, and sedimentation and/or erosion. The composition of the helophyte belts varied among the areas as the result of the differing times at which regulation occurred. Based on historical data, hydrological regulation of the estuaries has resulted in deterioration of intertidalSchoenoplectus andBolboschoenus stands, due to erosion and predation.P. australis stands have been invaded by terrestrial plant species or have been replaced byT. angustifolia. A scheme is presented of helophyte vegetation development under the influence of changes in the hydrological regime. 相似文献
52.
We present a parsimonious wave‐equation travel‐time inversion technique for refraction waves. A dense virtual refraction dataset can be generated from just two reciprocal shot gathers for the sources at the endpoints of the survey line, with N geophones evenly deployed along the line. These two reciprocal shots contain approximately 2N refraction travel times, which can be spawned into refraction travel times by an interferometric transformation. Then, these virtual refraction travel times are used with a source wavelet to create N virtual refraction shot gathers, which are the input data for wave‐equation travel‐time inversion. Numerical results show that the parsimonious wave‐equation travel‐time tomogram has about the same accuracy as the tomogram computed by standard wave‐equation travel‐time inversion. The most significant benefit is that a reciprocal survey is far less time consuming than the standard refraction survey where a source is excited at each geophone location. 相似文献
53.
We invert three-dimensional seismic data by a multiscale phase inversion scheme, a modified version of full waveform inversion, which applies higher order integrations to the input signal to produce low-boost signals. These low-boost signals are used as the input data for the early iterations, and lower order integrations are computed at the later iterations. The advantages of multiscale phase inversion are that it (1) is less dependent on the initial model compared to full waveform inversion, (2) is less sensitive to incorrectly modelled magnitudes and (3) employs a simple and natural frequency shaping filtering. For a layered model with a three-dimensional velocity anomaly, results with synthetic data show that multiscale phase inversion can sometimes provide a noticeably more accurate velocity profile than full waveform inversion. Results with the Society of Exploration Geophysicists/European Association of Geoscientists and Engineers overthrust model shows that multiscale phase inversion more clearly resolves meandering channels in the depth slices. However, the data and model misfit functions achieve about the same values after 50 iterations. The results with three-dimensional ocean-bottom cable data show that, compared to the full waveform inversion tomogram, the three-dimensional multiscale phase inversion tomogram provides a better match to the well log, and better flattens angle-domain common image gathers. The problem is that the tomograms at the well log provide an incomplete low-wavenumber estimate of the log's velocity profile. Therefore, a good low-wavenumber estimate of the velocity model is still needed for an accurate multiscale phase inversion tomogram. 相似文献
54.
Sediment yield of the lower Tana River,Kenya, is insensitive to dam construction: sediment mobilization processes in a semi‐arid tropical river system 下载免费PDF全文
Naomi Geeraert Fred Ochieng Omengo Fredrick Tamooh Paolo Paron Steven Bouillon Gerard Govers 《地球表面变化过程与地形》2015,40(13):1827-1838
Dam construction in the 1960s to 1980s significantly modified sediment supply from the Kenyan uplands to the lower Tana River. To assess the effect on suspended sediment fluxes of the Tana River, we monitored the sediment load at high temporal resolution for 1 year and complemented our data with historical information. The relationship between sediment concentration and water discharge was complex: at the onset of the wet season, discharge peaks resulted in high sediment concentrations and counterclockwise hysteresis, while towards the end of the wet season, a sediment exhaustion effect led to low concentrations despite the high discharge. The total sediment flux at Garissa (c. 250 km downstream of the lowermost dam) between June 2012 and June 2013 was 8.8 Mt yr‐1. Comparison of current with historical fluxes indicated that dam construction had not greatly affected the annual sediment flux. We suggest that autogenic processes, namely river bed dynamics and bank erosion, mobilized large quantities of sediments stored in the alluvial plain downstream of the dams. Observations supporting the importance of autogenic processes included the absence of measurable activities of the fall‐out radionuclides 7Be and 137Cs in the suspended sediment, the rapid lateral migration of the river course, and the seasonal changes in river cross‐section. Given the large stock of sediment in the alluvial valley of the Tana River, it may take centuries before the effect of damming shows up as a quantitative reduction in the sediment flux at Garissa. Many models relate the sediment load of rivers to catchment characteristics, thereby implicitly assuming that alterations in the catchment induce changes in the sediment load. Our research confirms that the response of an alluvial river to external disturbances such as land use or climate change is often indirect or non‐existent as autogenic processes overwhelm the changes in the input signal. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
55.
We reformulate the equation of reverse‐time migration so that it can be interpreted as summing data along a series of hyperbola‐like curves, each one representing a different type of event such as a reflection or multiple. This is a generalization of the familiar diffraction‐stack migration algorithm where the migration image at a point is computed by the sum of trace amplitudes along an appropriate hyperbola‐like curve. Instead of summing along the curve associated with the primary reflection, the sum is over all scattering events and so this method is named generalized diffraction‐stack migration. This formulation leads to filters that can be applied to the generalized diffraction‐stack migration operator to mitigate coherent migration artefacts due to, e.g., crosstalk and aliasing. Results with both synthetic and field data show that generalized diffraction‐stack migration images have fewer artefacts than those computed by the standard reverse‐time migration algorithm. The main drawback is that generalized diffraction‐stack migration is much more memory intensive and I/O limited than the standard reverse‐time migration method. 相似文献
56.
Thecamoebians were examined from 71 surface sediment samples collected from 21 lakes and ponds in the Greater Toronto Area
to (1) elucidate the controls on faunal distribution in modern lake environments; and (2) to consider the utility of thecamoebians
in quantitative studies of water quality change. This area was chosen because it includes a high density of kettle and other
lakes which are threatened by urban development and where water quality has deteriorated locally as a result of contaminant
inputs, particularly nutrients. Fifty-eight samples yielded statistically significant thecamoebian populations. The most diverse
faunas (highest Shannon Diversity Index values) were recorded in lakes beyond the limits of urban development, although the
faunas of all lakes showed signs of sub-optimal conditions. The assemblages were divided into five clusters using Q-mode cluster
analysis, supported by Detrended Correspondence Analysis. Canonical Correspondence Analysis (CCA) was used to examine species-environment
relationships and to explain the observed clusterings. Twenty-four measured environmental variables were considered, including
water property attributes (e.g., pH, conductivity, dissolved oxygen), substrate characteristics, sediment-based phosphorus
(Olsen P) and 11 environmentally available metals. The thecamoebian assemblages showed a strong association with phosphorus,
reflecting the eutrophic status of many of the lakes, and locally to elevated conductivity measurements, which appear to reflect
road salt inputs associated with winter de-icing operations. Substrate characteristics, total organic carbon and metal contaminants
(particularly Cu and Mg) also influenced the faunas of some samples. A series of partial CCAs show that of the measured variables,
sedimentary phosphorus has the largest influence on assemblage distribution, explaining 6.98% (P < 0.002) of the total variance. A transfer function was developed for sedimentary phosphorus (Olsen P) using 58 samples from
15 of the studied lakes. The best performing model was based on weighted averaging with inverse deshrinking (WA Inv, rjack2 = 0.33, RMSEP =
102.65 ppm). This model was applied to a small modern thecamoebian dataset from a eutrophic lake in northern Ontario to predict
phosphorus and performed satisfactorily. This preliminary study confirms that thecamoebians have considerable potential as
quantitative water quality indicators in urbanising regions, particularly in areas influenced by nutrient inputs and road
salts. 相似文献
57.
58.
Impact of Berm Breaching on Hyperbenthic Macroinvertebrate Communities in Intermittently Closed Estuaries 总被引:1,自引:0,他引:1
Adrian W. T. Lill Gerard P. Closs Marc Schallenberg Candida Savage 《Estuaries and Coasts》2012,35(1):155-168
Berm breaching of intermittently closed estuaries, either naturally or due to management practices, results in a physicochemical
disturbance due to the flushing of water, material and biota into the ocean and the subsequent tidal influx. In 2007 and 2008
comparative and controlled studies were employed in three New Zealand estuaries: Sawmill Creek (46°04′ S 170°12′ E), Otokia
Creek (45°57′ S 179°20′ E) and Kaikorai Lagoon (45°56′ S 170°23′ E), to investigate the impact of berm breaching on the hyperbenthic
macroinvertebrate community in intermittently closed estuaries. Significant changes in community structure occurred in both
the weekly comparative and the controlled studies (ANOSIM P < 0.01). Furthermore, the catch per unit effort of both total
and key invertebrate taxa significantly increased after breaching (ANOVA P < 0.01). However, substantial numbers of individuals were expelled into the ocean (5,800 individuals, 20-min sample) while
new taxa immigrated into the estuaries. Along with migration-related changes to community structure, berm breaches also resulted
in the loss of ∼80% of inundated habitat and the concentration of existing populations. Consequently, the management of intermittent
estuary connections to the ocean has implications for the ecology of individual, managed estuaries and also for regional coastal
populations of epibenthic invertebrates. 相似文献
59.
Gerard W. Hazeu Arnold K. Bregt Allard J.W. de Wit Jan G.P.W. Clevers 《International Journal of Applied Earth Observation and Geoinformation》2011
Land cover and land use are important information sources for environmental issues. One of the most important changes at the Earth's surface concerns land cover and land use. Knowledge about the location and type of these changes is essential for environmental modeling and management. Remote sensing data in combination with additional spatial data are recognized as an important source of information to detect these land cover and land use changes. 相似文献
60.
Iron-hydroxide clogging of public supply wells receiving artificial recharge: near-well and in-well hydrological and hydrochemical observations 总被引:2,自引:2,他引:0
Diego A. Bustos Medina Gerard A. van den Berg Boris M. van Breukelen Maria Juhasz-Holterman Pieter J. Stuyfzand 《Hydrogeology Journal》2013,21(7):1393-1412
Clogging of water wells by iron-hydroxide incrustations due to mixing of anoxic and oxic groundwater is a common well-ageing problem. The relation between well operation (on and off), the spatial and temporal variations in hydrochemistry outside and inside a supply well, and the distribution of clogging iron-hydroxides were studied in an artificial recharge well field in the Netherlands. Camera inspection, high-resolution multi-level water sampling outside the well and detailed in-well pH/EC/O2 profiles revealed remarkable patterns. During pumping, the top of the upper well screen abstracted oxic filtrate, although the larger part of the in-well water column was anoxic. The column rapidly turned oxic after shutdown due to a downward short-circuiting of oxic water via the well. Within 15 d it became anoxic due to the slow advance of anoxic lake filtrate created by local changes in flow direction as the neighboring wells continued to pump. Severe clogging occurred where the oxic filtrate entered the well, while half-clogging of the upper well screen occurred due to less inflow of oxic filtrate on the lake side. Transport of iron flocs and bacterial slimes after shutdown seemed to clog the lower part of the well screen. Frequent on/off switching should be avoided in iron-clogged wells. 相似文献