全文获取类型
收费全文 | 243篇 |
免费 | 11篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 12篇 |
地球物理 | 48篇 |
地质学 | 135篇 |
海洋学 | 6篇 |
天文学 | 34篇 |
综合类 | 4篇 |
自然地理 | 4篇 |
出版年
2022年 | 5篇 |
2021年 | 2篇 |
2020年 | 4篇 |
2019年 | 2篇 |
2018年 | 17篇 |
2017年 | 17篇 |
2016年 | 24篇 |
2015年 | 8篇 |
2014年 | 18篇 |
2013年 | 15篇 |
2012年 | 19篇 |
2011年 | 15篇 |
2010年 | 12篇 |
2009年 | 24篇 |
2008年 | 19篇 |
2007年 | 12篇 |
2006年 | 11篇 |
2005年 | 6篇 |
2004年 | 9篇 |
2003年 | 1篇 |
2001年 | 2篇 |
2000年 | 1篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1991年 | 1篇 |
排序方式: 共有257条查询结果,搜索用时 171 毫秒
101.
Rajesh Nune Biju A. George Andrew W. Western Kaushal K. Garg Sreenath Dixit Ragab Ragab 《水文研究》2021,35(8):e14305
The clearest signs of hydrologic change can be observed from the trends in streamflow and groundwater levels in a catchment. During 1980–2007, significant declines in streamflow (−3.03 mm/year) and groundwater levels (−0.22 m/year) were observed in Himayat Sagar (HS) catchment, India. We examined the degree to which hydrologic changes observed in the HS catchment can be attributed to various internal and external drivers of change (climatic and anthropogenic changes). This study used an investigative approach to attribute hydrologic changes. First, it involves to develop a model and test its ability to predict hydrologic trends in a catchment that has undergone significant changes. Second, it examines the relative importance of different causes of change on the hydrologic response. The analysis was carried out using Modified Soil and Water Assessment Tool (SWAT), a semi-distributed rainfall-runoff model coupled with a lumped groundwater model for each sub- catchment. The model results indicated that the decline in potential evapotranspiration (PET) appears to be partially offset by a significant response to changes in rainfall. Measures that enhance recharge, such as watershed hydrological structures, have had limited success in terms of reducing impacts on the catchment-scale water balance. Groundwater storage has declined at a rate of 5 mm/y due to impact of land use changes and this was replaced by a net addition of 2 mm/y by hydrological structures. The impact of land use change on streamflow is an order of magnitude larger than the impact of hydrological structures and about is 2.5 times higher in terms of groundwater impact. Model results indicate that both exogenous and endogenous changes can have large impacts on catchment hydrology and should be considered together. The proposed comprehensive framework and approach demonstrated here is valuable in attributing trends in streamflow and groundwater levels to catchment climatic and anthropogenic changes. 相似文献
102.
The effect of raindrops striking water surface and their role in modifying the prevailing sea-surface roughness is investigated. The work presents a new theoretical formulation developed to study rain-induced stress on sea-surface based on dimensional analysis. Rain parameters include drop size, rain intensity and rain duration. The influences of these rain parameters on young and mature waves were studied separately under varying wind speeds, rain intensity and rain duration. Contrary to popular belief that rain only attenuates surface waves, this study also points out rain duration under certain condition can contribute to wave growth at high wind speeds. Strong winds in conjunction with high rain intensity enhance the horizontal stress component on the sea-surface, leading to wave growth. Previous studies based on laboratory experiments and dimensional analysis do not account for rain duration when attempting to parameterize sea-surface roughness. This study signifies the importance of rain duration as an important parameter modifying sea-surface roughness. Qualitative as well quantitative support for the developed formulation is established through critical validation with reports of several researchers and satellite measurements for an extreme cyclonic event in the Indian Ocean. Based on skill assessment, it is suggested that the present formulation is superior to prior studies. Numerical experiments and validation performed by incorporating in state-of-art WAM wave model show the importance of treating rain-induced surface roughness as an essential pre-requisite for ocean wave modeling studies. 相似文献
103.
The present study pertains to the development of a foundation model for predicting the behavior of geosynthetic reinforcement railway track system rested on soft clay subgrade. The ballast and sub‐ballast layers have been idealized by Pasternak shear layer. The geosynthetic layer is represented by a stretched rough elastic membrane. Burger model has been used to characterize the soft clay subgrade. Numerical solutions have been obtained by adopting the finite difference scheme combined with non‐dimensioning the governing equations of the proposed model. The results confirm that the present model is quite capable of predicting the time‐dependent settlement response of geosynthetic reinforcement railway track system placed on soft clay subgrade. The surface settlement profile and mobilized tensile load of geosynthetics has been evaluated by considering variation in the wheel load, sleeper width, thickness of ballast and sub‐ballast layers and shear modulus of ballast and sub‐ballast layers. It has been observed that an increase in the sleeper width by 24% results in the reduction in central settlement and mobilized tensile load by 6.5% and 20.1%, respectively. It was found that with a 50% increase in the thickness of the ballast layer, the central settlement has decreased by 7.3% and the mobilized tension at the zone of maximum curvature has increased by 24.6%. However, with an increase in the thickness of the sub‐ballast layer, a considerable reduction in both central settlement and the mobilization of tension on geosynthetic has been noticed. The pattern of variation of settlement and mobilized tension for an increase in the shear modulus of ballast and sub‐ballast material was found to be almost similar. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
104.
<正>1.Introduction The continental crust,covering nearly a third of the Earth's surface,is dominantly made up of granites and granodiorites(Rudnick and Gao, 2003).Although the vast majority of these granitoids are amphiboleand /or biotite-bearing,orthopyroxene-bearing granitoids form a minor but important component of the lower continental crust in many high-grade terrains(e.g.,Bohlender et al.,1992;Kilpatrick and Ellis,1992;Sheraton et al.,1992;Berger et al.,1995;Zhou et al., 1995;Peucat et al.,1996;Duchesne and Wilmart,1997;Hughes et al.,1997;Prame.1997;Frost et al.,2000;Janasi,2002;Mendes 相似文献
105.
Geosynthetic is widely used to reinforce the weak rock mass, mine waste dump, soil slopes road cut slopes, etc. The present
paper discusses the effect of geogrids on the stability of mine waste dump. The stability of mine waste dump has been done
by Fast Langrage Analysis of Continua (FLAC) slope software, which is based on finite difference method. Reinforcement by
geogrids mainly depends on the tensile strength, aperture size of geogrids, and particle size distribution of dump rock mass.
Different permutations and combinations of spacing between two geogrid sheets have been taken into consideration to study
the stability of mine waste dump. The factor of safety is calculated to quantify the effect of geogrids on waste dump slope.
It has been observed from numerical modeling that the maximum slope angle is 45° at a height of 10 m. The scope of increasing
slope angle from 45 to 60° is evaluated using geogrids. It has been found from the study that the factor of safety increases
as the spacing between geogrids decreases. Maximum strain is also plotted of each case to identify the slip circle. The positions
of geogrids modify the probable slip circle or failure plane of mine waste dump. Using ten geogrids at a spacing of 1 m, the
slope angle can be increased up to 60° with factor of safety of 1.4. 相似文献
106.
Influence of hydrogeochemical processes on temporal changes in groundwater quality in a part of Nalgonda district, Andhra Pradesh, India 总被引:4,自引:3,他引:1
Geochemical processes that take place in the aquifer have played a major role in spatial and temporal variations of groundwater
quality. This study was carried out with an objective of identifying the hydrogeochemical processes that controls the groundwater
quality in a weathered hard rock aquifer in a part of Nalgonda district, Andhra Pradesh, India. Groundwater samples were collected
from 45 wells once every 2 months from March 2008 to September 2009. Chemical parameters of groundwater such as groundwater
level, EC and pH were measured insitu. The major ion concentrations such as Ca2+, Mg2+, Na+, K+, Cl−, and SO4
2− were analyzed using ion chromatograph. CO3
− and HCO3
− concentration was determined by acid–base titration. The abundance of major cation concentration in groundwater is as Na+ > Ca2+ > Mg2+ > K+ while that of anions is HCO3
− > SO4
2− > Cl− > CO3
−. Ca–HCO3, Na–Cl, Ca–Na–HCO3 and Ca–Mg–Cl are the dominant groundwater types in this area. Relation between temporal variation in groundwater level and
saturation index of minerals reveals the evaporation process. The ion-exchange process controls the concentration of ions
such as calcium, magnesium and sodium. The ionic ratio of Ca/Mg explains the contribution of calcite and dolomite to groundwater.
In general, the geochemical processes and temporal variation of groundwater in this area are influenced by evaporation processes,
ion exchange and dissolution of minerals. 相似文献
107.
We determine the degree of variation of model fitness, to a true model based on amplitude variation with angle (AVA) methodology
for a synthetic gas hydrate model, using co-operative fuzzy c-means clustering, constrained to a rock physics model. When
a homogeneous starting model is used, with only traditional least squares optimization scheme for inversion, the variance
of the parameters is found to be comparatively high. In this co-operative methodology, the output from the least squares inversion
is fed as an input to the fuzzy scheme. Tests with co-operative inversion using fuzzy c-means with damped least squares technique
and constraints derived from empirical relationship based on rock properties model show improved stability, model fitness
and variance for all the three parameters in comparison with the standard inversion alone. 相似文献
108.
109.
Siby Kurian Rajesh Agnihotri D. V. Borole S. Wajih A. Naqvi Ana M. Ferreira Carlos Vale 《第四纪科学杂志》2009,24(2):109-116
Using multiple geochemical proxies including specific biomarkers (dinosterol, phytol, stigmasterol and β‐sitosterol) measured in a high‐sedimentation rate core collected from the inner shelf (depth ~45 m) off Goa (India), we reconstruct surface productivity, which is mainly controlled by the monsoon upwelling in this region, during the last ca. 700 a. Surface productivity appears to have varied in tandem with the Konkan–Goa rainfall and sunspot activity during the instrumental period (last 250 a). The productivity proxies also covary with the total solar irradiance reconstructed for the period beyond the instrumental era, within the considerable uncertainty of the age model. This suggests that solar forcing may control coastal upwelling intensity and biological productivity in the eastern Arabian Sea on decadal to centennial timescales. During the late Anthropocene (last ca. 50 a), steep increases in all four biomarkers indicate greatly enhanced productivity in response to high solar irradiance as well as anthropogenic inputs of nutrients. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
110.
Extremely low frequency (ELF)/Very low frequency (VLF) hiss is whistler mode wave that interacts with energetic electrons
in the magnetosphere. The characteristics features of ELF/VLF hiss observed at low latitude ground station Jammu (Geomag.
lat. 22°16′ N, L=1.17) are reported. It is observed that most of hiss events first propagate in ducted mode along higher L-values (L = 4–5), after reaching lower edge of ionosphere excite the Earth-ionosphere waveguide and propagate towards equator to be
received at low-latitude station Jammu. To understand the generation mechanism of ELF/VLF hiss, incoherent Cerenkov radiated
power from the low-latitude and mid-latitude plasmasphere are evaluated. Considering this estimated power as an input for
wave amplification through wave–particle interaction, the growth rate and amplification factor is evaluated which is too small
to explain the observed wave intensity. It is suggested that some non-linear mechanism is responsible for the generation of
ELF/VLF hiss. 相似文献