全文获取类型
收费全文 | 312篇 |
免费 | 25篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 13篇 |
大气科学 | 30篇 |
地球物理 | 60篇 |
地质学 | 161篇 |
海洋学 | 16篇 |
天文学 | 45篇 |
综合类 | 10篇 |
自然地理 | 14篇 |
出版年
2023年 | 4篇 |
2022年 | 6篇 |
2021年 | 11篇 |
2020年 | 9篇 |
2019年 | 6篇 |
2018年 | 37篇 |
2017年 | 29篇 |
2016年 | 36篇 |
2015年 | 15篇 |
2014年 | 17篇 |
2013年 | 26篇 |
2012年 | 13篇 |
2011年 | 12篇 |
2010年 | 11篇 |
2009年 | 13篇 |
2008年 | 7篇 |
2007年 | 10篇 |
2006年 | 7篇 |
2005年 | 8篇 |
2004年 | 8篇 |
2003年 | 5篇 |
2002年 | 1篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 3篇 |
排序方式: 共有349条查询结果,搜索用时 281 毫秒
31.
A multivariate statistical strategy employing cluster analysis, discriminant analysis, and ordination was used to classify and interpret depositional environments of Barataria Bay, Louisiana, from grain-size data tabulated by Krumbein and Aberdeen (1937). Weight-percent whole-phi variables for 69 samples were tested for redundancy using R-mode cluster analysis. These samples were partitioned into five environmentally significant facies using Q-mode cluster analysis: (1) beach-ridge sand; (2) shallow wave-zone sand; (3) channel sand and silty sand; (4) channel-margin silty sand; and (5) low-energy sandy silt and clayey silt. Then the classification was extended with discriminant analysis to the remaining 29 samples which exhibited incipient flocculation and whose constituents did not total unity. Environments of deposition were associated with each facies following the method of Visher (1969) and using gradient analysis on a two-dimensional Q-mode ordination. Wilk's lambda and discriminant analysis tested the statistical significance of these facies; they were further tested by comparing their distribution with field relations and known environmental processes. 相似文献
32.
N. Hussain N. Bhandari K. R. Ramanathan B. L. K. Somayajulu 《Journal of Earth System Science》1980,89(1):23-29
The miliolite deposits of Saurashtra have been dated by234U,230Th,231Pa and14C methods. Concordant ages of 105 years using the U decay series isotopes are obtained which agree with the ages of the coral reefs of Okha-Dwaraka coast suggesting
a contemporaneous origin for both. The lower14C ages (≤40,000 years) may be due to a recent influx of seawater or ground water. Quartz and clay minerals together constitute
only ≤10% by weight, as such the aeolian characteristics of quartz grains may not be relevant to the origin of the miliolites. 相似文献
33.
Lead levels in the indicator mussel Mytilus edulis, exceeds the WHO food standards in 19 of the 22 locations sampled in Port Phillip Bay, Melbourne. While this seriously reflects contamination of the food chain, LD50 experiments on Mytilus indicate that the lethal effect of lead on this organism is much less severe than excessive doses of mercury, cadmium, copper or zinc. Little is known about the long term effects of lead on Mytilus. 相似文献
34.
刘群 《中国海洋湖沼学报》2015,33(4):966-973
Catch and effort data were analyzed to estimate the maximum sustainable yield(MSY) of King Soldier Bream, Argyrops spinifer(Forssk?l, 1775, Family: Sparidae), and to evaluate the present status of the fish stocks exploited in Pakistani waters. The catch and effort data for the 25-years period 1985–2009 were analyzed using two computer software packages, CEDA(catch and effort data analysis) and ASPIC(a surplus production model incorporating covariates). The maximum catch of 3 458 t was observed in 1988 and the minimum catch of 1 324 t in 2005, while the average annual catch of A. spinifer over the 25 years was 2 500 t. The surplus production models of Fox, Schaefer, and Pella Tomlinson under three error assumptions of normal, log-normal and gamma are in the CEDA package and the two surplus models of Fox and logistic are in the ASPIC package. In CEDA, the MSY was estimated by applying the initial proportion(IP) of 0.8, because the starting catch was approximately 80% of the maximum catch. Except for gamma, because gamma showed maximization failures, the estimated results of MSY using CEDA with the Fox surplus production model and two error assumptions, were 1 692.08 t(R 2 =0.572) and 1 694.09 t( R 2 =0.606), respectively, and from the Schaefer and the Pella Tomlinson models with two error assumptions were 2 390.95 t( R 2 =0.563), and 2 380.06 t( R 2 =0.605), respectively. The MSY estimated by the Fox model was conservatively compared to the Schaefer and Pella Tomlinson models. The MSY values from Schaefer and Pella Tomlinson models were the same. The computed values of MSY using the ASPIC computer software program with the two surplus production models of Fox and logistic were 1 498 t(R 2 =0.917), and 2 488 t( R 2 =0.897) respectively. The estimated values of MSY using CEDA were about 1 700–2 400 t and the values from ASPIC were 1 500–2 500 t. The estimates output by the CEDA and the ASPIC packages indicate that the stock is overfished, and needs some effective management to reduce the fishing effort of the species in Pakistani waters. 相似文献
35.
Macrobenthic faunal composition was studied at six different depth ranges (30–50, 51–75, 76–100, 101–150, 151–175 and >176 m) in five transects (off Karaikkal, Parangipettai, Cuddalore‐SIPCOT, Cheyyur and Chennai) in the continental shelf of southeast coast of India. Eleven diverse taxa were found, comprising 113 species of polychaetes, 14 species of bivalves, 10 species of amphipods and ‘others’ (five tanaids, five crabs, four isopods, three echinoderms, two shrimps, two cnidarians, two fishes and one cephalochordate). Polychaetes were the dominant taxa, constituting 88.5% of the total abundance and 30.7% of the total biomass. The number of species (seven per 0.2 m2 at >176 m depth range in Chennai to 46 per 0.2 m2 at 30–50 m in Cheyyur), abundance (216 per 0.2 m2 at >176 m in Karaikkal to 353 per 0.2 m2 at 30–50 m in Cheyyur) and biomass (0.09 g per 0.2 m2 at 151–175 m in Karaikkal and 4.6 g per 0.2 m2 at 30–50 m in Cheyyur) of macrobenthos decreased with increase in depth. DO decreased gradually from 30 m depth; beyond 150 m, the decrease was pronounced due to the presence of the oxygen minimum zone. Using the distance based linear model (DISTLM), it was found that the environmental variables explained about 73.3% of the total variability in macrofaunal distribution. The heavy metals cobalt and mercury, as well as water pressure (proxy for depth), showed a significant relationship with macrofauna, explaining respectively 9, 7.3 and 7% of the total variability. The contribution of other variables was smaller. 相似文献
36.
37.
38.
Hydrochemistry and Classification of Groundwater Resources of Ishwardi Municipal Area,Pabna District,Bangladesh 总被引:2,自引:0,他引:2
Golzar Hossain M. Farhad Howladar Lutfun Nessa Syed Samsuddin Ahmed Chowdhury Quamruzzaman 《Geotechnical and Geological Engineering》2010,28(5):671-679
The chemical property of groundwater depends largely on the mineralogical composition of the rocks through which the water
has moved and the rate of movement and these characteristics of surface water depend on organic and inorganic reactions, industrial
effluents, rainfall and temperature etc. The underground water tends to contain more dissolved materials than those in surface
water because of their more intimate and longer contact with organic materials of soil and rock particles. The groundwater
of the studied area is dominant of alkaline earth’s (Ca2+ and Mg2+) and weak acids (HCO3
−) which may be classified as Magnesium-Bicarbonate and Calcium Carbonate types. Genetically, the groundwater of the area belongs
to both “Normal Chloride” “Normal Sulphate” and “Normal Carbonate” to “Super Carbonate” group. Based on EC, SAR and RC, the
groundwater of the area varies from good–excellent quality for irrigation purposes with low alkali hazard and medium salinity
hazard. 相似文献
39.
Groundwater samples (n = 163) were collected across Kashmir Valley in 2010 to assess the hydrogeochemistry of the groundwater in shallow and deep aquifers and its suitability for domestic, agriculture, horticulture, and livestock purposes. The groundwater is generally alkaline in nature. The electrical conductivity (EC) which is an index to represent the total concentration of soluble salts in water was used to measure the salinity hazard to crops as it reflects the TDS in groundwater ranging from 97 to 1385 μS/cm, except one well in Sopore. The average concentration of major ions was higher in shallow aquifers than in deeper aquifers. In general, Ca2+ is the dominant cation and HCO \(_{3}^{-}\) the dominant anion. Ca–HCO3, Mg–HCO3, Ca–Mg–HCO3, Na–HCO3 were the dominant hydrogeochemical facies. High concentration of HCO3 and pH less than 8.8 clearly indicated that intense chemical weathering processes have taken place in the study area. The groundwater flow pattern in the area follows the local surface topography which not only modifies the hydrogeochemical facies but also controls their distribution. The groundwater in valley flows into four directions, i.e., SW–NE, NE–W, SE-NW and SE–NE directions. The results suggest that carbonate dissolution is the dominant source of major ions followed by silicate weathering and ion-exchange processes. The concentrations of all the major ions determined in the present study are within the permissible limits of WHO and BIS standards. The results of Total Hardness, SAR, Na%, Kelly Index, USDA classification, Magnesium absorption ratio, residual sodium carbonate, and PI suggested that groundwater is good for drinking, livestock, and irrigation purposes. 相似文献
40.
Multi-factor impact analysis of agricultural production in Bangladesh with climate change 总被引:1,自引:0,他引:1
Alex C. Ruane David C. Major Winston H. Yu Mozaharul Alam Sk. Ghulam Hussain Abu Saleh Khan Ahmadul Hassan Bhuiya Md. Tamim Al Hossain Richard Goldberg Radley M. Horton Cynthia Rosenzweig 《Global Environmental Change》2013,23(1):338-350
Diverse vulnerabilities of Bangladesh's agricultural sector in 16 sub-regions are assessed using experiments designed to investigate climate impact factors in isolation and in combination. Climate information from a suite of global climate models (GCMs) is used to drive models assessing the agricultural impact of changes in temperature, precipitation, carbon dioxide concentrations, river floods, and sea level rise for the 2040–2069 period in comparison to a historical baseline. Using the multi-factor impacts analysis framework developed in Yu et al. (2010), this study provides new sub-regional vulnerability analyses and quantifies key uncertainties in climate and production. Rice (aman, boro, and aus seasons) and wheat production are simulated in each sub-region using the biophysical Crop Environment REsource Synthesis (CERES) models. These simulations are then combined with the MIKE BASIN hydrologic model for river floods in the Ganges-Brahmaputra-Meghna (GBM) Basins, and the MIKE21 Two-Dimensional Estuary Model to determine coastal inundation under conditions of higher mean sea level. The impacts of each factor depend on GCM configurations, emissions pathways, sub-regions, and particular seasons and crops. Temperature increases generally reduce production across all scenarios. Precipitation changes can have either a positive or a negative impact, with a high degree of uncertainty across GCMs. Carbon dioxide impacts on crop production are positive and depend on the emissions pathway. Increasing river flood areas reduce production in affected sub-regions. Precipitation uncertainties from different GCMs and emissions scenarios are reduced when integrated across the large GBM Basins’ hydrology. Agriculture in Southern Bangladesh is severely affected by sea level rise even when cyclonic surges are not fully considered, with impacts increasing under the higher emissions scenario. 相似文献