全文获取类型
收费全文 | 400篇 |
免费 | 6篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 27篇 |
大气科学 | 22篇 |
地球物理 | 63篇 |
地质学 | 198篇 |
海洋学 | 13篇 |
天文学 | 73篇 |
综合类 | 2篇 |
自然地理 | 11篇 |
出版年
2022年 | 7篇 |
2021年 | 8篇 |
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 21篇 |
2017年 | 25篇 |
2016年 | 19篇 |
2015年 | 10篇 |
2014年 | 18篇 |
2013年 | 34篇 |
2012年 | 20篇 |
2011年 | 27篇 |
2010年 | 27篇 |
2009年 | 19篇 |
2008年 | 12篇 |
2007年 | 12篇 |
2006年 | 12篇 |
2005年 | 12篇 |
2004年 | 6篇 |
2003年 | 8篇 |
2002年 | 7篇 |
2001年 | 8篇 |
2000年 | 4篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1967年 | 1篇 |
1966年 | 2篇 |
1965年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有409条查询结果,搜索用时 15 毫秒
231.
Evidence of lacustrine sedimentation in the Upper Permian Bijori Formation, Satpura Gondwana basin: Palaeogeographic and tectonic implications 总被引:1,自引:0,他引:1
The Upper Permian Bijori Formation of the Satpura Gondwana basin comprising fineto coarse-grained sandstone, carbonaceous
shale/mudstone and thin coal bands was previously interpreted as the deposits of meandering rivers. The present study documents
abundance of wave ripples, hummocky and swaley cross-stratification and combined flow bedforms in the Bijori Formation, suggesting
that a significant part of the formation was deposited in a wave-agitated environment. Evidence of near-emergent depositional
conditions provided by repeated occurrence of rootlet beds and hydromorphic paleosols, local flooding surfaces denoting rapid
fluctuation of water level, occurrences of temnospondyl vertebrate fossils, and absence of tidal signatures and marine fossils
suggest a lacustrine rather than marine depositional regime. Five facies associations recognised within the Bijori Formation
are inferred to represent fluvial channels and associated floodplains (FA1), lake shorelines (FA2), subaqueous distributary
channels and associated levees (FA3), waveand storm-affected delta front (FA4), and open lacustrine/lower shoreface (FA5)
deposits. The planoconcave fluvial channel-fill sandbodies with unidirectional cross-beds are clearly distinguishable from
the delta front bars that show a convexo-plan or bi-convex sandbody geometry and dominance of wave and combined flow bedforms.
Some of the distributary channels record interaction of fluvial and wave-dominated basinal processes. Major distributary sandbodies
show a north to northwest flow direction while wave-affected delta front sandbodies show very complex flow patterns reflecting
interaction between fluvial discharge and wave processes. Wave ripple crest trends show that the lake shoreline had an overall
east-northeast to west-southwest orientation. The lack of documented contemporaneous lacustrine or marine sediments in the
Satpura Gondwana basin posed a major problem of basin-scale palaeogeographic reconstruction. The existence of Bijori lake
solves the problem and the lake is inferred to have acted as repository for the contemporaneous alluvial drainage. Development
of the large Bijori lake body implies generation of accommodation space exceeding the rate of sediment supplied and thus represents
locus of high tectonic subsidence. Transition of fluvial sediments with red mudstone and calcareous soil profile in the lower
part of the succession to carbonaceous shale and coal-bearing lacustrine sediments in the upper part, denote a change from
a warm semi-arid climate with seasonal rainfall to a more humid one. 相似文献
232.
The statistical behaviour of the sudden enhancement in signal strength (SES) in relation to solar X-ray flares has been studied for the near east-west propagation of 40 kHz radio waves from Sanwa (36°11’N; 139°51’E) in Japan to Calcutta (22°34’N; 88°24’E) over a long distance path of 5100 km for a period of two years. The period has been divided into four phases - P1, P2, P3 and P4, according to the position of the overhead sun. The change in signal strength during X-ray flares is dependent on the solar zenith angle and climatic conditions. The statistical modal values of the time lag of the SES peak with respect to that solar X-ray flare is found to increase as solar zenith angle increases. The relative rates of increase and decrease of the signal strength (RRISS and RRDSS respectively) have been evaluated for a number of SES which are related to large X-ray flares. Their characteristics have also been investigated. The modal values of the relaxation time have been found to be highly correlated with climatic conditions like temperature and humidity of the propagation path. 相似文献
233.
The problem of selecting the optimum operating frequency of a scatterometer, used for remote sensing of sea surface wind speed
has been addressed by applying the criteria of maximum sensitivity of backscattering coefficient to wind speed as well as
its correlation with wind speed. The backscattering coefficient values for sea surface were computed by the two-scale scattering
theory. To compute the atmospheric transmittance, 753 clear sky atmospheres over Indian Ocean were used. While the correlation
coefficient was uniform (0.94) throughout the frequency range of 1 to 30GHz, only frequencies above 5GHz were found sensitive
enough to yield a wind speed accuracy of ±2msec−1 and better, the accuracy improving with frequency. 相似文献
234.
The paper deals with the relationship between the sea surface wind speed (U) and the backscattering coefficient (σ0) for microwave frequencies and quasi specular range of angles. Valenzuela’s effective reflection coefficient based on the theory of scattering of electromagnetic waves from rought dielectric surfaces has been introduced in the expression of backscattering coefficient and the final result compared with the σ0-U relationship derived fromSEASAT scatterometer and concurrent sea truth data. 相似文献
235.
236.
Applying the method of ‘statistical linear regression’, atomspheric water vapour over oceanic areas has been estimated from
the 19GHz and 22 GHz data of the satellite microwave radiometer (SAMIR) system onboard the Bhaskara II satellite. In the absence of any simultaneousin situ measurements on water vapour over ocean, theSAMIR-derived water vapour data have been compared with like data from theNOAA-7 satellite. It is suggested that a positive bias seen in theSAMIR data could be due to calibration errors in the basic data. In view of the observed bias, the original regression equation
is modified and then used to obtain water vapour distributions over ocean for winter and south-west monsoon seasons usingSAMIR data of several orbits. 相似文献
237.
Results of investigations of local earthquakes in the region of the Main Central Thrust (MCT) in the Kumaon Himalaya, between and adjacent to the valleys of the Bhagirathi and Yamuna rivers, are presented. Records of over 250 earthquakes were analysed and the following facts emerged:
- 1. (1) Earthquakes in the Himalayas occur in specific areas and belts. One such belt has been identified in the region under investigation, hypocentral estimates being more reliable for earthquakes occurring in the middle segment approximately 70 km long of this belt, crossing the Yamuna river between the villages of Barkot and Syanachatti.
- 2. (2) All but a few epicentres in this middle segment, lie to the southwest of the surface trace of the MCT in a zone with a width of 10–30 km.
- 3. (3) Most of the earthquakes in this segment occur at depths of less than 10 km below the ground surface, the maximum estimated depth being 32 km.
238.
239.
Landslides are one of the most destructive phenomena of nature that cause damage to both property and life every year, and therefore, landslide susceptibility zonation (LSZ) is necessary for planning future developmental activities. In this paper, apart from conventional weighting system, objective weight assignment procedures based on techniques such as artificial neural network (ANN), fuzzy set theory and combined neural and fuzzy set theory have been assessed for preparation of LSZ maps in a part of the Darjeeling Himalayas. Relevant thematic layers pertaining to the causative factors have been generated using remote sensing data, field surveys and Geographic Information System (GIS) tools. In conventional weighting system, weights and ratings to the causative factors and their categories are assigned based on the experience and knowledge of experts about the subject and the study area to prepare the LSZ map (designated here as Map I). In the context of objective weight assignments, initially the ANN as the black box approach has been used to directly produce an LSZ map (Map II). In this approach, however, the weights assigned are hidden to the analyst. Next, the fuzzy set theory has then been implemented to determine the membership values for each category of the thematic layer using the cosine amplitude method (similarity method). These memberships are used as ratings for each category of the thematic layer. Assuming weights of each thematic layer as one (or constant), these ratings of the categories are used for the generation of another LSZ map (Map III). Subsequently, a novel weight assignment procedure based on ANN is implemented to assign the weights to each thematic layer objectively. Finally, weights of each thematic layer are combined with fuzzy set derived ratings to produce another LSZ map (Map IV). The maps I–IV have been evaluated statistically based on field data of existing landslides. Amongst all the procedures, the LSZ map based on combined neural and fuzzy weighting (i.e., Map IV) has been found to be significantly better than others, as in this case only 2.3% of the total area is found to be categorized as very high susceptibility zone and contains 30.1% of the existing landslide area. 相似文献
240.
Bhabesh C. Sarkar Bashab N. Mahanta Kalyan Saikia Pradip R. Paul Gurdeep Singh 《Environmental Geology》2007,51(7):1177-1196
A study on geo-environmental quality assessment in Jharia coalfield, India, has been attempted using multivariate statistical
analysis and geographic information system (GIS) modelling techniques. Water quality index, calculated for each sample network
station in the study area to assess the suitability of water for human consumption, revealed very poor to poor quality surface
water and mine water. Air quality indexing indicated that there is no sample station with clean air as per the Indian standards,
which indicate the hazardous air quality. Multi-criteria evaluation (MCE), a potential GIS tool, has been applied to the delineation
of various degrees of stressed villages in terms of quality of life (QoL). The role of various geo-environmental parameters
such as quality of groundwater, surface water, mine water and air together with village population densities has been emphasized
for delineation of the environmentally stressed villages in Jharia coalfield. The integrated cluster analysis and MCE approach
provide an improved means to geo-environmental quality assessment in Jharia coalfield in terms of QoL. The assessment study
is aimed to be used for future coal mining, ensuring ecologically sustainable industrial development, particularly in a coalfield. 相似文献