全文获取类型
收费全文 | 7267篇 |
免费 | 1371篇 |
国内免费 | 38篇 |
专业分类
测绘学 | 188篇 |
大气科学 | 347篇 |
地球物理 | 3608篇 |
地质学 | 2603篇 |
海洋学 | 429篇 |
天文学 | 1059篇 |
综合类 | 25篇 |
自然地理 | 417篇 |
出版年
2022年 | 34篇 |
2021年 | 125篇 |
2020年 | 146篇 |
2019年 | 255篇 |
2018年 | 376篇 |
2017年 | 445篇 |
2016年 | 579篇 |
2015年 | 524篇 |
2014年 | 587篇 |
2013年 | 691篇 |
2012年 | 515篇 |
2011年 | 508篇 |
2010年 | 441篇 |
2009年 | 362篇 |
2008年 | 379篇 |
2007年 | 291篇 |
2006年 | 244篇 |
2005年 | 207篇 |
2004年 | 209篇 |
2003年 | 186篇 |
2002年 | 161篇 |
2001年 | 164篇 |
2000年 | 131篇 |
1999年 | 64篇 |
1998年 | 71篇 |
1997年 | 48篇 |
1996年 | 30篇 |
1995年 | 28篇 |
1994年 | 40篇 |
1993年 | 32篇 |
1992年 | 41篇 |
1991年 | 35篇 |
1990年 | 50篇 |
1989年 | 37篇 |
1988年 | 23篇 |
1987年 | 27篇 |
1986年 | 30篇 |
1984年 | 29篇 |
1983年 | 23篇 |
1982年 | 27篇 |
1981年 | 25篇 |
1980年 | 24篇 |
1979年 | 26篇 |
1978年 | 24篇 |
1977年 | 25篇 |
1976年 | 25篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1973年 | 29篇 |
1971年 | 36篇 |
排序方式: 共有8676条查询结果,搜索用时 15 毫秒
101.
L. G. Medaris Jr. E. D. Ghent H. F. Wang J. H. Fournelle E. Jelínek 《Mineralogy and Petrology》2006,86(3-4):203-220
Summary In the Kutná Hora Complex, the Běstvina Formation, which is similar to Gf?hl granulite, contains eclogite that has escaped
widespread retrograde recrystallization. The eclogite assemblage, garnet + omphacite + quartz + rutile ± plagioclase, yields
an estimate for peak metamorphic conditions of 18–20 kbar and 835–935 °C, which is comparable to that determined from felsic
granulite, 14–20 kbar and 900–1000 °C. Garnet in eclogite exhibits both prograde and retrograde compositional zoning, from
which constraints on thermal history of the Gf?hl terrane can be derived by diffusion modelling. At 900 °C, a garnet grain
of 800–1000 μm radius would homogenize in 7.5–11.7 million years, but the existence of compositional gradients on a length
scale of 100–200 μm suggests that the duration of peak metamorphism may have been limited to ∼500,000 years. Diffusion modelling
of retrograde zoning in garnet yields a cooling rate of 150–100 °C/m.y. for a radius of 800–1000 μm and initial temperature
of 900 °C. The relatively brief duration of high-pressure/high-temperature metamorphism and rapid cooling and exhumation of
the Gf?hl terrane may be a consequence of lithospheric delamination during Early Carboniferous collision of Bohemia (Teplá-Barrandia)
and Moldanubia (Franke, 2000). 相似文献
102.
The U.S. and U.K. literatures have discussed “food deserts,” reflecting populated, typically urban, low‐income areas with limited access to full‐service supermarkets. Less is known about supermarket accessibility within Canadian cities. This article uses the minimum distance and coverage methods to determine supermarket accessibility within the city of Edmonton, Canada, with a focus on high‐need and inner‐city neighborhoods. The results show that for 1999 both of these areas generally had higher accessibility than the remainder of the city, but six high‐need neighborhoods had poor supermarket accessibility. We conclude by examining potential reasons for differences in supermarket accessibility between Canadian, U.S., and U.K. cities. 相似文献
103.
The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt 总被引:4,自引:0,他引:4
J. Sánchez España E. López Pamo E. Santofimia Pastor J. Reyes Andrés J. A. Martín Rubí 《Aquatic Geochemistry》2006,12(3):269-298
This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD)
waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters
were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO4=) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously
recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and
precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5
(stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates
of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially
sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO4=
anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous
free cations (Pb2+, Zn2+, Cu2+, Cd2+, Mn2+, Co2+, Ni2+) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely
scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10−1) of the SO 4= ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH
< 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co,
Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB. 相似文献
104.
A debris flow originating from the Alcamayo River on 10th April 2004 destroyed a part of the town of Aguas Calientes, resulting
in 11 victims, and with serious affects to the tourist flow to the Machupicchu inka citadel. On the same day, as well as in
January and March 2004, other similar phenomena occurred on the Cedrobamba and Leonchayoq Rivers, affecting the railway and
an electrical tower, and disrupting the train service. 相似文献
105.
Skier-triggered avalanches are the main cause of avalanche accidents in backcountry skiing. The risk of accidents during backcountry
skiing was analysed statistically and related to factors such as elevation level, aspect, stability rating and the time of
the year. The analysis is based on a database about terrain usage and avalanche accidents from a large heli-skiing operator
in Canada, which makes it possible to study the conditional probability of accidents given the recorded pattern of terrain
usage. This study shows that the historical risk of accidentally triggering an avalanche greater than size 1 depends highly
on the stability rating, with the highest risk occurring during “poor” stability. The risk is greater at high elevations,
and it is lower during the late season than earlier on. Skier risk does not depend as much on aspect as may be indicated from
avalanche data alone. However, it is relatively high in the N–NE–E sector. These factors are not independent of each other
and therefore analyses of combined factors were also performed. Questionnaires and interviews were used to gain knowledge
about the terrain selection of professional mountain guides. These results indicate that when selecting terrain, guides first
look at the overall shape and size of the terrain, but avalanche history of terrain and inclination are also important factors.
Finally, remarks in avalanche reports were analysed, and common human factors identified. 相似文献
106.
S-S. Xu A. F. Nieto-Samaniego S. A. Alaniz-Álvarez L. G. Velasquillo-Martínez 《International Journal of Earth Sciences》2006,95(5):841-853
The power-law exponent (n) in the equation: D=cL
n
, with D = maximum displacement and L = fault length, would be affected by deviations of fault trace length. (1) Assuming n=1, numerical simulations on the effect of sampling and linkage on fault length and length–displacement relationship are done in this paper. The results show that: (a) uniform relative deviations, which means all faults within a dataset have the same relative deviation, do not affect the value of n; (b) deviations of the fault length due to unresolved fault tip decrease the values of n and the deviations of n increase with the increasing length deviations; (c) fault linkage and observed dimensions either increase or decrease the value of n depending on the distribution of deviations within a dataset; (d) mixed deviations of the fault lengths are either negative or positive and cause the values of n to either decrease or increase; (e) a dataset combined from two or more datasets with different values of c and orders of magnitude also cause the values of n to deviate. (2) Data including 19 datasets and spanning more than eight orders of fault length magnitudes (10−2–105 m) collected from the published literature indicate that the values of n range from 0.55 to 1.5, the average value being 1.0813, and the peak value of n
d (double regression) is 1.0–1.1. Based on above results from the simulations and published data, we propose that the relationship between the maximum displacement and fault length in a single tectonic environment with uniform mechanical properties is linear, and the value of n deviated from 1 is mainly caused by the sampling and linkage effects. 相似文献
107.
The analysis of the time and space distribution of specular (reflecting) points in bistatic altimetry between GPS and CHAMP
satellites or SAC-C (taken as examples) is extended from Wagner and Klokočník (2003 J. Geod 77: 128–138). We demonstrate a
significantly higher number and density of reflecting points in bistatic altimetry in comparison with traditional monostatic
altimetry. After an outline of our older accuracy assessment for the vertical position of the reflecting point, we add a new
independent derivation and compare both approaches. We account for orbit errors of both the transmitters (GPS) and receiver
(CHAMP) satellites, and the measurement (delay) error. We found that the accuracy of the vertical position of the reflecting
point decreases only slowly with increasing off-nadir angle and that the orbit errors must be accounted for if decimeter and
better accuracy is required. In this paper, we do not study errors such as state of the ocean, technical parameters of the
receiving system, and atmospheric corrections. 相似文献
108.
Mariano Barriendos Javier Martín-Vide Juan Carlos Peña Roberto Rodríguez 《Climatic change》2002,53(1-3):151-170
Meteorological observations in the city of Cádiz are acknowledged as having been made from the middle of the 18th century onwards although they were only recorded and preserved in documentary form from 1789 onwards. Data readings were taken at the new Naval Observatory in San Fernando, ten kilometres from Cádiz, from 1797 onwards. Continuous series for temperature and atmospheric pressure at a daily resolution have been compiled and constructed from 1817 until 1996. The data series is composed of thrice daily observations made at Cádiz (1821–1880) by local observers and hourly data from Naval Observatory at San Fernando (1870–1996). 相似文献
109.
This work presents a novel neural network‐based approach to detect structural damage. The proposed approach comprises two steps. The first step, system identification, involves using neural system identification networks (NSINs) to identify the undamaged and damaged states of a structural system. The partial derivatives of the outputs with respect to the inputs of the NSIN, which identifies the system in a certain undamaged or damaged state, have a negligible variation with different system errors. This loosely defined unique property enables these partial derivatives to quantitatively indicate system damage from the model parameters. The second step, structural damage detection, involves using the neural damage detection network (NDDN) to detect the location and extent of the structural damage. The input to the NDDN is taken as the aforementioned partial derivatives of NSIN, and the output of the NDDN identifies the damage level for each member in the structure. Moreover, SDOF and MDOF examples are presented to demonstrate the feasibility of using the proposed method for damage detection of linear structures. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
110.
An analytical model for high damping elastomeric isolation bearings is presented in this paper. The model is used to describe mathematically the damping force and restoring force of the rubber material and bearing. Ten parameters to be identified from cyclic loading tests are included in the model. The sensitivity of the ten parameters in affecting the model is examined. These ten parameters are functions of a number of influence factors on the elastomer such as the rubber compound, Mullins effect, scragging effect, frequency, temperature and axial load. In this study, however, only the Mullins effect, scragging effect, frequency and temperature are investigated. Both material tests and shaking table tests were performed to validate the proposed model. Based on the comparison between the experimental and the analytical results, it is found that the proposed analytical model is capable of predicting the shear force–displacement hysteresis very accurately for both rubber material and bearing under cyclic loading reversals. The seismic response time histories of the bearing can also be captured, using the proposed analytical model, with a practically acceptable precision. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献