全文获取类型
收费全文 | 859篇 |
免费 | 21篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 63篇 |
地球物理 | 211篇 |
地质学 | 324篇 |
海洋学 | 57篇 |
天文学 | 114篇 |
综合类 | 3篇 |
自然地理 | 72篇 |
出版年
2020年 | 10篇 |
2018年 | 16篇 |
2016年 | 15篇 |
2015年 | 8篇 |
2014年 | 22篇 |
2013年 | 29篇 |
2012年 | 34篇 |
2011年 | 27篇 |
2010年 | 19篇 |
2009年 | 40篇 |
2008年 | 36篇 |
2007年 | 27篇 |
2006年 | 34篇 |
2005年 | 26篇 |
2004年 | 28篇 |
2003年 | 16篇 |
2002年 | 29篇 |
2001年 | 19篇 |
2000年 | 8篇 |
1999年 | 14篇 |
1998年 | 15篇 |
1997年 | 8篇 |
1996年 | 14篇 |
1995年 | 9篇 |
1994年 | 8篇 |
1992年 | 13篇 |
1991年 | 12篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 9篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1972年 | 11篇 |
1970年 | 8篇 |
1968年 | 8篇 |
1955年 | 7篇 |
1954年 | 9篇 |
1950年 | 8篇 |
1948年 | 8篇 |
排序方式: 共有884条查询结果,搜索用时 343 毫秒
91.
92.
93.
Two ocean profiles from the Peru Basin from regions with different surface productivities were analyzed for total210Pb and201Po to evaluate the influence of particulates in the water column on their distribution. Comparison with a published226Ra profile for the region was made. The profile closest to the coast, where upwelling and productivity are high, shows depletion of210Pb relative to226Ra at all depths, with particularly marked excursions from radioactive equilibrium at the surface and in the bottom water.210Po appears to be deficient relative to210Pb at depth as well. Mean residence times in the deep water, relative to particulate removal from the water column to the sediments, of about 100 years for210Pb and about two years for210Po are indicated. The profile northwest of the upwelling region shows the226Ra210Pb210Po system close to equilibrium at all depths to 1500 m (except for the effect of atmospheric210Pb input seen at the surface. 相似文献
94.
Bruno Ribbat Wolfgang Roether Karl Otto Münnich 《Earth and Planetary Science Letters》1976,32(2):331-341
A14C balance for the Eastern Caribbean deep water indicates the average inflow of Atlantic water into the basin to be 2.3 × 105 m3/sec (±30%), or about 2–4 times the values estimated previously. The balance uses a model representation of the deep-water turnover, and is based on14C concentrations at a station in the Venezuelan Basin which average Δ14C= 89‰ below 800 m depth with a total range of only 9‰, as well as on a14C concentration of the Atlantic inflow of Δ14C= ?71%. as obtained from measurements outside the Antilles Arch. The turnover time of the basin water below 2500 m depth is 55 years, which corresponds to an average upwelling velocity at this depth of about 35 m/year. With such upwelling, the temperature profile below 1800 m (the depth of the sill determining the inflow of new water) requires a vertical eddy diffusivity of about 5 cm2/sec. The oxygen consumption, and silica and CO2 regeneration, rates below 2500 m depth are obtained as ?0.18, + 0.08, and + 0.2 μmole kg?1 yr?1, respectively. The CO2 regeneration has but a negligible effect on the14C balance. 相似文献
95.
Fourteen whole-rock samples from three traverses through the contact aureole of the Stillwater Complex were analyzed for Rb, Sr and87Sr/86Sr. Twelve of these samples yielded an age of 2750 ± 45m.y.; (87Sr/86Sr)0 = 0.705 ± 0.003 (2σ). In addition one whole rock and a biotite separate derived from it gave an age of 2544 m.y.; (87Sr/86Sr)0 = 0.714. These data support a minimum age of 2750 m.y. for the intrusion of the complex. 相似文献
96.
Very Long Baseline Interferometry (VLBI) is one of the new techniques which will probably dominate geodesy and geophysics
in the near future. Its main advantage lies in the fact that it brings the accuracy of direction measurements to a level previously
possible only for range measurements. This closes the gap between powerful range determination techniques such as laser ranging
and the much less accurate determination of directions through photographic tracking of artificial earth satellites.
The technique is geometric in the sense that the relevant observations are independent of the gravity field of the earth.
However, the “orbits” of the observed extragalactic radio sources with respect to an earth-fixed system are dominated and
perturbed by the rotation of the earth with respect to inertial frame. This allows the determination of polar motion, precession-nutation
and length-of-the-day variations, and the technique becomes also “dynamic” in this respect.
The capability of determining the geometry of a network of stations within a short time interval and with a centimeter level
accuracy also allows the study of the variation of network geometry with time caused by earth tides and other periodic or
secular station drifts.
The main objective of the present work is the exploration of the capabilities of VLBI for the recovery of earth rotation and
network geometry parameters. For this purpose, a number of characteristic experimental designs based on present and candidate
for the near future station locations is chosen. The results from the analysis of simulated observations for each particular
design are presented in the paper.
Presented at IAG International Symposium on “Optimization of Design and Computation of Control Networks”, Sopron, Hungary,
4–10 July 1977. 相似文献
97.
The spaceborne laser ranging (or lasering) system provides a method of precise positioning of a large number of points on
the earth's surface in a short period of time. That is, a measure of the relative location of geodetic markers from a space
platform can maintain horizontal and vertical control to 2 to 5 cm. At this level of control, small earth surface crustal
motions should be detectable. Development of a model for the strain field can be constructed. Furthermore, the spaceborne
lasering system can survey an area in a very short period of time (one to two weeks) and resurvey the area as required.
System design parameters are now being established by NASA for a possible test flight aboard the Shuttle in 1982. These include
design specifications of economical corner cubes for ground retroreflectors coupled with the evolution of engineering model
to flight model development. If the experiment of the Shuttle proves to be successful, it is hoped to put the laser in a free
flight satellite. This paper presents the results of a simulated analysis for this latter case.
The system is conceived as an orbiting ranging device with a ground base grid of reflectors or transponders (spacing 10 to
30 km), which are projected to be of low cost (maintenance-free and unattended) and which will permit the saturation of a
local area to obtain data useful to monitor crustal movements. The test network includes 75 stations with roughly half of
them on either side of the San Andreas fault zone. Critical study comparatively evaluates various observational schemes and
statistically analyzed crustal motion recovery.
The study considers laser radar as the main ranging system pending final selection from many possible candidates. The satellite
orbit is inclined at 110° and slightly eccentric (e=0.04) with orbital altitudes varying from 370 km to 930 km.
The results indicate that the geometric mode (simultaneous ranging) with a minimum of five grid and three distant (fundamental)
stations and mixed ranging to satellite and airplane seems to be most promising. The fundamental stations are distinguished
from the grid station in their location and this location should be “distant” enough from the area of crustal movement so
that they can be considered stationary over the time span of the motion involved.
Presented at the 1977 I.A.G. International Symposia on Satellite Geodesy, Budapest, Hungary, June 28–July 1, and on Recent
Crustal Movements, Palo Alto, California, USA, July 25–30. 相似文献
98.
Dr. Christian Sonntag Prof. Dr. Eberhard Klitzsch Prof. Dr. E. M. El Shazly Christel Kalinke Prof. Dr. Karl Otto Münnich 《International Journal of Earth Sciences》1978,67(2):413-423
Zusammenfassung In den14C-Altern der Saharawässer spiegelt sich die wechselnde Abfolge von Feucht- und Trockenphasen während des späten Pleistozäns und Holozäns wider. Außerdem zeigt sich ein West-Ost-Gefälle in ihrem D- und18O-Gehalt, das als Kontinentaleffekt im Grundwasser gedeutet wird. Dieser Kontinentaleffekt setzt voraus, daß das nordafrikanische Klimageschehen vergangener Feuchtphasen von der Westdrift bestimmt wurde, die regenbringende atlantische Luftmassen weit ins Saharainnere getrieben hat. Ein einfaches mathematisches Modell, das den Kontinentaleffekt durch eine Rayleigh-Kondensation beschreibt, ergibt die beste Anpassung an die Meßwerte, wenn für die Sahara (ähnlich wie heute in Europa) ein West-Ost-Gefälle in den Paläo-Winterniederschlagsmengen von –30% pro 1000 km angenommen wird. Unter der Annahme ehemaliger Winterniederschläge (Sept.-März) von 600 mm (entspricht Lissabon heute) für das Gebiet an der nordafrikanischen Atlantikküste ergibt diese Modellrechnung z. B. 250 mm für die Paläo-Winterniederschläge im Gebiet des Murzuk-Beckens.
The14C-age distribution of Saharian groundwaters shows the alternating sequence of humid and arid periods in the late Pleistocene and Holocene. The observed west-east decrease of the D- and18O-content of the Sahara waters is interpreted to be the Continental Effect in rain and groundwater. The paleoclimatic situation during ground-water formation therefore was controlled by the western drift carrying wet Atlantic air masses across the Sahara. The best fit of a simple Rayleigh condensation model of the continental effect to the experimental data is obtained with a west-east decrease of the Saharian paleo-winter precipitation of –30% per 1000 km (similar to the one in Europe today). With an annual winter precipitation rate (Sept.-March) of 600 mm (corresponding to the one of Lisboa today) for the Northafrican coastal area at the Atlantic Ocean a previous winter precipitation rate of 250 mm is obtained for the Murzuq Basin.
Résumé Une succession de périodes humides et arides à la fin du Pléistocène et à l'Holocène se réflète dans la distribution des âges14C d'eaux souterraines du Sahara. Les teneurs en D et18O diminuent d'Ouest en Est, ce qui est interprété comme un effet continental dans l'eau souterraine. Celui-ci signifie que la climatologie des phases pluviales passées, dans l'Afrique du Nord, était déterminée par des vents d'Ouest qui apportaient des masses d'air atlantique humide à l'intérieur du Sahara. Un modèle mathématique simple décrivant l'effet continental par une condensation du type Rayleigh, reproduit avec la meilleure conformité les valeurs expérimentales, pour autant qu'on admette que les pluies hivernales au Sahara diminuaient anciennement d'Ouest en Est de 30% par 1000 km (comme à présent en Europe). En supposant qu'autrefois la quantité de pluie hivernale, de septembre à mars à la côte atlantique de l'Afrique du Nord était de 600 mm (comme aujourd'hui à Lisbonne), on obtient à l'aide de ce modèle une précipitation hivernale ancienne de 250 mm dans le bassin du Mourzouk.
, 14C, . , 18O, , . , , , , . , Rayleigh, — , — — 30 % 1000 . , - , . . , 600 , — —, 250 .相似文献
99.
Karl F Ehrlich 《Marine pollution bulletin》1977,8(10):228-229
The hatching success of California grunion (Leuresthes tenuis) was significantly reduced by a non-thermal component of the effluent from an electricity generating station. The embryos appeared to develop normally morphologically but showed reduced viability with respect to hatching. 相似文献
100.
Disposal of radioactive waste in the sea floor of fracture zones associated with the flanks of the Mid-Atlantic Ridge may
be a satisfactory alternative to land disposal. Effective physiographic, sedimentary, chemical, and oceanographic barriers
exist in these aseismic deep canyons, especially in the eastern Atlantic. In addition, the major producers of radioactive
wastes are likely to be near the Atlantic Ocean. If such a disposal strategy is adopted, intensive study of the sedimentologic
and oceanographic properties of oceanic fracture zones will be necessary. 相似文献