全文获取类型
收费全文 | 118527篇 |
免费 | 1633篇 |
国内免费 | 990篇 |
专业分类
测绘学 | 2853篇 |
大气科学 | 8074篇 |
地球物理 | 22862篇 |
地质学 | 42375篇 |
海洋学 | 10741篇 |
天文学 | 27374篇 |
综合类 | 354篇 |
自然地理 | 6517篇 |
出版年
2022年 | 770篇 |
2021年 | 1317篇 |
2020年 | 1401篇 |
2019年 | 1570篇 |
2018年 | 3333篇 |
2017年 | 3107篇 |
2016年 | 3742篇 |
2015年 | 1974篇 |
2014年 | 3621篇 |
2013年 | 6208篇 |
2012年 | 3848篇 |
2011年 | 5032篇 |
2010年 | 4521篇 |
2009年 | 5776篇 |
2008年 | 5062篇 |
2007年 | 5130篇 |
2006年 | 4750篇 |
2005年 | 3492篇 |
2004年 | 3477篇 |
2003年 | 3298篇 |
2002年 | 3211篇 |
2001年 | 2791篇 |
2000年 | 2688篇 |
1999年 | 2174篇 |
1998年 | 2255篇 |
1997年 | 2061篇 |
1996年 | 1823篇 |
1995年 | 1785篇 |
1994年 | 1533篇 |
1993年 | 1423篇 |
1992年 | 1369篇 |
1991年 | 1395篇 |
1990年 | 1388篇 |
1989年 | 1173篇 |
1988年 | 1124篇 |
1987年 | 1273篇 |
1986年 | 1152篇 |
1985年 | 1411篇 |
1984年 | 1584篇 |
1983年 | 1470篇 |
1982年 | 1377篇 |
1981年 | 1275篇 |
1980年 | 1166篇 |
1979年 | 1133篇 |
1978年 | 1079篇 |
1977年 | 915篇 |
1976年 | 892篇 |
1975年 | 882篇 |
1974年 | 844篇 |
1973年 | 913篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A. G. Lyne R. S. Pritchard F. Graham-Smith 《Monthly notices of the Royal Astronomical Society》2001,321(1):67-70
Between 1997 August and October, the radio pulses from the Crab pulsar were followed by discrete moving echoes, which appear to be reflections from part of an ionized shell in the outer part of the Crab Nebula, crossing the line of sight to pulsar. Similar events have now been recognized in recordings from the past 30 yr, and it seems that the Nebula must contain a large number of ionized shell-like surfaces on a much finer scale than recognized hitherto. 相似文献
992.
A. G. Mavraganis 《Celestial Mechanics and Dynamical Astronomy》1991,51(4):395-405
The almost constant-speed motion of a mass acted upon by a Newtonian attraction and a resisting force is treated. The equation of orbit is derived for a specific type of resistance which covers the familiar case of Danby's drag(=r
–2) whilst the vector invariants are obtained by direct operation on the vector form of the equation of motion. 相似文献
993.
Aicheng ZHANG Yunbin GUAN Weibiao HSU Yang LIU Lawrence A. TAYLOR 《Meteoritics & planetary science》2010,45(2):238-245
Abstract– A metamorphosed lithic clast was discovered in the CM chondrite Grove Mountains 021536, which was collected in the Antarctica by the Chinese Antarctic Research Exploration team. The lithic clast is composed mainly of Fe‐rich olivine (Fo62) with minor diopside (Fs9.7–11.1Wo48.3–51.6), plagioclase (An43–46.5), nepheline, merrillite, Al‐rich chromite (21.8 wt% Al2O3; 4.43 wt% TiO2), and pentlandite. Δ17O values of olivine in the lithic clast vary from ?3.9‰ to ?0.8‰. Mineral compositions and oxygen isotopic compositions of olivine suggest that the lithic clast has an exotic source different from the CM chondrite parent body. The clast could be derived from strong thermal metamorphism of pre‐existing chondrule that has experienced low‐temperature anhydrous alteration. The lithic clast is similar in mineral assemblage and chemistry to a few clasts observed in oxidized CV3 chondrites (Mokoia and Yamato‐86009) and might have been derived from the interior of the primitive CV asteroid. The apparent lack of hydration in the lithic clast indicates that the clast accreted into the CM chondrite after hydration of the CM components. 相似文献
994.
Hydrologic precursors to earthquakes: A review 总被引:4,自引:0,他引:4
Evelyn A. Roeloffs 《Pure and Applied Geophysics》1988,126(2-4):177-209
This review summarizes reports of anomalous flow rates or pressures of groundwater, oil, or gas that have been interpreted as earthquake precursors. Both increases and decreases of pressure and flow rate have been observed, at distances up to several hundred kilometers from the earthquake epicenter, with precursor times ranging from less than one day to more than one year. Although information that might rule out nontectonic causes does not appear in many published accounts of hydrologic anomalies, several recent studies have critically evaluated the possible influences of barometric pressure, rainfall, and groundwater or oil exploitation. Anomalies preceding the 1976 Tangshan, China, and the 1978 Izu-Oshima-Kinkai, Japan, earthquakes are especially well-documented and worthy of further examination.Among hydrologic precursors, pressure head changes in confined subsurface reservoirs are those most amenable to quantitative interpretation in terms of crustal strain. The response of pressure head to earth tides determines coefficients of proportionality between pressure head and crustal strain. The same coefficients of proportionality should govern the fluid pressure response to any crustal strain field in which fluid flow in the reservoir is unimportant. Water level changes in response to independently recorded tectonic events, such as earthquakes and aseismic fault creep, provide evidence that a calibration based on response to earth tides may be applied to crustal strains of tectonic origin.Several models of earthquake generation predict accelerating stable slip on part of the future rupture plane. If precursory slip has moment less than or equal to that of the impending earthquake, then the coseismic volume strain is an upper bound for precursory volume strain. Although crustal strain can be only crudely estimated from most reported pressure head anomalies, the sizes of many anomalies within 150 kilometers of earthquake epicenters appear consistent with this upper bound. In contrast, water level anomalies at greater epicentral distances appear to be larger than this bound by several orders of magnitude.It is clear that water level monitoring can yield information about the earthquake generation process, but progress higes on better documentation of the data. 相似文献
995.
Vega-Jorquera P. Lazzús J. A. Tamblay L. Palma-Chilla L. Salfate I. Pacheco R. 《Geomagnetism and Aeronomy》2021,61(2):287-292
Geomagnetism and Aeronomy - The total solar eclipse on July 2, 2019, has been studied by using data of the La Serena ground magnetic station in Chile located in the path of the totality (at... 相似文献
996.
Man's engineering activities are concentrated on the uppermost part of the earth's crust which is called engineering-geologic zone. This zone is characterized by a significant spatialtemporal variation of the physical properties status of rocks, and saturating waters. This variation determines the specificity of geophysical and, particularly, geoelectrical investigations. Planning of geoelectric investigations in the engineering-geologic zone and their subsequent interpretation requires a priori) geologic-geophysical information on the main peculiarities of the engineering-geologic and hydrogeologic conditions in the region under investigation. This information serves as a basis for the creation of an initial geoelectric model of the section. Following field investigations the model is used in interpretation. Formalization of this a priori) model can be achieved by the solution of direct geoelectric problems. An additional geologic-geophysical information realized in the model of the medium allows to diminish the effect of the “principle of equivalence” by introducing flexible limitations in the section's parameters. Further geophysical observations as well as the correlations between geophysical and engineering-geologic parameters of the section permit the following step in the specification of the geolectric model and its approximation to the real medium. Next correction of this model is made upon accumulation of additional information. The solution of inverse problems with the utilization of computer programs permits specification of the model in the general iterational cycle of interpretation. 相似文献
997.
Water Resources - The basic indicators of regional water use have been identified and assessed as a component of a single social–natural–technogenic system. These indicators have been... 相似文献
998.
Pyroclastic density currents (PDC) related to paroxysmal eruptions have caused a large number of casualties in the recent history of Stromboli. We combine here a critical review of historical chronicles with detailed stratigraphic, textural, and petrographic analyses of PDC deposits emplaced at Stromboli over the last century to unravel the origin of currents, their flow mechanism and the depositional dynamics. We focus on the 1930 PDC as they are well described in historical accounts and because the 1930 eruption stands as the most voluminous and destructive paroxysm of the last 13 centuries. Stromboli PDC deposits are recognizable from their architecture and the great abundance of fresh, well-preserved juvenile material. General deposit features indicate that Stromboli PDC formed due to the syn-eruptive gravitational collapse of hot pyroclasts rapidly accumulated over steep slopes. Flow channelization within the several small valleys cut on the flanks of the volcano can enhance the mobility of PDC, as well as the production of fine particles by abrasion and comminution of hot juvenile fragments, thereby increasing the degree of fluidization. Textural analyses and historical accounts also indicate that PDC can be fast (15–20 m/s) and relatively hot (360–700 °C). PDC can thus flow right down the slopes of the volcano, representing a major hazard. For this reason, they must be adequately taken into account when compiling risk maps and evaluating volcanic hazard on the Island of Stromboli. 相似文献
999.
Chung-Lin SHIE Long S. CHIU Robert ADLER Eric NELKIN I-I LIN Pingping XIE Feng-Chin WANG R. CHOKNGAMWONG William OLSON D. Allen CHU 《大气科学进展》2009,26(6):1071-1080
Accurate sea surface flux measurements are crucial for
understanding the global water and energy cycles. The oceanic
evaporation, which is a major component of the global oceanic fresh
water flux, is useful for predicting oceanic circulation and
transport. The global Goddard Satellite-based Surface Turbulent
Fluxes Version-2 (GSSTF2; July 1987--December 2000) dateset that was
officially released in 2001 has been widely used by scientific
community for global energy and water cycle research, and regional
and short period data analyses. We have recently been funded by NASA
to resume processing the GSSTF dataset with an objective of
continually producing a uniform dataset of sea surface turbulent
fluxes, derived from remote sensing data. The dataset is to be
reprocessed and brought up-to-date (GSSTF2b) using improved input
datasets such as a recently upgraded NCEP/DOE sea surface
temperature reanalysis, and an upgraded surface wind and microwave
brightness temperature V6 dataset (Version 6) from the Special
Sensor Microwave Imager (SSM/I) produced by Remote Sensing Systems
(RSS). A second new product (GSSTF3) is further proposed with a
finer temporal (12-h) and spatial (0.25ox0.25o)
resolution. GSSTF2b (July 1987--December 2008) and GSSTF3 (July
1999--December 2009) will be released for the research community to
use by late 2009 and early 2011, respectively. 相似文献
1000.
P. A. Lozovik A. K. Morozov M. B. Zobkov T. A. Dukhovicheva L. A. Osipova 《Water Resources》2007,34(2):204-216
A relationship between indirect chemical indices of organic matter content of surface waters and light absorption in the visible and ultraviolet ranges is discussed. The allochthonous and autochthonous types of organic matter are found to essentially differ in the rate of light absorption. Techniques are proposed for the calculation of the concentrations of allochthonous and autochthonous organic matter by the integral rate of light consumption in the visible region and the bichromate oxidability of water, as well as by three indirect indices: chemical oxygen demand, permanganate oxidability, and water color index. The mean concentrations of allochthonous and autochthonous organic matter in large lakes (Ladoga, Onega, and Baikal) and in rivers and smaller water bodies in Karelia (>300 water objects) are analyzed. Allochthonous organic matter was found to predominate in most surface waters of Karelia (>80%), while allochthonous organic matter predominates only in lakes with a small specific catchment area (supposedly, <5) and in highly eutrophic lakes. 相似文献